Storage

Storage routing is the simplest form of hydrologic routing. It relies on the assumption that the storage in a reach is related to the depth of flow based on the outflow from the reach. As we previously said, hydrologic routing is based on the continuity equation. The continuity equation states

where V is the volume. When the equation is rewritten over a finite interval, it becomes the more familiar form.

The formula is rearranged to the following form:

The storage in the channel is dependent on the average cross sectional area of the reach for a given flow rate. It can be determined by developing a flow rate – area relationship based on the Manning’s equation. The channel length multiplied by the average cross section yields the storage in the reach at a given flow rate.

Generally, the average flow velocity is computed at either bank full condition or at a flow of 75% of the hydrograph peak. The procedure is similar to the Storage Indication Reservoir routing technique. A channel properties table is first developed. The table lists stage, flow, storage, S-Odt/2 and S+Odt/2.

With the properties table, the right side of the above equation is solved. The value is just a lookup in the properties table for the previous stage. With the right side of the above equation now known, simply look up the stage, flow and storage for the left side of the equation.