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GEOLOGIC HAZARDS OF VIRGINIA 1 2 Author contacts 3 4 L. Scott Eaton, Department of Geology and Environmental Science, James Madison 5 University, Harrisonburg, VA 22807, [email protected] 6 7 R. Craig Kochel, Department of Geology, Bucknell University, Lewisburg, PA 17837, 8 [email protected] 9 10 David A. Hubbard, Jr., Virginia Department of Mines, Minerals and Energy’s Division of 11 Mineral Resources, 900 Natural Resources Dr., Suite 500, Charlottesville, VA 22903, 12 [email protected] 13 14 Fiorella V. Simoni de Cannon, Department of Chemistry and Biochemistry, George 15 Mason University, Fairfax, VA 22030, [email protected] 16 17 Douglas G. Mose, Department of Chemistry and Biochemistry, George Mason 18 University, Fairfax, VA 22030, [email protected] 19 20 21 22
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INTRODUCTION: AN OVERVIEW OF GEOLOGIC 24
HAZARDS OF VIRGINIA 25
L. Scott Eaton 26
In the United States, natural hazards are annually responsible for hundreds of deaths, 27
disruption of commerce, destruction of homes and infrastructure, and billions of dollars 28
in disaster aid (USGS, 2007). The U.S. Geological Survey includes earthquakes, floods, 29
hurricanes, landslides, tsunamis, volcanoes, and wildfires as natural hazards. Although 30
the number of lives lost to natural hazards each year generally has declined, the economic 31
cost of major disaster response and recovery continues to rise. Each decade, property 32
damage from natural hazards events doubles or triples. The United States is second only 33
to Japan in economic damages resulting from natural disasters (USGS, 2007). 34
Geologic hazards, a subset of natural hazards, are defined as a geologic condition or 35
phenomenon, natural or brought about by human activity, that represents a threat to 36
human life, welfare, and property (Neuendorf et al., 2005). In the Commonwealth, the 37
threat of natural hazards continues to increase as more rural counties become 38
suburbanized. In 2005, three of the top ten fastest growing counties in the United States 39
were in Virginia (Christie, 2006). Loudoun County’s population increased nearly 60% 40
from 2000 to 2006, making it the fourth fastest-growing county in the United States 41
(Francis, 2007). Additionally, sparsely populated regions of Virginia are not immune to 42
geologic hazards. The remnants of Hurricane Camille in 1969 resulted in landslides and 43
flooding that killed over 150 people in Nelson County, equating to approximately 1% of 44
the county population. Other geologic hazards, although not as energetic, pose risk1
1 Throughout the section on radon, the term risk is used to mean the possibility of unwanted, adverse consequences to human life, health, property, or the environment. (Source: The Society for Risk Analysis, www.sra.org.)
to 45
humans and property. For example, many rural counties in western Virginia have 46
extremely high radon gas concentrations, and counties in karst regions can be at risk of 47
sinkhole collapse and groundwater contamination. Regardless of the population density, 48
recognizing these potential hazards in the Commonwealth, and disseminating the 49
information to its citizens is of utmost importance. 50
3
The Virginia Department of Emergency Management lists geologic hazards within the 51
Commonwealth as including landslides, flooding, subsidence from sinkholes and mining, 52
radon, shoreline erosion, earthquakes, and soil movement. This chapter focuses on the 53
first four of the listed topics; the topics of shoreline erosion, earthquakes, and subsidence 54
from mining are addressed in other chapters of this volume. Soil movement, including the 55
processes of frost heave and expandable soils, are notably present in Virginia, and are 56
discussed in the Soils of Virginia chapter of this volume. 57
58
LANDSLIDE AND FLOODING HAZARDS IN VIRGINIA 59
L. Scott Eaton and R. Craig Kochel 60
INTRODUCTION 61
Landslides and flooding events pose a threat to both property and life throughout the 62
United States. Annually, landslides cause approximately $3.5 billion in damage (valued 63
in year 2001 dollars), and kill between 25 and 50 people annually, primarily from rock 64
falls, rock slides, and debris flows (USGS, 2006). Flooding is even more costly in the 65
loss of life and property, where over $6 billion in property damage and 140 deaths occur 66
annually. Virginia experienced 13 flood-related federally declared disasters between 1996 67
and 2005, during which 12 hurricanes tracked across the state, including Fran, Bonnie, 68
Floyd, Isabel, Jeanne, and Dennis (VDEM, 2007). Surprisingly to some, Virginia has 69
experienced several of the most catastrophic geomorphic flooding events in the history of 70
the United States. Two floods in recent memory that were notorious for their intensity 71
and destructiveness are the Hurricane Camille event in Nelson County in 1969, and the 72
Rapidan flood in Madison County in 1995 (Fig. 1). In Nelson County, nearly 1% of the 73
County’s population perished in this deluge; 113 confirmed dead with 39 missing, and 74
damages amounting to more than $1.4 billion. In Madison County, the loss of life was 75
limited to one fatality; and destruction of property reached $110 million. Geologically, 76
these storms are remembered for accomplishing over a thousand year’s worth of erosion 77
in a single day, and for altering mountain front landscapes to a degree rivaling anything 78
observed in the nation since European settlement (Eaton et al., 2003a,b). Rainfall in both 79
of these events reached approximately 760 mm (30 in) in a day, and the resulting peak 80
4
discharges of streams rank among the largest in the nation, based on the USA maximum 81
flood envelope (Fig. 2). Virginia’s geographic position as a common collision zone 82
between extratropical and tropical air masses; and a wide distribution of orographic-83
triggering mechanisms on the slopes of the Appalachians (Michaels, 1985) combine to 84
give Virginia one of the most dramatic hydroclimatic flood-producing terrains in the 85
eastern USA. 86
Flooding has the potential to occur across Virginia. Notably, slow moving coastal 87
storms, such as Northeasters, can cause widespread disturbances to seaside communities, 88
as evident by the Ash Wednesday storm of 1962 (e.g., Zhang et al., 2002). Chincoteague 89
Island experienced the brunt of this storm, where a combination of northeasterly winds 90
from a stalled low pressure system and a tidal surge further enhanced from spring tide 91
conditions, impacted the island community. This event still remains the largest flood to 92
impact Chincoteague Island in recent memory. Coastal flooding is important, and is 93
addressed in the chapter on Coastal Processes and Offshore Geology in this volume. 94
Occasionally, large geographic areas of Virginia are inundated by storms of both 95
tropical and extratropical origins, including Hurricane Agnes in 1972 and Hurricane Fran 96
in 1996. These storms were most remembered for their impact on the lowlands, where 97
many communities were flooded by steadily rising water from mainstem rivers and large 98
tributaries. Generally, these large storms bring periods of steady precipitation over 99
several days, and usually allows citizens to seek shelter as flood waters are predicted to 100
rise. In contrast, the scenario that continues to be of increasing concern is storms that are 101
less predictable, intense, of long duration, and have the ability to strike rapidly in 102
mountainous terrain and trigger landslides. While the authors do not want to minimize the 103
importance of flooding in the lowlands or along the coastline, this section examines the 104
real and largely unrecognized hazards created by the combination of 1) heavy, prolonged 105
rainfall; 2) steep mountainous topography of the central Appalachians; and 3) the 106
encroachment of human development onto these landslide hazard areas. Specifically, this 107
section of the paper examines both the geologic and practical considerations of 108
catastrophic flooding and landslides, and how they affect human activity in the 109
Commonwealth of Virginia. 110
111
5
STORM-GENERATED DEPOSITS AND LANDFORMS IN VIRGINIA 112
The term landslide is a general expression for the downslope movement of rock, soil, 113
or artificial fill under the influence of gravity. Specifically, mass movements can be 114
broadly categorized as slides, flows, or heaves (Carson and Kirkby, 1972). In slides, 115
cohesive blocks of rock or soil move along a well-defined failure surface, and minimal 116
internal disruption of the material takes place while in motion. In comparison, flows 117
move entirely by differential shearing within the material, and no clear failure plane can 118
be defined at the base of the moving mass (Ritter et al., 2002). Debris flows are a subset 119
of this class, and includes a complex group of gravity-induced rapid mass movements 120
that contain a variety of grain sizes from boulders to clay; and have sediment 121
concentrations that range from 70 to 90% by weight (Costa, 1988) (Fig. 3). 122
In Virginia, geomorphic processes responsible for transporting sediment from steep, 123
mountainous terrain to alluvial fans and valley bottoms include water floods (streamflows), 124
debris flows, and hyperconcentrated flows, the latter defined as streamflows carrying 40 to 70% 125
of sediment by weight (Costa, 1988) (Fig. 3). All types of flow processes have been documented 126
in Virginia, but there appears to be some regional geologic factors that promote dominance of 127
one process over the other, and the resulting alluvial fan types. In general, the fans along the 128
western slopes of the Blue Ridge (i.e., eastern margins of the Shenandoah Valley) tend to be 129
dominated by streamflow processes; whereas those of the interior and eastern Blue Ridge, along 130
the flanks of Massanutten Mountain, and the western Appalachian front tend to be dominated by 131
debris flows (Fig. 4). These distinctions are based on stratigraphy and morphology of the 132
landforms. The formative processes and resulting landforms appear to be both a function of the 133
watershed hydrology, basin lithology, and the lateral accommodation space for the deposits. 134
The western flank of the Blue Ridge is bordered by extensive sand and gravel of fluvial origin 135
that form a nearly continuous apron, or bajada, between the Blue Ridge and the low, hilly ground 136
stretching to the Shenandoah River or its tributaries (Fig. 4). Gravel deposits were described by 137
King (l950) in the Elkton vicinity, and more broadly by Hack (l965). More detailed studies have 138
been undertaken by Kochel and Johnson (1984), Kochel (1987, 1990, 1992), Duffy (1991), Kite 139
(1992), Whittecar and Duffy (1992, 2000), Morgan et al. (2003), Wieczorek et al. (2006); and by 140
thesis studies of Bell (1986), Wilson (1987), Simmons (1988) and Mason (1992). Together these 141
studies have demonstrated that an extensive plexus of alluvial fan deposits extend with gentle 142
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slopes of usually less than 6° from the mountain front to the Shenandoah River (Fig. 4). The 143
sharp demarcation of bedrock resistance that exists between the siliciclastic-based mountains and 144
the carbonate lowlands allows these fans to grow unrestricted, in the lateral sense, into the 145
Shenandoah Valley. These fans are comprised of imbricated, well-sorted quartz arentite gravels 146
and sands, display a broad fan shape in plan view, and range in area from approximately 2 to 10 147
km2 (Simmons, 1988) (Fig. 5). The aerial extent of the fans is proportional to the drainage basin 148
area, similar to the fans in the southwestern Unites States (Mills et al., 1987). Typically, fan 149
thickness is greatest in the mid-fan region, but overall fan thickness varies depending upon 150
vertical accommodation space from the dissolution of underlying carbonates, that is strongly 151
influenced by the dip of the carbonate bedrock underlying the fans (Simmons, 1988). The older 152
fans have collapsed into the karst so that accumulations of alluvial deposits commonly reach 30 153
m (100 ft) (King, 1950); and drill records reveal that they can be as much as 180 m (600 ft) thick 154
in places (Simmons, 1988). Some debris flow sediments are common in the proximal areas of 155
these fans, but most of the fans are formed by streamflows and hyperconcentrated flows. 156
Watersheds feeding these fans are typically larger than those directly producing debris flows 157
(e.g., basins draining the eastern flank of the central Blue Ridge (Kochel, 1990)), and thereby 158
able to dilute sediment yields with enough water volume to retard debris flow transport. Flash 159
floods of significant magnitude are historically common on these fans, as exemplified in the 160
floods produced by the remnants of Hurricanes Juan (1985), Fran (1996), and, most recently, 161
Isabel in 2003 (Fig. 6). 162
In contrast, debris fans have drainage basins that are usually smaller and steeper than fans 163
formed by streamflow (Fig. 7). Fan shapes are often irregular (Kochel 1987, 1990) because of 164
their restricted lateral accommodation space, having formed within rocks of low solubility in the 165
high-relief mountain regions of the Blue Ridge and the western Appalachian front. In the Blue 166
Ridge, debris fan slopes average 5-17°, and are composed of poorly-sorted sediments that range 167
in size from boulders of several meters in diameter to clay (Eaton et al., 2003a). These deposits 168
are relatively thin (several meters thick), and possess both matrix-supported and clast-supported 169
units (Fig. 7). Stratification of these units is usually lacking, but the boundary between individual 170
deposits is typically sharp. Paleosols are partially preserved at some contacts between these 171
units, indicating a period of quiescence of debris flow activity sufficiently long enough to create 172
soil profiles. Lower magnitude floods are usually incapable of remobilizing the largest of the 173
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material, leaving it to weather in-situ or to be remobilized by the next event of similar or greater 174
magnitude. 175
Fans and associated landforms in montane and mountain-front areas appear to be little 176
impacted by frequent low magnitude storm events. Rather, these landforms which include 177
boulder berms, boulder bars, boulder levees, and fan sediments are only altered by extreme high 178
magnitude events of low frequency like those mentioned previously in this paper. In these steep 179
environments, thousands of years of geomorphic work occurs episodically during a single event. 180
Measurements from the 1969 and 1995 events suggest that the long-term transport of sediment 181
