1

Spillway Ratings Spillway Ratings andand

Stability Design ProceduresStability Design Procedures

____________________________________________________SITES 2005SITES 2005

INTEGRATED DEVELOPMENT ENVIRONMENTINTEGRATED DEVELOPMENT ENVIRONMENTfor

WATER RESOURCE SITE ANALYSIS

2

OBJECTIVEOBJECTIVE

Provide understanding of:Provide understanding of:• Principal spillway ratingPrincipal spillway rating• Auxiliary spillway ratingAuxiliary spillway rating• Allowable stress approachAllowable stress approach• Cover discontinuitiesCover discontinuities

3

1st entry - principal spillway crest elevation 1st discharge = 0 cfsElev and discharge must increase

Structure TableStructure Table

4

PS TypePS Type

5

Inlet

Conduit

PS SchematicPS Schematic

6

PS InletPS Inlet

7

Example HelpExample Help

8

PS ConduitPS Conduit

9

Principal Spillway DataPrincipal Spillway Data• Number of conduits, Number of conduits,

Identical Conduits (if > 1) Identical Conduits (if > 1) • Conduit(s) size (length and diameter)Conduit(s) size (length and diameter)• Manning’s Manning’s nn value for the conduit(s), value for the conduit(s), • Hydraulic grade line at the outlet Hydraulic grade line at the outlet

(tailwater elevation)(tailwater elevation) Elevation of the center of the pipe outlet Elevation of the center of the pipe outlet Tailwater is assumed constant Tailwater is assumed constant

• SITES develops rating table SITES develops rating table spillway crest spillway crest maximum elevation in the structure tablemaximum elevation in the structure table

10

Op

tion

al

Aux Spwy EntryAux Spwy Entry

11

•

Auxiliary Spillway RatingAuxiliary Spillway Rating

• Flow resistanceFlow resistance varies varies by reach• Mixed vegetal and non-vegetal Mixed vegetal and non-vegetal

flow resistance• Seeks hydraulic control sectionSeeks hydraulic control section

12

InletNaturalGroundRch1

ConstructedInlet Channel

Rch2

ConstructedExit Channel

Rch3

Exit Natural Ground

Rch4

Tie

Sta

tion

Auxiliary Spillway ProfileAuxiliary Spillway Profile

13

Tie StationTie Station• Downstream end of the level crest Downstream end of the level crest

section of the auxiliary spillwaysection of the auxiliary spillway• Same coordinate system as Same coordinate system as

geologic materialsgeologic materials• Locates auxiliary spillway template Locates auxiliary spillway template

relative to geologic materials relative to geologic materials • Not where cowboys put horses when in Not where cowboys put horses when in

the saloonthe saloon

14

AS TemplateAS Template• Natural or existing ground profile - Natural or existing ground profile -

geology input geology input • SITES computes intersection SITES computes intersection

natural ground – constructed natural ground – constructed channel channel

• Level crest section is required!Level crest section is required!• Flow resistance may vary by reachFlow resistance may vary by reach• Vegetation or earthen lining may Vegetation or earthen lining may

change by reachchange by reach

15

SITES AS ComputationSITES AS Computation

• SITES seeks the hydraulic control SITES seeks the hydraulic control section for each discharge section for each discharge

• Backwater analysisBackwater analysis• Does exit channel reach or Does exit channel reach or

downstream natural ground downstream natural ground reaches sustain subcritical flow at reaches sustain subcritical flow at given discharge?given discharge?

16

Hydraulic Control Hydraulic Control

Avoid short steep slopes between mild slopes!Avoid short steep slopes between mild slopes!

• SITES Hydraulic Control: upstream end of 1st supercritical reach downstream of crest at given discharge

• Reach length not checked

• Short steep reach submerged by downstream mild slope reach – possible improper rating

Actual Hydraulic control

Not here

17

N vs VRN vs VRA

E

DC

B

10.0

5.664.442.88

7.6

AH-667

TP-61

Man

nin

g’s

n

VR Product ft2/s0.1 1 10 100

1

0.1

0.01

18

Flow ResistanceFlow Resistance

1.1. Manning’s Manning’s nn

2.2. SCS TP-61 Retardance (A-E)SCS TP-61 Retardance (A-E)

3.3. Ag Handbook 667 Retardance Ag Handbook 667 Retardance Curve (CCurve (Cii))

19

CCII = 5.6 = 5.6

n = 0.04n = 0.04

Head-DischargeHead-Discharge

20

TR-60 AS Criteria TR-60 AS Criteria

DurationDuration

Stability Stability CheckCheck ShortShort 6 hour6 hour

IntegrityIntegrity

CheckCheckShort & LongShort & Long 6 hour &6 hour &

24 hour24 hour

21

Stability vs IntegrityStability vs Integrity

• StabilityStability Check Check Protect the aux spillway Protect the aux spillway SURFACESURFACE No surface erosion!No surface erosion!

