USDA 97-Earth Spillway Design

7/24/2019 USDA 97-Earth Spillway Design

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National Engineering Handbook

Earth Spillway DesignChapter 50

5015(210-vi-NEH, August 1997)

United StatesDepartment ofAgriculture

NaturalResourcesConservationService

Part 628 DamsNational Engineering Handbook

Chapter 50 Earth Spillway Design


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5016 (210-vi-NEH, August 1997)

Issued August 1997

The United States Department of Agriculture (USDA) prohibits discrimina-tion in its programs on the basis of race, color, national origin, sex, religion,age, disability, political beliefs, and marital or familial status. (Not all pro-hibited bases apply to all programs.) Persons with disabilities who requirealternative means for communication of program information (Braille, largeprint, audiotape, etc.) should contact USDAs TARGET Center at (202) 720-2600 (voice and TDD).

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Contents:

7i50i

628.5000 Scope 501

628.5001 Basic concepts of earth spillway design 501

(a) Hydraulic and structural concerns ........................................................... 501

(b) Stability analysis ......................................................................................... 501

(c) Integrity analysis ......................................................................................... 502

628.5002 Basic concepts of spillway performance 502(a) Hydraulic performance .............................................................................. 502

(b) Structural performance.............................................................................. 503

628.5003 Spillway investigation guidance 504

(a) Topographic surveys .................................................................................. 504

(b) Geologic investigation................................................................................ 504

628.5004 Spillway design guidance 505

(a) Layout and alignment ................................................................................. 505

(b) Inlet............................................................................................................... 505

(c) Hydraulic control ........................................................................................ 505

(d) Outlet ............................................................................................................ 505

(e) Earth material erodibility......................................................................... 5012

(f) Hydrologic and hydraulic routing results .............................................. 5012

(g) Stability analysis ....................................................................................... 5012

(h) Integrity analysis ....................................................................................... 5012

628.5005 Maintenance considerations 5013

(a) Vegetation .................................................................................................. 5013

(b) Flow disturbance ...................................................................................... 5013

628.5006 References 5013


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Figures Figure 501 Erosion of spillway and retaining dike resulting 506

in failure of dike and erosion of toe of dam

Figure 502 Early failure of retaining dike 506

Figure 503 Erosion of spillway and retaining dike resulting 506

in breach of spillway, dike, and dam

Figure 504 Spillway gully resulting in breach of spillway 506

Figure 505 Outlet of spillway exit channel discharges to hill slope 509

Figure 506 Outlet of spillway exit channel extended to flood plain 5010

Figure 507 Lateral bulk requirements 5011


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6 2 8 .5 0 0 0 S c o p e

Chapter 50 describes the design considerations andprocesses involved in earth spillway design. It doesnot contain the detailed hydrologic or hydraulic proce-dures used to synthesize the anticipated storm flowconditions. It also does not contain details required forgeologic investigation or laboratory testing and analy-sis.

6 2 8 . 5 0 0 1 B a s i c c o n c e p t s

o f e a r t h s p i l l w a y d e s i g n

Spillway stability is determined by comparing thespillways allowable erosion resistance to the appliedhydraulic stresses for a given design storm. If theapplied stresses do not exceed the allowable values,the spillway is considered stable. Spillway integrity isthe measure of a spillway's resistance to breachingfailure. It is the most fundamental consideration in anearth (soil, rock, or both) spillway design. Althoughlarge discharges may cause significant erosion, the

spillway must not breach during passage of the free-board hydrograph.

(a ) Hydra u l i c an d s t r u c tu ra lc o n c e r n s

Open channel earth spillways must perform satisfacto-rily both hydraulically and structurally. From a hydrau-lic standpoint, the spillway must convey the requiredrange of flows with a predictable stage versus dis-charge relationship. From a structural standpoint thedesign of an earth spillway considers both the stability

and integrity of the spillway. The structural analysis isbased on the concepts that some erosion or scour ispermissible if its occurrence is infrequent, mainte-nance is provided, and if the spillway will not breachduring passage of the freeboard hydrograph.

