Resumen Pag 1-55 - Inglés

8/15/2019 Resumen Pag 1-55 - Inglés

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DISEÑO DE OBRASHIDRÁULICAS PRESAS Y EMBALSES – PT 60

EARTH DAMS AND

RESERVOIRS TECHNICAL RELEASE NUMBER 60

GENERAL

DAM CLASSIFICATIONIn determining dam classication, a number offactors must be considered. Considerationmust be given to the damage that might occurto existing and future developments shouldthe dam suddenly release large quantities of

water downstream due to a breach, failurlandslide into the res ervoir.

The stability of the spilay materiphysical ch aracteristics of the site and the

valley downstream, and the relatsite to industrial and residential areasincluding controls of future development allhave a bearing on the amount of potentialdamage in the even t of a failure.

CLASSES OF DAMS

The following classes of dams have beenestablished by policy and repeated here forconvenience of the u ser.

• Low Hazard Class— dams located in rural oragricultural areas where failure may damagefarm buildings, agricultural l and, or t ownshipand country roads.

• Signicant Hazard Class— dams located inpredominantly rural or agricultural areas

where failure may damage isolated homes,main highways or minor railroads, or c auseinterruption of use or service of relativelyimportant p ublic utilities.• High Hazard Class— dams located wherefailure m ay cause loss of life, serious d amageto homes, industrial and commercial

buildings, important public utilithighways, or rai lroads.

PEAK BREACH DISCHARGE

CRITERIA

The minimum peak discharge of the breachhydrograph, regardless of the technique usedto analyze the downstream inundation area,is:

1. For depth of water at the dam at the time offailure where H w ≥ 103 ft

Q max = (65 ) H w1.85

2. For depth of water at the dam at the time offailure where H w < 103 ft

Q max = (1,100 )B r1.35 Where

B r =(V s )( H w )

A

but not less tan Q max= (3.2 ) H w2.5 nor

more than Q max= (65 ) H w1.85

3. When the width of the valley, L, at the water surface elevation corresponding to

the depth, H w , is less than,

T =(65 ) H w

0.35

0.416 replace the equation,

Q max = (65 ) H w1.85

, in 1 and 2 above with,

Q max = (0.416 ) ( L) H w1.5

Where:Q max = the peak breach discharge, ft 3 /secB r = breach factor, acre

V s = reservoir st orage at the time of failure,acreftH w = depth of water at the dam at the time offailure; however, if the dam is overtopped,depth is set equal to th e h eight of dam, ft

A = cross-sectional area of embankment at theassumed location of breach, usually thetemplate section (normal to the dam

longitudinal ax is) at the general ood plainlocation, ft 2

1 ESCUELA PROFESIONAL DE INGENIERÍA CIVIL - UNPRG


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T = theoretical breach width at the watersurface el evation corresponding to the depth,H w , for t he e quation, Q max H w = (65 ) 1.85 , ft

L = width of the valley at the water su rfaceelevation corresponding to th e d epth, H w , ft

UTILITY CABLES AND PIPELINES

Existing pipelines, cables, and conduits of a wide variety of sizes, at

are f requently en countered at dam sites. Theseconduits a re u sually located at sh allow depthsin the ood plain. They constitute a hazard tothe safety of the dam and must be either

relocated away from the site or recon structedor m odied to provide th e d urability, strength,and exibility equal in all aspects to theprincipal s pillway designed for the site, inaccordance with service criteria andprocedures.Conduits permitted to remain under any partof the embankment below the crest of t heauxiliary sp illway are t o b e:• Provided with seepage control againstpotential piping;• Properly articulated on all yieldingfoundations;• Encased in concrete or otherwise t reated toensure durability and strength equal to that ofthe p rincipal spillway; an d• Made watertight aga inst l eaking either intoor ou t of the p ipe.

CUT SLOPE STABILITY

Natural and excavation cut slopes must beplanned and formed in a stable and safemanner.

JOINT USE OF RESERVOIR

CAPACITY

A reservoir site may be used more efficiently where hydrologic conditions permit joint

of storage capacity by ood water andconservation storage. The followingrequirements m ust be met for joint use st orage

dams.

• There is reasonable assurance that water will be available to meet objectives.

• F lood protection objectives of t he project aresatised.

• Spillway conditions are such that t he dam will perform safely.Special hydrologic studies must be made toshow that the requirements can be met. Thismay include hydrometeorologicinstrumentation and analysis.

