Storm Water Management Model Sensitivity to Different Design Storm Types and Parameters: The Case of Tangier Experimental Basin, Morocco

Abstract

Rainfall records with time steps less than 15 min, which are required for designing storm water sewerage in urban areas, are rarely available throughout the Moroccan territory. The study was conducted with the objective to tackle this problem through the use of design storm structures with time steps that vary from 5 to 15 min. For this purpose, we used two structural families of design storms: (i) design storms derived from Intensity-Duration-Frequency (IDF) curves of Chicago, Composite, Desbordes, Watt, Weibull, Uniform, and Triangular design storm types; (ii) design storms from the Soil Conservation Service (SCS) synthetic rainfall. The selected design storm structures are those adapted to the rainfall distribution of Tangiers experimental basin and their resulted peak flows are comparable to the flow generated using the synthetic rainfall events. We also did sensitivity analysis for the Storm Water Management Model (SWMM) to changes of different design storm structural parameters, which includes storm duration, time steps, and intensity peak position. We evaluated how changes on these parameters affect peak flows and runoff volumes. In addition, Principal Component Analysis (PCA) method was used to validate the sensitivity analysis by identifying key storm parameters that significantly affect peak flows and Runoff volumes. SWMM model calibration results showed that the Desbordres and Chicago structures best fitted observed flow. Moreover, comparison of different SCS synthetic rainfall forms to observed rainfall events retains that of type 1. Furthermore, the Weibull and watt storms have both a shape parameter that has been calibrated by using the observed events. Overall, simulation results confirmed that peak flow was greatly affected by design storm structures, but was not closely correlated with rainfall intensity.