from the mountains to the lowland floodplains is episodic; that is nearly half of the sediment that 182
would normally be expected be transported gradually over a few thousand years is moved by a 183
single event (Eaton et al., 2003b). Episodic, high magnitude events appear to be the dominant 184
agents of landscape change and geomorphic work in mountain regions (e.g., Hack and Goodlett, 185
1960; Wolman and Miller, 1960; Williams and Guy, 1973; Kochel, 1987; Jacobson et al., 1989; 186
Miller, 1990; Eaton et al., 2003b). 187
188
RECENT HISTORY OF COUPLED FLOODING AND MAJOR LANDSLIDE 189
ACTIVITY 190
Numerous storms that produced torrential rain and associated flooding have struck the 191
Commonwealth. Four storm events between 1949 through 1995 were intense enough to produce 192
significant numbers of landslides, primarily debris flows, which modified the landscape, 193
damaged property, and in most cases took lives. Two of the storms impacted the central Virginia 194
Blue Ridge, whereas the other two affected the Valley and Ridge province along the West 195
Virginia–Virginia border near the central Shenandoah Valley. What is noteworthy about all four 196
of these storms in both the Valley and Ridge and Blue Ridge provinces is that catastrophic 197
flooding was generated by a variety of extreme weather conditions, rather than by exclusively 198
hurricanes. The most infamous of these was Hurricane Camille. In the late evening of August 19, 199
1969, the remnants of Hurricane Camille crossed the Blue Ridge from the west, collided with a 200
southeastward-advancing cold front, and stalled in the rugged foothills of Nelson County, 201
Virginia. As much as 711 mm (28.0 in) of rain fell over a 7–8 hour period during the early 202
morning hours of August 20 (Camp and Miller, 1970), although one unofficial reading of nearly 203
1020 mm (40.2 in) of rainfall was made at a single locality (Simpson and Simpson, 1970). The 204
8
deluge triggered thousands of debris flows and killed more than 150 people, and still ranks as 205
Virginia’s most costly natural disaster (Fig. 8). 206
Not all of these events, however, were hurricane derived. In contrast, the June 27, 1995 storm 207
centered over the Rapidan River basin in Madison County developed from the combination of a 208
stalled cold front and westward-flowing, moisture-laden air moving toward the eastern slopes of 209
the Blue Ridge Mountains (Smith et al., 1996). Maximum rainfall totals for the storm system 210
reached 775 mm (30.5 in) during a 16 hour period (Wieczorek et al., 2000). The deluge triggered 211
more than 1000 debris flows, and flooding in the region was catastrophic (Fig. 9). Major 212
flooding from this event also affected the North Fork of the Moormans River in western 213
Albemarle County, located 45 km southwest of the Rapidan Basin. The rainfall exceeded 279 214
mm (11.0 in) (Morgan and Wieczorek, 1996), but may have been as great as 63 mm (25 in) 215
(Carlton Frazier, 1996, pers. comm.). Nearly 100 debris flows were documented in the basin. 216
This same storm also impacted a third area near Buena Vista, where over a dozen debris flows 217
were mobilized and entered the Maury River (Sas and Eaton, 2006a). Unfortunately, no rainfall 218
estimates exist for this cell of the storm. Both the 1969 and 1995 storms rank near the edge of the 219
USA maxima flood envelope (Fig. 2), illustrating the extreme flash flood index potential (i.e.; 220
Beard, 1975) of the Blue Ridge Province (Fig. 2). 221
West of the Blue Ridge, catastrophic storms struck the Valley and Ridge province along the 222
West Virginia–Virginia border in 1949 and 1985 (Fig. 1). In both storms, nearly all of the 223
fatalities were from flooding in the lowlands rather than from debris flow impacts. The June 224
1949 storm was the result of convective storm cells limited to only a few mountainous basins in 225
Augusta and Rockingham Counties. The torrential rainfall produced as much as 229 mm (9.0 in) 226
of rainfall in western Virginia and 380 mm (15.0 in) in eastern West Virginia (Stringfield and 227
Smith, 1956), and triggered dozens of debris slides and flows (Hack and Goodlett, 1960). In 228
contrast, the November 1985 storm covered a much larger area, and was noted for rainfall at a 229
moderate intensity and a long duration of three days. The storm produced as much as 250 mm 230
(9.8 in) of rain, and was dominated primarily by two low-pressure systems and, to a lesser extent, 231
the remnants of Hurricane Juan (Colucci et al., 1993). This storm initiated thousands of debris 232
flows and occurred over the same region as areas affected by the 1949 deluge. Large-scale flood 233
events are typically associated with debris flow-producing events and may affect significant 234
areas downstream of the mountainous region along piedmont rivers. For example, the 1969 235
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Nelson County flood produced one of the largest discharges in the past 400 years on the James 236
River at Richmond. 237
238
HAZARDS AND DEVELOPMENT ON FANS IN VIRGINIA 239
Overview 240
Several aspects of debris flows in the mountainous terrain of Virginia make them especially 241
problematic, resulting in potentially dangerous situations for humans. First, the recurrence of 242
debris flows at an individual site is episodic over the span of several thousand years, but occurs 243
somewhere across the whole of the southern and central Appalachians approximately every 5-10 244
years. Second, steep slopes in mountainous terrain tend to focus human development on the fans 245
rather than neighboring hillslopes. Third, when events do occur they are typically catastrophic, 246
impacting significant parts of the fan surface. Finally, there has been an increasing trend of 247
suburban sprawl targeting mountain-front developments in piedmont areas such as the 248
episodically-active fans in Virginia. Comprehension of these factors is essential for assessing the 249
level of risk associated with development on alluvial and debris fans in the Appalachians. 250
Recognition of Debris Fans and Debris Flow Frequency 251
Debris fans are ubiquitous mountain-front landforms along the eastern slopes of the Virginia 252
Blue Ridge and the western Appalachian front. Prior to the Hurricane Camille (1969) and 253
Rapidan storm (1995) events, debris fans in Virginia were not recognized as active landforms 254
due to the combination of their atypical fan morphology; and that they are commonly forested or 255
cultivated as orchards, thus disguising their presence. Recent geomorphic mapping illustrates the 256
location of debris fans, and the high frequency of debris fans that have not experienced historic 257
activity along segments of the Blue Ridge (e.g., Eaton et al., 2001) (Fig. 10). Radiometric dating 258
of these fan surfaces shows a history of late Pleistocene and Holocene activity; and they will 259
likely see similar catastrophic events in the future. In basins where mountain hollows are filled 260
with colluvium, and the fan deposits show a paucity of recent debris flow activity, it is even 261
more likely the conditions are primed for debris flows when the next intense rainfall occurs in 262
their contributing mountain watersheds. These are the sites where considerable suburban 263
development is occurring and will likely continue in the future. 264
Recent work on Virginia debris fans activated in the 1969 and 1995 storm events provide 265
pertinent information on the long-term recurrence intervals of these events (Kochel and Johnson, 266
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1984; Eaton and others, 2003a, b). Figure 11 depicts radiocarbon-dated debris flows in Madison 267
County. Return intervals for debris flows vary between 1,800 – 3,000 years at-a-site. Similar 268
debris flow return intervals were found in Nelson County (Kochel, 1987). The presence of 269
debris flows occurrence during the Holocene post-glacial climate is reasonable warning that they 270
are active processes capable of generation by modern hydroclimatic conditions; and not relicts of 271
a former climate. A fact worth noting is that while at-a-site recurrence intervals are measured in 272
millennial timescales, significant historic debris flow events have occurred somewhere in 273
Virginia once each decade, and throughout the Appalachians on average of every three years 274
(Eaton et al., 2003a; Clark, 1987). Thus, the hazard and risk of debris flows increases 275
significantly as development spreads to new locations throughout Virginia and the Appalachians. 276
Another factor to consider is that conditions capable of producing debris flows occur even more 277
frequently at a site than the 1,800 – 3,000 year interval as suggested, because a significant 278
recovery period is necessary to refill the hillslope hollows and stream channels with colluvium so 279
that there is ample material available for mobilization by the next intense rainfall (Fig. 12). A 280
good example of the importance of event ordering is illustrated by the succession of storms in 281
Madison County in 1995, and Hurricane Fran in 1996. The 1995 event produced over 1,000 282
debris flows, whereas not a single debris flow resulted from up to 432 mm (17 in) of rain from 283
the remnants of Hurricane Fran in 1996 (Eaton, 1999). The possibility exists that all of the 284
unstable hillslope colluvium was mobilized in 1995, as many of the debris flows in 1995 were 285
triggered around this threshold rainfall value. In contrast, the 1996 event produced massive 286
runoff from recently exposed bedrock slopes evacuation by debris flows in 1995; and resulted in 287
major floodplain and channel morphological changes downstream from the fans. Similar 288
contrasts in geomorphic response to subsequent rainfalls were also observed in Great Britain by 289
Newson (1980). Interestingly, in 2003 Hurricane Isabel delivered up to 513 mm (20.2 in) of rain 290
near Waynesboro (Wieczorek et al., 2006). This region had no historic record of debris flow 291
activity in the past, and no debris flows or large scale slope failures were observed from 292
Hurricane Isabel. Perhaps not enough time has passed in this part of the Blue Ridge to refill the 293
hollows with sediment to the critical threshold required to mobilize debris flows; or that the 294
triggering threshold of rainfall/duration may not have been exceeded. 295
The level of risk from debris flow hazards can be reduced by detailed bedrock and 296
surficial mapping of the geomorphic landforms within the landscape (e.g., Eaton et al., 2001; 297
11
Mills et al., 2005; Heller and Eaton, 2010). This knowledge can assist in alerting land managers 298
and home owners of the potential risks of debris flows and flooding. Figure 10b depicts surficial 299
mapping of the Graves Mill area on the Rapidan River, and shows numerous dwellings residing 300
on debris fans that were activated during the 1995 storm. Fortunately, only several of the homes 301
were destroyed and loss of life was minimal. Based on the growth trends in Virginia, surficial 302
mapping will become increasingly important as more of the fans are considered for developed. 303
304
GEOLOGIC FACTORS INFLUENCING DEBRIS FLOW LOCATION AND 305
FREQUENCY 306
Although rainfall events like those in 1969 and 1995 are likely to result in widespread debris 307
flow activity, the distribution of debris flows in these areas compared to spatial patterns of 308
rainfall does not perfectly correlate; suggesting that there are other factors such as geologic 309
structure that may exert an influence on the localization of debris flow activity. Several 310
investigators examined nearly 50 debris flows triggered by the Camille 1969 event in Nelson 311
County (Terranova, 1987; Terranova and Kochel, 1987; Gryta and Bartholomew, 1989). They 312
found that the morphology of slope failures varied according to hillslope orientation and its 313
intersection with structural elements in the granite-gneiss bedrock (Fig. 13). The research noted 314
that where slope aspect coincided with dominant foliation and joint strikes and dips, residual 315
soils around the margin of failure scarps showed sandier soils of lower cohesion compared to 316
sites where slope aspects did not parallel structural lineations. Kochel (1987) suggested that the 317
more cohesive, clay-rich soils reflected areas of lower frequency of debris flow activity. Jurgens 318
(1997) conducted a similar survey of three hollows in Madison County and concluded that areas 319
where foliation and major joint trends coincided had significantly more debris flows than other 320
regions with similar rainfall. Figure 14 is an example of this asymmetry. Here, the dominant 321
foliation and a dominant joint plane both dip toward the southeast. Debris flow scars are seen on 322
these southeast-facing slopes, whereas no debris flows occurred on the northwestern slope of 323
Kirtley Mountain, or on the northwestern slope of the small drainage to its west. In another 324
study, Sas and Eaton (2006a) examined geologic controls of slope failures from the 1995 debris 325
flows in Rockbridge County and found preferential failure along joint and bedding planes. In 326
summary, these studies indicate that bedrock geologic mapping may prove quite useful in 327
12
delineating regions of highest risk for debris flow in areas where a variety of slope orientations 328
occur, such as in major topographic hollows common in the Blue Ridge Province. 329
330
RAINFALL THRESHOLDS 331
One of the applied products that is beginning to emerge from debris flow research are 332
threshold curves that document the level at which rainfall intensity and duration are sufficient to 333
mobilize debris flows. Wieczorek et al. (2000) determined a minimum continuous rainfall 334
intensity-duration envelope for the granitic-gneissic terrains of the central Blue Ridge, including 335
sites impacted from the 1969 and 1995 storms. The threshold curve indicates that sustained 336
intensities of 70 mm/hr for 2 hours, 50 mm/hr for 4 hours, 40 mm/hr for 6 hours, and 25 mm/hr 337
for 12 hours are sufficient for triggering debris flows in the Blue Ridge of central Virginia. The 338
research notes that the Blue Ridge has the highest recognized rainfall thresholds when compared 339
to other studied regions, including Puerto Rico, Hawaii, and the San Francisco Bay area. Possible 340
explanations for the high values may include high permeability and storage capacity of the thick 341
regolith. It also appears that these intensity-duration threshold curves are probably rock type 342
dependent. One site in the Blue Ridge near Waynesboro received 513 mm (20.2 in) of rainfall in 343
less than 24 hours during Hurricane Isabel, clearly placing this event above the threshold curve 344
(Wieczorek et al., 2006). The small basin is underlain by highly fractured quartzite bedrock, and 345
