Optimal Best Management Practices (BMPs) placement strategies for nonpoint source pollution control at a watershed scale

The purpose of this study is to establish an optimization model for the optimalplacement of structural Best Management Practices (BMPs) at the watershed scale. It consists of three interacting components, i.e. a watershed stormwater simulation model(VAST/BASINS), a zero-dimensional reservoir water quality model (unsteady, time variable Vollenweider model), and an optimization module (the genetic algorithms, GAs). Three major tasks are completed in this work: 1) the investigation of how the watershed hydrology and water quality is affected during the storm period; 2) develop a reliable and effective
optimization framework coupling the optimization tool and the reservoir water quality model to explore the optimal placement strategies of BMPs system; and 3) Both total suspended solids (TSS) and total phosphorus (TP) are taking into account and three kinds of BMPs are
considered.
A post treatment program is developed which combines BMPs simulation, channel routing, reservoir water quality models and GAs. The results analyzed by watershed stormwater models, such as VAST and BASINS models, are taken as a response matrix. The program test results show that a single small storm does not cause significant effect on the reservoir water quality, i.e. no BMPs is required under such situation. The program also finds
the optimal solutions in an acceptable time depending on the chromosome population. By changing the pollution concentrations in the constraints, three scenarios are analyzed in this study. The TP concentrations should satisfy oligotrophic, mesotrophic and eutrophic
conditions while the TSS concentrations should be kept below 25 mg/l at the same time.
Real storm events during 03/28/2003-04/14/2003 and 2002-2003 are adopted to calibrate and validate VAST and BASINS models respectively. The modeling results demonstrate thatVAST and BASINS are good stormwater analyzing tools in Fei-Tsui Reservoir watershed.
Although there is a pollutant settling rate in the unsteady, time variable Vollenweider model, the TSS and TP concentration variations in 2002 are also compared with the field data observed by Taipei Fei-Tsui Reservoir Bureau.
There are three different time scale scenarios analyzed in this study, they are short term, long term and single design storm. The results analyzed by design storms cost much more money than the other two. Because the 2-year/ 2-hour and 5-year/ 4-hour storms are the critical storms found by Hsieh et al. (2003) that cause the serious damages on the water body. During 1988 to 2004, the 2002 is the driest year so that the annual costs analyzed by the
program are relatively smaller. The near optimal solutions found in this study are compared with former research in
Fei-Tsui Reservoir watershed. This study also uses their placement strategies as one of the initial chromosome. The peak pollutant concentrations are larger than the averaged values they used in former studies. Due to the advantage of unsteady state model, more economical placement strategies are found and suggested in this study. The decision makers or designers can adapt the BMPs placement strategies found in this study and should be good at the
limitations of BMPs. Several non-structural BMPs proposed by Fan and Chang (2006) in Fei-Tsui Reservoir watershed can be good alternatives for nonpoint source pollution control when there are topographic and land ownership problems in the future.