Implications of Various Hydromodification Management Criteria on LID System Performance and Sizing
This study explores various implications of meeting a range of hydromodification management criteria on storm runoff. Specifically, this study evaluates results of meeting Section 438 of the Energy Independence and Security Act (EISA) criteria, San Francisco Bay Area flow-duration criteria and Phase II NPDES requirements. Results are presented to illustrate effects of a range of precipitation and infiltration conditions. Differences in surface area and total volume used to meet the criteria are compared. Performance of the various scenarios is presented through flow-duration plots, long term infiltration benefit and effectiveness of 100-year flood impact mitigation. The study concludes that the LID systems have a dramatic impact on which storms generate peak runoff. In predevelopment conditions, short duration, high intensity storms control peak local runoff. However, with the implementation of LID and hydromodification management, low intensity, long duration (high volume) event cause peak local runoff.
The presentation starts with a review of various flow management objectives and presents how criteria are worded to meet these objectives. Then, the process used to evaluate the effects of meeting these criteria is presented along with a summary of the scenarios being compared. One key issue is how much volume is necessary to meet specific criteria because volume generally corresponds to costs. The results of the presentation illustrate how different configurations of similar volumes can produce significantly different hydrologic performance. A better understanding of these relationships can lead to better use of maximum feasible detention/retention volume to meet watershed objectives.
The analysis is based on continuous simulation using one of Clear Creek Solution’s HSPF based hydrology models. The results demonstrate the critical significance of the selection of the low flow threshold and low important even a low infiltration rate can be on system sizing and performance. Related to this, the potential inappropriate applications local time of concentration on low flow threshold determination is presented.
The study concludes that evaluating LID system performance for flood management credit requires careful consideration of initial conditions and that even low infiltration rates can be significant to overall system performance. Issues with criteria, infiltration, infiltration testing, orifice sizing and system maintenance are also discussed.