VISUAL RESOURCE DESIGN

The public generally prefers lake or waterscapescenery.

Therefore, when permanent pools are created by dam construction, they can enhance the visual resource if the water views are

emphasized. A visual design objective mustfocus public views toward the permanent pooland reduce the visual focal effects of thestructural elements.

SAFETY AND PROTECTION

Many dams are hazardous to the public.

Features designed for recreation or sh and wildlife are especially attractive to the publicsince they provide an opportunity to use the

water.

WATER SUPPLY PIPES

Water supply pipes or conduits for otherpurposes installed under any part of t heembankment below the crest of the auxiliaryspillway are t o:• provide durability, st rength, a nd exibilityequivalent to th e p rincipal spillway;• b e w atertight agai nst anticipated pressures;• be adequate for t heir intended use; and• be provided with seepage control againstpotential piping.

STREAMFLOW DIVERSION DURING

CONSTRUCTION

A hazard exists during dam construction

beginning when the embankment, cofferdam,



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or other ancillary structures obstruct thenatural streamow.During construction, a greater risk usuallyexists for some time than after the dam is

completed.

The risk is different for m because the varying factors of construction time,climate, watershed size, and diversioncapacity. An evaluation should be made of therisk from embankment failure by overtoppingand other similar hazards duringconstruction.

The risk involved in overtopping duringconstruction increases with the following

factors: • Dams of higher hazard class• Greater vol ume of reservoir storage• Dams with larger watersheds• Lo nger cri tical construction time p eriods• Smaller diversion “release” rat es (less unitdischarge per u nit watershed a rea)

RESERVOIR CONSERVATIONSTORAGE

Reservoirs with water st ored for co nservationpurposes must be analyzed using a water

budget to determine a dependable watersupply.

HYDROLOGY

This section describes hydrologic criteria fordetermining spillway discharges andoodwater storage volumes.

PRECIPITATION AND RUNOFF AMOUNTSPRINCIPAL SPILLWAY

Precipitation data must be obtained from themost recent National Weather Service (NWS)reference which is ap plicable to the area u nderstudy. References that contain precipitationdata for r eturn periods u p to 100 years an d fordurations u p to 10 days are l isted in sections

A and B of table 2-1.

The return period for design precipitation amounts is dependent on the damclassication, purpose, size, location, and typeof auxiliary spillway. Table 2-2 showsminimum return period. The minimumallowable areal adjustment ratios for 1- an d10-day precipitation amounts a re t abulated intable 2 -3 (a).

The procedure for estimating runoff volumesmust be selected based on which one requires

the higher a uxiliary spillway crest el evation when the principal spillway hydrograph isrouted through the structure. Procedures to

be used to estimate runoff volumes include:• The runoff curve number (CN) proceduredescribed in NEH-4. Use average antecedentrunoff conditions (ARC II) or gre ater u nless aspecial study shows tha t a d ifferent conditionis justied. The CN adjustment for 10-daystorm is est imated from table 2 -3(b).• Runoff volumes, based on stream gagestudies which also account for snow melt,from gures 2 -1(a) and (b) or 2 -2(a) and (b).

AUXILIARY SPILLWAY AND FREEBOARD

The most recent NWS references applicable tothe location of the dam site shall be used todetermine precipitation amounts, spatialdistributions, and temporal distributions.

Table 2-1 provides references current as ofdate of this publication. See the NWS Website,http://www.nws.noaa.gov/oh/hdsc/index.html , for t he m ost current references.



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The discharge capacity, stability (surfaceerosion potential), and integrity (breachingpotential) of the auxiliary spillway shall b e

evaluated as follows:• Both a short duration (6 hour or l onger) anda long duration (24 hour or longer) storm shall

be analyzed and the most critito check the discharge capacity and theintegrity of the a uxiliary spillway.• Only the sh ort duration storm shall be u sedto check the stab ility of t he a uxiliary spillway.• For l ocations where NWS references provideestimates of local storm and general s torm

values, both storms shall be analyzed. For

other locations, at least a 6-hour an d a 24-hour duration storm shall be an alyzed.

DESIGN HYDROGRAPHSPRINCIPAL SPILLWAY HYDROGRAPHS

When the area above a proposed dam ishydrologically complex, the area should bedivided into two or more hydrologicallyhomogeneous sub-basins for developing thedesign hydrograph.