its regolith may be more conducive to efficiently storing and expelling into the subsurface 346
rainwater to minimize slope failures. As more automated weather stations are activated and are 347
able to report rainfall data in near real-time format, scientists and emergency officials will 348
hopefully have a greater ability to forecast the potential for debris flows during the course of a 349
storm. 350
351
CONSIDERATIONS FOR WATERSHED PLANNING AND MANAGEMENT 352
Since the seminal work of Wolman (1967) and Leopold (1968), it has been recognized that 353
land use can have significant impacts on the nature of water runoff and sediment yield to stream 354
channels and floodplains. Suburban development into remote mountain watersheds will 355
undoubtedly increase runoff rates, and hence increase the unit hydrograph for rainfalls of most 356
recurrence intervals. Major debris flow events like that of the Rapidan event in 1995 can also 357
significantly alter runoff characteristics for decades after the event. Bedrock, primarily exposed 358
13
in headwater streams from debris flow events, can increase the routing efficiency of streams and 359
changes in channel morphology in large rivers may reduce their conveyance efficiency, such that 360
water is routed more rapidly from the mountain streams and drains more slowly from the 361
lowland and valley bottom streams. Conversations with people residing along the Rapidan River 362
indicate that significant floods now occur in the Rapidan watershed from lower magnitude 363
rainfall events than those prior to June 1995 (i.e., Randall Lillard, Douglas Graves, 2006, pers. 364
comm.). A prime example is the volume of runoff from Tropical Storm Fran in September 1996, 365
when discharges nearly equaled that of 1995 but from half of the rainfall in 1996 (Fig. 15). In 366
addition to problems of flooding, there is an increased volume of coarse bedload in transport 367
during flows following the 1995 debris flow event. Sediments released from the slopes in 1995 368
are now being flushed through mid-and-downstream reaches of the Rapidan River by subsequent 369
floods like the 1996 event and other smaller floods. Culverts that were replaced from the 1995 370
event were quickly overwhelmed by the bedload transport in 1996 along many of the tributaries 371
of the Rapidan (Eaton, 1999). These examples highlight the importance of factoring in 372
significant adjustments in water and sediment fluxes when designing upgrades or replacements 373
to infrastructures following a major disturbance within a watershed. 374
Similarly, changes in discharge and sediment yield need to be incorporated into the planning 375
of stream restoration projects in a system-wide basin-scale approach. Stream channel geometry 376
is greatly influenced by discharge and sediment delivered to the channel from the upstream 377
watershed. Changes in land use (e.g., channelization, deforestation) and by major geomorphic 378
events such as debris flows will result in downstream adjustments in channels that often require 379
decades to re-establish equilibrium. A stream restoration project on the Rapidan River near 380
Graves Mill is a prime example of the kinds of problems than can occur when these issues are 381
not taken into account in project design. First, the Rapidan River has been channelized multiple 382
times for agricultural and transportation purposes since the region was first settled in the late 383
1700s, thus altering its system from a multiple-channel braided pattern to a single meandering 384
channel. Channels transporting large quantities of coarse bedload are better served by braided 385
courses with wide and shallow channels to maximize bed shear stress. Thus, when large supplies 386
of sediment are delivered to the Rapidan during major floods and debris flow events, the channel 387
reverts back to its braided condition, resulting in valley-wide inundation, scour, and deposition. 388
This happened in 1995 and again in 1996 (Fig. 15) (Eaton, 1999). Exposures of the floodplain 389
14
uncovered during these events revealed numerous wide and shallow paleochannels that were 390
likely the high-flow anabranches of the Rapidan prior to channelization following European 391
settlement. In 2002 the Virginia Department of Transportation initiated a restoration project on a 392
reach of the Rapidan River, 1 km south of Graves Mill, to help maintain the course of the river 393
under the State Route 767 bridge. Within a year a major stream avulsion had occurred because 394
the restored channel was not designed to transport the higher supply of bedload the reach was 395
receiving, largely due to the continued adjustments to the 1995 debris flow event (Fig. 16). Most 396
natural channel design projects do not account for long-term adjustments that may be occurring 397
in streams due to past land use and/or major geomorphic changes. Kochel et al. (2005) observed 398
a failure rate of more than 70% for natural channel design projects in North Carolina after 399
experiencing their first significant flood. 400
401
LANDSLIDES AND TRANSPORTATION 402
Road construction on steep hillslopes can increase landslide susceptibility by 1) 403
adding weight to the slope with fill material; 2) steepening the slope on both cut and fill 404
surfaces; 3) removing support of the cutslope; and 4) rerouting and concentrating 405
drainage water (see review by Sidle et al., 1985). Increased landslide susceptibility from 406
roads can impact downslope and downstream areas, and interrupt transportation. The 407
literature shows that landslide hazards along transportation corridors in the central 408
Appalachians are numerous (e.g., Watts and Whisonant, 1992; Sas and Eaton, 2006b; 409
Douglas et al., 2007; Lantham et al., 2007). Some of the larger landslide events in the 410
central Appalachian region in recent memory include large rockslides along the Interstate 411
40 corridor in 1985 (Winchester, 1985) and debris flows originating from road fill 412
failures along the Blue Ridge Parkway from Hurricanes in 2004 (Sas and Eaton, 2006b). 413
These road-related landslides often have a larger volume and can increase the risk of 414
debris flows to downstream areas (May, 2002). Recent advances in remote sensing and 415
terrain analysis are likely to increase the ability of scientists and land managers to detect 416
high risk areas along transportation corridors. 417
418
419
420
15
CLIMATE CHANGE 421
Because of the common linkage of floods and debris flows with tropical storms, the concern 422
about global climate change could be significant in the evaluation of flood hazards for Virginia. 423
If tropical storm frequency increases in the mid Atlantic, or if the hurricane tracks are altered so 424
that these storms collide with the central Appalachians, a higher frequency of debris flow events 425
may result in some locations in Virginia. Kochel (1987) suggested that debris flow activity in 426
the Appalachians could be correlated to the retreat of the polar front as the Pleistocene climate 427
waned; although Eaton et al. (2003a) found activity since the Last Glacial Maximum. What is 428
not known is, if discrete warming trends that occur as documented in the Blue Ridge pollen 429
record (Litwin et al., 2001) may also correlate to these debris flow events. It is clear from Clark’s 430
(1987) research that more debris flow events have impacted the southern Appalachians, 431
presumably from their closer proximity to tropical air masses and steep, mountainous terrain. 432
Whether or not these events become more common from south to north over time as the 433
incursion of tropical air masses would likely have become increasingly common is still 434
undetermined, but worthy of further investigation. 435
436
437
KARST HAZARDS IN VIRGINIA 438
David A. Hubbard, Jr. 439
INTRODUCTION 440
Karst is a terrain that develops by the action of water with soluble bedrock and 441
characteristically features karren, sinkholes, caves, and subsurface drainage. Soluble 442
bedrock that has developed karst in Virginia occurs in all five of the major physiographic 443
provinces: in Pliocene indurated shelly sand of the Yorktown Formation of the Coastal 444
Plain province; in Cambrian to Ordovician marble and limestone of the Piedmont and 445
Blue Ridge provinces; in Cambrian to Mississippian-age limestone, dolostone, gypsum, 446
and salt in the Valley and Ridge province; and as the Mississippian limestone in the 447
Appalachian Plateaus province. The most significant karst in the Commonwealth, and the 448
focus of this discussion, extends over twenty-six counties in the Valley and Ridge 449
physiographic province (Fig. 17). Karst mapping of the Valley and Ridge province 450
defined areas of karst by carbonate and non-carbonate rock boundaries and sinkholes 451
16
(Hubbard, 1983, 1988, 2001). Approximately 48,800 sinkholes were remotely sensed 452
using stereographic interpretation of low-altitude aerial photography and are plotted on 453
the three karst maps. Because Virginia karst is a cover karst, wherein a mantle of soil and 454
sediment covers most of the solutional patterns of bedrock (karren), sinkholes are the 455
most readily observable surface feature. The natural processes active in karst result in 456
some phenomenon recognized as hazards: subsidence, sinkhole flooding, and the 457
recharge of karst groundwater aquifers with contaminated surface water. Unfortunately, 458
land-use modifications in karst may induce changes to the local hydrology than can 459
trigger and exacerbate these hazards. 460
461
KARST HAZARDS 462
Three phenomena that are commonly recognized as karst hazards are subsidence, 463
sinkhole flooding, and the recharge of the aquifer by surface waters. Each of these 464
hazards is the result of natural processes and water-rock interactions in karst. Historically, 465
the hazard of greatest concern to karstland residents is catastrophic sinkhole collapse. 466
Subsidence Hazards 467
Water-rock interactions in a landscape underlain by soluble bedrock, which may range 468
from 60 to 99 percent soluble, almost guarantee that subsidence will occur. Anecdotal 469
evidence suggests that most people who reside in karstlands are relatively at ease with 470
existing sinkholes that gradually deepen or slowly grow in extent; and many are 471
undaunted by sudden collapses within alluviated subsidence features. However, residents 472
are concerned when un-patterned catastrophic sinkholes develop without warning; 473
particularly at locations without a previous history of cover subsidence. The collapse of 474
bedrock into cave passages is relatively rare in karst, as the soils and sediments that 475
mantle bedrock in a covered karst can temporarily span enlarging soil-bedrock interface 476
voids that form at epikarst drains. These voids may expand beyond the strength of the 477
arching soils and catastrophically fail to the surface of the landscape and form a sinkhole 478
(Fig. 18). 479
The soluble bedrock surface is not equally exposed to water, so most dissolution 480
occurs along preferred flow paths at the bedrock surface and within rock partings and 481
fractures. The solutional patterns of the bedrock, both on exposed surfaces and under soil 482
17
or sediment cover, are known as karren. Unfortunately, the preferred flow paths hidden 483
by covering sediments are not necessarily mirrored by the landscape surface. Most water 484
flow is not along surface drainage features, but along channels eroded on the covered 485
bedrock surface and in the solution-enlarged partings and fractures, known as conduits 486
and caves, within the bedrock. Some subsidence sinkholes are the result of the direct 487
dissolution of the bedrock and form by the gradual letdown of cover materials over 488
periods of human life-spans. 489
In contrast, collapse sinkholes result from erosion of soil and sediment covers at 490
epikarst drains, (the interval between the mostly unaltered bedrock and the topsoil), and 491
may be catastrophic in their failure. The water-rock processes of karst that sculpt this 492
dominantly erosional landscape are sensitive to the stability of climatic and land-use 493
conditions. Because the degree of activity of water-rock processes largely is veiled by the 494
mantle of soil and sediment, these unstable terrains are mistakenly perceived as inactive 495
or stable. Extreme weather events, including droughts, and land-use changes that result in 496
changes to the local hydrology can trigger and exacerbate subsidence hazards. 497
To help assist the risk of rapid subsidence, government officials have used subsidence 498
susceptibility mapping, primarily a GIS tool used by planners, to designate or model site 499
suitability for development. From the most simplistic perspective, areas of dense sinkhole 500
populations are more likely to experience future sinkhole formation than karst without 501
existing sinkholes. However, research suggests that the greatest influence in future 502
sinkhole formation is not the past land-use, nearest sinkhole, nearest stream, etc.; rather, 503
most new sinkholes form in response to very recent land-use changes in drainage 504
volumes and flow-paths (Hubbard, 2003). Hubbard noted that sinkholes frequently form 505
over an existing epikarstic drain, and the subsequent surface collapse is accelerated by 506
alteration of the localized drainage volume or flow-path of surface runoff. The additional 507
volume of water provides a positive feedback situation, which scours out sediment within 508
the void and contemporaneously increases the conduit size, thus allowing for greater flow 509
capacity. The covered subsurface drain is gradually enlarged and leads to the subsequent 510
collapse of the soil mantle. 511
512
513
18
Sinkhole Flooding Hazards 514
Throughout most of the Commonwealth, sinkhole flooding is a minor hazard. 515
Sinkholes, blind valleys, and losing surface streams are input points for surface water, 516
especially storm-generated runoff, to enter groundwater conduits of the karst aquifer. All 517
karst sinkholes are drained by solution-enlarged flow-paths that lead to the karst aquifer, 518
or to base-level streams and rivers. Under normal hydraulic conditions, most sinkholes 519
serve as drains for surface water runoff and subsurface water, draining along the soil-520
bedrock interface, to access the karst aquifer. There are three conditions under which 521
sinkholes and other karst depressions flood (Fig. 18). The first two conditions are 522
characterized by under-drainage occurring at lesser rates than drainage into the sinkhole, 523
whereas the final condition is one of back-flooding due to high-head pressures in the 524
groundwater conduits under-draining the sinkhole. The first condition typically occurs 525
