Water Resource Planning; an alternative to Expensive BMPs

FOR 595N: Surface Water Investigation l wye station

sarah artuso

final project analysis

IntroductionFor the past 10+ years, the City of Raleigh, the Triangle Transit Authority (TTA) and the State of North Carolina have been strongly considering the idea of constructing a rail system that will circulate throughout the Triangle. They insist that transportation of this kind has the ability to enhance and change the future of this area in an extremely powerful way. There are many ideas for where the locations of rail stops should be, but one seems most sensible for a central hub location. The site in question is comprised of 12 different parcels totalling approximately seven acres, and lies in the heart of the warehouse district. Some time ago, the TTA acquired the land and entered into a joint agreement with Cherokee Investment Company (also located in downtown Raleigh) for the redevelopment of this space. In the future, the site is foreseen to link ‘The Triangle Area’ with the rest of the Char-lanta region, as well as the eastern seaboard.

Project SummaryThe site is located in the warehouse district of downtown Raleigh at latitude 35.7773619413 and longitude -78.6470260808. The parcel numbers associated with the site include: 1703487885, 1703489817, 1703488736, 1703488631, 1703488450, 1703488314, 1703488250, 1703488076, 1703478969, 1703486115, 1703485081, and 1703475886.

The site has been used for industrial purposes for decades, resulting in extreme soil degradation and groundwater contamination. These areas of course must be remediated before any kind of development can occur. The site is also composed of 65% impervious surface (including all roofs and paved areas). As a result, surface runoff is having a severe impact on the water quality of streams to which the site drains (including 303d listed Rock Branch Creek).

This project strives to achieve multiple objectives:

The first goal is to better explain the process of generating appropriate information for permitting and regulation to landscape architects and designers. Because landscape architects generally do not speak the language valuable to concrete scientific analysis, finding a way to better explain critical information is key to design decision-making.

The second goal of this project is to investigate the before-and-after effects of development based on impervious surface levels, drainage area and the defined watershed boundary. Both existing conditions and those proposed through a series of design phases will be explored.

This analysis also aims to answer the following questions:

(1) What is the drainage area for the site in questions? How large is it? How was channel initiation threshold established?(2) In what direction are flowlines moving? How far is the site from the nearest RPW and TNW?(3) What is the impact of the existing conditions found on the site based on 65% of impervious surface?(4) What could be the impact of 5, 10 and 25-year designs proposed for the site?(5) What recommendations could be made to developers for future use of the site?

* RPW - Relatively Permanent Water

* TNW - Traditionally Navigable Water

Data CollectionData collected for use in this analysis included:

(1) National Hydrography Dataset, Neuse River Basin High Resolution Data (nhdftp.usgs.gov/SubRegions/High/NHD0302.zip) Shapefile name: NHD Flowline

(2) NCOneMap, NCDWQ Statewide NHD High-Resolution Dataset (Hydrography- 1:24,000) Shapefile names: hydro24k arc hunc_poly (for use in referencing 14-digit HUC Code used by EEP)

(3) NCOneMap, Topo Quads & Counties (http://dataateway.nrcs.usda.gov/NexPage.aspx) Shapefile name: download Wake County, TOPO QUADS & COUNTIES

(4) NCDOT USGS Topographic Map, 1:24,000-scale SID Image (http://www.ncdot.org/it/gis/DataDistribution/USGSTopoMaps/default.html) Shapefile name: Wake County

(5) NCSU Libraries, Aerial Photography: Color, B&W, Color Infrared NCSU Libraries, LiDAR Digital Elevation Models: 20-foot (www.lib.ncsu.edu/gis/)

(6) NRCS Spatial Data Gateway, Wake County Soil Information (http://datagateway.nrcs.usda.gov.NextPage.aspx) Shapefile Name: soils_nc183

(7) Web Soil Survey, Hydrologic Soil Map (websoilsurvey.nrcs.usda.gov/)

(8) National Wetlands Inventory (NWI) (http://wetlandsfws.er.usgs.gov/nwi/download.html) Shapefile Name: nwi_poly (Neuse River Basin)

(9) Southeast GAP Analysis Project (http://www.basic.ncsu.edu/segap/DataDownload.html) Shapefile Name: lc_segap_nc2

1.0 Locating the Site + Aerial PhotographyTo begin the process of analyzing the Wye Station site, it was first important to do some base mapping. The process of collecting various amounts of data necessary to perform the analysis was just as critical as the analysis itself. As mentioned earlier, the site is located in downtown Raleigh, North Carolina at latitude 35.7773619413 and longitude -78.6470260808. One of the most interesting features about the site is the future transformation of the space from ‘the back-door’ to the ‘main entrance’ of downtown.