Streamow records may be u sed to develop theprincipal spillway hydrograph where a specialstudy shows they are adequate for thispurpose

DAMS IN SERIES

UPPER DAM

The hydrologic criteria and design of an upper dam in a system of dams inseries must be the same as, or moreconservative than, those for da ms d ownstreamif failure of the u pper da m could contribute tofailure of the lower da m.

LOWER DAM

For the design of a lower dam, hydrographsshall be developed for t he areas controlled bythe upper dams based on the same hydrologiccriteria as the lower dam. The hydrographsare routed through the spillways of the

upstream dams and the outows routed to thelower dam where they are combined with the

hydrograph from the intermediateuncontrolled drainage area. The combinedprincipal spillway hydrograph is used todetermine the capacity of the principal

spillway and the oodwater retarding storagerequirement for t he lower si te. The combinedstability design (auxiliary spillway) hydrographis used to evaluate the stability (erosionresistance) of an y vegetated or earth spillwayat the lower site. The combined freeboardhydrograph is u sed to determine the height ofdam and to evaluate the integrity of any

vegetated or earth spillway at the lower site.

LARGE DRAINAGE AREAS When the area above a proposed dam

approaches 5 0 square m iles, it is d esirable todivide the area into hydrologicallyhomogeneous sub-basins for developing thedesign hydrographs. Generally, the drainagearea for a sub-basin should not exceed 20square m iles.If the T c for t he entire d rainage area i s g reaterthan 24 hours, storm durations longer than

the T c should be tested to determine theduration that gives the maximum reservoirstage for t he routed stability design (auxiliaryspillway) and freeboard hydrographs.Precipitation amounts may exhibit marked

variation in a large watershed. This variationis based upon topographical andmeteorological parameters such as aspect,orientation, mean elevation of sub-basin, andstorm orientation. A special study also m ay be

warranted in areas where signicant snowmeltcan occur during the design storms.

SEDIMENTATION

Reservoirs u sed to store ru noff water s tored alarge part of the sediments in runoff water.

Therefore, you must assign storage capacity tosediment accumulation calculated during thedesign life of the t ank. The ge neral criteria andprocedures for determining the volume of



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sediment accumulation are contained in NEH-3, sedimentation:

• sediment yield for present conditionsand for the future after planned land

treatment and other measures areapplied in the drainage a rea of the dam;

• trap efciency of the reservoir;• distribution and types of sediment

expected to accumulate;• proportion of the sediment that will be

continuously submerged vs. thataerated; and

• densities to which the sediment will become compacted.

If the amount of sediment accumulationcalculated exceeds two watershed inches in 50

years for the uncontrolldam, reevaluate the entire watershed todetermine if more economical methods ofreducing sediment yield or trapping sedimentmay b e feasible an d applicable.

GEOLOGIC INVESTIGATIONS

The intensity and detail of geologic siteinvestigations shall be consistent with theclass of da m, com plexity of site geology, andthe data needed for design. Following are thegeologic conditions that require specialconsideration beyond the minimuminvestigations spelled out in the abovereference.

SEISMIC ASSESSMENT

Should prepare a map with the location andintensity of magnitude of all intensity V ormagnitude 4 or greater earthquakes of record,and any historically active faults, w ithin a100-kilometer (62-mile) radius of the si te. Thereport s hould also summarize other p ossibleearthquake hazards such as groundcompaction, landslides, excessive shaking ofunconsolidated soils, sei ches, an d in coastalareas, tsunamis.

SUBSIDENCE

Investigate the potential for surfacesubsidence due to past or f uture solid, liquid(including ground water) or gaseous mineralextraction. Evaluate the impact of the

preemption of mineral deposits, includingsand and gravel, by dams an d reservoirs.In arid and semiarid areas and in eoliandeposits, determine the potential of moisturedecient soil materials to collapse uponsaturation or wetting. I f the potential ex ists,make extensive and intensive siteinvestigations to provide quantitativeinformation for desi gn and construction.

AUXILIARY SPILLWAYSLarge dams with auxiliary spillways in softrock or cem ented soil materials t hat cannot beclassied as soil or as rock, as generallydened for engineering purposes, andspillways in rocks with extraordinary defectsrequire a special individual evaluation.

MASS MOVEMENTSEvaluate landslides a nd landslide potential atdam and reservoir si tes, or ot her ad verse rockattitudes occur. Summarize the history ofmass m ovement in the project area. Auxiliaryspillway cuts and reservoir effects must begiven careful consideration.