during exceptional precipitation events whereby surface water run-off topographically 526
funneled to sinkholes greatly exceeds the subsurface drain’s capacity to transmit water to 527
the karst aquifer. This type of flooding is usually temporary and represents a lag or 528
ponding of drainage. The second condition is usually due to a change in the land-use and 529
results from increased sediment yield in the development of the area topographically 530
draining to a sinkhole, and the partial choking of the sinkhole drain by sediment. Under 531
such circumstances, storm-generated runoff can pond in the sinkhole due to the reduced 532
efficiency of the sediment-clogged drain. This condition is usually further complicated by 533
a secondary effect of land-use change, an increase in surface runoff. Most anthropogenic 534
induced land-use modification creates more runoff by changes in vegetation, soil 535
compaction, and the application of impermeable surfaces such as roofs and pavements. 536
Increased runoff to a sediment-clogged sinkhole drain greatly enhances sinkhole ponding. 537
Sedimentation from such ponded waters, which may carry additional sediment to the 538
sinkhole, tends to increase the period of ponding. The third type of sinkhole flooding is 539
caused by a non-passive back-flooding phenomenon and truly is a flooding hazard. This 540
type of flooding occurs when the under-draining conduit or cave system is overwhelmed 541
by down-system sinkhole drainage during an exceptionally large precipitation event. 542
Under this situation, flow of water from other sinkholes downstream in the conduit 543
system results in high head pressures reversing conduit flow. Water resurges from the 544
19
sinkhole drain and floods the sinkhole. Under these circumstances, a sinkhole may not 545
only flood, but water may fill and overflow from the sinkhole and create additional 546
flooding of other adjacent sinkholes or lowlands. 547
548
Karst Aquifer Contamination Hazards 549
The most pervasive karst hazard is of aquifer contamination by recharge of polluted 550
surface waters. Perhaps the greatest hazard in Virginia karst is the routing of highway 551
run-off to sinkhole drains. Not only does this type of design contribute heavy metals, road 552
salts, nutrients, bacteria, and hydrocarbons carried in highway run-off (Stephenson and 553
Beck, 1995) to aquifer recharge, but such designs assure that hazardous materials and 554
other potential contaminant cargos that are transported along highways are potential 555
aquifer contaminants (Hubbard, 1999). Data from both the U.S. Department of 556
Transportation and the Environmental Protection Agency indicate that highway 557
transportation of hazardous materials is a relatively high-risk industry (Padgett, 1993). 558
Crop and livestock operations can pollute karst groundwater directly by sinkholes, as 559
well as through diffuse non-point source entry of bacteria, leaching nutrients, pesticides, 560
or other contaminants (Berryhill, 1989; Boyer and Pasquarell, 1994, 1999). Some of the 561
contaminants that have degraded Virginia’s karst aquifers include: the leachate of 562
improperly disposed wastes; and spills or leaks of petroleum products, herbicides, 563
solvents, fertilizers and poultry waste, sheep and cattle dip, sewage, and milk (Hubbard 564
and Sterrett, 1994). 565
566
HAZARD MITIGATION AND REMEDIATION 567
The successful mitigation and remediation of karst hazards is problematical due to the 568
out-of-sight nature of the water-soil-rock processes that form karst and its attendant 569
hazards. That karst in an unstable terrain, and that land-use changes to it generally 570
enhance hazard realization, does not bode well for the idea of hazard mitigation. 571
Although the basic concepts for hazard mitigation and remediation are presented in the 572
following sections, most mitigation and remediation in karst addresses the manifestations 573
of the hazards and solutions to stop and correct subsidence, flooding, and aquifer 574
pollution. Successful mitigation and remediation is more likely accomplished through 575
20
professionals experienced with karst hazards, and the use of appropriate methodologies 576
that address the site-specific processes and conditions. 577
578
Subsidence Hazard Mitigation and Remediation 579
Catastrophic sinkhole collapse is undoubtedly the hazard of greatest concern to 580
karstland residents. The most commonly recognized land-use triggers of sinkhole 581
collapse are water-well drilling and increased run-off. In most situations where water-582
well drilling induces one or more sinkholes, three conditions are present: 1) a water level 583
is encountered above bedrock; 2) a saturated mud-seam (a mud-filled cave), is 584
intercepted at some depth in the bedrock; and 3) the driller “blows mud” to develop a 585
water-filled void in the saturated mud-fill. The cause of ground failure is the creation of 586
an open water-filled void in rock and its failure through overlying rock and soil straight 587
to the surface. Most catastrophic karst sinkholes do not involve rock failure. Collapse 588
sinkholes are formed when voids at the soil-bedrock surface fail through the sediment-589
soil cover to the ground-surface. When the groundwater level is above the soil-bedrock 590
interface, saturated cave fills have continuity within the cave to the saturated sediment 591
above the sediment covered cave entrance. Most caves are presently entrance-less or are 592
not enterable, and probably not recognized by humans. In the folded and faulted 593
carbonates of Virginia’s Valley and Ridge province, extensive mud seams may wind 594
through bedrock to a sediment covered-rock opening within hundreds of feet of the well-595
head. When groundwater is shallower than bedrock, drillers should avoid blowing-out 596
saturated mud-seams. 597
Increased run-off is the other most commonly recognized cause of new sinkhole 598
collapses. Extending drainage outflows as far from buildings as possible will reduce the 599
risk of new sinkhole formation near structures. Increases in run-off and new flow paths 600
trigger more rapid growth of soil voids at epikarst drains, which form new sinkholes as 601
the voids extend in size beyond critical soil strength and progressively collapse to the 602
land surface. 603
The best way to remediate a collapsed sinkhole, and not induce additional new 604
sinkholes, is to excavate the feature to the bedrock surface and build a reverse filter that 605
bridges the rock throat of the drain so that material cannot wash into the drain, but 606
21
subsurface water can continue to drain. An aggregate size too large to fit into the bedrock 607
drain is used as a base. Successively finer aggregate, that is too coarse to fit in the voids 608
between clasts in the underlying layer, is used in adjacent layers to construct a stable 609
filter that allows water to pass, but prevents voids from forming and collapsing. Methods 610
utilizing materials that can erode into under-draining conduits may undergo further 611
subsidence. Grout plugs that are not emplaced on bedrock may undergo settling as the 612
underlying soil is eroded into under-draining conduits. 613
Effective grout sealing of a solution-enlarged epikarst drain restricts groundwater 614
movement, which may create addition problems. Subsoil water that previously drained at 615
this site may collect and saturate adjacent soils, or propagate new sinkholes proximal to 616
the site by moving along the soil/bedrock interface into adjacent epikarst drains and 617
enhances void expansion and collapse. In rare circumstances, grout sealing may extend 618
into the under-draining cave system, resulting in back flooding of up-system sinkholes 619
drained by the grout-occluded conduit. 620
621
Sinkhole Flooding Hazard Mitigation and Remediation 622
Sinkhole flooding can be mitigated by reducing the amount of run-off flowing to the 623
sinkhole, and by eliminating siltation and sedimentation problems in these drainage 624
features. Whereas most individuals that work and/or reside in karst landscapes understand 625
that building within a closed contour feature may lead to flooding, the presence of large 626
complex sinkholes and blind valleys, many of which extend more than a half-kilometer in 627
length in Virginia, can make the recognition and delineation of potential flood zones 628
extremely difficult. The modification of sinkhole drains, also referred to as dry wells and 629
drainage wells, to better accommodate surface water run-off and minimize sinkhole 630
flooding, may result in new sinkhole formation and non-passive back-flooding of other 631
sinkholes adjacent or remote to a remediated site. This type of modification commonly 632
contributes to groundwater contamination of karst aquifers by enhancing recharge of 633
contaminated surface waters. 634
635
636
637
22
Karst Aquifer Contamination Hazard Mitigation and Remediation 638
Karst aquifer contamination is the most extensive and serious karst hazard in Virginia. 639
The folded and faulted nature of the sedimentary rocks in the Valley and Ridge province 640
can compromise thin shale and other thin non-carbonate beds as potential aquitards that 641
would normally partition carbonate sequences into separate aquifers in less tectonically-642
affected sedimentary sequences. The first step in the mitigation of a karst groundwater 643
contaminant is to determine the source and stop the flow of the contaminant. This 644
concept may be much easier to state than to implement. Karst aquifers in dense, low 645
primary porosity carbonate and evaporite rocks are principally composed of conduits that 646
are difficult to intercept by drilling. Investigators unfamiliar with karst commonly 647
interpret the absence of a contaminant at a newly drilled monitoring well as evidence that 648
a contaminant is contained, or that the plume is limited to up-gradient of the 649
uncontaminated well. Drilling into the right conduit is akin to the proverbial search for a 650
needle in a haystack. Tracers, commonly dyes, are utilized in tracking karst aquifers and 651
determining the routes of contaminants. In karst, the question is not did the contaminant 652
move, but where did it go? Existing water wells and springs are common monitoring 653
points to determine flow paths, which may demonstrate surprisingly rapid transport. 654
Contaminant remediation in karst aquifers generally is more complicated than in 655
homogeneous aquifers. Remedial technologies have continued to advance in the past few 656
decades. In-situ treatment by biodegradation can be effective, as can extractive methods. 657
Extremely toxic and refractory organic contaminants that can destroy aquifer resources 658
remain problematic. Karst aquifer contamination sites are contaminant and site specific, 659
and their remediation is beyond the scope of this summary. Complexities include the 660
nature of the contaminants in terms of density, solubility, stability, toxicity, volubility, 661
reactivity with rock and sediment as well as complexities of the nature of the particular 662
karst aquifer (Freeze and Cherry, 1979; Fetter, 1988). 663
The contamination of karst aquifers continues as land is shifted from agricultural to 664
residential, commercial, and industrial land-uses. Perhaps the most significant 665
contaminant disasters involve tanker releases of hazardous and toxic cargoes along 666
Virginia’s karstland highway corridors. A major initiative is needed to trace and map the 667
run-off flow routes from Interstate Highways and other major transportation routes 668
23
through Virginia karst. Such a project could enable the development of early warning 669
communication systems for emergency responders to alert groundwater users along major 670
hazardous and toxic transportation corridors; and could alleviate the potential of 671
avoidable waterborne lethal exposures and poisonings. 672
673
SINKHOLES OTHER THAN KARST 674
Non-karst collapse sinkholes typically form by two processes: 1) soil piping, or 2) 675
collapse of anthropogenically-made voids, such as underground mines. Numerous 676
examples of these failures exist in the Commonwealth. Examples of sinkholes resulting 677
from underground mine failures exist in karst rocks such as salt, gypsum, and carbonate 678
rocks. The failure processes probably include some degree of dissolution of the host rock, 679
but blast damage, existing fractures, weak strata above ceilings, excessive ceiling spans, 680
and robbed pillars are more likely the critical culprits in these failures (Figs. 20a,b). Most 681
historic mines were not designed for longevity and post-mining failures are inevitable at 682
many of these underground mines. An example of a subsurface mine failure in a non-683
karst setting is this coalfield sinkhole shown in Fig. 21, , which resulted from the collapse 684
of coalfield workings. 685
686
687
RADON HAZARDS IN VIRGINIA 688
Fiorella V. Simoni de Cannon, Douglas .G. Mose, and L. Scott Eaton 689
INTRODUCTION 690
Naturally occurring radioactive radon gas (Radon-222) is always present in the air that 691
we breathe (Lowder, 1985). However, indoor radon in high levels has been a recent 692
addition to the list of natural health hazards that pose serious risk to human health and 693
property value. Radon has always been present in air, water and soil, but in 1924 Ludwig 694
and Lorenser postulated radon as an important factor in lung cancer (Samet, 1994). By 695
the mid 1960s, many reports of lung cancer mortality among U.S. uranium miners 696
emerged (e.g., Wagoner et al., 1965; Sevc et al., 1976; Kunz and Sevc, 1978; Harley, 697
1984, 1989; Proctor, 1995; NAS, 1999). The focus of the problem was transferred from 698
24
miners to the general public in the 1980s, when a home in Pennsylvania was found to 699
have radon levels over 600 times the threshold level recommended by the Environmental 700
Protection Agency (Lafavore, 1987). The elevated concentrations at this site as well as 701
other localities throughout the United States created nationwide concern of radon, and 702
initiated a series of state and federal programs to assess and understand the problem 703
(Gunderson and Wanty, 1991). 704
In 1994, the National Toxicology Program (NTP) of the U.S. Department of Health 705
and Human Services first listed radon in its Seventh Annual Report on Carcinogens. In 706
1999, the National Academy of Sciences BEIR VI (BEIR-Committee on the Biological 707
Effects of Ionizing Radiation) Report concluded that radon is the second leading cause of 708
lung cancer after smoking (NAS, 1999). Today, NTP (2005) lists radon in its 11th Annual 709
Report on Carcinogens as a known human carcinogen based on evidence of 710
carcinogenicity in humans presented by the Agency for Toxic Substances and Disease 711
