Essay: Maximising peak flow reduction which located in the outlet discharges of Puri Mas

Each chromosome in the population is evaluated according to the objective functions. As it is mentioned in Section 7.2, the objective functions of this study are maximising benefits and minimising costs. The benefit is maximising peak flow reduction which located in the outlet discharges of Puri Mas. The outlet discharges are taken from SWMM model results. By implementing BMPs in the subcatchments, it will decrease the peak flows in the outlet discharges of Puri Mas. Therefore, the first fitness evaluation is peak flow reduction in outlet discharges. The second objective function is minimising costs of BMP implementation as described in Section 7.2. If the unit area of BMPs has fulfilled the objective functions (convergency of population), the GA stops and the optimum solution is selected from the current population. Otherwise, new population is generated for doing same processes as the previous population.
In the fitness evaluation, each solution is assessed on how it performs according to the objective functions. The better performing solutions will be Pareto optimal if no other solution can improve the performance. To identify the performing solutions, each solution in population is ranked. NSGA-II compares two solutions according to the rank. Lower rank is assigned to be better solution. If two solutions are in the same rank then it compares based on crowding distance. Higher crowding distance is better solution. After completing these processes, new generation are produced by crossover and mutation.
For this study, a program that links NSGA-II to SWMM model developed by Dr. Solomon is used. This program was developed using C++ and it can be used to optimise the unit area of BMPs application. The input data for this program are: input file of SWMM model, NSGA-II parameters, and data of unit BMPs. The NSGA-II parameters that need to be defined are: population size, number of generation, mutation rate, and crossover rate. Five scenarios of BMP application are proposed for this study. Scenario A, B, C, and D will implement bio-retention cell, green roof, infiltration trench, and rain barrel individually and secenario E will implement the combined all BMPs (See Table 7.2).

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