( b ) S ta b i l i t y a n a l y s i s

The stability analysis is an evaluation of the erosionpotential for a given design storm for the constructedspillway exit channel. The designer selects the spill-

way dimensions and layout required to maintainhydraulic stresses below the erosion threshold for thedesign hydrograph. The erosion threshold is the stresslevel associated with the initiation of erosion of thespillway surface. For flows larger than the designhydrograph, spillway erosion may occur. The erodedspillway will probably require maintenance.


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( c ) I n t e g r i t y a n a l y s i s

The integrity analysis is an evaluation of the breachpotential of the spillway in passing the freeboardhydrograph. A spillway is considered breached if thespillway crest is degraded by erosion and floodwater isreleased through the spillway below the crest eleva-tion. Breach potential is a function of the spillwaysystem, the characteristics of the spillway outflowhydrograph, the erodibility of the earth materials, thespillway layout and bottom width, and maintenance.Designers should conduct the integrity analysis all theway to the valley floor. The integrity analysis indicateswhether the spillway design must be modified toensure that the spillway will not breach. Once integrity

is assured, anticipated maintenance costs, associatedwith the passage of the freeboard hydrograph, maycontrol the spillway design.

6 2 8 . 5 0 0 2 B a s i c c o n c e p t s

o f s p i l lw ay pe r f o r m an c e

An earth spillway is an open channel that generallyconsists of an inlet channel and an exit channel. Theinlet channel includes the spillway crest, or levelsection, and the constructed spillway upstream fromthe crest. The exit channel is the constructed spillwaydownstream from the crest. A spillway may be de-signed with or without a control section. If a controlsection is used, the flow is subcritical in the inletchannel and supercritical in the exit channel. A spill-

way that conveys only subcritical flow is acceptable.Proper spillway function is considered in terms ofhydraulic and structural performance.

( a ) H y d r a u l i c p e r f o r m a n c e

For optimum hydraulic performance, the spillwaylayout should convey uniformly distributed flowacross the entire spillway cross section. To accom-plish this, layouts should not have flow-concentratingfeatures, such as short radius curves and nonlevelsections perpendicular to the direction of flow.

The layout must ensure a predictable reservoir stageversus discharge relationship. This predictability maybe accomplished by using a control section such thatinlet channel flow will be subcritical and the exitchannel flow will be supercritical for most of theexpected range in discharges.

Computer technology and computational methodsallow determination of stage versus discharge relation-ships even when the control section shifts with dis-charge. Thus, a forced control section is not essential

for predicting the rating of the spillway. Therefore,topography, stability, and integrity may govern thedesign of the spillway surface profile.

Topographic features, such as depressions, naturaldrainage patterns, and steep slopes, can significantlyaffect hydraulic performance by concentrating flowand inducing erosion. Although this erosion mayinitiate in areas outside the spillway, it can migrateinto the spillway and adversely affect spillway perfor-mance. Align spillways to avoid topographic features


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that concentrate flow. Divert natural drainagewaysaround the spillway and release in a manner that

avoids potential erosion of the spillway, gutters, re-taining dikes, or the dam.

(b ) S tr u c tu ra l pe r f o rm an ce

The structural performance of an earth spillway is afunction of its vegetal and geotechnical resistance tohydraulic erosion. Good spillway design depends uponproper geologic investigation and assessment of theerosion resistance of the vegetation and earth materi-als at the site.

The erodibility of an earth material is contingent uponboth material properties and in-place mass properties.For soils, material properties include plasticity, grain-size distribution (particularly percent clay), bulkdensity, consistency, and strength. Mass propertiesinclude the occurrence of discontinuities in soils, suchas soil joints, desiccation cracks, and root or animalholes. Discontinuities significantly weaken the integ-rity of an earth material and increase its erodibility.

Material properties of rock or rocklike material in-clude rock type, constituent grain size, hardness, and

unconfined compressive strength. Mass properties ofrock or rocklike material include stratigraphic andstructural discontinuities. Important stratigraphicbreaks include abrupt lateral or vertical changes inrock type; such breaks may place very dissimilarmaterials adjacently with widely contrasting erodibil-ity characteristics. Structural discontinuities, espe-cially persistent, intersecting joint sets that are system-atically spaced and oriented, increase the erodibility ofthe mass. The engineering significance of variousattributes of discontinuities is described in detail in

TR-78, The Characterization of Rock for HydraulicErodibility.