KARSTIC AREASLimestone and gypsum in reservoirs and atdam sites require special investigationalmethods and careful evaluation of subsidence ,leakage hazards, and construction costs.Multipurpose structures in these areas areespecially critical.

MULTIPURPOSE DAMSInvestigate the ground water regime andhydraulic c haracteristics of t he e ntire reservo irarea of water storage dams and evaluate forleakage.

OTHER



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Special studies and evaluations may benecessary where compaction shales; sometypes of siliceous, calcareous o r p yritic s hales;rebound joints; d ispersed soils; or artesian

waters occur at a sit

EARTH EMBANKMENTS AND

FOUNDATIONS

Earth embankments constructed of soil androck are the principal means of impounding

water. The earth embankment and itsfoundation must withstand the anticipatedloads without movements leading to failure.Measures must be provided for adequateseepage control.

HEIGHT The design height of an earth embankment

must be sufcient to prevent overtoppingduring passage of either the freeboardhydrograph or stability design hydrographplus th e freeboard required for f rost conditionsor w ave action, whichever i s l arger. The d esign

height must also meet the requirements forminimum auxiliary sp illway depth. The designheight of the dam. Must be increased by theamount needed to compensate for settlement.

TOP WIDTH The minimum top width of embankment is

shown in table 1.

Table 1- Minimum top width of

embankment

Top Width (ft) Total height

ofembankment

,H, (ft)

Alldams

Singlepurpose

oodwater

retarding

Multipurposeor

otherpurposes

14 or less 8 N/A N/A15-19 10 N/A N/A20-24 12 N/A N/A

25-34 14 N/A N/A35-95 N/A 14 (H+35)/5

Over 95 N/A 16 26

The width may need to be greater than theabove minimums to:

• meet state and local standards;• accommodate embankment zoning;• provide roadway access and trafc

safety; and• provide structural stability

An increase in top width is a major designfeature in preventing breaching afterembankment slumping caused by earthquakeground motion.

When the embankment top is used as a publicroadway, the minimum width shall be 16 feetfor one-way and 26 feet for two-way trafc.Guardrails or other safety measures shall beused and must meet the requirements of theresponsible road authority.

EMBANKMENT SLOPE STABILITY All analyses including assumptions regarding

shear st rength parameters for each zone of theembankment and each soil type. They shouldtake into account the design featuresnecessary t o p rovide requ ired safety factors.

Use t he appropriate degree of conservatism inthe a nalysis. Minimum required safety factorsare su mmarized in table 2 for each conditionanalyzed.Evaluate the effect of seismicity on each site.Otherwise, u se the horizontal accelerationfactors in a pseudo-static stability calculationusing conditions su mmarized in table 2.

Analyze embankment stability for each of thefollowing conditions in the design life of the

structure t hat are a ppropriate to the site. If acondition is not analyzed, clearly documentthe reasons.

END OF CONSTRUCTION

It must be analyzed when either embankmentor foundation soils (or both) are predicted todevelop signicant pore pressures duringembankment construction. Factorsdetermining the likelihood of this occurring

include the height of the plannedembankment, the speed of construction, the



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saturated consistency of foundation soils, andothers. Perform Appropriate shear tests tomodel placement conditions of embankmentsoils, as su mmarized in table 2 .

RAPID DRAWDOWN

Analyze the stability of the upstreamembankment slope for the condition created

by a rapid drawdown of the water level reservoir from the reservoi r l evel from which aphreatic line is l ikely to d evelop. The u pstreamdrawdown condition should be analyzedassuming that soils are saturated fromtemporary pool storage to the elevation of theauxiliary s pillway. Select sh ear p arameters f or

the analyses according to table 2, asillustrated in gure 1 .

Figure 1 - Mohr-Coulomb Envelope forupstream drawdown

STEADY SEEPAGE WITHOUT SEISMIC

FORCES

Subject sat urated soils below the phreatic lineto an uplift force simulated by a phreaticsurface developed from the auxiliary spillwaycrest. Use innite slope equations and

appropriate safety factors as under RapidDrawdown condition.

STEADY SEEPAGE WITH SEISMIC FORCES

Apply a horizontal acceleration constantappropriate to the seismic zone in which thesite is l ocated. Do not u se uplift forces due toa reservoir stage at the auxiliary spillway

because the likelihood of a simultaneousoccurrence of an earthquake and an auxiliary

spillway ow event is ext remely rem ote.

