1 Spillway Ratings and Stability Design Procedures __________________________ SITES 2005 INTEGRATED DEVELOPMENT ENVIRONMENT for WATER RESOURCE SITE ANALYSIS

2 OBJECTIVE 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 cfs Elev and discharge must increase Structure Table

4 PS Type

5 Inlet Conduit PS Schematic

6 PS Inlet

7 Example Help

8 PS Conduit

9 Principal Spillway Data Number of conduits,Number of conduits,  Identical Conduits (if > 1) Conduit(s) size (length and diameter)Conduit(s) size (length and diameter) Manning’s n value for the conduit(s),Manning’s n value for the conduit(s), Hydraulic grade line at the outlet (tailwater elevation)Hydraulic grade line at the outlet (tailwater elevation)  Elevation of the center of the pipe outlet  Tailwater is assumed constant SITES develops rating tableSITES develops rating table  spillway crest  maximum elevation in the structure table

10 Optional Aux Spwy Entry

11 Auxiliary Spillway Rating Flow resistancevariesFlow resistance varies by reach Mixed vegetal and non-vegetalMixed vegetal and non-vegetal flow resistance Seeks hydraulic control sectionSeeks hydraulic control section

12 Inlet Natural Ground Rch1 Constructed Inlet Channel Rch2 Constructed Exit Channel Rch3 Exit Natural Ground Rch4 Tie Station Auxiliary Spillway Profile

13 Tie Station Downstream end of the level crest section of the auxiliary spillwayDownstream end of the level crest section of the auxiliary spillway Same coordinate system as geologic materialsSame coordinate system as geologic materials Locates auxiliary spillway template relative to geologic materialsLocates auxiliary spillway template relative to geologic materials Not where cowboys put horses when in the saloonNot where cowboys put horses when in the saloon

14 AS Template Natural or existing ground profile - geology inputNatural or existing ground profile - geology input SITES computes intersection natural ground – constructed channelSITES computes intersection natural ground – constructed 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 change by reachVegetation or earthen lining may change by reach

15 SITES AS Computation SITES seeks the hydraulic control section for each dischargeSITES seeks the hydraulic control section for each discharge Backwater analysisBackwater analysis Does exit channel reach or downstream natural ground reaches sustain subcritical flow at given discharge?Does exit channel reach or downstream natural ground reaches sustain subcritical flow at given discharge?

16 Hydraulic Control 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 VR A E D C B AH-667 TP-61 Manning’s n VR Product ft2/s

18 Flow Resistance 1.Manning’s n 2.SCS TP-61 Retardance (A-E) 3.Ag Handbook 667 Retardance Curve (C i )

19 C I = 5.6 n = 0.04 Head-Discharge

20 TR-60 AS Criteria Duration Stability Check Short 6 hour IntegrityCheck Short & Long 6 hour & 24 hour

21 Stability vs Integrity Stability CheckStability Check  Protect the aux spillway SURFACE  No surface erosion! Integrity CheckIntegrity Check  Some erosion accepted/expected  Protect the aux spillway LEVEL CREST from breaching  Protect dam from overtopping

22 Stability vs Integrity Analysis Hydro- graph Parameter StabilitySDH Surface erosion in aux spwy? Peak stress IntegrityFBH Level crest aux spwy breached? Cumulative erosion

23 Peak SDH vs Cumulative FBH  SDH Peak Outflow Stability Check Peak SDH Integrity Check Cumulative FBH

24 After the Storm Hydro- graph Anticipated Local Sponsor Action StabilitySDH Verify no surface erosion in aux spwy IntegrityFBH Verify dam level crest not breached Fill erosion in aux spwy

25 Stability Analysis Definition Tractive stress of earth or vegetated spillway computed from SDH 6-hour peak dischargeTractive stress of earth or vegetated spillway computed from SDH 6-hour peak discharge

26 TR-60 Stability Policy Aux Spillway Frequency of Use Allowable Stress > 1 in 50 years 1.0 X AH667  1 in 50 years 1.2 X AH667  1 in 100 years 1.5 X AH667 

27 SITES Auxiliary Spillway Stability Analysis

28 Stability Analysis in SITES Design variable optionsDesign variable options  Allowable stress ( , psf)  Permissible velocity (fps) Compute optionsCompute options 1.User enters AS width   SITES compute AS stress 2.User enters allowable stress   SITES compute AS width