1.1 2005 Color Aerial Photography

A series of tiles had to be downloaded in order to view the aerial photography of the site in ArcGIS. Most recent images were retrieved from the 2005 Color Air Photo Collection (NCSU GIS Webpage). The 2005 Wake County Index was also downloaded in order to determine what tile numbers were needed. In the end the following were projected to NAD 1983 North Carolina State Plane Feet:

0793_006-8, 10-12, 14-16, 18-20; 0794_10-12, 14-16, 18-20; 1703_005-20; 1704_009-15, 17-20; 1713_006, 005, 009, 010, 013, 014, 017, 018; 1714_009, 10, 13, 14, 16-18

Figure 1 on the following page displays the color air photo at scale 1:24,000, and will be used throughout this analysis. Figure 2 displays the same aerial photograph at scale 1:5,000 in order to gain a better understanding of the immediate site context and conditions.

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nDowntown Raleigh, NC 2005scale: 1” = 24,000’

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nWye Station Site, 2005scale: 1” = 5,000’

1.2 1993 Black & White Aerial Photography

While it is important to look at current aerial photography, viewing older images is equally as important. This process enables one to map change over time, and determine pre-existing conditions in a more accurate manner. The1993 Black and White Aerial Photography was retrieved for this purpose from the NCSU GIS Webpage. The process required the download of the Wake County Index which breaks the county into various city quads. Those required for this analysis included:

(1) cary1; (2) cary2; (3) cary3; (4) cary4; (5) raleie1; (6) raleie2; (7) raleie3; (8) raleie4

Figure 3 displays this imagery at a more immediate scale, again with the Wye Station site highlighted in red.

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nWye Station Site, 1993 B & Wscale: 1” = 5,000’

1.3 1998 Color Infrared

The third type of aerial photography critical for download in this analysis was the color infrared. It is only in through this type of photography that one can distinctly see the differences between water, forested areas and urban development. For this area, the most recent color infrared photo was taken in 1998. Figures 4 & 5 on the following pages display the results of projecting this data. The photography was again broken into quads, and retrieved from NCOneMap. It was projected to NAD1983 North Carolina State Plane meters in order to display properly. Tiles downloaded included:

(1) raleie1; (2) raleie2; (3) raleie3; (4) raleie4; (5) raleiw1; (6) raleiw2; (7) raleiw3; (8) raleiw4

Regulators are always extremely interested in these images, because of their unique ability to highlight water bodies and wetland areas (which appear as a blue-green color on the aerial). Before visiting a site, this type of aerial is also critical for site orientation and understanding of hydrologic patterns.

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nDowntown Raleigh, NC 1998scale: 1” = 24,000’

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nWye Station Site, 1998scale: 1” = 5,000’

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2.0 Basic Mapping

2.1 USGS Topographic Map + NHD + FEMA Flood Map + NWI

In order to gain a general understanding of the site’s hydrologic patterns, the first map generated for the project was one displaying these patterns. While no streams or wetlands occur within the site boundary, water draining from the site does flow into 303d-listed Rocky Branch Creek (less than one mile south). Figure 6 was created by overlaying the following: (1) a USGS Topographic Map; (2) a shapefile containing all major streams and water bodies obtained from the National Hydrography Dataset (NHD); (3) a shapefile identifying all existing wetlands obtained from the National Wetland Inventory (NWI); and (4) a shapefile containing critical floodplain information obtained from FEMA. This map is extremely important for submission to a regulating agency.

Figure 7 represents a more abstract depiction of the site’s hydrologic patterns. Black and white images are shown, highlighting areas on the site to which water is currently flowing (low areas). A closer view of the site is shown on the right, capturing areas where groundwater and soil contamination exist. Overall, this image further emphasizes the southern movement of water from the site to Lake Benson and the Neuse River.

