Using Annual Hydrographs to Determine Effective Impervious Area
Reducing the amount of directly connected impervious areas improves watercourse health and increases the potential for sustainable fish communities in streams. It also reduces the impacts of frequently-occurring rainfall-runoff events, and shifts watercourse hydrology closer to pre-development conditions. There is increasing focus on low impact development (LID) strategies as a means of improving watercourse health. Accurately determining the reduction of effective impervious area (EIA) has become crucial for assessing the effectiveness of these strategies. Quantifying EIA is also essential in developing and calibrating hydrologic models. In addition, it is important to test the effectiveness of LID strategies over the entire year, to take into account the large variability in antecedent conditions. Continuous simulation models focus on all events throughout the year, and not just a large design event.
Although several methods exist for measuring EIA, they tend to overestimate the effectiveness of LID strategies in wet climates when soils are saturated for a significant portion of the year. An annual hydrograph method is proposed to determine EIA. This involves using existing gauged creek systems and rainfall records from rainfall gauging networks. This enables the use of available data to determine a watershed’s response to rainfall throughout the year, and thereby compute a year-round EIA. The results of this type of analysis will help to assess the effect of implementing LID strategies for existing or proposed developments, particularly for areas with wet climates.
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