Investigating potential of storm water management through rooftop water harvesting technology.

Abstract

The hydrological cycle describes water as a renewable and recyclable resource. It is a key determinant in the existence of life and agricultural activity. It is seemingly abundant on earth but in case of any scarcity, it is thought of as a precious resource. Being an integral of urban life, it is used in homes for several purposes such as drinking, cooking and even watering gardens. Away from homes, it is used for fishing, navigation and recreation. Storm water runoff is a part of this cycle. It is described as that portion of precipitation that is not lost to interception, surface detention, evaporation and infiltration. Urban drainage systems of municipalities in Uganda are overwhelmed with runoff, most of which are generated from rooftops and left uncaptured, largely due to the presence of impervious surfaces resulting from increasing urbanization, surfaces that used to absorb water; vegetation and topsoil, are now covered with building tops, sidewalks, parking lots, paved and cemented roads causing water to run off them. Unfortunately, the existing drainage systems are not even in functioning conditions to hold the large runoff volumes. Most of this runoff ends up in stream channels and other receiving surface water bodies thus immensely affecting water quality. This study was aimed at investigating potential of storm water management through rooftop water harvesting technology in Western division, Tororo municipality. This was achieved through hydrological modelling in HEC-HMS, the Rational method for roof areal runoff quantification and land use monitoring for modelling the effects of increasing urbanization on the watershed runoff potential rates. The output of the study includes generated hydrographs, peak discharge volumes and the water harvesting potential of the area. The results of the study from the hydrological model gives a peak discharge of 14m3/s whereas that captured by roofs is 12.58m3/s indicating an 89.8% decrease in the amount that would otherwise runoff. The area also has an annual water harvesting potential of 2,022,178.568m3. This could be used to supplement the existing sources of water supply and it is in itself a cheap technology if adopted, reducing on the cost of water bills as well as mitigating the occurrences of floods in the region. With increasing urbanization levels, the peak runoff discharges are also bound to increase because of increase in the percentage of impervious surfaces, thus increase even in the water harvesting potential.