Registry (ATSDR) and by the International Agency for Research on Cancer (IARC). In 712
the United States, it is estimated that radon causes between 15,000 to 22,000 deaths from 713
lung cancer each year. That is, approximately 12% of all lung cancer deaths are linked to 714
radon exposure (NAS, 1999). By U.S. Environmental Protection Agency (US-EPA) 715
(2005) estimates, indoor radon causes lung cancer fatalities in hundreds of Virginia 716
residents each year. 717
Radon concentrations in air are measured in picoCuries per liter (pCi/L). By 718
definition, a Curie is the rate of decay of one gram of radium, that is, 37 billion decays 719
per second (Cohen, 1989). Because this is a very large quantity, radioactivity in the 720
environment is usually measured in units of one millionth of a millionth of a Curie (10-12 721
Curie), or a picoCurie (pCi). In effect, in a room with a radon concentration of 1 pCi/L, 722
two alpha-particles would be emitted about every minute from radon atoms per liter of air 723
(Brookins, 1990). While the US-EPA (2006) has established 4 pCi/L as a general 724
guideline for maximum acceptable indoor radon concentration when buying a home, it 725
established 2 pCi/L as the limit for people living in a home. It also recommends testing 726
all homes and urges mitigating action by increasing ventilation and preventing soil gas 727
entry into the home. The average home in the U.S. has an indoor radon concentration of 728
25
1.3 pCi/L; the average outdoor radon concentration is 0.3 pCi/L (Cohen, 1989; 729
UNSCEAR, 2000; US-EPA, 2006). 730
731
VARIATION IN RADON CONCENTRATION 732
Uranium is a naturally occurring radioactive element that is present in varying 733
concentrations in all rocks and soils throughout the United States. Studies in Virginia 734
and other states show that particular geologic units and the soil above these units are 735
associated with elevated indoor radon concentrations (Mose and Mushrush, 1997a; Mose 736
and Mushrush, 1999). For example, granite has relatively high uranium content and so 737
granite and granitic soil tend to generate more radon than do other geological materials 738
(Brookins, 1990). 739
Indoor radon concentrations can vary in response to weather on hourly, daily, and 740
seasonal time scales. The most important weather factors are wind, barometric pressure 741
changes, and soil moisture changes (Mose et al., 1992a,b; Mose and Mushrush, 1997b). 742
Of all the weather variables, a change in soil moisture is the most important. As 743
precipitation infiltrates and saturates the surface horizons of the soil profile, the saturated 744
zone temporarily serves as a confining unit and is effective at slowing the release of gases 745
from within the profile. Therefore, on rainy days the wet soil around a home slows the 746
rate at which radon is released into the atmosphere, thus increasing its concentration in 747
the profile around a home relative to non-precipitation days. Conversely, on hot sunny 748
days the soil is dry, allowing the release of soil gases through the surface horizons into 749
the atmosphere; and less radon is available in the soil for potential migration into the 750
home. 751
The mechanism of radon entry into buildings is well understood. Large to microscopic 752
wall and floor cracks, floor and wall penetrations (wires, pipes, sump holes) and floor-to-753
wall corner joints are all common entry points. A negative atmospheric pressure in a 754
home relative to the soil produces a soil-to-home pressure gradient that draws radon in 755
through entry points in soil-facing walls and floors. Atmospheric pressure in a home can 756
also be reduced by furnace combustion, ventilation devices, and the stack effect (the 757
26
rising and escape of warm air from the upper floors of the building) during cold winter 758
months. This negative pressure draws radon into a home from the underlying soil. 759
Mose and Mushrush (1988, 1997b; Mose et al., 1991) found that variations in home 760
construction are related to indoor radon levels in homes with otherwise similar weather 761
and geology. That is, factors related to home construction affect indoor radon levels. For 762
example, they found that higher indoor radon tends to occur in homes with basements, 763
probably because homes with basements have more soil-touching entry points for radon, 764
a more pronounced stack effect, and lower air pressure relative to the surrounding soil 765
than no-basement homes (i.e., homes without a basement or with a crawl space). The 766
authors note that basements with concrete block walls tend to have higher indoor radon 767
concentrations than basements with poured concrete walls, probably because concrete 768
blocks are likely to be more permeable and to develop more fractures. The research also 769
documents that homes with electrical heating systems (heat pumps) tend to have higher 770
indoor radon than homes with combustion heating systems, probably because a fuel-771
burning furnace significantly removes home air for combustion, which pulls in low-radon 772
air from outside the home. 773
774
WATERBORNE RADON AND CANCER 775
The pores between the grains of soil and the cracks in rock usually contain a mixture 776
of air and water (Brady and Weil, 1996). Often, a radon atom will come to rest in the 777
water and remain there (Wilkening, 1990). In many homes in Virginia, domestic water is 778
drawn through a water well from underground sources. Surface water (e.g., from a water 779
reservoir) normally has radon concentrations too low to measure (less than 100 pCi/L), 780
but ground water tends to accumulate radon generated within the groundwater aquifer 781
(Hess et al., 1982; Nazaroff and Nero, 1988; Gosink et al., 1990; Mose and Mushrush, 782
1997a). Radium is also in groundwater, but because radium is not as soluble as radon, 783
the concentration of radon is usually 10-1,000 times the concentration of radium 784
dissolved in ground water (Milvy and Cothern, 1990; Mose et al., 1990; Mose and 785
Mushrush, 1997b). 786
27
Waterborne radon enters the air in a home primarily as bursts of radon released into 787
the air by mechanical sprays during a shower or by the heating and agitation of water that 788
occur during laundering, washing, and cooking (Mose et al., 2005). All of these 789
contribute to an increase in indoor radon concentration. Hess et al. (1982) first reported 790
that indoor radon levels can be correlated with concentrations of radon in the water 791
supply, suggesting that in a typical home about 1 pCi of radon per liter would be added to 792
indoor air from the outgassing of water containing 10,000 pCi/L of radon. Many 793
subsequent studies have used this 1:10,000 ratio, though in small homes the ratio may be 794
as low as 1:100 (Mose et al., 2005) This can be a problem in Virginia homes where the 795
waterborne radon is sometimes as high as 10,000 pCi/L. 796
The ingestion of radon-enriched and RDP (radon decay product)-enriched water may 797
also be a serious health concern. Gosink et al. (1990) reported that radon ingested from 798
drinking well water is not rapidly eliminated by metabolic respiration and can remain in 799
the body for 12 hours depending on physical activity. During this time interval, radium, 800
radon, and RDPs carried by ingested water can move through the body and may produce 801
an effect at cancer prone sites (Mose and Mushrush, 1997b). 802
803
RADON IN VIRGINIA 804
According to Schumann (1993), the rock types and overlying soils that are most likely 805
to cause indoor radon problems in the U.S. are carbonaceous shales, sandstones, certain 806
fluvial sediments, phosphorites, carbonate rocks, uranium-rich granitic rocks, 807
metamorphic rocks of granitic origin, and sheared and faulted rocks. Those least likely to 808
produce indoor radon problems are marine quartz sandstone, non-carbonaceous shales 809
and siltstones, and silica-poor metamorphic and igneous rocks. However, localized 810
uranium deposits cause exceptions within all these categories and can produce high-radon 811
concentrations in homes. Within the Commonwealth of Virginia, the varied geology in 812
each of the geologic provinces creates different radon potentials, and these conditions are 813
examined further in the following text. 814
815
28
Coastal Plain 816
The Coastal Plain of Virginia is underlain largely by fluvial and marine sediments that 817
date from the Cretaceous to the Late Holocene, and many of these sediments were 818
deposited in beach or near shore environments. Under these conditions, quartz (which 819
contains essentially no uranium) is chiefly the residual surviving product of the intense 820
mechanical and chemical weathering (Gundersen et al., 1992). The uranium-bearing 821
minerals are mostly dissolved and the uranium is carried away. This process is thought to 822
explain the general trend of low indoor radon readings in the Coastal Plain (i.e., most 823
houses tested in this area have concentrations of less than 4 pCi/L). Interestingly, some 824
small areas have shown high indoor radon, possibly due to accumulations of less-825
weathered river-deposited sediments. Higher values were also found in samples 826
originating from phosphatic fossil units and glauconitic sands in the Aqui, Brightseat, and 827
Calvert Formations in Maryland and Virginia (Otton, 1992). In one extreme example, the 828
Yorktown Formation had an average radon concentration of 1050 PCi/l, and Goodwin et 829
al. (1989) suggests that fossilized whale bones in the formation may be the source of the 830
radon. Additionally, heavy mineral deposits in the Virginia Coastal Plain have also 831
created localized high values of radon (Gundersen, 1993). 832
Mose and Mushrush (1987) first reported on the comparatively low indoor radon 833
measurements characteristic of the Coastal Plain in 1987. The US-EPA reports the 834
average indoor radon concentrations from 1986-1989 were <1 pCi/L for most parts of the 835
Outer Coastal Plain (Gundersen et al., 1992), and that indoor radon measurements from 836
the Inner Coastal Plain averaged 2.3 pCi/L. Earlier, Berquist et al. (1990) found similar 837
trends within Virginia in his study of radon potential with respect to geologic province, 838
finding that the probability of a house exceeding 4 pCi/l in the Coastal Plain is less than 839
10%. He attributed the fact that homes on the Outer Coastal Plain tend to have the lowest 840
indoor radon due to 1) the deep burial of uranium-bearing basement rocks as the clastic 841
wedge of sediments thickens to the east; and 2) the largely weathered nature of the 842
sediments residing in the Coastal Plain. In summary, it appears that with the exception of 843
localized elevated radon concentrations, the Coastal Plain has the lowest potential of the 844
provinces for radon. 845
846
29
Piedmont and Blue Ridge Provinces 847
In contrast to the Coastal Plain, the complexity and varied geology found in the 848
Piedmont and Blue Ridge provinces creates a patchwork of high and moderate zones of 849
concentrated radon. The region is underlain by Proterozoic and Paleozoic igneous and 850
metamorphic rocks that have undergone differing levels of metamorphism and structural 851
deformation; and also include sedimentary and contact metamorphic rocks deposited or 852
emplaced during Mesozoic rifting. The literature originating from both Virginia and the 853
larger mid Atlantic region indicates that granites, pegmatites, and monazites have the 854
greatest propensity to contain trace amounts of uranium (e.g., Grauch and Zarinski 1976; 855
Mose and Mushrush, 1987; Gundersen and Wanty, 1991). In general, several known 856
localities exist in the province have elevated levels of radioactivity. These sites include 857
1) the inner Piedmont and Goochland Terrane; 2) plutons northwest of Fredericksburg 858
(Neuschel et al., 1971); and 3) the region near Spotsylvania (Neuschel, 1970) (Fig. 22). 859
Very high uranium concentrations have been documented in the Petersburg Granite near 860
Richmond (Baillieul and Dexter, 1982), the Maidens Gneiss of the Goochland Terrane 861
near Powhatan (Krason et al., 1988), the Old Rag Granite in Rappahannock County 862
(Baillieul and Daddazio, 1982), the Crozet Granite in Albemarle County (Baillieul and 863
Daddazio, 1982), and the Swanson Uranium deposit near Pittsylvania County (Halladay, 864
1987). Gundersen (1993) provides an excellent review on the numerous plutons 865
throughout the Commonwealth that also have elevated concentrations of uranium and the 866
potential for radon. 867
Regional metamorphism (heat, pressure, and chemically active fluids) can produce 868
conditions conducive to the segregation and concentration of radioactive minerals. 869
Within the Piedmont and Blue Ridge, the foliated metamorphic rocks, including phyllites, 870
schist and gneisses, have the potential to contain radioactive minerals (e.g., allanite, 871
monazite, zircon, and titanite). One of the best studied examples of the above conditions 872
is in northern Virginia, where researchers document very high aerial radioactivity, soil-873
gas radon, and indoor radon levels are associated with the Peters Creek Schist in Fairfax 874
County (Mose et al., 1988a,b; Otton et al., 1988; Schumann and Owen, 1988). 875
Additionally, sheared fault zones have been found to be generating high amounts of 876
radon. Mylonitization of the rock can increase the radon levels through 1) a volume loss 877
30
and grain size reduction of uranium accessory minerals, thereby leaving the rock enriched 878
in uranium; and 2) the foliation imparted on the rock during shearing increases the 879
permeability, allowing the introduction of uraniferous fluids and gases into the shear zone 880
(Gundersen, 1991). Several Piedmont sites that report high radioactive anomalies or the 881
presence of uranium are in shear zones (e.g., Baillieul and Dexter, 1982; Halladay, 1987; 882
Gundersen, 1991) Additionally, some of the highest indoor radon concentrations 883
documented in the United States have been within shear zones (Gundersen et al., 1987, 884
1988; Smith et al., 1987; Henry et al., 1991). 885
Postdating the regional Paleozoic metamorphism of the Piedmont and Blue Ridge are 886
the rocks of the Mesozoic basins (Fig. 22). In numerous locations, these rocks, chiefly 887
siltstones, sandstones, and conglomerates, were transformed to uranium-enriched 888
hornfels when they were locally metamorphosed by igneous intrusions. When faulting 889
occurred as best seen in the Danville and Richmond basins, zones of high-uranium 890
mylonites were created (Gundersen, 1993). Particular geological units now known to be 891
higher than average in uranium include the Manassas Sandstone of the Culpepper Basin, 892
the Cow Branch Formation of the Danville Basin (Gundersen et al., 1992), and hornfels 893