The ability of vegetation to protect erodible geologicmaterial at the spillway surface is dependent on thelength, density, and uniformity of the vegetal cover.For vegetal protection to be effective, adequate root-ing depth must be available. Cover uniformity is espe-cially important since locally weak areas may substan-tially reduce the protective value. An adequate depthof suitable topsoil is necessary to provide an erosionresistant sod. In cases where only a few inches of

topsoil is placed over dense erosion resistant materi-als, flow tends to cause large sections of this sod to be

rafted out of the spillway. Generally, a 1-foot thicknessof topsoil is adequate to prevent this action. Vegetalerosion protection is most effective for flow depthsless than 3 feet and for exit channel slopes less than 6percent.

Where spillway integrity is questionable, considerincorporating a barrier into the spillway channel as anadditional defense against headcut breach. The barriermust consist of erosion resistant material in a configu-ration that will remain stable when subjected to thehydraulic attack associated with the anticipatedheadcut erosion. Where earth spillway layouts cannot

ensure stability or integrity, or both, a structuralspillway is required.


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6 2 8 . 5 0 0 3 S p i l l w a y i n v e s ti -

g a t i o n g u i d a n c e

(a ) To po gr ap h ic su rve ys

A satisfactory hydraulic design and layout stronglydepend upon accurate and complete topographicsurveys of all practical sites for the spillway in thevicinity. The surveys must cover an area large enoughto enable proper layout studies for geologic investiga-tion, preparation of final grading plans, and estimatesof quantities of soil and rock excavation. Topographic

maps on a scale of 1 inch equals 20, 40, 50, or 100 feetand a contour interval of 1, 2, or 5 feet are satisfactoryfor most spillway layouts. For areas under consider-ation for a spillway, a 1- or 2 -foot contour interval isrecommended where abutment slopes are less than 30percent, and a 5-foot interval where slopes exceed 30percent.

(b ) Geo lo g i c in ves t i ga t io n

Requirements for geologic investigation and samplingmust be consistent with policy. The requirements vary

according to the size and purpose of the dam, eco-nomic and safety considerations, and geologic com-plexity of the site.

Before initiating a field investigation, the geologistshould study all pertinent technical materials, such asregional geologic maps, published and unpublishedreports, topographic maps, site surveys, aerial photo-graphs, soil survey reports, geotechnical maps, andreports of the site or similar sites. Local outcrops androadcuts should be inspected to ascertain majorstructural features and dominant earth materials.

These preliminary studies aid in determining theappropriate level of intensity for the detailed fieldinvestigation.

The field investigation consists of whatever borings,test pits, or test trenches are necessary to identify,classify, log, and correlate earth materials using stan-dard Natural Resources Conservation Service (NRCS)(formerly the Soil Conservation Service, SCS) proce-dures. The geologist may use geophysical methods tosupplement the investigation process. Assessment of

the erodibility of soil materials may include the use ofa submerged jet erosion tool (see ASTM D5852 Jet

Index Method). The geologist: Develops an engineering geologic map at an

appropriate scale showing the location, extent,and distribution of earth materials at the site.

Makes the mapping area large enough to encom-pass all probable layouts as well as any areasdirectly downstream of the constructed exitchannel that flow may impact.

Develops representative profiles and crosssections of all mappable earth materials.

Differentiates mapping units by the headcuterodibility index.

Refer to Chapter 52, Field Procedures Guide for theHeadcut Erodibility Index, TR-78; and other pertinentdocuments for guidance on assessing earth materialfor hydraulic erodibility.

The maps may indicate zones of highly erodible mate-rials that should be avoided in the design layout.Conversely, zones identified as highly erosion-resistantmay be exploited in the design and layout of the spill-way, particularly in the outlet channel, crest, and othersensitive areas.

Determine the elevation and seasonal range of thewater table or location of water-bearing strata tofacilitate the design of any required abutment slopedrainage, relief drainage within the spillway, or addi-tional slope stability protection.

Collect representative samples as needed for labora-tory analysis in accordance with requirements inGeology Note 5, Sample Size Requirements, to supportengineering decisions regarding:

The suitability of earth materials in the spillwayexcavation for use in construction of the dam.

The suitability of earth materials at and directly

below the spillway grade line for establishing andmaintaining good vegetative cover.

