Ponding at Approaches to Sag Locations

Because a sag location has a different amount of flow approaching from both directions, the gutter to each side of the inlet has a different ponded width and depth. At sag locations, the hydraulic designer must consider sag inlet capacity and allowable ponding using the following steps:
  1. Estimate the apportionment of runoff to the left and right approaches. Next compute the discharge to the sag location based on the entire drainage area. Then determine the approximate fraction of area contributing to each side of the sag location. Multiply the total discharge by each fraction to determine the discharge to each side.
  2. Determine the longitudinal slope of each gutter approach. For sawtooth profiles, the slopes will be the profile grades of the left and right approaches. However, if the sag is in a vertical curve, the slope at the sag will be zero, which would mean no gutter capacity. In reality there is a three-dimensional flow pattern resulting from the drawdown effect of the inlet. As an approximation, assume a longitudinal slope of one half of the tangent grade.
  3. For each side of the sag, calculate the ponded depth and width using the appropriate flow apportionment, longitudinal slope, and
    Equation 10-1
    . Compute the ponded width using
    Equation 10-2
    .