Double-counting of volumes in the Full unsteady hydraulic model

The Full unsteady hydraulic calculations in DRAINS treat overflow routes as unsteady flow channels and allow for the volumes that water can occupy in these. This can sometimes lead to double counting of the available volumes when overflow routes are incorrectly connected to detention basins or sag pits below ponding water levels, which could lead to an underestimation of outflow rates. For example, consider this model of a detention basin:

Three drainage lines discharge into the open basin through inlet pipes that are shown to scale. To link these to the basin object, located near the basin's outlet pipe, three overflow routes are shown. These have the following characteristics:

This arrangement is acceptable in Lite hydraulic model calculations, where the overflow route is simply used to transfer flows from the inlet pipes to the basin. However, in Full unsteady hydraulic model calculations, the channel is assumed to occupy volume that is already defined in the elevation-volume relationship for the basin, resulting in a double counting of storage.

This problem might be minimised by making the overflow routes shorter than the nominal lengths of 10m specified, but this can cause instabilities in calculations. Two better solution are to eliminate the dummy overflows altogether, as shown below, at the expense of displaying the inlet pipes with an incorrect end position or to set the downstream invert level of the overflow route to be above the basin water level.

Minor problems can also occur in Full unsteady hydraulic model calculations when overflows in streets interact with sag pit storages into which they flow. The link between two sag pits can be replaced by a special weir, which does not include a volume.