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2.11 Hydrologic Investigation

Through investigation of the NHD and the USGS Topographic maps, there appears to be no evidence of a stream or ditch within the site boundary. In fact, the closest stream to the site is Rocky Branch Creek which is between .4 and .5 miles southwest of the southernmost parcel #1703475886. A thorough description of the Upper Neuse River Basin, as well as all streams contributing to this Basin from the review area is described below:

The Wye Station is located within the Upper Neuse River Basin, Cataloging Unit: 03020201 (aka: 8-Digit HUC Number). Flow paths from the site trace first to Relatively Permanent Water (RPW) Rocky Branch Creek beginning at latitude 35.770771, longitude -78.648978 (Dam at Lake Raleigh), and travel for approximately 4.18 miles. RPW Rocky Branch then flows into RPW Walnut Creek which begins at latitude 35.757514, longitude -78.638596, and travels approximately 7.43 miles directly to Traditionally Navigable Water (TNW) Neuse River. This information is summarized below:

Total distance from Wye Station site to closest TNW: 11.61 miles Closest identified TNW: Neuse River

Path from Wye Station Site to Neuse River:

(1) Wye Station directly contributes to RPW Rocky Branch: GNIS Name: Rocky Branch GNIS Number: 00993565 Stream Reach Code: 03020201001186 FCode: Intermittent Origin: latitude 35.756070, longitude -78.637396 Total Distance: Approx. 7,43 miles from Rocky Branch to Walnut Creek DWQ Class: C;NSW (Nutrient Sensitive Water) Other reach codes associated with stream (beginning in 03020201): 000791, 002117 &002648 (consisting of both perennial and intermittent reach sections)

(2) RPW Rocky Branch flows into RPW Walnut Creek: GNIS Name: Walnut Creek GNIS Number: 00996737 Stream Reach Code: 03020201000641 (at junction point with Rocky Branch) FCode: Perennial (at junction point with Rocky Branch) Origin: latitude 35.791920, longitude -78.688354 Total Distance: Approx. 4.2 miles from point of beginning to Neuse River

Other reach codes associated with stream (beginning in 03020201): 000640, 000639, 000638, 000637, 000636, 000635, 000634, 000633, 000632 (all considered perennial)

2.12 FEMA Floodmap Investigation

It is extremely important to investigate flood information to be sure the site is above the 100-year flood elevation (at least 5-feet above this threshold in order to meet LEED requirements). In the case of the Wye Station site (shown in Figures 1 and 2), it is obvious that the site sits well above this established elevation. The closest areas prone to flooding occurs to the south-southwest along Western Boulevard, and within the property boundary of the Correctional Facility (each about 1/2 mile away). While there are a few depressions within the site itself, the soils that comprise these areas (which will be discussed later in this analysis) are extremely well drained, preventing the formation of any sort of permanent wetland. A visit to the site for analysis would most definitely reveal the same findings.

2.13 Wetland Investigation

Based on information acquired from the National Wetlands Inventory (Figure 6), there are no mapped wetlands within the limits of the Wye Station site. However, storm water is leaving the space, flowing south into Rocky Branch Creek (as described earlier). This storm water contributes to the closest mapped wetland site located at latitude 35.760851, longitude -78641682. The NWI_ID number for this wetland is 117506, and its NWI_Name (classification) is PFO1/4A; this translates to be Palustrine, Forested, Broad-leaved Deciduous & Needle-Leaved Evergreen, Temporarily Flooded (a Bottomland Hardwood wetland type according to the NCDCM Description- as it does occur within the floodplain).

2.2 Soils Mapping

Another map critical to the success of this project is a soils map. Again, while there are no identified wetlands or streams within the site boundary, it is important to investigate the soils found on the site and confirm that they possess upland characteristics. While there are many different ways to obtain soil information, Figure 8 is a map generated in ArcGIS by importing the soils shapefile downloaded from the NRCS website.

The map clearly shows that only two types of soils can be found within the site. These include:

(1) CeC2: Cecil Sandy loam, 2 to 6 percent slopes, moderately eroded (comprising approx. 20% of the site)(2) CeB2: Cecil Sandy loam, 6 to 10 percent slopes, moderately eroded (comprising approx. 80% of the site)

Based on the soils descriptions, neither of the two soils are considered to be hydric. Each soil is also considered to be well drained. Therefore, it is unlikely that frequent ponding will occur (further substantiated through Figure 9). One can see the location of the Wye Station site in the center of the graphic. All soils colored in blue are considered non-hydric; all soils in green are considered relatively hydric. There is a direct correlation between where relatively hydric soils occur and thelocation of FEMA flood mapping boundaries.