Figure 2 - Mohr-Coulomb Envelope fordownstream steady seepage

Consider following additional guidance inanalyzing s lope stab ility.

• Only effective stress (CD) parameters areappropriate for s oils that w ill consolidateas rap idly a s l oad is a pplied.

• The end of construction condition isusually the one that con trols the designof embankments when it is applicable.

• Total stress parameters used in theconstruction of composite sh ear strengthenvelopes should not consider largenegative pore pressures that maydevelop during sh ear testing.

SEEPAGE To the extent needed, an analysis shall be

made of anticipated seepage rates andpressures through the embankment,foundation, abutments, and reservoirperimeter (when storage is desired). Controlsand treatment s hould be adequate to:

• accomplish the intended reservoirfunction;

• provide a safely operating structure; and• prevent damage to downstream property.

ZONINGEmbankment zoning can be used whenneeded to:

• obtain a stable structure with the mosteconomical use o f available m aterials;

• control seepage in a saf e manner; or



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• reduce to a minimum the uncertaintiesof material strengths and resultantstability.

Embankment zones should be a minimum of

10 feet wide exce pt for lters an d drains withspecied and controlled gradation. Drains an dlters sh ould meet the requirements con tainedin Soil Mechanics Notes.Soil materials which exhibit signicantshrinkage, swell or d ispersion should be u sedonly with extreme care. If possible, theyshould not be used for embankmentconstruction. When there is no economicalalternative to th eir use, they m ust be:

• treated to improve their performance;• placed in zones where effects will not be

detrimental; or• protected by use of lters and drains or

self-healing transition zones.

SURFACE PROTECTIONEmbankment surfaces must be protectedagainst su rface erosion. Protection may be

vegetative, gravel, rock r structural, or similar treatment of durablequality an d proven satisfactory p erformance.

VEGETATIVE PROTECTION

Vegetative protection may be used on ur where the following conditions

• inundation of the surfaces is of suchfrequency that veget ative growth will not

be inhibited;• vigorous growth can be sustained under

average climatic conditions by normalmaintenance w ithout irrigation; and

• stable protection can be designedaccording to the p rocedures in TR-56.

STRUCTURAL PROTECTION

Protection against wave erosion by riprap orother st ructural m easures shall be providedfor:

• dams where vegetation will not provideeffective co ntrol;

• multiple purpose dams; and• dams with uctuating normal water

levels.Protection must extend from the lowest

drawdown elevation that presents an erosionhazard, to a few feet above the crest of thelowest ungated spillway. The upper l imit sh all

be based on an analysis of anticipated waveheight and run up.Quality of riprap and other structuralprotection must be consistent with theanticipated life of the dam and designed to bestructurally stable.

PRINCIPAL SPILLWAYS

CAPACITY OF PRINCIPAL SPILLWAYLa capacidad requerida del aliviadero p rincipaldepende de:• The required capacity of the principal

spillway depends on the:• purpose of the dam;• amount of storage provided by the

retarding p ool;• kind of auxiliary spillway;• stream channel capacity and stability

down-stream;• potential damage from prolonged storage

in the ret arding p ool;• potential damage downstream from

prolonged h igh outow rates;• possibility of substantial runoff from two

or more storms in the time required to

empty the retarding pool;• limitations imposed by water rights or

other l egal requirements;• environmental concerns;• planned or potential alterations of the

channel downstream; and• necessity to pass base and ood ows

during construction.

ELEVATION OF PRINCIPAL SPILLWAYS



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SINGLE-PURPOSE FLOODWATER

RETARDING DAMS The crest of the princ

stage inlet o f a two-stage principal sp illwayshall be set at the submerged sediment poolelevation. For d ry dams, the elevation of theprincipal sp illway inlet sh all be placed asdescribed above and provisions made to drainthe reservoir in a reasonable time and, thus,satisfy th e functional or legal requirements ofthe dam.

