Slide 45 of 49
Notes:
The primary purpose of the overbank flood protection volume sizing criteria is to prevent an increase in the frequency and magnitude of out-of-bank flooding generated by development. The local review authority may require that the peak discharge rate from the ten year storm event be controlled to the pre-development rate. Other options are the 2, 5, or 25 year storm, but the 10 year storm is most commonly used. The choice of storm event will depend on the capacity of the upstream drainage system. Generally, the less frequent the storm event, the higher the cost of flood control as well as flood damage. If channel protection storage is provided, it is generally not advisable to also require 2 or 5 year peak discharge control. In these cases, the 10 or 25 year peak control is more warranted.