Cecil Series Soil Description:The Cecil Series consists of very deep, well drained moderately permeable soils on ridges and side slopes of the Piedmont uplands. They are deep to saprolite and very deep to bedrock. They formed in residuum weathered from felsic, igneous and high-grade metamorphic rocks of the Piedmont uplands. Slopes range from 0 to 25 percent. Mean annual precipitation is 48 inches, and mean annual temperature is 59 degrees Fahrenheit near the type location.

Taxonomic Class: Fine, kaolinitic, thermic Typic KanhapludultsTypcial Pedon: Cecil sandy loam - forested

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nBasic Soils Mapscale: 1” = 24,000’

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Natural ResourcesNatural ResourcesNatural ResourcesNatural ResourcesConservation ServiceConservation ServiceConservation ServiceConservation Service

Web Soil Survey 2.1National Cooperative Soil Survey

11/23/2008Page 1 of 5

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scale: 1” = 10,000’

3.0 LiDAR Digital Elevation Models

3.1 Choosing LiDAR Data + Analysis Software

To begin, there are several programs that can be used within ArcGIS to analyze a site in the way this project describes. Several have been used throughout the semester, including TAUDEM, ArcHydro, and TAS to fill sinks within a DEM (explained below). This project utilizes ArcHydro for generating contours, but TAUDEM for the rest of the project due to the program’s ability to produce the most accurate results for the Wye Station site. Deciding which program to use for analysis varies project-to-project based on desired results.

The path of water through the landscape is controlled by topography, and digital elevation models (DEMs) are the best means of depicting and analyzing topography in a geographic information system (GIS) (Colson, 2008). While the USGS remains the primary creator and distributor of DEMs in the United States, they often require a significant amount of pre-processing. LiDAR DEMs produced by the North Carolina Floodplain Mapping Program on the other hand, have been proven to represent surveyed elevations significantly better. For this analysis, 20-foot LiDAR DEMs were obtained from the NCSU Libraries GIS webpage. Figure 10 displays these DEMs mosaiced together at a site limit appropriate for this project’s requirements. Because of their resolution, the processing of these DEMs is more accurate for the representation of first order streams.

DEMs do require a small amount of preprocessing (as mentioned above) before they can be used for analysis. Often they contain what are known as spurious pits, “defined as pixels with no neighboring pixels of lower elevations. Some pits are the result of errors in the source data used to interpolate the DEM, or an artifact of the interpolation process. Other pits, however, represent actual depressions in the surface, and in the case of LiDAR DEMs, many micro-depressions can be reflected in the DEM surface. These depressions inhibit the prediction of stream network location as they have no drainage, and the direction of flow through them cannot be resolved” (Colson, 2008). As a result, the depressions must be removed. Figure 11 is the same DEM as that shown in Figure 10, however the sinks have been filled using the Basic Grid Analysis - Fill Pits function found within TAUDEM. Outlined in yellow are areas where one can see a notable difference in the DEM’s surface where sinks have and have not been filled.

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n20-Foot LiDAR DEMscale: 1” = 24,000’

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n20-Foot LiDAR DEM: Sinks Filledscale: 1” = 24,000’

3.2 Hillshading DEMs + Producing Topography

To better understand just what a DEM looks like in relation to the topography, GIS has the ability to hillshade DEMs in avisually appealing and understandable way. Figure 12 is an image of the DEM pictured in Figure 11 with a hillshade applied. Already, one can better understand the topography and get a better feel for what is happening in and around the Wye Station site without even visiting it.

One other feature of ArcHydro that helps with the analysis of any site is its ability to generate contours from a DEM. Figure 13 is an example of this; one can see that again the site becomes easier to understand. While flowlines and patterns have yet to be generated, topography lines help in understanding the direction in which water is flowing and where it is flowing to.