OTHER DAMS When conservation storage will be provided,

the el evation of the crest of the lowest ungatedinlet of the p rincipal spillway is d etermined bythe volume, area, or d epth of water requiredfor the planned purpose or pu rposes and therequired sediment storage. The lowest crestmay be the crest of the low-stage inlet, singlestage inlet, or a n open spillway.ROUTING OF PRINCIPAL SPILLWAY

HYDROGRAPHSReservoir ood routing used to proportiondams an d associated spillways sh all be basedon the assumption that all sedimentationexpected in the reservoi r d uring its design lifehas occurred. The reservoir stage storagecurve used for routing should reect theanticipated accumulation of sediment. Theinitial reservo ir stage for principal sp illwayhydrograph routing sh all be a t the crest of thelowest u ngated inlet or ( if not su btracted from

the stage-storage curve) the anticipatedelevation of the s ediment storage, whichever i shigher, except as provided below.• For dams with signicant base ow,

principal spillway hydrograph routingsmust start not lower t han the elevation ofthe water surface associated with the

base ow.• For dams with joint use storage capacity,

when one of the uses is oodwaterdetention

• Single purpose, low hazard classirrigation dams with gated outlets andearth or veget ated auxiliary sp illways.

DESIGN OF PRINCIPAL SPILLWAYSHYDRAULICS

The principal spillway must be designed tocarry the planned ow for expected head andtailwater conditions.

RISERSRisers for drop inlet spillways must bedesigned to maintain the reservoi r pool level ator near the inlet cr est el evation during low

ow periods, to establish full pipe ow at aslow a head over the crest as practical, and tooperate without excessive surging, noise,

vibration, or vortex action at any reservoirstage.

CONDUIT The conduit should be straight in alignment when viewed in plan. Changes from straight

alignment, if required, must be accomplished

by watertight angle changes at joints or byspecial elbows having a radius equal to orgreater than the diameter or width of theconduit.

JOINTSConduit joints shall be designed andconstructed to remain watertight undermaximum anticipated hydrostatic head andmaximum probable joint opening as computedfrom Standard Drawing ES-146 and relatedprocedures of TR-18, including the effects of

joint rotation and the required margin ofsafety.

Piping and seepage control - Use a lter and

drainage diaphragm around any structurethat extends through the embankment to thedownstream slope. Design the diaphragm withsingle or multiple zones to meet therequirements of NEH-633, chapter 23.

OUTLETS



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The choice of outlet iconsideration of all site and ow conditionsthat may affect operation and energydissipation.

Plunge p ools sh all be d esigned to d issipate theenergy and be st able. Unless t he pool is to bein bedrock or very erosion resistant m aterials,riprap will be n ecessary to i nsure st ability.

TRASH RACKS Trash racks shall be designed to provide

positive protection against clogging of thespillway under any operating level. Theaverage velocity of ow through a clean trash

rack must not exceed 2.5 feet per secondunder the full range of stage and discharge.

Velocity must be computed on the basis of net area of opening through the rack.

ANTIVORTEX DEVICE All closed conduit spillways designed for

pressure ow must have adequate antivortexdevices.

HIGH SULFATE AREASUnder certain conditions, concrete issusceptible to deterioration from sulfate ions,especially those derived from sodium andmagnesium sulfates. In areas whereexperience or soil tests indicate the potentialfor problems, t he following shall be used fordesign purposes.

Sulfateconcentration

(ppm)Hazard Corrective measures

0 – 150 Low None

150 – 1,000 Moderate

Use Type II Cement.(ASTH C-150). Adjustmix to p rotectagainst sulfate a ction.

1,000 – 2,000 High

Use Type V Cement(ASTM C-150). Adjustmix to p rotectagainst su lfate a ction.Use so ils i n contact

with concretesurfaces t hat are l ow insulfates.

2,000 – UP

Do not use concretematerials unlessmeasures ar e takento protect con cretesurfaces f rom sulfates.Product manufacturersshould be con sulted.

AUXILIARY SPILLWAYS

Auxiliary spillways are provided to conveyexcess water through, over, or around a dam.

They are usually open channels excavated innatural earth, earthll, rock, or co nstructed ofreinforced concrete.

CLOSED TYPE SPILLWAYS An open channel auxiliary spillway must be

provided for each dam except as provided below:

• Low hazard class dams with a product ofstorage times the effective height of t hedam of less than 10,000

• Dams with drainage areas of 10 square

miles or l ess (except t hose covered by item

1) – a closed conduit principal sp illwayhaving a conduit with a cross-sectionalarea of 20 square feet or m ore,

• Dams with drainage areas greater than 10square m iles (except t hose covered by item1) – a cl osed type p rimary a uxiliary sp illwaymay be used in lieu of an open channelauxiliary sp illway.

