Wetland crossings for flood mitigation and groundwater enhancement : case of the Attanagalu Oya river basin

Abstract

Recurrent flooding mostly experienced in urbanized lower floodplains is a major problem faced by many of the river basins. In the developed countries, flood retention/detention is effectively practiced as a flood management option which also enhances groundwater recharge. However, this approach requires sufficient land with provision for storage and acquisition of land for incorporation of these facilities is a major issue in built up areas. Present modelling work analysed the possibility of utilising the distributed temporary storage zones that exist at wetland crossing locations of the road network. The mathematical model calibrated and verified for the Attanagalu Oya watershed of Gampaha District at Karasnagala and extended to the outlet at Ja-ela, investigated the possibility of storage enhancement by raising the road surface together with an engineered drainage structure to detain the floods. Model simulations for the rainfall event of May 2010, revealed that the trial interventions of 0.3 m road formation level increase with a 0.2 m drainage outlet sill level increase at feasible wetland crossings lead to a 14.2% reduction in peak flow while the enrichment of groundwater was 8.7%, and the flooded area reduction at Ja Ela/Elenegoda exit point was 9.2%. This demonstrated that a network of engineered wetland crossings provides an eco efficient infrastructure solution for sustainable management of floods and groundwater enhancement.