HYDROLOGIC DESIGN OF A WETLAND - ADVANTAGES OF CONTINUOUS MODELING

A continuous hydrologic model for constructed or depressional wetlands, intended as a design tool to supplement event-based hydrologic methods, uses reservoir routing methods and is driven by daily rainfall and watershed inflows. Simulated daily hydrology provides annual and monthly water balances, hydroperiod distributions, flood frequency distributions, soil exceedance values for hydrologic plant suitability and retention time distributions. The model was applied to a standard stormwater wetland design. The hypothetical wetland performed as designed, providing good flood reduction and an adequate supply of water even in dry years. The analysis of drawndown regimes showed that for this wetland - watershed system, the water supply was so reliable from the watershed (423 cm/year +/- 130 cm/year), that, without water level management, 40% of the wetland would likely develop into an open-water pond with low species diversity and an additional 40% would only support late-season emergents. Retention time distributions showed that an orifice outlet design would retain and provide longer treatment (98% of all runoff stays > 9 days) than would a weir outlet structure (60% of all runoff stays > 9 days).