Detention does not change the total runoff volume; however, detention does change the rate of discharge, depending on the characteristics of the runoff and the detention facility. Such facilities may be in the form of holding reservoirs, large borrow ditches, or underground storage.

In the past, detention was not typically incorporated into the design because the Department's policy was to remove and dispose of runoff as quickly and effectively as possible. However, with increased development in Texas, runoff rates and volumes have increased, causing the need for larger and more costly drainage structures. The greater rates and quantities of runoff may also damage downstream development.

A detention facility may decrease facility costs or diminish potential damages due to the increased runoff rates and volumes. With this intent, many municipalities, counties, and other entities in Texas have begun to require detention as an integral part of drainage design. While not specifically required to comply with municipal and county requirements, hydraulic designers for the Department should consider the need for detention in the design to avoid the risk of damage to adjacent properties. A detention system may also be necessary for water quality control.