Sea outfall disposal of stormwater in Doha Bay : risk assessment based on dispersion modelling

A 3D numerical hydrodynamic and dispersion study was carried out using the MIKE3 Hydrodynamic model to assess the dispersion of stormwater effluents discharged from the four sea outfalls into Doha bay area in Qatar. Dispersion simulations were carried out for three scenarios: dry weather, representative storm event and first flush. Input parameters (such as temperature, salinity, total suspended solids, total Nitrogen and Faecal coliform) for the dispersion simulation were defined from measurements obtained during representative events for each scenario of interest. Results show that variation of temperature and salinity mostly occurred at the vicinity of disposal point. The net sedimentation of solids was observed above 25 g/m2 within about 250 m from the outfalls. Concentration of Faecal coliform was found >500 count/100 mL >90% of the simulation time within 50 m from one of the outfalls, which reduced to 250-500 count/100 mL at 100 m distance. Simulation of exceedance for total N of 0.2 mg/L shows that during the dry weather flow condition, overall in the Southern Doha Bay, total N exceeded from 70% to >90% of the time. However, when exceedance of 0.6 mg/L of total N was modelled, the vicinity of only one outfall showed exceedance >90% of the simulation period. A risk assessment was carried out using results of the numerical dispersion modelling based on negligible, minor, moderate and major risk categories. Criteria for risk assessment were developed based on field observation of local ambient sea water condition. Risk of eutrophication, impact on swimming water quality and sedimentation of hazardous contaminants was evaluated. The developed risk criteria are applicable for similar bays in Qatar and in other GCC countries near Arabian Gulf.