The suitability of available topsoil for establish-ing and maintaining good vegetative cover.

The erodibility of all earth materials in the profileof the spillway and along the flow path to thevalley floor.

The stability of the side slopes of the spillway. The optimum methods for correcting or stabiliz-

ing slides or for precluding potential slides in thevicinity of the spillway cut.


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6 2 8 .5 0 0 4 S p i l l w a y d e s i g n

gu i da n c e

(a ) Layo u t an d a l ig n me n t

Earth spillways may be located either in the abutmentat one or both ends of an earth embankment orthrough a topographic saddle at some point along theperiphery of the reservoir. The layout and alignment ofa spillway should take into consideration the engineer-ing geologic map of the site that delineates zones ofearth materials with similar erodibility characteristics.

To the extent practicable, the layout should avoidzones of highly erodible earth materials, particularly inthe following areas: the crest, exit channel sectionswith changes in slope, locations where hydraulic

jumps may form, any point on the constructed exitchannel, and on the natural slope downstream of anexit channel. Conversely, the layout should, to theextent possible, take advantage of zones of earthmaterial that are erosion resistant with respect to theanticipated hydraulic energy. Reinforce, protect, orreplace earth material having unacceptable erodibilitycharacteristics. The reinforcement or protection mayconsist of many options, such as engineered earth or

barriers.

( b ) I n l e t

If a control section is used, the inlet channel is to belevel for a minimum distance of 30 feet upstream fromthe control section. This level part of the inlet channelis to be the same width as the exit channel, and itscenterline is to be straight and coincident with thecenterline of the exit channel. A curved centerline ispermissible in the inlet channel upstream from the

level section, but it must be tangent to the centerlineof the level section.

A large cross sectional area of flow in the inlet chan-nel, in comparison with the flow area at the spillwaycrest section, facilitates a uniform distribution of flowwithin the inlet channel and minimizes energy losses.

The flow area in the inlet channel may be increased bywidening, deepening, or both, the inlet channel. How-ever, deepening the inlet channel reduces the quantityof earth material available to resist potential headcut

movement; therefore, consider all aspects in designingthe inlet. Minimize energy losses and flow concentra-

tions in the inlet channel by prohibiting obstructions,such as trees, structures, and debris accumulations.

(c ) Hydr au l i c co n tro l

If a control section is desired at the spillway crest, theslope of the exit channel immediately downstream ofthe control section must be steep enough to ensurethat all significant flows will be supercritical. Thisrequirement ensures that the position of the controlsection will remain fixed and that the stage versusdischarge relationship will be unique.

The control section may be positioned anywhere alongthe spillway. However, the further downstream it islocated, the higher will be the reservoir stage for anygiven discharge because of increased head loss result-ing from an increase in length of the inlet channel. Anyincrease in head loss requires an increase in embank-ment height. Generally, the further downstream thecontrol section is located, the more spillway bulk isprovided to help resist spillway breaching.

(d ) Ou t le t

When a control section is desired, the exit channelmust have a slope that ensures supercritical flow forall significant flows. However, slopes greater than 4percent generally meet this requirement. Significantflows are flows equal to or greater than 25 percent ofthe peak discharge associated with passage of thefreeboard hydrograph. All flows that would require aslope greater than 4 percent to ensure supercriticalflow may be considered insignificant.

Uniform flow distribution must be maintained for

supercritical flows, particularly if overtopping of aretaining dike poses a risk of erosion to the embank-ment or gutter (fig. 501). Therefore, if supercriticalflow may occur, the channel alignment should bestraight to that point downstream where overtoppingof the dike would not cause erosion of the dam em-bankment or gutters. Curvature in the exit channelalignment causes flow concentrations and enhanceserosion in the exit channel.


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Figure 501 Erosion of spillway and retaining dikeresulting in failure of dike and erosion of toeof dam

Ensure that the exit channel, including cross section,slope, alignment, and enclosing cut slopes or dikes, is

sufficient to contain the discharge for the freeboardhydrograph and to release the flow without erodingthe embankment or gutters (fig. 502 and 503).