SPILLWAY REQUIREMENTS

CAPACITY OF AUXILIARY SPILLWAYS

Auxiliary spillways must be proportioned sothey will pass the stability design hydrographat th e safe velocity determined for the site.

They must have sufcient capacity to pass thefreeboard hydrograph with the water surfacein the reservo ir at or b elow the el evation of thedesign top of the dam. In no case is thecapacity of the auxiliary spillway to be less

than 200ft3/sec or 237 DA0.493.



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ELEVATION OF THE CREST OF THE

AUXILIARY SPILLWAY

The minimum retarding storage volume andthe associated principal spillway dischargemust be such that the discharge through theauxiliary spillway will not occu r during therouting of the principal sp illway hydrographand the 10-day drawdown requirement is metor th e c rest elevation of t he a uxiliary spillwayis rai sed as noted under C apacity of PrincipalSpillway.

AUXILIARY SPILLWAY ROUTINGS

The stability design and the freeboardhydrographs must be routed through thereservoir st arting with the water su rface a t theelevation of the lowest ungated principalspillway inlet, the anticipated elevation of thesediment storage, the elevation of the watersurface associated with signicant ba se owor the pool elevation after 10 days ofdrawdown from the maximum stage attained

when routing the principal spillway

hydrograph, whichever is h igher.HYDRAULIC DESIGN

The relationship between the water surfaceelevation in the reservoir and the dischargethrough the auxiliary spillway shall beevaluated by computing the head losses i n theinlet c hannel upstream of the control sectionor, if a control section is not used, bycomputing the water surface prole through

the full length of the spillway. B ernoulli’sequation and Manning’s formula shall be usedto evaluate friction losses, com pute watersurface proles and determine velocities.Policy on the selection of ow resistance

values is given in types of auxiliary s pillways.

STRUCTURAL STABILITY

The spillway must be investigated, analyzed

designed and constructed adequately toestablish and maintain stability during the

passage of des ign ows without blockage or breaching. Excavated open cut spillways must

have cut and ll slopes in earth and rock which are stable against sliding. Cut slope

stability must b e evaluated for the long-term weathered, natural moisture condition and for

adverse moisture conditions associated withrapid drawdown from the auxiliary spillwaydesign discharge.

VEGETATED AND EARTH AUXILIARY

SPILLWAYS

Vegetated and earth auxiliary spillways are

open channels a nd usually consist of an inletchannel, a control section, and an exitchannel. S ubcritical ow exists in the inletchannel and the ow is usually supercriticalin the ex it channel.

Vegetated auxiliary spillways are usuallytrapezoidal i n cross-sect ion and are protectedfrom damaging erosion by a grass cover. Theyare adapted to sites where a vigorous grassgrowth can be sustained by normalmaintenance without i rrigation.Earth spillways are u sed in those areas w here

vegetative growth cannot be maintained. Theyare similar to vegetated spillways, b ut aredesigned for lower velocities, l ower stresses,and less frequent u se. Normally they requiremore maintenance after a ow occurs.

LAYOUT

Guidance on the layout of auxiliary spillwaysis provided in NEH-628.50, Earth SpillwayDesign. Spillways m ust be located away fromthe dam whenever possible. The layout andprole of vegetated or ea rth spillways shouldprovide safety against breaching of thespillway during the passage of the freeboardhydrograph. Both extending the length andattening the grade of the exit channel todelay or prevent headcut formation, and

maximizing the bu lk of material to con tain anyheadcut advancement should be considered.



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STABILITY DESIGN OF EARTH AND

VEGETATED EARTH SPILLWAYS

Limitations during routing of the stabilitydesign (auxiliary spillway) hydrograph – themaximum stress limitations given below for

vegetated or earth spillways pplchannel. They must not be exc eeded in thereach where an exit channel failure mightcause the ow to impinge on the toe of thedam. The stress limitations are based on themaximum discharge in routing the stabilitydesign (auxiliary spillway) hydrograph and theassumption that uniform ow conditions exist

in the ex it channel.

