Low lying site-
Runoff water may not discharge effectively by gravity from low lying area. Therefore, rainfall with small intensity or small duration may lead to significant depth of flooding or significant inundated area at such locations. If there is no waterbody nearby the project site with bed level suitable to discharge the runoff water from the project site, then gravity drains may not be feasible at such locations. Backfilling the entire site land to avoid the flooding may not be economical, may not good for foundations and may take significant time for backfilling. Mechanical dewatering (pumping) may be the only option in such a case, however cost of pumping station, fuel/electricity may be significant. Failure in pumping system due to any reason may lead to flooding. As the overall area in and around the project site is low lying and flooded, disposal of site runoff outside the project boundary may not additionally impact the adjacent area. However, it need to be verified before planning the mechanical dewatering.
Uniform outward slope–
In such case, even if significant external catchment area is governing runoff to the project site, runoff water can be drain outside the project site by gravity. Therefore, depth of flooding at such site may not be significant or if it is significant, wetness time may not be significant. However, there may water logging in local depressions. Local depressions can be treated by cutting and filling to avoid drainage system. However, if the site strata are rocky or soil for the backfilling is need to bring from outside, then cost of cutting and backfilling may be significant.
Highly mild slope or flat ground-
Due to flat land, there may not be defined flow path and therefore, there may not be natural streams exist at such site. Therefore, it may be difficult to discharge the site runoff from such mild sloping or flat land. Pumping water outside the project boundary may increase the flood significance in the adjacent land and may lead to property damage. Therefore, mechanical dewatering (pumping) may also not feasible to drain the site runoff. In such condition, infiltration ponds and infiltration trenches may be a solution to explore. Infiltration ponds and trenches may be created in the entire project site at several locations uniformly. After each rainfall event, the pond and trenches may get filled with water and its shall get infiltrate and evaporate completely or partially before the next rainfall event. However, this solution may require huge land, good infiltration rate of soil, high temperature, low humidity and adequate gap between consecutive rainy days etc.
Steep gradient land in and around the project site–
Provision of drainage system may minimize the flood significance. On steep gradient land, drains can be provided with higher slope and higher velocity. Higher velocity may require lower drain section and good lining material. However, if the external and site catchment area is very huge and land has steep gradient, then huge runoff may govern in short duration. Therefore, drain sectional requirement may increase significantly despite maintaining high velocity. Increase in drainage section and requirement of good lining material may increase the cost. Cost of drain at such site condition may be minimized by providing detention pond. Runoff water reached to the project site from the external catchment area may be collected in detention pond. Small outlet may be provided at the bottom of the detention pond and may be discharged into the site drain. As the outlet to the detention pond will be small, small section of drain may adequate to cater the discharge from the detention pond. As the outlet to the detention pond will be small and volume and rate of runoff water entering into the detention pond will be huge, water level in the detention pond may raise rapidly. Therefore, area and depth of detention pond may require huge. Water level in the detention pond may slowly lowered down after the rainfall event and detention pond may completely dry before receiving runoff water from the next rainfall event. Detention pond may be simply excavated to minimize the cost, however, higher requirement of land area for the detention pond may increase the cost. Also security fence or barricade may need to provide all around the detention pond.