in the Culpeper basin (Otton et al., 1988; Schumann and Owen, 1988). Also known as 894
high-uranium units are black shales in the Mesozoic basins (e.g., the Cow Branch 895
Member, the Balls Bluff Siltstone, the Catharpin Creek Formation, the Waterfall, Turkey 896
Run, and Midland Formations, the upper portions of the Danville basin lacustrine 897
sequence, and the Vinita Beds Member of the Tuckahoe Formation). All appear to have 898
locally elevated uranium (J.P. Smoot, USGS, oral comm.). 899
In general, the Inner Piedmont and Goochland Terrane subprovinces have the greatest 900
potential for radon (Fig. 22). Goodell (1989) reports that homes in the Piedmont have a 901
30% probability of high radon values, with local values exceeding 50% in homes located 902
in dark gray phyllites, graphite schists, slates, and hornfels, many of which likely fall 903
within these two subprovinces. Homes located in the Mesozoic basins have radon values 904
equivalent to the rest of the Piedmont (Goodell, 1989). 905
906
907
908
31
Valley and Ridge and Appalachian Plateau Provinces 909
The rocks of the Valley and Ridge geologic province of Virginia include Paleozoic 910
sedimentary rocks dominated by the carbonates, shales, and sandstones that have been 911
folded and faulted. Similar are the rocks present in the Appalachian Plateau province, 912
with the addition of bituminous coal and a minimal amount of structural deformation to 913
the rocks. In both provinces, the dark shales and soils derived from carbonate and dark 914
shale bedrock show the highest concentrations of radon in the province (e.g., Schultz et 915
al., 1992; Gundersen, 1993). In general, most carbonate rocks have low concentrations of 916
uranium, but the residual soils developed from the underlying bedrock are commonly 917
elevated in uranium. Research by Schultz et al. (1992) in the Great Valley of West 918
Virginia suggest that the deepest, most mature soils have the highest radium and radon 919
concentrations, presumably from the dissolution of the carbonates and the concentration 920
of base metals, including uranium. Elevated concentrations of soil-gas radon have been 921
found in the Elbrook, Conococheague, and Beekmantown formations, all of which are 922
interbedded limestones and dolomites; and in soils underlain by the Martinsburg 923
Formation, characterized by zones of dark shales (Schultz et al., 1992). Gundersen 924
(1993) reports the findings of aerial radiometric data in Virginia (Texas Instruments 925
Incorporated, 1980), where uranium anomalies are associated with Devonian black shales 926
to which the Martinsburg belongs. Specifically, the anomalies were present in 1) the 927
sandstones and shales of the Chemung Formation; 2) shale and sandstone of the 928
Hampshire Formation; 3) sandstone, shale, and coal of the Pocono Formation; 4) 929
limestone and shale of the Greenbrier Group; and 5) with some of the Pennsylvanian 930
sandstones, shales, and coals, where the upper Devonian-Pennsylvanian sandstone units 931
in Virginia have only local areas of high radioactivity (Baillieul and Daddazio, 1982; 932
Gundersen, 1993). Additionally, the enlargement of fractures and cave systems would 933
likely increase the advection of radon, although Schultz et al. (1992) found no increase in 934
radon concentration with respect to proximity to fractures, joints, or faults in the Great 935
Valley. 936
The numerous studies listed previously indicate that some localities in the Valley and 937
Ridge contain some of the highest concentrations of radon in the Commonwealth. 938
Additionally, Goodell (1989) found the highest probabilities of radon exceeding the 939
32
threshold value in the Valley and Ridge, where they average 50%, with values exceeding 940
70% in houses located on black or dark gray shales. In general, the US-EPA has 941
designated the Valley and Ridge as having a high potential for radon (Fig. 22). 942
943
RADON POTENTIAL MAPS FOR VIRGINIA 944
The preceding section illustrates the general usefulness of geologic maps for providing 945
a good first approximation of delineating areas that may have the presence of 946
radioactivity. However, the previous research mentioned shows the heterogeneity of 947
radon concentrations within a geologic province, and even within a specific rock 948
formation. These studies illustrate some of the shortcomings of only relying on geologic 949
maps or soil surveys for predicting the levels of radioactivity, as anomalies are often 950
missed. Several factors that limit the usefulness of geologic maps include 1) uranium is 951
usually not uniformly distributed in each geologic unit shown on maps; 2) generalized 952
geologic maps may not show fault zones, which can be enriched in uranium; 3) fault 953
zones may be soil covered and inconspicuous to bedrock mappers, leading to their 954
omission on geologic maps; and 4) the complexity of the soil physics that cannot be 955
measured by mapping. In short, geologic maps alone can, at best, be used to produce 956
generalized radon-risk maps. 957
Most radon potential maps such as those developed by the U.S. Geological Survey 958
(USGS) for the US-EPA are very dependent on geological maps. The US-EPA in 959
cooperation with the USGS published Open-File Report 93-292, titled Geologic Radon 960
Potential of EPA Region 3, which includes Virginia (Schumann, 1993). The purpose and 961
intended use of this report was 1) to help identify areas where states can target their radon 962
program resources; 2) to provide guidance in selecting the most appropriate building code 963
options for areas; and 3) to provide general information on radon and geology for each 964
state for federal, state, and municipal officials dealing with radon issues. In this report 965
(EPA Region 3), the radon potential map is based on countywide generalizations of the 966
bedrock geology. The map is also based on a database of indoor radon measurements to 967
verify the report’s conclusions. The EPA’s national indoor radon database consisted of 968
about 100,000 home-tests, of which about 1,100 measurements were from the 969
33
Commonwealth of Virginia. The EPA map is also based on a consideration of 970
aeroradioactivity. 971
As a follow up to the USGS Geologic Radon Potential study, the US-EPA (2006) 972
published a series of Radon Zones Maps, including one for Virginia (Fig. 22). They 973
developed three tiers of potential radon zones derived from the results of their national 974
radon survey program in conjunction with the USGS. The zones show areas with a high, 975
an intermediate, or a low percentage of homes with an indoor radon problem (Fig. 22), 976
where: 977
• Zone 1 has the highest indoor radon potential and shows counties where untested 978
homes probably have an average indoor radon level greater than 4 pCi/L; 979
• Zone 2 has moderate indoor radon potential and shows counties where untested 980
homes probably have an average indoor radon level between 2 and 4 pCi/L; 981
• Zone 3 has the lowest indoor radon potential and shows counties where untested 982
homes probably have an average indoor radon level less than 2 pCi/L. 983
Homes possessing elevated levels of radon have been documented in all three zones; 984
and serves as a reminder that these maps are generalized and should only be used as a 985
first approximation of radon potential. These maps were only intended to aid in the 986
resource-allocation-decision-making-process of national, state, and local organizations as 987
radon-resistant building codes are implemented (US-EPA, 2006). 988
989
SUMMARY 990
The diversity of Virginia’s geology, geography, and climatic setting, when combined 991
with its rapidly increasing population, has created an environment prone to geologic 992
hazards. The primary hazards include lowlands flooding, landslides and debris flows in 993
steep terrain, subsidence from sinkholes in karst areas, and radon gas emissions into 994
homes. Nationwide, flooding is the leading weather-related killer in the U.S., costing an 995
average of $3.7 billion annually; and flooding is the cause of 90 percent of all natural 996
disaster damage, excluding agricultural losses due to drought. In Virginia, combined 997
flooding and landslides are responsible for the greatest loss of life and property in recent 998
34
history. These hazards are created by the combination of 1) heavy and prolonged rainfall; 999
2) steep mountainous topography of the central Appalachians; and 3) the encroachment 1000
of human development onto these flood-prone landscapes. The hazard of sinkhole 1001
development is primarily found in terrain underlain by carbonate rock. Geomorphic 1002
processes active in karst result in subsidence, sinkhole flooding, and surface water 1003
recharge of groundwater aquifers. Human induced changes to the local hydrology can 1004
trigger and exacerbate subsidence, flooding, and groundwater contamination hazards. 1005
Successful mitigation and remediation of karst hazards require mitigation that address the 1006
karst processes. In addition to land surface processes, radon is also a risk in Virginia. 1007
Radon is a naturally occurring radioactive gas formed as a decay-product of uranium, a 1008
radioactive element that is a known human carcinogen. Radon may accumulate inside a 1009
home due to a number of factors, including rock and soil types, weather conditions, soil 1010
moisture levels, construction style of the home, and from negative air pressure inside a 1011
home. Radon is present in varying concentrations in nearly all rocks and soils throughout 1012
the U.S., but has a tendency to be highest in granitic rocks, dark marine shales and 1013
limestones, and environments that have been severely structurally deformed. All five 1014
geologic provinces contain specific formations that have elevated concentration of radon. 1015
The provinces of the Valley and Ridge, as well as the Piedmont show a greater propensity 1016
for high radon concentrations in homes. Current radon potential maps are useful as a first 1017
approximation for assessing radon concentrations, but more data of in-situ radon 1018
concentrations and detailed geologic mapping are necessary to assess the radon levels of 1019
individual dwellings. 1020
1021
ACKNOWLEDGEMENTS 1022
Virginia Division of Geology and Mineral Resources colleague James A. Lovett 1023
graciously provided the photograph of the sinkhole associated with an underground 1024
gypsum mine. The authors express their appreciation for the helpful comments and 1025
suggestions offered on this manuscript by colleagues Christine May, Karen Hostettler, 1026
and William Lassetter. Funding for radiocarbon dates was provided by the U.S. 1027
Geological Survey. 1028
1029
35
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National Uranium Resource Evaluation Report PGJ/F-114 (82), 46 p. 1034 1035 Baillieul, A., and Dexter, J.J., 1982, Evaluation of uranium anomalies in the Hylas zone 1036 and northern Richmond basin, east-central Virginia: in Goodknight, C.S., and Burger, 1037 J.A., eds., Reports on investigations of uranium anomalies: U. S. Department of 1038 Energy, National Uranium Resource Evaluation Report GJBX-222 (82). 97 p. 1039 1040 Beard, L.R., 1975, Generalized evaluation of flash flood potential. Austin: Univ. Texas 1041 Center Water Res. Tech. Rpt. CRWR-124., p. 1-27. 1042 1043 Bell, A.M., 1986, Morphology and stratigraphy of terraces in the upper Shenandoah 1044 Valley, Virginia: M.S. Thesis, West Virginia University, Morgantown, 160 p. 1045 1046 Berquist, C.R., Jr., Cooper, J.M., and Goodwin, B.K., 1990, Radon from Coastal Plain 1047 sediments, Virginia: Preliminary results: Geological Society of America, Abstracts 1048 with Programs, v. 22, no. 2, p. 4-5. 1049 1050 Berryhill, W.S., Jr., 1989, The impact of agricultural practices on water quality in karst 1051 regions, in Beck, B.F., ed., Engineering and Environmental Impacts of Sinkholes and 1052 Karst, Proc. Third Multidiscipinary Conference on Sinkholes and the Engineering and 1053 Environmental Impacts of Karst. St. Petersburg Beach, FL. A.A. Balkema, 1054 Rotterdam, p. 159-164. 1055
Boyer, D.G., and Pasquarell, G.C., 1994, Agricultural land use impacts on karst 1056 groundwater quality. American Water Resources Association, Proceedings of the 1057 Symposium on Effects of Human-Induced Changes on Hydrologic Systems, p. 791- 1058 799. 1059 1060 Boyer, D.G., and Pasquarell, G.C., 1999, Agricultural land use impacts on bacterial water 1061 quality in a karst groundwater aquifer. Journal of the American Water Resources 1062 Association, 35(2): 291-300. 1063 1064 Brady, N.C., and Weil, R.C., 1996, The nature and properties of soils: Prentice Hall, New 1065 Jersey. 1066 1067 Brookins, D., 1990, The indoor radon problem: Columbia University: New York. 1068 1069 Camp, J.D., and Miller, E.M., 1970, Flood of August, 1969 in Virginia: U.S. Geological 1070 Survey Water Resources Division Open-File Report, 120 p. 1071 1072 Carson, M.A., and Kirkby, M., 1972, Hillslope form and process. London: Cambridge 1073 University Press. 1074 1075
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Christie, L., 2006, 100 fastest growing counties, CNNMoney.com 1076 http://money.cnn.com/2006/03/15/real_estate/fastest_growing_US_counties/index.htm 1077 1078 Clark, G.M., 1987, Debris slide and debris flow historical events in the Appalachians 1079 south of the glacial border in J.E. Costa and G.F. Wieczorek, (eds), Debris 1080 Flows/Avalanches: Process, Recognition and Mitigation: Geological Society 1081 of America Reviews in Engineering Geology, Vol. VII, p. 125-179. 1082 1083 Cohen, B.L., 1989, Radon, a homeowner’s guide to detection and control: Consumers 1084 Union, New York. 1085 1086 Colucci, S.J., Jacobson, R.B., and Greco, S., 1993, Meteorology of the storm of the 1087 November 3–5, 1985, in West Virginia and Virginia, in Jacobson, R.B., ed., 1088 Geomorphic studies of the storm and flood of November, 1985, in the upper Potomac 1089 and Cheat River basins in Virginia and West Virginia: U.S. Geological Survey 1090 Bulletin 1981, p. B1–B31. 1091 1092 Costa, J.E., 1988, Rheologic, geomorphic, and sedimentologic differentiation of water 1093 floods, hyperconcentrated flows, and debris flows. in Baker, V.R., Kochel, R.C., and 1094 Patton, P.C., (eds) Flood Geomorphology, p. 113-122., John Wiley, New York. 1095 1096 Douglas, T.J., Wooten, R.M., Gillon, K.A., Bauer, J.B., 2007, Geohazards and 1097 transportation aspects of sulfidic rock in North Carolina, (abs.): 7th Technical Forum, 1098 Geohazards in Transportation in the Appalachian Region, Asheville, NC. 1099 1100 Duffy, D.L., 1991, The geomorphology of alluvial fan deposits on the west flank of the 1101 Blue Ridge, Augusta County, Virginia: M.S. thesis, Old Dominion University, 1102 Norfolk, VA, 118 p. 1103 1104 Eaton, L.S., 1999, Debris flows and landscape evolution in the Upper Rapidan Basin 1105 Blue Ridge Mountains, Central Virginia: Ph.D. Thesis, University of 1106 Virginia, 154 p. 1107 1108 Eaton, L.S., Morgan, B.A., and Blair, J.L., 2001, Surficial geology of the Fletcher, 1109 Madison, Stanardsville, and Swift Run Gap 7.5-minute quadrangles, Madison 1110 Greene, Albemarle, Rockingham and Page Counties, VA: U. S. Geological 1111 Survey Open File Reports 01-92, 4 maps 1:24,000-scale. 1112 1113 Eaton, L.S., Morgan, B.A., Kochel, R.C., and Howard, A.D., 2003a, Quaternary deposits 1114 and Landscape evolution of the central Blue Ridge of Virginia: Geomorphology, 1115 v. 1351, p. 1-16. 1116 1117 Eaton, L.S., Morgan, B.A., Kochel, R.C., and Howard, A.D., 2003b, Role of debris 1118 Flows in long-term landscape denudation in the central Appalachians of 1119 Virginia: Geology, v. 31, p. 339-342. 1120 1121
