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 San Antonio
 Unified Development Code
 Appendix H. STORM WATER DESIGN CRITERIA MANUAL
 Chapter 10. CULVERTS
 Appendix 10.3. Design Guidelines

  § 10.3.13. Roadway Overtopping  

Latest version.

  • When roadway overtopping occurs on an existing roadway, the design engineer should check the depth of flow over the roadway for the design storm and compare the depth to Figure 4.3.1C. A new development should not increase the depth of flow from the "Proceed with Caution" to "Dangerous" conditions. If this condition occurs then some culvert or other drainage improvements may be needed to mitigate this "Dangerous" condition.

    35-H10-3-12-A.png

    Figure 10.3.13.A - Culvert with Overtopping Flow
    (Source TxDOT, Hydraulic Design Manual, 2011)

    When the calculation of the culvert headwater, assuming the total discharge is going through the culverts, is above the low point of the roadway, a weir condition will develop. The calculation of the amount of flow that passes through the culvert and the remaining portion of flow that overtops the roadway is an iterative process.

    Use the Weir Equation 10.3.13 to determine the average depth between headwater and low roadway elevation (H h ) for the roadway. The normal discharge coefficient for roadways should be 2.9.

    (Equation 10.3.13)

    Q = k t CLH h (1.5)

    Q = discharge (cfs)
    k t = over-embankment flow adjustment factor (see Figure 10.3.11.B)
    C = discharge coefficient
    L = horizontal length of overflow, ft. This length should be perpendicular to the over-flow direction.
    H h = average depth between headwater and low roadway elevation (ft.)

    If the tail water is sufficiently high, the adjustment factor k t would reduce the discharge over the roadway. For values of H t /H h below 0.8, the adjustment factor k t is one (1). For roadway embankments as shown in Figure 10.3.13.D may need to be broken down into segments for the computation of the weir flow.

    The use of HEC-RAS or other approved model can be used to determine the flow through the culvert and over the roadway.

    35-H10-3-13-B.png

    Figure 10.3.13.B - Over-Embankment Flow Adjustment Factor
    (Source TxDOT, Hydraulic Design Manual, 2011)

    35-H10-3-13-C.png

    Figure 10.3.13.C - Roadway Overtopping with High Tailwater
    (Source TxDOT, Hydraulic Design Manual, 2011)

    35-H10-3-13-D.png

    Figure 10.3.13.D - Cross Section of Flow over Embankment
    (Source TxDOT, Hydraulic Design Manual, 2011)