he proposed flood water detention tank shall have a minimum effective storage capacity of 500 KL (500, 000 litres or 500 m³) and shall be constructed as an underground reinforced cement concrete (RCC) watertight structure. The tank shall be designed based on the site-specific stormwater catchment area, rainfall intensity, peak runoff, permissible discharge rate, detention period, groundwater level and soil bearing capacity. The indicative internal dimensions may be approximately 20 m × 10 m × 2.5 m effective water depth, subject to final hydraulic and structural design. The tank shall be constructed using minimum M30 grade concrete with Fe500D/Fe550D reinforcement steel, or as specified by the structural consultant. The base slab, side walls and roof slab shall be designed to withstand hydrostatic pressure, earth pressure, surcharge loads, groundwater pressure, buoyancy/uplift and applicable imposed loads under both full and empty tank conditions. All RCC construction joints shall be properly treated with suitable water bars and joint-sealing systems to ensure watertightness. The tank shall be provided with suitable waterproofing treatment, including integral waterproofing admixture and external/internal waterproofing wherever required.
The tank shall be connected to the site's stormwater drainage system through suitably sized inlet and outlet arrangements based on hydraulic calculations. A suitable silt trap/desilting chamber shall be provided upstream of the tank to prevent excessive sediment accumulation. The tank shall have a controlled outlet arrangement, including suitable valves, orifice/weir arrangement or flow-control device, to restrict discharge to the approved downstream stormwater drainage system. An adequately sized emergency overflow shall also be provided to safely discharge excess stormwater during extreme rainfall events. The tank floor shall be suitably sloped towards a sump or collection point to facilitate cleaning, dewatering and removal of accumulated sediment. Heavy-duty, lockable access manholes shall be provided at suitable locations, together with internal ladders or access arrangements for inspection and maintenance. Adequate ventilation shall be provided to prevent pressure or vacuum buildup inside the tank.
Where gravity discharge is not feasible, suitable submersible drainage/dewatering pumps shall be provided, complete with level sensors, automatic/manual controls, control panel and necessary electrical protection. The tank shall be equipped with suitable high-level and low-level monitoring arrangements and an alarm system where required. All electrical installations associated with pumps and level-control equipment shall be properly earthed and protected against moisture and flooding. The complete tank system shall be designed, constructed and tested in accordance with the relevant BIS/IS codes, applicable NBC provisions and approved structural, hydraulic and geotechnical design requirements. On completion, the tank shall be subjected to water-retention/leakage testing, inspection and commissioning to demonstrate its structural integrity, watertightness and proper functioning of the inlet, outlet, overflow, pumping and control systems. All materials, workmanship and construction shall be of approved quality and shall conform to the requirements of the project specifications and instructions of the Engineer-in-Charge.