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n20-Foot LiDAR DEM with Hillshadescale: 1” = 24,000’

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n20-Foot LiDAR DEM with Contoursscale: 1” = 24,000’

3.3 Terrain Analysis Using Digital Elevation Models (TAUDEM)

As mentioned earlier, this analysis utilizes TAUDEM due to limitations within ArcHydro related to the accuracy of streamlines and the delineation of watershed boundaries. TAUDEM’s options are based upon scientific validity versus ease of use (Colson, 2008). Unlike ArcHydro, TAUDEM must be “nudged” in the right direction (Colson, 2008), but the results are much more desirable. This document will not show a resulting image from each of the steps described below, but selectively choose those thought be most revealing about the process (as there are steps within TAUDEM that are more related to the processing of the DEM than anything else). The steps below are also described in an order necessary for TAUDEM to produce accurate results.

Step One: Fill Pits

Figure 11 is a result of the first step that must be completed within TAUDEM before any analysis can take place (filling pits). Again, this so that sinks can be filled, resulting in more accurate flowlines.

Step Two: D8 Flow Directions

The second step in determining streamlines is the process of assigning D8 Flow Directions. The actual definition of what this means will not be included in this document, but just know it is a critical step in the analysis process. This is achieved through Basic Grid Analysis - D8 Flow Directions. In this stage, “a binary value will be assigned to each grid cell indicating one of eight flow directions. In addition, slope, in the path of steepest decent in one of the 8 directions will also be calculated” (Colson, 2008). The flow direction values differ from ArcHydro output in that possible values range from 1-8, and not 1-128.

Step Three: Dinf Flow Direction

In this step, “a floating point value will be assigned to each grid cell indicating one of the infinity flow directions based upon the relationship of the grid celll and surrounding grid cells based upon planar triangular facets. In addition, slope, in the path of steepest decent in infinite directions is also calculated” (Colson, 2008); a function unique only to TAUDEM. Again, this step is performed through Basic Grid Analysis - Dinf Flow Directions.

Step Four: D8 Contributing Area

For this step, select Basic Grid Analysis - D8 Contributing Area. “A binary value will be assigned to each grid cell indicat-ing the upstream accumulation of grid cells at each grid cell” (including the analysis cell itself which ArcHydro would not count).

Step Five: D-infinity Contributing area

Select Basic Grid Analysis - D-Infinity Contributing Area. In this step, “a floating point value will be assigned to each grid cell indicating the upstream accumulation of grid cells at each grid cell” (including the analysis cell itself) (Colson, 2008). “The difference between the output of this function and that of the D8 Contributing Area function is that flow from a portion of call is considered to be able to flow into another cell based on the flow direction values of each cell” (Colson, 2008).

Step Six: Grid Order and Flow Path Lengths

Select Basic Grid Analysis - Grid Order and Flow Path Lengths. Output of this function is a flow path network order grid, a longest upslope length grid, and a total upslope length grid. Figure 14 is an image of the result of this function. At this point in the process, networks and streamlines begin to define themselves.

Step Seven: Full River Network Raster

It was this step in the analysis that it was decided point-processing was the most effective means of determining stream networks in and around the review area (which will eventually lead to a defined watershed boundary). The same process is achieved as if a general analysis was being performed, but a point is chosen in an effort to narrow the scope of what is being analyzed. The point for this analysis was chosen based on the results of Figure 14, and the obvious merging of network lines below the Wye Station site. The location of this point will become more obvious in Step Eight.

Select Basic Grid Analysis - Full River Network Raster. For this processing step there is a handful of inputs, however, the most important step in initiating the function is defining the ‘Contributing Area Threshold.’ Under “Stream Delineation Method,” Contributing Area Threshold must be checked and set to 2500. This number was chosen based on previous research done by T.P. Colson on the Neuse River Basin and Wake County Area. This number varies project-to-project based on desired results and size of the review area.

Step Eight: Network Delineation: Stream Order Grid and Network Files

For this step, select Network Delineation - Stream Order Grid and Network Files. Again, the input for this function is lengthy, but the result is an image that clearly shows the beginning of streamlines with an order defined. Because the Wye Station site is so small, there are very few lines. However, the two colors help show the direction in which water is flowing based on contributing areas. The result of this processing step is shown in Figure 15.

