ROLE OF URBAN STORM-FLOW VOLUME IN LOCAL DRAINAGE PROBLEMS

At culvert entrances and other drainage obstructions the area upstream of the obstruction acts as a reservoir, holding arriving runoff while earlier arriving waters are still passing through. Urban development in a watershed increases storm-flow volume and peak Tate, increasing the potential accumulation of water and hence rising of stage and overflowing. Storm hydrographs with different flow volumes and peak rates were routed through a computer model of a culvert entrance, with the upstream area functioning as a reservoir. Maximum stage rose with increasing flow volume for more than half of the modeled combinations of conditions. Thus in an urbanizing watershed, for a wide range of conditions, the tendency to overflow at an obstruction increases unless volume of flow is suppressed, whether or not peak rate is suppressed by detention. Storm-water infiltration, which controls both flow volume and peak rate, would be a more complete solution. Storm-water management policy that is aimed to prevent overflows at drainage obstructions should consider flow volume control in addition to peak rate control.