• IntegrityIntegrity Check Check Some erosion accepted/expectedSome erosion accepted/expected Protect the aux spillway Protect the aux spillway LEVEL LEVEL

CRESTCREST from breaching from breaching Protect dam from overtoppingProtect dam from overtopping

22

Stability vs IntegrityStability vs Integrity

AnalysisAnalysis Hydro-Hydro-graphgraph ParameterParameter

StabilityStability SDHSDHSurface Surface erosion in erosion in aux spwy?aux spwy?

Peak stressPeak stress

IntegrityIntegrity FBHFBHLevel crest Level crest aux spwy aux spwy breached?breached?

Cumulative Cumulative erosionerosion

23

Peak SDH vs Peak SDH vs Cumulative FBHCumulative FBH

SDH Peak Outflow

Stability CheckPeak SDH

Integrity CheckCumulative FBH

24

After the StormAfter the StormHydro-Hydro-graphgraph

Anticipated Local Sponsor Anticipated Local Sponsor ActionAction

StabilityStability SDHSDHVerify no surface Verify no surface erosion in aux spwyerosion in aux spwy

IntegrityIntegrity FBHFBH

Verify dam level crest Verify dam level crest not breachednot breached

Fill erosion in aux spwyFill erosion in aux spwy

25

Stability Analysis Stability Analysis DefinitionDefinition

• Tractive stress of earth or Tractive stress of earth or vegetated spillway computed from vegetated spillway computed from SDH 6-hour peak dischargeSDH 6-hour peak discharge

26

TR-60 Stability PolicyTR-60 Stability Policy

Aux Spillway Aux Spillway Frequency of UseFrequency of Use Allowable StressAllowable Stress

> 1 in 50 years> 1 in 50 years 1.0 X AH667 1.0 X AH667

1 in 50 years1 in 50 years 1.2 X AH667 1.2 X AH667

1 in 100 years1 in 100 years 1.5 X AH667 1.5 X AH667

27

SITESSITES

Auxiliary SpillwayAuxiliary SpillwayStability AnalysisStability Analysis

28

•

Stability Analysis in SITESStability Analysis in SITES

• Design variable options Design variable options Allowable stress (Allowable stress (, psf), psf) Permissible velocity (fps)Permissible velocity (fps)

• Compute optionsCompute options1.1. User enters AS widthUser enters AS width

SITES compute AS stress

2.2. User enters allowable stressUser enters allowable stress SITES compute AS width

29

SITES Summary – 0.341 psfSITES Summary – 0.341 psf

30

SITES Text OutputSITES Text Output

31

EffectiveEffective Stress Stress = 0.314 psf= 0.314 psf

32

0.85”

AH 667 AH 667 Fig 3.1Fig 3.1 0.314 psf

• 1” grain size allowable stress ~ 0.4 psf

• 0.9” grain size meets stability requirements

33

Vegetal Erosion ProtectionVegetal Erosion Protection

total

veg

soil

34

= Total Hydraulic Stress – lb/ft2

= Vegetal Stress + Effective Soil Stress

= ve + e

Total Hydraulic StressTotal Hydraulic Stress

35

va = 0.75 Ci

Ci = Retardance Index

@ Ci = 5.6

va = 4.2 lbs/ft2

Allowable Vegetal StressAllowable Vegetal Stress

36

Vegetal Stress - LimitVegetal Stress - Limit

37

Allow Eff Soil StressAllow Eff Soil StressErodibilityErodibility Allow Eff Allow Eff

Soil StressSoil Stress Soil TypeSoil Type

Easily ErodedEasily Eroded 0.02 lb/ft0.02 lb/ft22 Weak sandyWeak sandy0<PI<80<PI<8

