Storage Routing

Runoff storage occurs in many different ways such as: reservoirs, small basins in developed areas, parking lots in industrial areas, or where embankments for roads and other structures impede the passage of water.  The end result in each of these cases is the temporary (or in some cases long term) storage of flood waters.  Such storage reduces the peak flow by spreading the runoff out over time.  In cases like large reservoirs the runoff may be stored for long periods of time so that it can be released as a water supply during dry months.

The fundamental mechanism of storage routing is the same for each of these cases and is defined by the equation:

ΔS = Inflow - Outflow

This is illustrated in the following animation.  When runoff entering a detention basin, or behind an embankment is greater than the capacity of a low-level outlet or culvert the excess water is stored.  As the storage increases so will the water surface elevation.  Increased water surface elevation produces a greater hydraulic head on the low level outlet which will increase the outflow, but as the detention basin is filling it is still not at the rate of inflow.  Finally, the water surface elevation may increase to the point that water spills over an emergency spillway or the top of the embankment as weir flow.  As the flood waters recede the outflow will gradually become less than the inflow and the detention basin will empty.

In order to numerically simulate storage in a reservoir relationships between elevation and volume of storage and elevation and discharge must be defined.