Laserfiche WebLink
ORDINANCE N0. 91-38 <br />This standard basically requires peak flow to be reduced in <br />proportion to increased runoff volume. If runoff volume is not <br />increased, peak flows are only required to match predevelopment <br />peak runoff rates for equivalent size storms for all storms from <br />one (1) year to one-hundred 100 year storms. This criteria is <br />designed to require more control when the change in the runoff <br />pattern caused by development is more drastic. The quantity and <br />cost of preventative control features will need to be in proportion <br />to the level of increase in runoff caused by the change in land use <br />- a fair equitable criterion. <br />Concern is frequently expressed in regard to the extremes of <br />the accelerated storm water control standard - the one- and the <br />one-hundred-year frequency storms. Justification for using the <br />one-year storm is based on the erosivity of stream flow and on what <br />is known as the "routing problem". Small or frequent storms are <br />very erosive simply because they occur so often. Damage doesn't <br />have time to heal before the next storm comes. Bankfull channel <br />flow is essentially the most erodible state of stream flow on the <br />channel itself. Any increase spills out over the flood plain with <br />very little increase in the velocity of channel flow. Any flow <br />less than bankfull generally has a corresponding decrease in <br />velocity, and therefore, less erosion damage to the channel. Where <br />there is a significant increase in runoff volume, it is very <br />important to have the runoff occur at something less than bankfull <br />flow, especially in watersheds approaching complete development. <br />5