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 metho ds, uses reservoir routing methods and is driven by daily rainfall and watershed inflows. Simulated daily hydrology provides annual and mont hly water balances, hydroperiod distributions, flood frequency distrib utions, soil exceedance values for hydrologic plant suitability and re tention time distributions. The model was applied to a standard stormw ater wetland design. The hypothetical wetland performed as designed, p roviding good flood reduction and an adequate supply of water even in dry years. The analysis of drawndown regimes showed that for this wetl and - watershed system, the water supply was so reliable from the wate rshed (423 cm/year +/- 130 cm/year), that, without water level managem ent, 40% of the wetland would likely develop into an open-water pond w ith low species diversity and an additional 40% would only support lat e-season emergents. Retention time distributions showed that an orific e 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).