The goal of this criterion is to prevent flood damage to conveyance systems and infrastructure and reduce minor flooding caused by overbank floods. Overbank floods are defined as floods which exceed the bankful capacity of the channel and spill over to the floodplain where they can damage property and structures. The key management objective is to protect downstream structures (houses, businesses, culverts, bridge abutments, etc.) from increased flows and velocities from upstream development.
Most local reviewing authorities establish an overbank design storm that is matched with the same design storm used to design open channels, culverts, bridges, and storm drain systems. Most localities in Minnesota require that post-development peak discharge rates from the 10-year and/or 25-year, 24-hour design storm event be controlled to pre-development rates.
In general, the storage volume needed to manage the 25-year return design storm is much greater than the 10-year design storm. Modeling has shown that control of the 10-year storm coupled with control of the 100-year storm effectively attenuates storm frequencies between these two events (e.g., the 25-year storm). Even without attenuation of the 100-year event, 10-year control provides a significant control for the 25-year storm (approximately 70 to 80 percent).
Consequently, most communities across the state have adopted the 10-year design storm control for overbank protection, since it requires less storage volume and provides some de-facto control for the 25-year storm. The choice of what design storm(s) to target for overbank control is always a local decision, and normally depends on whether the 10- or 25-year design storm has historically or currently been used as the basis for the design of conveyance systems and culverts.