29 SITES Summary – psf

30 SITES Text Output

31 Effective Stress = psf

” AH 667 Fig psf 1” grain size allowable stress ~ 0.4 psf 0.9” grain size meets stability requirements

33 Vegetal Erosion Protection  total  veg  soil

34  = Total Hydraulic Stress – lb/ft2  = Vegetal Stress + Effective Soil Stress  =  ve +  e Total Hydraulic Stress

35  va = 0.75 C i C i = Retardance C i = 5.6  va = 4.2 lbs/ft 2 Allowable Vegetal Stress

36 Vegetal Stress - Limit

37 PI=NP-8 Allow Eff Soil Stress Erodibility Soil Type Easily Eroded 0.02 lb/ft 2 Weak sandy 0<PI<8 Erodible 0.03 lb/ft 2 CL: PI~10 Erosion Resistant 0.05 lb/ft 2 CL: PI~15 Very Erosion Resistant 0.07 lb/ft 2 PI >20

38 Total Stress Exceeded  actual >  allow

39 CoverSlope % Erosion Resistant Easily Eroded Bermudagrass > Kentucky Bluegrass > Weeping Lovegrass Permissible Velocity (ft/s)

40 COVER COVER FACTOR C F Bermudagrass Kentucky Bluegrass Weeping Lovegrass Vegetal Cover Factor Effective Stressused to compute Effective Stress

41 AH 667 TP 61 ORDINARY FIRM LOAM SILT LOAM SANDY LOAM EROSION RESISTANT EASILY ERODED FLOW DEPTH, ft AVERAGE VELOCITY, ft/s Vel vs 

42 Maintenance Code Input

43 Maintenance Code Factor Definitions 1.Uniform cover or surface 2.Minor discontinuities 3.Major discontinuities

44 Maintenance Code 1 Uniform vegetal coverUniform vegetal cover Standard assumption for stability designStandard assumption for stability design

45 Maintenance Code 2 Minor discontinuities in vegetative coverMinor discontinuities in vegetative cover Max dimension parallel to flow:Max dimension parallel to flow:  Flow depth  Stem length of vegetation Examples:Examples:  Tire tracks perpendicular to flow  Individual small trees in spillway

46 Maintenance Code 3 Major discontinuities in the coverMajor discontinuities in the cover Concentrated flow in the area of discontinuityConcentrated flow in the area of discontinuity Negates value of vegetal coverNegates value of vegetal cover ExampleExample  Reservoir access road in spillway  Cattle trails up/down slope Impractical to design stable spillway with maintenance code 3Impractical to design stable spillway with maintenance code 3

47 Maintenance Code without Vegetation No vegetation cover presentNo vegetation cover present Maintenance code reflects the uniformity of non-vegetated surfaceMaintenance code reflects the uniformity of non-vegetated surface  Does surface have rills/gullies?  Is the surface relatively smooth?

48 Maintenance code effect Maintenance code effect  e =  ds(1-C f ) (n s /n) 2 (AH-667) Effective Stress MC CfCfCfCf nnsnns00nnsnns

49 Rooting Depth

50 Rooting Depth Root penetration < 1 footRoot penetration < 1 foot  Areas with no vegetation  Sod stripped or rafted from surface at low stresses  SITES warning at shallow topsoil (potential rooting) depths Rooting depth also considered in integrity analysisRooting depth also considered in integrity analysis Potential rooting depth >1 foot-- minimal impact on spillway performancePotential rooting depth >1 foot-- minimal impact on spillway performance

51 BASED ON PEAK OF SDH BASED ON PEAK OF SDH APPLIED TO EXIT CHANNEL APPLIED TO EXIT CHANNEL VELOCITY OR STRESS calculations VELOCITY OR STRESS calculations COMPUTE V,  e, or BED WIDTH COMPUTE V,  e, or BED WIDTH MAINTENANCE CODE for stress calculations MAINTENANCE CODE for stress calculations Stability Analysis Summary 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,  e, or AS WIDTHCOMPUTE V,  e, or AS WIDTH Maintenance Code for stress calculationsMaintenance Code for stress calculations

52 End