Storage Outflow

While the storage capacity curve is used to define the relationship between volume and elevation, a rating curve that defines the elevation vs. discharge is required in order to determine the outflow from the storage basin.  Generally discharge is controlled by low level outlets and spillways.  Culverts and other pipe openings are typically used for low level outlets.  Spillways are part of any dam construction, but in the case of a roadway or other similar embankment that may cause flood waters to be temporarily stored, the roadway itself acts as a spillway once the water surface elevation reaches the top.

Rating curves for hydraulic control structures like culverts and spillways can be computed from simplified equations such as the orifice and weir equations.  More sophisticated programs such as HY8 or HEC-RAS can also be used to generate rating curves where inlet and outlet losses and improved structures are considered.  Such rating curves can be exported for use in detention basin analysis.