Role and significance of silt detention dams to sustain the functional life of a rapidly silting lake at Chandigarh

A detailed study was conducted during 2010–2011 to find out the present Storage Capacity (SC) of all the water bodies (WB) created behind the Silt Detention Dams (SDDs), their siltation rate, amount of silt arrested and possibilities of release of water to augment lake storage. 150 functional WBs were identified having combined SC of 254.2 hectare meters (ha-m) which was 48% of Lake SC of 532 ha-m. In the good monsoon season of 2010 when 1332 mm of rainfall was recorded against the average of 1120 mm, these WBs retained 217 ha-m or 41% of Lake storage indicating that a substantial part of runoff was retained by the SDDs. The subsequent loss of 137 ha-m through seepage and evaporation till May, 2011 was loss of much needed water by the Lake. Out of 150 WBs, 28 were totally silted, 38 had less than one and 36 between 1 to 2 ha-m storage and likely to silt in next 6 to 8 years. The silt load to these WBs was still around 40 tha'yr"¹ necessitating the desiltation of these 102 WBs so that silt flow to the Lake remains under check. There were 48 WBs with SC between 2 to 7 ha-m which would not need desiltation for long and could donate with proper mechanism around 83.3 ha-m in good, around 60 in normal and 40 in low rainfall years after retaining sufficient water for wildlife and also keep tourist interest alive. The quantity of silt arrested by SDDs was worked out as 137.76 ha-m or 20, 66, 400 tons. The study concluded that the degraded micro-catchments continued to release silt load of 401 ha'yr"¹ inspite of best treatment and hence the provision of SDDs was necessary to bring down initial silt loads to the Lake. The desiltation of these WBs would be necessary on reducing scale to save the Lake from further siltation. It was possible to release water to the Lake depending upon rainfall situation from 48 larger WBs.