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Quadrangle, Virginia: Virginia Division of Geology and Mineral Resources 1203 Publication Open File Report 10-2, 1:24,000 scale. 1204
1205 Henry, M.E, Kaeding, M.E., and Monteverde, D.H., 1991, Radon in soil gas and gamma-1206 ray activity of rocks and soils at the Mulligan Quarry, Clinton, New Jersey: in 1207 Gundersen, L.C.S., and Wanty, R.B., (eds.) , Field studies of radon in rocks, soils, and 1208 water, U.S. Geological Survey Bulletin 1971, 334 p. 1209 1210 Hess, C.T., Weiffenbach, C.V., and Norton, S.A., 1982, Variations of airborne and 1211 waterborne Rn-222 in houses in Maine: Environment International, v.8, p. 59-66. 1212 1213
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Hubbard, D.A., Jr., 1983, Selected karst features of the northern Valley and Ridge 1214 province, Virginia. Virginia Division of Mineral Resources, Publication 44, one sheet. 1215 1216 Hubbard, D.A., Jr., 1988, Selected karst features of the central Valley and Ridge 1217 province, Virginia. Virginia Division of Mineral Resources, Publication 83, one sheet. 1218 1219 Hubbard, D.A., Jr., 1999, Highways in Virginia karst: Resource and hazard 1220 considerations. 50th Annual Highway Geology Symposium & TRB Karst Meeting, 1221 Proc. & Field Trip Guide, 13-22. 1222 1223 Hubbard, D.A., Jr., 2001, Selected karst features of the southern Valley and Ridge 1224 province, Virginia. Virginia Division of Mineral Resources, Publication 167, one 1225 sheet. 1226 1227 Hubbard, D.A., Jr., 2003, Use of regional sinkhole mapping for sinkhole susceptibility 1228 maps, in, Beck, B.F., ed., ASCE Geotechnical Special Publication No. 122, p. 61-71. 1229 1230 Hubbard, D.A., Jr. and Sterrett, R.M., 1994, Ground-water contamination in Virginia 1231 karst: Case studies (abs.). Program and Abstracts, 37th Annual Meeting of the 1232 Association of Engineering Geologists, Williamsburg, VA, p. 49. 1233 1234 Jacobson, R.B., Miller, A.J., and Smith, J.A., 1989, The role of catastrophic geomorphic 1235 events in central Appalachian landscape evolution: Geomorphology, v. 2, p. 257–284. 1236 1237 Jurgens D.M., 1997, An investigation of the controls of slope stability in the 1995 debris 1238 flows of Madison County, Virginia. BS thesis, Bucknell University, Lewisburg, 1239 Pennsylvania, 34 p. 1240 1241 King, P.B., 1950, Geology of the Elkton area, Virginia: U. S. Geological Survey 1242 Professional Paper 230, 82 p. 1243 1244 Kite, J. S., 1992, Geomorphology research in the Shenandoah Valley: West Virginia 1245 University Geology Program: in Whittecar, G.R., editor, Alluvial Fans and boulder 1246 streams of the Blue Ridge Mountains, west-central Virginia: Southeastern 1247 Friends of the Pleistocene, 1992 Field Trip, p. 113-118. 1248 1249 Kochel, R.C., 1987, Holocene debris flows in central Virginia in Costa, J.E., and 1250 Wieczorek, G.F., (Eds.), Debris flows/avalanches: Process, recognition, and 1251 mitigation: Geological Society of America Reviews in Engineering Geology, v. 7, p. 1252 139-155. 1253 1254 Kochel, R.C., 1990, Humid fans of the Appalachian Mountains, in Rachocki, A.H., and 1255 Church, M. (Eds.), Alluvial Fans: A field approach: Wiley, New York, p.109-129. 1256 1257 1258 1259
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Kochel, R.C., 1992, Geomorphology of alluvial fans in west-central Virginia: in 1260 Whittecar, G.R., (Ed.), Alluvial fans and boulder streams of the Blue Ridge 1261 Mountains, west-central Virginia: Southeastern Friends of the Pleistocene, 1992 Field 1262 Trip, p.47-60. 1263 1264 Kochel, R.C., and Johnson, R.A., 1984, Geomorphology and sedimentology of humid 1265 Temperate alluvial fans, central Virginia. in Gravels and Conglomerates, E. Koster 1266 and R. Steel, (eds.) Canadian Society of Petroleum Geologists Memoir 10, p. 109-122. 1267 1268 Kochel, R C., Miller, J.R, Lord, M., and Martin, T., 2005, Geomorphic 1269 problems with in-stream structures using natural channel design strategy for stream 1270 restoration projects in North Carolina, Abstracts with Programs, Geological Society of 1271 America, vol.37, no.7, p. 329. 1272 1273 Krason, J., Johnson, S.S., Finley, P.D., and Marr, J.D., Jr., 1988, Geochemistry and 1274 radioactivity in the Powhatan area, Virginia: Virginia Division of Mineral resources 1275 Publication 78, 60 p. 1276 1277 Kunz E., and Sevc J., 1978, Lung cancer mortality among uranium miners 1278 (methodological aspects): Health Physics, v.35, p.579. 1279 1280 Lafavore, M., 1987, Radon: The invisible threat: Rodale Press, Pennsylvania, 256 p. 1281 1282 Latham, R.S., Wooten, R.M., Witt, A.C., Gillon, K.A., 2007, Big slow movers: a look at 1283 weathered rock slides in western North Carolina: (abs.), 7th Technical Forum, 1284 Geohazards in Transportation in the Appalachian Region, Asheville, NC. 1285 1286 Leopold, L.B., 1968, Hydrology for Urban Land Planning – A Guidebook on the 1287 Hydrologic Effects of Urban Land Use, U.S. Geological Survey Circular 554, 18p. 1288 1289 Litwin, R.J., Morgan, B.A., Eaton, L.S., Wieczorek, G.F., and Smoot, J.P., 1290 2001, Proxy climate evidence from Late Pleistocene Deposits in the Blue Ridge 1291 of Virginia: U.S. Geological Survey Open File Report 01-406, p. 79. 1292 1293 Lowder, W.M., 1985, Part One - Radon: Overview, in Indoor Air and Human Health: in 1294 Gammage, R.B. and Kaye, S.V., (eds.), Lewis Publishers: Michigan, p. 39-41. 1295 1296 Mason, J.E., 1992, Surficial geology of alluvial fans and terraces on the western 1297 Flanks of the Blue Ridge Mountains between Elkton and Port Republic, VA: M.S. 1298 thesis West Virginia University, Morgantown, WVA, 74 p. 1299 1300 May, C.L., 2002, Debris flows through different forest age classes in the central Oregon 1301 Coast Range: Journal of the American Water Resources Association 38(4), p. 1097-1302 1113. 1303 1304 1305
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Michaels, P.J., 1985, Virginia climate advisory: Charlottesville, University of Virginia, v. 1306 9, no. 2, 30 p. 1307 1308 Miller, A.J., 1990, Flood hydrology and geomorphic effectiveness in the central 1309 Appalachians: Earth Surface Processes and Landforms, v. 15, p. 119–134. 1310 1311 Mills, H.H., Campbell, E., Floyd, J., Hibbits, H., and Williams, S., 2005, Preliminary 1312
Surficial Geologic Map of the Grottoes Quadrangle, Virginia, Division of Mineral 1313 Resources Open File Report 05-05. 1314
1315 Mills, H.H, Brakenridge, G.R., Jacobson, R.B., Newell, W.L., Pavich, 1316 M.J., and Pomeroy, J.S., 1987, Appalachian mountains and plateaus, DNAG, Decade 1317 of North American Geology; Geological Society of America., Boulder, CO. 1318 1319 Milvy, P., and Cothern C.R., 1990, Scientific background for the development of 1320 regulations for radionuclides in drinking water; in Cothern, C.R., and Rebers, P.A., 1321 (eds.), Radon, radium and uranium in drinking water, Lewis Publishers, Michigan, p. 1322 1-16. 1323 1324 Morgan, B.A., and Wieczorek, G.R., 1996, Debris flows and landslides resulting from the 1325 June 27, 1995, storm on the North Fork of the Moormans River, Shenandoah National 1326 Park, Virginia: U.S. Geological Survey Open-File Report 96-503. 1327 1328 Morgan, B.A., Eaton, L.S., and Wieczorek, G.R., 2003, Pleistocene and Holocene 1329 colluvial fans and terraces in the Blue Ridge region of Shenandoah National Park, 1330 Virginia, U. S. Geological Survey, Report: OF 03-0410, 25 p., 1 Sheet. 1331 1332 Mose, D.G., and Mushrush, G.W., 1987, Correlation between indoor radon and geology 1333 in VA and MD: Geological Society of America, Abstracts with Programs, v. 19, p. 1334 779. 1335 1336 Mose, D.G., and Mushrush, G.W., 1988, The effect of home construction on indoor radon 1337 in Virginia and Maryland: Environment International, v.14, p. 223-230. 1338 1339 Mose, D.G. and Mushrush, G.W., 1997a, Carcinogenic risk associated with radon-1340 enriched well water: Energy Sources, v.19, p. 705-713. 1341 1342 Mose, D.G., and Mushrush, G.W., 1997b, Variable spatial and seasonal hazards of 1343 airborne radon: Atmospheric Environment, v. 31(21), p. 3523-3530. 1344 1345 Mose, D.G., and Mushrush, G.W., 1999, Influence of local geology on the concentration 1346 of indoor radon in Maryland: Energy Sources, v. 21, p. 715-721. 1347 1348 Mose, D.G., Mushrush, G.W., and Chrosniak, C., 1990, Radioactive hazard of potable 1349 water in Virginia and Maryland: Bulletin of Environmental Contamination and 1350 Toxicology, v. 44, p. 508-513. 1351
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1352 Mose, D.G., Mushrush, G.W., and Chrosniak, C., 1991, Seasonal indoor radon variations 1353 related to precipitation: Environmental and Molecular Mutagenesis, v. 17, p. 223-230. 1354 1355 Mose, D.G., Mushrush, G.W., and Chrosniak, C., 1992a, A two-year study of seasonal 1356 indoor air radon variations in southern Maryland: Environmental Pollution, v. 76, p. 1357 195-199. 1358 1359 Mose, D.G., Mushrush, G.W., and Chrosniak, C., 1992b, Soil radon, permeability and 1360
indoor radon prediction: Environmental Geology Water Science., v. 19, no. 2, p. 91-1361 96. 1362
1363 Mose, D.G., Mushrush, G.W., and Kline, S.W., 1988a, Geology and time dependent 1364 indoor radon variations in VA and MD: Geological Society of America, Abstracts 1365 with Programs, v. 20, p. 56-57. 1366 1367 Mose, D.G., Mushrush, G.W., and Kline, S.W., 1988b, The interaction of geology, 1368 weather and home construction on indoor radon in northern Virginia and southern 1369 Maryland: Northeastern Environmental Science, v. 7, p. 15-29. 1370 1371 Mose, D.G., Mushrush, G.W., and Simoni, F.V., 2005, Reduction of radon in municipal 1372 wells in Virginia and Maryland: Contaminated Soils, v. 9, p. 375-392. 1373 1374 NAS, 1999, Health Effects of Exposure to Radon: BEIR VI. National Academy of 1375 Sciences: Washington, D.C. 1376 1377 Nazaroff, W.W., and Nero, A.V., 1988, Radon and its decay products in indoor air, 1378 Wiley, New York. 1379 1380 Neuendorf, K.K.E., Mehl, J.P., Jr., and Jackson, J.A. (eds.), 2005, Glossary of Geology, 1381 AGI, Alexandria, VA, 779 p. 1382 1383 Neuschel, S.K., 1970, Correlation of aeromagnetics and aeroradioactivity with lithology 1384 in the Spotsylvania area, Virginia: Geological Society of America Bulletin, v. 81, no. 1385 12, p. 3575-3582. 1386 1387 Neuschel, S.K., Bunker, C.M., and Bush, C.A., 1971, Correlation of uranium, thorium, 1388 and potassium with aeroradioactivity in the Berea area, Virginia: Economic Geology, 1389 v. 66, no. 2, p. 302-308. 1390 1391 Newson, M., 1980, The geomorphological effectiveness of floods; A contribution 1392 simulated by two recent events in mid-Wales: Earth Surface Processes and Landforms, 1393 v. 5, p. 1-16. 1394 1395 NTP, [National Toxicology Program], 2005, 11th Report on Carcinogens: National 1396 Toxicology Program, North Carolina. 1397
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1398 Otton, J.K., 1992, Radon in soil gas and soil radioactivity in Prince Georges County, 1399 Maryland, U.S. Geological Survey Open File Report no. 92-11, 18 p. 1400 1401 Otton, J.K., Schumann, R.R., Owen, D.E., Thurman, N. and Duval, J.S., 1988, Map 1402 showing radon potential of rocks and soils in Fairfax County, Virginia: U. S. 1403 Geological Survey Miscellaneous Field Studies Map MF-2047, scale 1:62,500. 1404 1405 Padgett, D.A., 1993, Remote sensing applications for identifying areas of vulnerable 1406 hydrogeological and potential sinkhole collapse within highway transportation 1407 corridors: in Beck, B.F., (ed.), Applied Karst Geology, Proceedings of the Fourth 1408 Multidisciplinary Conference on Sinkholes and the Engineering and Environmental 1409 Impacts of Karst, Panama City, FL. A.A. Balkema, Rotterdam, p. 285-290. 1410 1411 Pierson, T.C., and Costa, J.E., 1987, A rheologic classification of subaerial sediment- 1412 water flows, in Costa, J.E., and Wieczorek, G.F., eds., Debris flows/avalanches: 1413 Process, recognition, and mitigation: Geological Society of America Reviews in 1414 Engineering Geology, v. 7, p. 1–12. 1415 1416 Proctor, R.N., 1995, Cancer Wars. How Politics Shapes What We Know and Don’t 1417 Know About Cancer: Basic Books, New York. 1418 1419 Ritter, D.F., Kochel, R.C., and Miller, J.E., 2002, Process Geomorphology, McGraw Hill, 1420 423 p. 1421 1422 Samet, J.M., 1994, Health Effects of Radon in Radon: prevalence, measurements, health 1423 risks and control: in Nagda, N.L., (ed.), ASTM Manual Series: MNL 15, 1 Publication 1424 Code 28-015094-17, ASTM: Philadelphia. 1425 1426 Sas, R.J. Jr., and Eaton, L.S., 2006a, Geologic controls of basin denudation from debris 1427 flows in Rockbridge County, Virginia, Abstracts with Programs - Geological Society 1428 of America, v. 38, no.2, p. 84. 1429 1430 Sas, R.J. Jr., and Eaton, L.S., 2006b, Landslide Warning Action Plan: Prioritizing Areas 1431 of High Risk Along Blue Ridge Parkway Using VisiData. Eos Transactions, 1432 American Geophysical Union, 87(52), H51B-0486. 1433 1434 Schultz, A., Wiggs, C.R, and Brower, S.D., 1992, Geologic and environmental 1435 implications of high soil-gas radon concentrations in the Great Valley, Jefferson and 1436 Berkeley counties, West Virginia: in Gates, A.E., and Gundersen, L.C.S. (eds.), 1437 Geologic controls on radon, Geological Society of America Special Paper, v. 271, 1438 p.29-44. 1439 1440 Schumann, R. R., 1993, Geologic radon potential of EPA region 3: Delaware, Maryland, 1441 Pennsylvania, Virginia, and West Virginia, 1993, U.S. Geological Survey Open-File 1442 Report 93-292-C, 206 p. 1443
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1444 Schumann, R. R., and Owen, D. E., 1988, Relationships between geology, equivalent 1445 uranium concentration, and radon in soil gas, Fairfax County, Virginia, U.S. 1446 Geological Survey Open-File Report 88-18, 28 p. 1447 1448 Sevc, J., Kunz, E., and Placek, V., 1976. Lung cancer in uranium mines and long-term 1449 exposure to radon daughters: Health Physics, v.30, p.433. 1450 1451 Sidle, R. C., A. J. Pearce, and C. L. O'Loughin. 1985. Hillslope stability and land use. 1452