INTEGRITY DESIGN OF EARTH AND

VEGETATED EARTH SPILLWAYS

The spillway shall be evaluated for headcutdevelopment and advancement during passageof the freeboard storm using the p rocedures inNEH-628.51, Earth Spillway Erosion Model,and NEH-628.52, Field Procedures Guide forthe Headcut Erodibility Index. The spillway

design must be su ch that the sp illway will not breach (i.e., headcut wil

the u pstream edge of the level part of the inletchannel) during passage of the freeboardstorm.Special precautions for high hazard classdams - Sp ecial consideration must be given tothe layout of s pillways on high hazard classdams to assure the spillway will not breachunder the most extreme conditions of ow. The

length of the ex it channel is t o be increase d tothe maximum extent possible so t hat the areamost susceptible to erosion is at aconsiderable distance from the dam. Withinthe limitations of the site, t he prole of thespillway is to be su ch that a maximum bulk ofmaterial is p rovided.

ROCK AUXILIARY SPILLWAY

Some of the principles u sed for the layout ofearth auxiliary s pillways are a pplicable to rockauxiliary spillways. Allowable average

frequency of use and permissible velocitiesmust be a scertained for t he sp ecic si te b asedon knowledge of the hardness, condition,durability, attitude, weatheringcharacteristics, and structure of the rockformation. An individual appraisal isnecessary to determine the proper roughnesscoefcient, n. The d esign is t o b e su ch that t heauxiliary spillway will not breach duringpassage of the freeboard storm.

STRUCTURAL AUXILIARY SPILLWAYS

Structural spillways sh all be designed so th atthe passage of the freeboard hydrograph willnot cause serious damage to the embankmentor the structures themselves. Theconguration of a structural spillway must becompatible with the foundation conditions atthe site, the channel stability downstream

from the spillway, the possible range oftailwater depth, and the proximity of thespillway to the em bankment. The inlet portionof a chute spillway shall consist of a straightinlet, a box drops inlet, an ogee crest, or o therappropriate hydraulic structure which willproduce critical ow at t he c rest and result ina determinate stage-discharge relationship.Structural auxiliary spillways must bedesigned to withstand lateral earth pressures,

uplift, seepage and other hydrostatic andhydrodynamic pressures. They must bestructurally designed for the full maximumfreeboard discharge with uplift and slidingsafety factors of not l ess than 1.0.



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ANEXOS

Tabla 2 - Criterios De Estabilidad De Taludes

Condiciones dediseño

Hipótesis Primaria ObservacionesResistencia al corte

que se utilizaráFactor de seguridad

mínimo

1. Fin de laconstrucción (taludde aguas arr iba o

aguas ab ajo)

Se espera que laszonas de terraplén o

capas de base paradesarrollar laspresionesintersticiales seansignicativas durantela co nstrucción.

Terrenos delterraplén que sonparcialmentepermeable debenprobarse concontenidos de aguaque son tan húmedacomo saturadoslentamente durantela construcción (porlo general enhumedad óptima), deterrenos decimentaciónpermeables que nose predicen paraconsolidarplenamente durantela construcción. Laszonas de terraplénpermeables y / oestratos de

fundación.

UU - incluye ensayostriaxiales UU,connamiento decompresión (qu),pruebas de campo yensayos de corte CU'o CD

1.4 para superciesde rotura que seextienden en lascapas decimentación1.3 para terraplenessobre bases sólidas,donde se encuentraen su totalidad lasupercie de falla enel t erra plén

2. Reducción rápida(talud de agu as

arriba)

Retirar fondos delembalse normal másalto a la salida más

baja.

Considere superciesde falla tanto en elterraplén y seextienden dentro dela fundación

Menor resistencia alcorte de unaenvolvente dematerial compuestode CU y CD (g. 5-1)

1.2

1.1 para análisis dependiente innitos

3. La ltraciónconstante, si n

fuerzas sí smicas(talud de agu as

abajo)

Línea freáticadesarrollada desde elembalse en la crestadel aliviaderoprincipal.Subpresión simulado

por la línea freáticadesarrol lado a partirde la cresta delaliviadero auxiliaraplicada a los suelosde cimentación deterrap lenessaturados.

Considere superciesde falla tanto en elterraplén y seextienden dentro dela fundación

Menor resistencia alcorte de una

envolvente dematerial compuestode Cu y (CU + CD) /2 (g. 5-2)

1.5

1.1 para análisis dependiente innitos

4. Filtraciónconstante con

sísmicidad (talud de

aguas ab ajo)

Línea freáticadesarrol lado a partirde la cresta delaliviadero principal

sin elevación.

Considere superciesde falla tanto en elterraplén y seextienden dentro de

la fundación

Menor resistencia alcorte de unaenvolvente dematerial compuesto

de Cu y (CU + CD) /2 (g. 5-2)

1.1


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