Spillway Ratings and Stability Design Procedures

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

2. OBJECTIVE Provide understanding of: • Principal spillway rating • Auxiliary spillway rating • Allowable stress approach • Cover discontinuities

3. Structure Table 1st entry - principal spillwaycrest elevation 1st discharge = 0 cfsElev and discharge must increase

4. PS Type

5. PS Schematic Conduit Inlet

6. PS Inlet

7. Example Help

8. PS Conduit

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

10. Aux Spwy Entry Optional

11. Auxiliary Spillway Rating Flow resistancevaries by reach Mixed vegetal and non-vegetal flow resistance Seeks hydraulic control section

12. Auxiliary Spillway Profile Inlet Natural GroundRch1 Constructed Inlet ChannelRch2 Constructed Exit ChannelRch3 Exit Natural GroundRch4 Tie Station

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

14. AS Template • Natural or existing ground profile - geology input • SITES computes intersection natural ground – constructed channel • Level crest section is required! • Flow resistance may vary by reach • Vegetation or earthen lining may change by reach

15. SITES AS Computation • SITES seeks the hydraulic control section for each discharge • Backwater analysis • 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 1 TP-61 A B C D 0.1 AH-667 E Manning’s n 10.0 7.6 5.66 4.44 2.88 0.01 100 10 1 0.1 VR Product ft2/s

18. Flow Resistance • Manning’s n • SCS TP-61 Retardance (A-E) • Ag Handbook 667 Retardance Curve (Ci)

19. Head-Discharge CI = 5.6 n = 0.04

20. TR-60 AS Criteria

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

22. Stability vs Integrity

23. Peak SDH vs Cumulative FBH Stability CheckPeak SDH Integrity CheckCumulative FBH SDH Peak Outflow t

24. After the Storm

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

26. TR-60 Stability Policy

27. Auxiliary SpillwayStability Analysis SITES

28. Stability Analysis in SITES Design variable options Allowable stress (t, psf) Permissible velocity (fps) Compute options User enters AS width SITES compute AS stress User enters allowable stress SITES compute AS width

29. SITES Summary – 0.341 psf

30. SITES Text Output

31. Effective Stress = 0.314 psf

32. AH 667 Fig 3.1 1” grain size allowable stress ~ 0.4 psf 0.9” grain size meets stability requirements 0.314 psf 0.85”

33. Vegetal Erosion Protection ttotal tveg tsoil

34. Total Hydraulic Stress t = Total Hydraulic Stress – lb/ft2 t = Vegetal Stress + Effective Soil Stress t= tve + te

35. Allowable Vegetal Stress tva = 0.75 Ci Ci = Retardance Index @ Ci = 5.6 tva= 4.2 lbs/ft2

36. Vegetal Stress - Limit

37. Allow Eff Soil Stress PI=NP-8

38. Total Stress Exceeded tactual > tallow

39. Permissible Velocity (ft/s) Cover Slope% Erosion Resistant Easily Eroded 0-5 5-10 >10 8 7 6 6 5 4 Bermudagrass 7 6 5 5 4 3 0-5 5-10 >10 Kentucky Bluegrass Weeping Lovegrass 2.5 0-5 3.5

40. Vegetal Cover Factor used to compute Effective Stress COVER FACTOR CF COVER Bermudagrass 0.9 Kentucky Bluegrass 0.87 0.5 Weeping Lovegrass

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

42. Maintenance Code Input

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

44. Maintenance Code 1 • Uniform vegetal cover • Standard assumption for stability design

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

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

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

48. Effective Stress • Maintenance code effect • e= ds(1-Cf) (ns/n)2 (AH-667)

49. Rooting Depth

50. Rooting Depth • Root 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 analysis • Potential rooting depth >1 foot-- minimal impact on spillway performance