A uniform flow condition does not concentrate hy-draulic attack within the channel. Figure 504 illus-trates how concentrated flow initiates gullies that canmigrate upstream through the exit channel and breachthe crest and inlet channel. The more uniform the

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Spillway

Erodedarea

Gutter

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Dike

Figure 502 Early failure of retaining dike

Figure 503 Erosion of spillway and retaining dikeresulting in breach of spillway, dike, and dam

Figure 504 Spillway gully resulting in breach of spillway

slope and cross section, the more uniform the flow,thereby reducing the maximum hydraulic attack.

To reduce tractive stress and erosion potential in theexit channel, the optimum layout for the exit channelis a uniform slope of 4 percent or less from the crestsection to the flood plain. If this configuration is notfeasible, design the slope to be as uniform as possibleand avoid major changes in slope that would inducethe formation of hydraulic jumps. Figure 505 showsgrades increasing in the downstream direction. An

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alternate layout could be assessed that uses a uniformgrade of about 7 percent from the crest to the flood

plain. The alternate layout will increase excavation.

The advantage of the spillway exiting to a hillslope is aflatter exit channel slope that results in less excava-tion, a larger volume of material to resist headcuterosion, and smaller erosive forces in the spillwayitself (fig. 505). When considering an exit to ahillslope, the investigators must carefully assess theerodibility of the earth materials in the exit channeland on the hillslope. If an evaluation of this configura-tion predicts the development of an overfall that couldadversely affect the function and safety of the spill-way, consider designing a steeper exit slope to either a

more stable outlet or to the flood plain (fig. 506).Other alternatives may be to locate the spillway intomore favorable earth materials or in a more favorabletopographic setting for the exit area.

For an earth spillway to function properly, the layoutmust meet the following requirements:

Design the layout to ensure that the energy lossis reasonably equal along all flow lines throughthe inlet (from the reservoir to the crest). Such alayout ensures that the discharge per unit width at the

crest is constant across the full width of the spillway.

Design and construct (within construction toler-ances) a level spillway crest perpendicular toflow.

Design a straight exit channel that extends be-yond the downstream toe of the dam. If curvatureof the exit channel is required, it shall be beyond thedownstream toe of the dam.

Divide the spillway width into bays of 200 feet orless. Divide the bays by dikes extending from at least

the upstream end of the crest to the downstream endof the exit channel. It may be advantageous to extendthe dikes further upstream in curved inlets to assist increating more uniform flow conditions. The top of thedikes, at all points along their lengths, must be at orabove the maximum water surface elevation attainedduring passage of the freeboard hydrograph. Designthe dikes to have a minimum top width of 10 feet,minimum side slopes of 2 horizontal to 1 vertical (2:1),and a constant base width to maintain a constantbottom width for each bay of the spillway.

Design the exit channel slope immediately down-stream of crest. If a hydraulically stable control

section is desired at the spillway crest, design the exitchannel slope immediately downstream of the crest sothat supercritical flow occurs for all discharges equalto or greater than 25 percent of the peak dischargeassociated with passage of the freeboard hydrograph.

If reinforcement barriers are used to stop antici-pated headcut movement, locate them near theupstream end of the crest. At this location thebarrier will be subjected to the least attack (hydraulicloading) for the least amount of time because a longertime is required for the headcut to reach the upstreamedge of the spillway crest.

Protect the dam and the gutter between thespillway and the dam by providing sufficientlateral bulk between the spillway and the gutterat all points along the spillway. Meet the followingrequirements for any cross section normal to thespillway centerline between the projected centerlineof the dam and the downstream toe of the dam:

On the side nearest the dam, make the spillwayside slope 3 horizontal to 1 vertical (3:1) orflatter for spillways in soil.

If a dike is required between the dam and the

spillway, do the following: At each cross section, design the top of thedike to be at or above the calculated maxi-mum water surface associated with passageof the freeboard hydrograph.

Design the top width of the dike to be at least12 feet.

In the plane of the cross section, ensure thata 2:1 slope projected downward from the topedge of the dike nearest the dam does not fallabove the ground at any point.

At any cross section where a dike is not required,ensure that the ground line is on or above a

hypothetical dike cross section as defined above.

Figure 507 illustrates these lateral bulk requirements.


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Figure 505 shows a spillway layout that outlets ontothe hillslope. Some features of this layout are:

A 70-foot-long level section (exceeds NEH628.5004 guidance).