Water Resources Monograph 11. American Geophysical Union, Washington, D.C. 1453 1454 Simmons, D.W., 1988, Geomorphology and sedimentology of humid-temperate 1455 Alluvial fans along the west flank of the Blue Ridge Mountains, Shenandoah 1456 Valley, Virginia: M.S. thesis, Southern Illinois University, Carbondale, IL, 1457 107 p. 1458 1459 Simpson, P.S., and Simpson, J.H., Jr., 1970, Torn land: Lynchburg, Virginia, J.P. Bell 1460 Co., 429 p. 1461 1462 Smith, J.A., Baeck, M.L., and Steiner, M., 1996, Catastrophic rainfall from an upslope 1463 thunderstorm in the central Appalachians: The Rapidan storm of June 27, 1995: 1464 WaterWater Resources Research, v. 32, p. 3099–3113. 1465 1466 Smith, R.C., II, Reilly, M.A., Rose, A.W., Barnes, J.H., and Berkheiser, S.W., Jr, 1987, 1467 Radon: A profound case: Pennsylvania Geology, v. 18, p. 1-7. 1468 1469 Stephenson, J.B., and Beck, B.F., 1995, Management of the discharge quality of highway 1470 runoff in karst areas to control impacts to ground water – A review of relevant 1471 literature: in Beck, B.F., ed., Karst GeoHazards, Proceedings of the Fifth 1472 Multidisciplinary Conference on Sinkholes and the Engineering and Environmental 1473 Impacts of Karst, Gatlinburg, TN. A.A. Balkema, Rotterdam, p. 297-321. 1474 1475 Stringfield, V.T., and Smith, R.C., 1956, The relation of geology to drainage, floods, and 1476 landslides in the Petersburg area, West Virginia: West Virginia Geologic and 1477 Economic Survey Report of Investigations 13, 19 p. 1478 1479 Terranova, T.F., 1987, Multivariate analysis of geological, hydrological, and soil 1480 mechanical controls on slope stability in central Virginia (MS Thesis), Southern 1481 Illinois University, Carbondale. 1482 1483 Terranova, T. F., and Kochel, R C., 1987 Multivariate analysis of factors related to debris 1484 avalanching in Nelson County, central Virginia, Abstracts with Programs - Geological 1485 Society of America, v.19, no.7, p.866. 1486 1487 1488
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UNSCEAR, [United Nations Scientific Committee on the Effects of Atomic Radiation], 1489 2000, Sources and Effects of Ionizing Radiation: United Nations Scientific Committee 1490 on the Effects of Atomic Radiation (UNSCEAR), 2000 Report to the General 1491 Assembly with Scientific Annexes: Set of two volumes. United Nations: June, 2001. 1492 1493 US-EPA, 2006, Radon (Rn) [Online]. November 17, 2006 [cited on November 20, 1494 2006].URL http://www.epa.gov/radon. 1495 1496 USGS, 2006, Flood Hazards: A National Threat, USGS Fact Sheet 2006-3026. 1497 http://landslides.usgs.gov/recent/archives/1997kinsey.php 1498 1499 USGS, 2007, Natural Hazards: A National Threat, USGS Fact Sheet 2006-3009. 1500 http://pubs.usgs.gov/fs/2007/3009/2007-3009.pdf 1501 1502 VDEM, (Virginia Department of Emergency Service), 2007, 1503 http://www.vaemergency.com/threats/flood/index.cfm 1504 1505 Wagoner, J.K., Archer, V.E, Lundin, F.E., Holaday, D.A., and Lloyd, J.W., 1965, 1506 Radiation as the cause of lung cancer among U.S. uranium miners: New England 1507 Journal of Medicine, v.273, p. 181-188. 1508 1509 Watts, C.F., and Whisonant, R.C., 1992, Slope stability problems along highway roadcuts 1510 in the Virginia Valley and Ridge Province: Geological Society of America Abstracts 1511 with Programs, v.24, no.2, p.72. 1512 1513 Wieczorek, G.F., Morgan, B.A., and Campbell, R.H., 2000, Debris-flow hazards in the 1514 Blue Ridge of central Virginia: Environmental and Engineering Geoscience, v. 6, p. 1515 3–23. 1516 1517 Wieczorek, G.F., Eaton, L.S., Yanosky, T.M., and Turner, E.J., 2006, Landslide activity 1518 along Meadow Run, Shenandoah Valley, Virginia, Journal of Landslides, v. 3, p. 95-1519 106. 1520 1521 Whittecar, G.R., and Duffy, D.L., 1992, Geomorphology and stratigraphy of Late 1522 Cenozoic alluvial fans, Augusta Co., Virginia, in Whittecar, G.R. (Editor), Alluvial 1523 Fans and Boulder streams of the Blue Ridge Mountains, west-central Virginia: 1524 Southeastern Friends of the Pleistocene, 1992 Field Trip, p. 79-112. 1525 1526 Whittecar, G.R., and Duffy, D.L, 2000, Geomorphology and stratigraphy of late 1527 Cenozoic alluvial fans, Augusta County, Virginia, U.S.A., Southeastern Geology, 1528 vol.39, no.3-4, p.259-279, Oct 2000 1529 1530 Wilkening, M., 1990, Radon in the Environment: Elsevier, New York. 1531 1532 1533 1534 1535
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Williams, G.P., and Guy, H.P., 1973, Erosional and depositional aspects of Hurricane 1536 Camille in Virginia, 1969: U.S. Geological Survey Professional Paper 804, 80 p. 1537 1538 Wilson, G.C., 1987, Debris flows and alluvial fan development in the Appalachians 1539 (M.S. Thesis), Southern Illinois University, Carbondale. 1540 1541 Winchester, P.W., Jr., 1985, Rockslide on Interstate 40 in North Carolina blocks both 1542 lanes approximately four miles from Tennessee-North Carolina state line, AEG 1543 Newsletter, vol.28, no.3, pp.16-18, Jul 1985. 1544 1545 Wolman, M.G., 1967, A cycle of sedimentation and erosion in urban river channels. 1546 Geogr. Annlr. 49-A:385-95. 1547 1548 Wolman, M.G., and Miller, J.P., 1960, Magnitude and frequency of forces in geomorphic 1549 processes: Journal of Geology, v. 68, p. 54–74. 1550 1551 Zhang, K., Douglas, B., Leatherman, S., 2002, Do storms cause long-term 1552 beach erosion along the U.S. east barrier coast?, Journal of Geology, vol.110, no.4, 1553 p.493-502, Jul 2002 1554 1555 1556 FIGURE CAPTIONS 1557 1558 Figure 1. Areas affected by debris-flow events in Virginia and West Virginia from 1949 1559 to 1996. 1—June 17–18, 1949, storm in western Virginia and eastern West Virginia; 2—1560 August 19–20, 1969, storm in western Nelson County, Virginia; 3—November 3–5, 1561 1985, storm in western Virginia and eastern West Virginia; 4—June 27, 1995 storm of 1562 the Blue Ridge Mountains. Site 4a corresponds to western Madison and Greene Counties; 1563 4b is western Albemarle; and 4c depicts eastern Rockbridge County. 1564
Figure 2. Flood envelope curve for United States, including discharge values 1565 for Hurricane Camille (Nelson County), the Rapidan Storm (Madison County), the 1949 1566 storm (western Virginia), and the 1985 storm (western Virginia and eastern West 1567 Virginia). 1568 1569 Figure 3. Rheologic classification of sediment-water flows. Vertical boundaries A, B, and 1570 C are rheologic thresholds, and are a function of grain size distribution (here assumed to 1571 be coarse, poorly-sorted mixture) and sediment concentration. Moving from left to right, 1572 boundary A marks onset of yield strength; boundary B marks sudden, rapid increase in 1573 yield strength that permits static suspension of gravel and onset of liquefaction behavior; 1574 boundary C marks cessation of liquefaction behavior. Horizontal velocity boundaries, 1575 also function of grain-size distribution and sediment concentration as well as particle 1576 density, are determined by how stress is transmitted between particles during flow (from 1577 Pierson and Costa, 1987) 1578 1579 Figure 4. Map of central Shenandoah Valley and Blue Ridge Mountains depicting 1580 dominant debris flow and alluvial fan types (indicated by arrows). 1581
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1582 Figure 5a. Topographic sketch map and longitudinal cross section of an alluvial fan in the 1583 Shenandoah Valley between Waynesboro and Elkton at One Mile Run basin. Evidence 1584 of the old age of this fan includes its being graded to a high terrace level and post-1585 depositional dissection. Erosion by the South Fork of the Shenandoah River has removed 1586 a significant portion of the northern toe of this fan. This can be seen clearly by comparing 1587 the two fan profiles. Fans appear to be of varying ages along the eastern Shenandoah 1588 Valley because others are graded to lower terrace levels. 1589 1590 Figure 5b. Schematic diagram of down-fan variation in facies in the fluvial dominated 1591 Shenandoah Valley fans. Thickness in the columns are not to scale, but the range of 1592 observed or inferred (from drillers’ logs) thicknesses are given in parentheses. Proximal 1593 fan facies are dominated by poorly-sorted, coarse-grained, angular-to-subangular 1594 bouldery material. Mid-fan facies contain interbedded sand and subrounded cobble 1595 gravel. Distal fan facies are dominated by cobble to granule gravels and well-stratified 1596 sheet sands. 1597 1598 Figure 6. (A) Bankful flow from Hurricane Isabel (2003) at Meadow Run near Grottoes; 1599 and (B) the resulting coarse grained braided stream pattern downstream of figure A. 1600 1601 Figure 7a. Stratigraphy and sedimentology of a debris fan along the North Prong of Davis 1602 Creek, Nelson County. A) Cross sections show irregular nature of 1969 deposits across 1603 fan, resulting from shifting loci of depositional lobes. Trends in variations in texture and 1604 matrix composition with depth are useful in determining boundaries between debris flow 1605 events. 1606 Figures 7B to 7E) Debris flow deposits exposed by the June 1995 storm in Madison 1607 County. Dashed lines shows boundary between two prehistoric debris flow deposits in 1608 photos B and D; and between saprolite and a debris flow unit in photo C. In photo E, the 1609 trowel easily penetrates a granitic clast of a debris flow deposit. 1610 1611 Figure 8: Debris flows and avalanches in Nelson County, Virginia, during 1969. A.) 1612 Flows deposited debris on small fans at the base of first-order hillslope channels near 1613 Lovingston. B.) Catastrophic erosion and impact forces from these flows removed some 1614 structures and devastated others (view from Davis Creek). Photos courtesy of Virginia 1615 Division of Mineral Resources. 1616 1617 Figure 9: Debris flows in Madison County, Virginia, from the 1995 storm. A) Two debris 1618 flows separated by 1.5 hours struck and moved a two story farm house; B) house was 1619 pushed forward by nearly 10 meters by the two events; C) large boulders deposited in 1620 1995 on the Generals fan, located 1 km northwest of Graves Mill. Arrow denotes person 1621 for scale. 1622 1623 Figure 10. A) Debris flow activity on Kirtley Mountain, western Madison County, 1624 Virginia, following Madison County 1995 storm. Debris-flow activity affected and 1625 denuded numerous low-order drainages. Arrows denote houses for scale. Dashed line 1626 denotes upper margins of debris fans. B) Surficial mapping of the Graves Mill area 1627
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(Eaton et al., 2001). Landforms on the simplified map are denoted as debris fans (df), 1628 terraces (t), and floodplains (fp). The letter ‘X’ marks the location of the dwelling 1629 destroyed in 1995 and is shown in figure 9b. 1630 1631 Figure 11. Ages of 11 debris flows in the upper Rapidan basin. Recurrence of debris 1632 flows was approximately every 2500 years (Eaton, 1999). The small circles represent 1633 samples from debris flow deposits, and their respective dates are listed in the table. Each 1634 vertical dashed line is interpreted as a discrete debris flow event. 1635 1636 Figure 12. A.) Cripple Creek, located in Nelson County, is typical of many first- and 1637 second-order tributaries in the Blue Ridge Mountains of Virginia. Note the abundance of 1638 cobbles and boulders. B.) An unnamed tributary of the upper Rapidan River denuded to 1639 bedrock and purged of nearly all boulders following the Madison County storm. 1640 1641 Figure 13. Schematic cross sections showing influence of bedrock structural 1642 characteristics on debris avalanche scar morphology. A.) Planar surfaces, where foliation 1643 and hillslope orientation are nearly normal. In these cases, failures were characterized by 1644 spalling of thin layers of bedrock along joint surfaces normal to slope. B.) Stepped 1645 surfaces, where foliation planes intersected hillslopes at small acute angles. These 1646 differences can be seen along the track of a single debris flow scar if its trend changes 1647 significantly downslope. 1648 1649 Figure 14. Topographic and structural controls of debris flows at Kirtley Mountain, 1650 Madison County. Note the dominant failure pattern to the southeast. 1651 1652 Figure 15. Figure 15. A) Rapidan River, 1 km downstream of Graves Mill during 1653 recession of the June 27, 1995 flood. Arrow denotes the remnants of the State Route 676 1654 bridge (view is upstream). Note the braided pattern of the channels emerging from the 1655 recession of the flow. B) This pattern resumed again during Hurricane Fran (Sept. 1996). 1656 The system has been repeatedly anthropogenically modified to a single narrow channel, 1657 and continues to be unstable. 1658 1659 Figure 16. Avulsion of Rapidan River at the State Route 676 bridge shortly after stream 1660 restoration in 2002. 1661 1662 Figure 17. A collapse sinkhole that formed during a rainfall event in Austinville, 1663 Virginia in September, 1989. 1664 1665 Figure 18. Sinkhole flooding that extended over part of a blind valley near Harrisonburg, 1666 Virginia after a rainfall event in October, 1996. 1667 1668 Figure 19a. A collapse sinkhole that formed above an underground limestone mine near 1669 Lowmoor, Virginia prior to 1990. 1670 1671 Figure 19b. A large collapse sinkhole that formed above an underground gypsum mine 1672 in Plasterco, Virginia in November, 1984. 1673
49
1674 Figure 20. A collapse sinkhole that formed over coalfield workings in the Richmond 1675 basin, Virginia in January 2007, (DMME photograph). 1676 1677 Figure 21. Geologic provinces of Virginia (after Gundersen, 1993). Gray areas indicate 1678 Mesozoic basins. Numbers indicate descending levels of radon potential (after US-EPA, 1679 2006). 1680
1681
50
Figure 1. 1682 1683
1684 1685 1686 Figure 2. 1687
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51
1691 Figure 3. 1692 1693
1694 1695 Figure 4. 1696 1697
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52
Figure 5. 1701 1702
1703 1704 Figure 6. 1705 1706
1707 1708 1709 1710 1711 1712 1713 1714 1715 1716 1717 1718
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Figure 7. 1719
1720 1721 Figure 8. 1722 1723
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54
Figure 9. 1727 1728
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1735 Figure 11. 1736 1737
1738 1739 Figure 12. 1740 1741
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56
Figure 13. 1747
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57
1759 1760 Figure 15. 1761 1762
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58
1768 1769 1770 1771 1772 Figure 17: 1773 1774
1775 1776 1777 1778 1779 1780 1781 1782 1783 1784 1785 1786
59
1787 Figure 18. 1788 1789
1790 1791 Figure 19. 1792 1793
1794
1795 1796 1797 1798
1799
60
Figure 20. 1800
1801 1802 1803 1804
61
Figure 21. 1805 1806
1807 1808 1809 Figure 22. 1810 1811
1812 1813 END OF FIGURES. 1814