Fig

ure

14

nTAUDEM Step Six: Flow Pathsscale: 1” = 24,000’

Fig

ure

15

n TAUDEM Step Eight: Stream Order based on Point Processingscale: 1” = 5,000’

Step Nine: Network Delineation: Stream Shapefile and Watershed Grid

Select Basic Grid Analysis - Stream Shapefile and Watershed Grid. The result of this function is similar to Drainage Line Processing and Catchment Grid Delineation in ArcHydro. It is in this step that the watershed boundary begins to define, and catchments that make it up are delineated. Important attributes that result from the execution of this process include: (1) Strahler Stream Order; (2) total number of sources upstream; (3) drainage area at the downstream end of the link; (4) drop in elevation from the start to the end of the link; (5) average slope of the link; and (6) drainage area at the upstream end of the link. This information will all be necessary in determining the volume of runoff coming from this site now and into the future.

Step Ten: Network Delineation: Watershed Grid to Shapefile

This is the final step necessary in completing the steps in TAUDEM. It defines the watershed boundary as a shapefile, and makes it possible to do further analysis on the site. The result of this step is shown in Figure 16. It is clear from this image that there are four catchments related to the Wye Station site that comprise its total watershed boundary (a boundary that will be utilized for the remainder of this analysis. Stream lines sit above these catchments, terminating at the point defined in Step Seven.

Fig

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16

n TAUDEM Step Nine: Watershed Boundary Determination

Catchment 2

Catchment 3

Catchment 4

scale: 1” = 5,000’

Catchment 1

4.0 Analysis: based on Soils, Precipitation + LandCover

In order to proceed with the rest of the analysis, a significant amount of data had to be collected and created for the Wye Station site (all projected to the NAD 1983 State Plane Feet Coordinate System unless otherwise noted).

4.1 Creating Land Cover Data based on LULC Standards

Land Use Land Cover Data is critical for this analysis to proceed. There are several databases that supply such information, and while much of what was used for this project was created, South East Gap (SEGAP) Data was initially downloaded. SE GAP has its own classification system, so much of what was downloaded had to be converted in a way that would align with MRLC LULC Codes. For more information on the classification of SEGAP Data, one may consult http://www.basic.ncsu.edu/segap.

Upon downloading the data from SEGAP, categories based on MRLC LULC standards had to be established. For this analysis it includes: (1) Open Water, (2) Urban, (3) Forested, and (4) Agricultural Land/ Open Space. From the downloaded raster, a shapefile had to first be created in order to make the layers selectable and combinable (Conversion Tools - From Raster - Raster to Polygon). Based on the GRIDCODE within the attribute table, Query Builder was utilized to combine layers into one of the four categories listed above. Once the four categories were built, each new layer was exported as a separate shapefile. This allowed for the manipulation of the layers for presentation purposes. Figure 17 depicts the new land covers categories at a scale of 1:10,000.

n

1for 595n: surface water analysis l wye station

land use map

urban forested open space + agriculture waterscale: 1” = 10,000’

n

Fig

ure

17

4.1 Creating Land Cover Data based on LULC Standards (Cont’d.)

Of course, data at this scale is not enough to generate any kind of conclusions about the Wye Station site. As a result, the next steps outline the process of creating land cover data based on MRLC LULC standards displayed at scale 1:5,000 (appropriate for analysis).

First, land use land cover based on three categories was created using 2005 Aerial Photography retrieved earlier for base mapping. These categories include: Urban Land Cover, Forested Land Cover, and Developed Open Space. Open Water was not included because within the watershed boundary there isn’t any. Land Use Land Cover Data was generated for the entire watershed boundary.

Figure 18 shows existing conditions of the Wye Station site and all features that lie within the defined watershed boundary. Again, separate shapefiles for each class had to be created (ArcCatalog - New - Shapefile - Polygon). Each layer is broken out below so that designers may understand exactly what is going on within each layer over time.

Existing Conditions - Urban Existing Conditions - Forested Existing Conditions - Open Space

Fig

ure

18

n Wye Station - LULC Existing Conditionsscale: 1” = 5,000’

URBANOPEN SPACEFORESTED

4.2 Land Use Land Cover: 5 + 10 + 25 year models

As mentioned, the designer for the Wye Station site is proposing three different ideas over the course of 5, 10 and 25 years. As a result, new land cover data had to be created for each design. Because this analysis strives to determined the effects of land use land cover over time related to the site as it lies within the watershed, only land cover within the site will change (not the entire watershed). This is important to note, because while it is recognized that LULC over the entire watershed WILL undoubtedly change in the future, there must be a control for this analysis to be conclusive.