A 95-foot-long straight section upstream of thecontrol section. Within practicable limits, thelonger this section, the better the opportunity todevelop uniform flow conditions after flowleaves the curved section of the inlet channel.

A gradual inlet curve having a long radius andlimited deflection. This will not disrupt uniformflow conditions as would a curve with a shortradius and substantially deflected alignment.

An upstream, inside dike to guide flow into thespillway entrance. If no dike were used and the

pool was at elevation 79, flow would enter thespillway laterally over the cut slopes to points Aand B. Flow entering over the right side slope

just upstream from point A would tend to inhibitthe formation of uniform flow because of itsangle of entry and its proximity to the crest. Theaddition of the dike forces the flow into themouth of the spillway (water surface at point C).

This causes the flowlines from B and C to thecrest to be approximately equal thus enhancinguniform flow conditions.

A straight exit channel that exits essentially

perpendicular to a contour line. This eliminatesthe concern for curvature in supercritical flow. Italso makes both sides of the exit channel essen-tially the same length thus eliminating the possi-bility of a flow concentration at the outlet of theshorter side.

An exit channel dike is not provided. The maxi-mum freeboard discharge would create flow overthe top of bank starting at station 4+40. Becausethis overbank flow is shallow, it will not flowtoward the dam, but will parallel the within-bankflow. A dike is not deemed necessary; however,in some circumstances an outside dike may be

needed to maintain the spillway flow within theland rights limits.

Note the short reach of 40 percent slope at theoutlet. This steep reach is undesirable because aheadcut will most likely start in this location.

This short, steep reach can be eliminated byshifting the spillway upstream until the exit slopedaylights at the head of the 12.1 percent reach, orby increasing the outlet slope to 4.6 percent, thuseliminating the 40 percent slope. Spillway layoutsthat create overfalls shall be avoided.

Figure 506 shows a spillway that extends to the floodplain. It has features similar to the spillway shown in

figure 505. They are: A gradual curve in the inlet channel. An extended level section. A straight outlet channel. An upstream dike. No downstream dike as the flow is contained

within the cut section. Spillways with variable slopes are not desirable.


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Figure 505 Outlet of spillway exit channel discharges to hill slope

Spillway plan

70

60

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80 80

B

CA

0+00

0+75.19PC

Ele

v:72

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S=1% S=0.0

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S=12.1%S=23.8%

70'Level

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Figure 506 Outlet of spillway exit channel extended to flood plain

Spillway plan

3:1

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Original ground line

S=3.64%S=6.11%

S=12.4%


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Figure 507 Lateral bulk requirements

Not acceptable

Not acceptable

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( e ) E a r t h m a t e r i a l e r o d i b i l i t y

The spillway erosion model in the SITES program(NRCS Water Resource Site Analysis program) dividesthe physical processes of spillway erosion into threephases. Because the processes acting in each phaseare unique, each phase uses a different approach todetermine the earth material erodibility. Phase I is thesurface failure stage, which includes the combinedeffect of the vegetation and earth material and predictsthe timing or rate of surface erosion. Phase II is aheadcut formation stage during which a surface de-tachment phenomenon is assumed to occur until aheadcut forms. Phase III predicts the rate and distanceof headcut movement.

Analysis of the erodibility of a spillway requires adetailed geologic investigation of the spillway site. Theinvestigation will gather information used as input tothe SITES program.

In the Phase I analysis, the model uses plasticity indexto represent the erodibility of the surface geologicmaterial. The model represents vegetated cover by aretardance curve index and vegetal cover factor (seeAgriculture Handbook 667, Stability Design of Grass-Lined Open Channels, and NEH 628.51 for details).

The user must also identify cover discontinuities.

In the Phase II analysis, the model represents geologicmaterial by particle size, bulk density, and percent clay(see NEH 628.51).

The Phase III analysis requires an earth materialerodibility index to predict spillway erosion. Theinvestigation to determine an erodibility index in-volves the measurement, description, and documenta-tion of specific geologic parameters (see TR-78, NEH628.51). The index is a measure of the hydraulic erod-ibility of any natural or engineered earth material,

including soil and rock. The geologic parameters thatconstitute the index include earth mass strength,particle or block size, discontinuity or interparticlebond strength, and orientation and shape of materialunits relative to the flow field. The index represents arational correlation between stream power and anerodibility classification of all earth materials.