ErodibleErodible 0.03 lb/ft0.03 lb/ft22 CL: PI~10CL: PI~10

Erosion Erosion ResistantResistant 0.05 lb/ft0.05 lb/ft22 CL: PI~15CL: PI~15

Very Erosion Very Erosion ResistantResistant 0.07 lb/ft0.07 lb/ft22 PI >20PI >20

38

Total Stress ExceededTotal Stress Exceeded

actualactual > > allowallow

39

Cover Slope%

Erosion Resistant

Easily Eroded

Bermudagrass 0-55-10>10

876

654

Kentucky Bluegrass

0-55-10>10

765

543

Weeping Lovegrass 0-5 3.5 2.5

Permissible Velocity Permissible Velocity (ft/s)(ft/s)

40

COVERCOVER FACTOR

CF

Bermudagrass

Kentucky Bluegrass

Weeping Lovegrass

0.9

0.87

0.5

Vegetal Cover FactorVegetal Cover Factor• used to compute Effective StressEffective Stress

41

AH 667TP 61

ORDINARY FIRMORDINARY FIRMLOAMLOAM

SILT LOAMSILT LOAM

SANDY LOAMSANDY LOAM

EROSION RESISTANT

EASILY ERODED

FLOW DEPTH, ft

AV

ER

AG

E V

EL

OC

I TY

, ft/

s

Vel vs Vel vs

42

Maintenance Code InputMaintenance Code Input

43

Maintenance Code Factor Maintenance Code Factor Definitions Definitions

1.1. Uniform cover or surfaceUniform cover or surface

2.2. Minor discontinuitiesMinor discontinuities

3.3. Major discontinuitiesMajor discontinuities

44

Maintenance Code 1Maintenance Code 1

• Uniform vegetal cover Uniform vegetal cover • Standard assumption for Standard assumption for

stability design stability design

45

Maintenance Code Maintenance Code 22

• Minor discontinuities in vegetative Minor discontinuities in vegetative cover cover

• Max dimension parallel to flow:Max dimension parallel to flow: Flow depthFlow depth Stem length of vegetationStem length of vegetation

• Examples:Examples: Tire tracks perpendicular to flowTire tracks perpendicular to flow Individual small trees in spillwayIndividual small trees in spillway

46

Maintenance Code Maintenance Code 33• Major discontinuities in the cover Major discontinuities in the cover • Concentrated flow in the area of Concentrated flow in the area of

discontinuitydiscontinuity• Negates value of vegetal coverNegates value of vegetal cover• ExampleExample

Reservoir access road in spillwayReservoir access road in spillway Cattle trails up/down slopeCattle trails up/down slope

• Impractical to design stable Impractical to design stable spillway with maintenance code 3 spillway with maintenance code 3

47

Maintenance Code Maintenance Code without without Vegetation Vegetation

• No vegetation cover present No vegetation cover present • Maintenance code reflects the Maintenance code reflects the

uniformity of non-vegetated uniformity of non-vegetated surfacesurface Does surface have rills/gullies?Does surface have rills/gullies? Is the surface relatively smooth?Is the surface relatively smooth?

48

• Maintenance code effectMaintenance code effect

e= ds(1-Cf) (ns/n)2 (AH-667)

Effective StressEffective Stress

MCMC CCff

22 00

33 00

nnnnss

49

Rooting DepthRooting Depth

50

Rooting DepthRooting Depth• Root penetration Root penetration << 1 foot 1 foot

Areas with no vegetation Areas with no vegetation Sod stripped or rafted from surface at low Sod stripped or rafted from surface at low

stresses stresses SITES warning at shallow topsoil (potential SITES warning at shallow topsoil (potential

rooting) depths rooting) depths • Rooting depth also considered in Rooting depth also considered in

integrity analysis integrity analysis

• Potential rooting depth >1 foot-- Potential rooting depth >1 foot-- minimal impact on spillway minimal impact on spillway performanceperformance

51

Stability AnalysisStability Analysis SummarySummary

• Based on peak of SDHBased on peak of SDH• Applied to exit channelApplied to exit channel• Velocity Or Stress calculationsVelocity Or Stress calculations

• COMPUTE V, COMPUTE V, ee, or AS WIDTH, or AS WIDTH

• Maintenance Code for stress Maintenance Code for stress calculationscalculations

52

EndEnd