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Review of Flood Routing

Review of Flood Routing. Philip B. Bedient Rice University. Lake Travis and Mansfield Dam. Lake Travis. Mansfield Dam, built in 1937. Lake Travis. Brays Bayou High Flow. 6 to 7 inches of Rainfall. T.S. Allison June 2001. Houston. Galveston Bay.

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Review of Flood Routing

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  1. Review of Flood Routing Philip B. Bedient Rice University

  2. Lake Travis and Mansfield Dam Lake Travis

  3. Mansfield Dam, built in 1937 Lake Travis

  4. Brays Bayou High Flow 6 to 7 inches of Rainfall

  5. T.S. Allison June 2001

  6. Houston Galveston Bay Hurricane Rita Landed on Sabine, TX On Sep 24, 2006

  7. Storage Reservoirs - The Woodlands

  8. Detention Ponds • These ponds store and treat urban runoff and also provide flood control for the overall development. • Ponds constructed as amenities for the golf course and other community centers that were built up around them.

  9. Reservoir Routing • Reservoir acts to store      water and release through      control structure later. • Inflow hydrograph • Outflow hydrograph • S - Q Relationship • Outflow peaks are reduced • Outflow timing is delayed Max Storage

  10. Inflow and Outflow

  11. Inflow and Outflow I1 + I2 – Q1 + Q2 S2 – S1 = 2 2 Dt

  12. Inflow & Outflow Day 3 = change in storage / time Re Repeat for each day in progression

  13. Determining Storage • Evaluate surface area at several different depths • Use available topographic maps or GIS based DEM sources (digital elevation map) • Outflow Q can be computed as function of depth for either pipes, orifices, or weirs or combinations

  14. Typical Storage -Outflow • Plot of Storage in acre-ft vs. Outflow in cfs • Storage is largely a function of topography • Outflows can be computed as function of elevation for either pipes or weirs Combined S Pipe Q

  15. Reservoir Routing LHS of Eqn is known Know S as fcn of Q Solve Eqn for RHS Solve for Q2 from S2 Repeat each time step

  16. Example Pond Routing Note that outlet consists • of weir and orifice. • Weir crest at h = 5.0 ft • Orifice at h = 0 ft • Area (6000 to 17,416 ft2) • Volume ranges from 6772 to 84006 ft3

  17. Example Pond Routing Develop Q (orifice) vs h Develop Q (weir) vs h Develop A and Vol vs h Storage - Indication 2S/dt + Q vs Q where Q is sum of weir and orifice flow rates.

  18. Storage Indication Curve • Relates Q and storage indication, (2S / dt + Q) • Developed from topography and outlet data • Pipe flow + weir flow combine to produce Q (out) Only Pipe Flow Weir Flow Begins

  19. S-I Routing Results I > Q Q > I See Excel Spreadsheet on the course web site

  20. S-I Routing Results I > Q Q > I Increased S

  21. Comparisons: River vs. Reservoir Routing Levelpool reservoir River Reach

  22. River Routing River Reaches

  23. River Rating Curves • Inflow and outflow are complex • Wedge and prism storage occurs • Peak flow Qp greater on rise limb • Peak storage occurs later than Qp

  24. Looped Rating Curves • Due to complex hydraulics • Higher peak Qp on inflow • Lower peak Qp on outflow • Due to prism and wedge • Red River results shown

  25. Wedge and Prism Storage • Positive wedge I > Q • Maximum S when I = Q • Negative wedge I < Q

  26. Muskingum Equations • Continuity Equation I- Q = dS / dt • S = K [xI + (1-x)Q] • Parameters are x = weighting and K = travel time - x ranges from 0.2 to about 0.5 • where C’s are functions of x, K, Dt and sum to 1.0

  27. Muskingum Equations C0 = (– Kx + 0.5Dt) / D C1 = (Kx + 0.5Dt) / D C2 = (K – Kx – 0.5Dt) / D Where D = (K – Kx + 0.5Dt) Repeat for Q3, Q4, Q5 and so on.

  28. Muskingum River X Select X from most linear plot Obtain K from line slope

  29. Hydraulic Shapes • Circular pipe diameter D • Rectangular culvert • Trapezoidal channel • Triangular channel

  30. Storage Indication Curve • Relates Q and storage indication, (2S / dt + Q) • Developed from topography and outlet data • Pipe flow + weir flow combine to produce Q (out) Only Pipe Flow Weir Flow Begins

  31. Storage Indication Inputs Storage-Indication

  32. Storage Indication Tabulation Time 3 - Note that 65.6 - 2(17.6) = 30.4 and is repeated for each one

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