The figures on the following page show the land cover in the same way that it was broken-out for the existing conditions of the site. The first row depicts that for 5 years; the second row for 10 year; and the third row for 25 years. Figures 19, 20 + 21 show these groups combined and the way they look within the watershed and site context. Again, all of this information is necessary for not only conclusions about runoff, but good design.

n


5-Year Design

Op

en

Sp

ac

eFo

rest

ed

Urb

an

10-Year Design 25-Year Design

Fig

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19

n Wye Station - LULC 5-Year Designscale: 1” = 5,000’


Fig

ure

20



Fig

ure

21



n


4.3 LULC + Soils = Runoff Curve Numbers (RCN’s)

Soils were discussed during the beginning of this analysis in Section 2.2. If you recall, there are only two Cecil soils that intersect the Wye Station site, including CeC2 and CeB2 (Figure 22). Through the use of Web Soils Survey and soil description information provided by the NRCS, it is known that both CeC2 and CeB2 lie within hydrologic soil group B. This is important to remember, because this information is critical in determining the Runoff Curve Numbers (RCN’s) for each type of soil when combined with LULC information. This analysis utilizes the “Urban Hydrology for Small Watersheds, TR-55” Manual for determining RCN’s.

The next step in determining volume of runoff requires referencing the TR-55 Manual. Beginning on Page 17, a list of ‘Cover Descriptions’ and ‘hydrologic soil groups’ is combined to determine the RCN’s for any type of land cover one can imagine.

Cover type, hydrologic condition and hydrologic soil group information were combined to generate the following RCN’s:

(1) Open SpaceCover Type: Open space (lawns, parks, golf courses, cemeteries, etc.)Hydrologic Condition: Existing- Poor; 5-Year- Fair; 10-Year- Good; 25-Year- GoodHydrologic Soils Group: BRCN: Existing: 79; 5-Year: 69; 10-Year: 61; 25-Year: 61

(2) UrbanCover Type: Urban Districts: Commercial and BusinessHydrologic Condition: N/AHydrologic Soils Group: BRCN: 92

(3) ForestedCover Type: WoodsHydrologic Condition: FairHydrologic Soils Group: BRCN: 60

CeB2

CeC2

CeB2

n Soils Mapscale: 1” = 5,000’

CeB2CeC2Watershed Boundary

Fig

ure

22

4.4 Intersecting LULC + Soils Information

At this point in the analysis, all LULC, Soil Information and RCN’s have been defined. Through the use of ArcGIS, the intersection of LULC and soils can now be determined. This action results in a series of maps identifying soil locations based on LULC. The total area of each one of these categories will result in a ‘weighted curve number’ necessary for determining volume of runoff for the Wye Station site’s existing conditions, 5 year, 10 year and 25 year designs.

4.41 Existing Conditions

Figure 23 shows the existing conditions of the Wye Station Watershed intersected with the soils information. By combining Worksheet 2: Runoff curve number and runoff found on pg 25 of the TR-55 Manual with area information generated from this map, weighted curve numbers and total volume of runoff were determined. Figure 24 shows a table based on all of this information. For this analysis, a 5-year, 24-Hour rainfall was used based on measurements taken from Centennial Campus of North Carolina State University. Take note of the CN used; as it will change with each design phase.

Fig

ure

23

n Existing Conditions: LULC + Soilsscale: 1” = 5,000’

CeB2, ForestedCeB2, OpenCeB2, UrbanCeC2, ForestedCeC2, OpenCeC2, Urban

5-Year, 24 Rainfall: 4.35 inchesTotal Site Area 149 Acres

Equation 2-3:

Q = [(P-.2S)^2] / (P+.8S)

Equation 2-4:

S = (1000/CN) - 10

*Where CN = Curve Number* Where S is related to the soil and cover conditions of the watershed through the CN. CN has a range of 0 to 100, and is determined through the use of Worksheet 2.

