Staged cost optimization of urban storm water drainage systems based on hydraulic performance in a changing environment

Storm water drainage system is a key infrastructure in an urban area. A large amounts ofinvestments is carried out in order to make the city liveable. Urban flooding, a majorcause which yields large economic losses, property damage and loss of lives is theconsequences of failure of urban drainage systems. It requires investments in differentphases of planning, designing, construction, operation and maintenance andrehabilitation during the entire life cycle of the asset for maintaining the systemperformance and accordingly maintaining the public health and safety from the potentialimpact of flooding on life and property.The impact of changes in catchments characteristics in the course of urbanization aswell as the impact of climate change leads to the increased runoff in the drainage system.The system thus must be able to cope with the changed environment.Number of interventions has to be carried out such as maintenance, upgrading ofnetwork or other mitigation measures during the life period in order to overcome thepossible damage and inconveniences caused by the induced flooding in the changingenvironment. Thus, a reliable method for decision making in order to allocate thelimited resource in the drainage sector is quite essential.The whole life cost optimization model is developed considering the changedenvironment, possible impacts of climate change and urbanization in the storm drainage,possible impacts of flooding within stipulated period of time and optimal interventionsthat can be done at different space and time dimensions. The model is developed for theeconomic evaluation of flooding as well as necessary interventions in the net presentvalue and put all these into a whole life cost optimization framework to get the optimalhydraulic performance of the network and provide the best alternative solution fordecision making. The provision of the detention storage at the flooded nodes are used tostore the excess flow beyond the capacity of the drainage system during rainfall andlater it is drained back again once the flood subsides down. The concept of utilizing theavailable open spaces like football ground, parks etc are used to manage during the highextent flooding due to larger rainfalls that happens occasionally.SWMM hydraulic simulator is used for hydraulic evaluation of the network, geneticalgorithm is used to optimize the trail solutions and coded program in C language isused as interlinking tool. The SWMM provides the flood volume in the network and theflood is converted into the flood cost as penalty cost. The flood cost due to rainfall withdifferent return periods are used to calculate the average annual flood cost. Theproposed detention volume at different time and place is used to calculate theintervention cost. The minimization of the total net present cost of flood cost andintervention cost is taken as the objective function for the GA to optimize in aprobabilistic basis.The developed tool is used to test for a case study area in Porto Alegre, Brazil. Theflooded nodes are managed to provide stepwise construction of detention storageswhich is most likely for investment of limited resources in developing countries. Theflooding scenarios for the optimal solutions for different costs are worked out and thecorresponding network flooding is observed to be significantly reduced as compared tothe no intervention case. The optimized cost is also compared with the static construction scenario and is found to be better performance throughout the time frame.The model is tested for range of input data sets for sensitivity. The model found to bemore sensitive with discount rate followed by change in population growth and bychange in rain rate. The model can be used for wide range of data depending upon thelocations