( f ) Hydro lo g i c an d h ydra u l i cro u t i n g re s u l ts

Refer to the requirements in Section 2, Hydrology inTR-60 Earth Dams and Reservoirs, for the hydrologiccriteria for determining spillway discharges and flood-water storage volumes. Use the SITES program toperform the routing of the design hydrographs used indetermining the proportioning of the dam and spill-ways. The SITES analysis is a multiple pass solutionusing spillway system alternatives.

( g ) S t a b i l i t y a n a l y s i s

Stability analysis is an evaluation of the spillwaydimensions, grades, and vegetation to maintain hy-draulic stresses below the threshold level for a givendesign storm. The SITES program output providesinformation for the stability analysis of the spillway.

This program follows the procedure in AgricultureHandbook No. 667 to determine the stability of thespillway for a given hydrograph.

(h ) In t eg r i ty a n a l y s i s

The integrity analysis is an evaluation of the breachpotential of a spillway during passage of a specifiedhydrograph. The SITES program provides an erosionand sediment transport analysis to indicate the extentand depth of any headcuts and thus assesses theintegrity of the spillway.

The Agricultural Research Service (ARS) developedthe model within SITES that makes this analysis. It isbased upon performance information for spillwaysthat experienced flows and on research conducted atthe Outdoor Hydraulics Laboratory in Stillwater,Oklahoma.

The evaluation of spillway breach potential requiresseparate evaluation of each reach of the spillway andnatural hillslope from the crest to the elevation oftailwater during flow. The model uses the SITESgenerated outflow hydrograph in predicting each ofthe three phases of spillway erosion and headcutmovement. In making these predictions, the modeluses data on the in-place earth materials as well as thespillway layout, vegetation, and maintenance. Themodel is described in detail in NEH 628.51.


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Part 628 Dams



6 2 8 .5 0 0 5 Ma i n t e n a n c e

c o n s i d e r a t i o n s

The quality of spillway maintenance can have a signifi-cant effect on the potential severity of spillway ero-sion. Experience indicates that for similar flow situa-tions, a well maintained spillway requires less repairafter a spillway flow than one that is poorly main-tained.

(a ) Veg eta t io n

A good, uniform cover of vegetation in the spillwayreduces the tractive stress applied to the soil. Tomaintain good cover condition, do not allow roads ortrails of any kind in any part of the spillway. Thisrestriction is particularly important for roads or trailsin the exit channel that are parallel to flow. Thesefeatures disturb the vegetative cover, reduce flowresistance, and cause significant flow concentrations.

Consult an agronomist for the vegetal species appro-priate for the local conditions.

(b ) Fl o w di s tu rb an ce

A flow disturbance is any local phenomenon thatcauses the flow to deviate from uniform flow condi-tions. A minor flow disturbance will occur at theupstream end of the longitudinal splitter dikes. This isa subcritical flow area, and the effect is negligible.

Generally, exit channel flow is supercritical. Anythingin the exit channel that disturbs flow and tends toconcentrate flow increases the risk of erosion. Fea-

tures that can create significant flow disturbances andconcentrations include trees, signs, pipelines, fences,boulders, roads, trails, debris associated with sideslope failure, and gullies. Regular maintenance isessential to restrict such encroachments in the spill-way.

6 2 8 .5 0 0 6 R e f e re n c e s

American Society for Testing and Materials. 1996. J etindex method. ASTM Standard D 5852, WestConshohocken, PA.

Temple, D.M., K.M. Robinson, R.M. Ahring, and A.G.Davis. 1987. Stability design of grass-lined openchannels. USDA Agriculture Handbook No. 667.

United States Department of Agriculture, NaturalResources Conservation Service. 1985. TR-60,Earth dams and reservoirs. NRCS, Washington,DC.

United States Department of Agriculture, NaturalResources Conservation Service. 1991. TR-78,

The characterization of rock for hydraulic erod-ibility. NRCS, Washington, DC.

United States Department of Agriculture, NaturalResources Conservation Service. 1997. NEH Part628.51.

Documents

USDA 97-Earth Spillway Design