Stormwater Detention

The modem stormwater strategy favours a temporary engineered storage of water in the basin in preference to immediate conveyance to the nearest designated water body or any other outfall point. It is obvious that this storage is over and above the natural detention that does take place in any given basin. It has been observed that ponding of rainwater by design on rooftops, or over parking lots, of course for a short duration, does amount to enough storage so that the peak runoff rates are reduced perceptibly. In this context, it may be pointed out that the physical features of roughness elements in a channel section, such as grass-line open drainage channels, are important to retard the flow of stormwater, thus contributing to the on-site stormwater detention in the form of valley storage.

The concept of constructing smaller reservoirs to detain, for sometime, water coming from streets, parking lots, etc., and then releasing under controlled conditions as required is gaining growing acceptance. These devices are best suited to manage the runoff from relatively short and intense local storms that would otherwise cause frequent flooding and severe erosion, with the consequent effects. Some particular benefits of stormwater storage through engineered detention can be listed as under:

(a)        Peak runoff rates are reduced.

(b)       Both the frequency and severity of flooding are attenuated.

(c)        Surface water quality is maintained to a satisfactory standard.

(d)       Sedimentation in streams gets reduced due to reduction of soil erosion.

(e)        While water stands detained, the chances of recharge of acquifer increase; however, much depends on the permeability of the soil.

However, the on-site stormwater detention basins need to be looked after for appropriate maintenance. They are subject to sedimentation and deposition of debris being the first stop that is applied on the surface flow; and their outlet structures, therefore, generally get easily clogged. Another problem relates to weed control and breeding of mosquitoes in stagnant waters; moreover, the safety of children of the locality cannot be left unattended. The beneficiaries must be motivated to share the responsibility of maintenance; or any appropriate agency, like area municipality, can attend to this aspect. A storage basin may retain water for all the time (i.e. with no outlets provided) forming a permanent pond, and can also be harnessed to provide aesthetic and recreational advantages to the community. Such storage is known as retention basin. There is a third type of storage structure, namely, recharge basin that allows the stored stormwater to percolate into the underlying aquifer, and thus recharging and replenishing the ground water reserves in addition to the primary function of controlling the storm runoff. It is, however, understood that the underlying soil must be permeable to an appropriate degree for relatively rapid infiltration. It is helpful if the seasonal high water table is at least 0.5 m below the bottom of the tank.