Fig

ure

24

4.42 5-Year Design

Figure 25 shows the 5-year design of the Wye Station Watershed intersected with the soils information. By combining Worksheet 2: Runoff curve number and runoff found on pg 25 of the TR-55 Manual with area information generated from this map, weighted curve numbers and total volume of runoff were determined. Figure 26 shows a table based on all of this information. For this analysis, a 5-year, 24-Hour rainfall was used based on measurements taken from Centennial Campus of North Carolina State University. Take note of the CN used for open space; it has changed to 69, assuming that remediation is working (thus decreasing runoff).

Fig

ure

25

n 5-Year Design: LULC + Soilsscale: 1” = 5,000’


Fig

ure

26


Equation 2-3:

Q = [(P-.2S)^2] / (P+.8S)

Equation 2-4:

S = (1000/CN) - 10


n


4.43 10-Year Design

Figure 27 shows the 10-year design of the Wye Station Watershed intersected with the soils information. By combining Worksheet 2: Runoff curve number and runoff found on pg 25 of the TR-55 Manual with area information generated from this map, weighted curve numbers and total volume of runoff were determined. Figure 28 shows a table based on all of this information. For this analysis, a 5-year, 24-Hour rainfall was used based on measurements taken from Centennial Campus of North Carolina State University. Take note of the CN used for open space; it has changed to 61, assuming that remediation is working (thus decreasing runoff).

Fig

ure

27




Equation 2-3:

Q = [(P-.2S)^2] / (P+.8S)

Equation 2-4:

S = (1000/CN) - 10


Fig

ure

28

4.44 25-Year Design

Figure 29 shows the 25-year design of the Wye Station Watershed intersected with the soils information. By combining Worksheet 2: Runoff curve number and runoff found on pg 25 of the TR-55 Manual with area information generated from this map, weighted curve numbers and total volume of runoff were determined. Figure 30 shows a table based on all of this information. For this analysis, a 5-year, 24-Hour rainfall was used based on measurements taken from Centennial Campus of North Carolina State University. Take note of the CN used for open space; it remains 61, assuming that remediation is still working (thus decreasing runoff).

Fig

ure

29



n


Fig

ure

30


Equation 2-3:

Q = [(P-.2S)^2] / (P+.8S)

Equation 2-4:

S = (1000/CN) - 10


5.0 ConclusionsAs a result of this analysis, there are several conclusions that can be drawn:

To begin, it should first be mentioned that the designer had every intent of increasing open space between the existing conditions plan and the 5-year site design. During the 10-year design; however, urban land use was increased as the development of the train station was implemented (thus reducing open space that was create in the 5-year design). It is development between 5 and 10 years that has the greatest amount of change of pervious to impervious cover. Therefore, one would assume runoff would increase. However, as explained below this is NOT the case. At the 25-Year design phase, density increases by building up (not out), and greenroofs are added to some building structures. Comparing theses very different ways of development proves extremely interesting.

Because remediation strategies play a large role in each phase of the design, runoff from open space decreases as soil is given the ability to regenerate and repair itself. In other words, soil on the site as it sits today is considered to be extremely compacted and contaminated (as are most soils inside the beltline), preventing water from infiltrating as it normally would. For this reason, even though the 5-year design has the greatest amount of open space, it actually does the least for decreasing runoff. In fact, it is actually the 25-year design that generates the lowest amount of runoff. Even the 10-year design, as a result of soil remediation, generates less runoff than the 5-year even with the addition of several buildings and the train station.

Below is a chart comparing LULC for each design phase, and the associated area based on percentage:

Existing Area (ac) 5-Year Area (ac) 10-Year Area (ac) 25-Year Area (ac)Forested 5.6 acres (3.7%) 5.6 acres (3.7%) 5.6 acres (3.7%) 7.1 acres (4.8%)Open 26.9 acres (18.1%) 33.7 acres (22.6%) 30 acres (20.1%) 29.5 acres (19.8%)Urban 116.4 acres (78.1%) 109.6 acres (73.6%) 113.3 acres (76%) 112.4 acres (75.4%)

In closing, one final goal of this report was not only to compare runoff volumes, but to draw some kind of conclusion about the site’s design. Generally, designers are taught that if you can increase open space, you can decrease runoff. This document proves that this is not always the case. Soil plays a major role in design and should never take the back seat in development. Yes, it is nice to protect sacred trees and open spaces, but appropriate and sound decision making are equally as important. In this case, future development and increased density actually have the ability to decrease runoff volumes. This report makes a strong case for density and development, proving that quality is far more important than quantity.

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