Modeling Low Impact Development Water Quality BMPs with IDEAL

Abstract

IDEAL (Integrated Design, Evaluation and Assessment of Loadings) is an object based model developed to predict water quality benefits of LID (Low Impact Development) and/or BMPs (Best Management Practices) in urban post construction watersheds. The paper describes scenarios from Greenville County, SC, using local rainfall and soils information. Results of this modeling effort are intended to show the effectiveness of various treatment trains. LID concepts and BMPs modeled include disconnecting storm drains, use of rain gardens, bioretention cells, bioswales, sand filters, and VFS. Comparison is made with the use of LID concepts and BMPs with undeveloped/natural land uses. IDEAL features the capability of predicting distributed source area generation of runoff, sediment, nutrients, and pathogens in post construction watersheds. Procedures are also available to apply the model to the urban fringe areas. The graphical user interface features drop and drag capability and works for large numbers of sub-watershed source areas, conveyances, and BMPs arranged in treatment trains. Numbers and sizes of sub-watersheds are limited only by computational time and appropriateness of the conveyance objects. BMPs are modeled with process-based algorithms that take into account mass continuity and the processes of infiltration, settling, and sorption for chemicals and pathogens as well as mortality and growth for pathogens. Interarrival time of storms is considered, and rainfall input can be either single storm or a matrix of conditional probability of rainfall amounts, seasons, and infiltration or curve numbers.