TN-8 : Study of reach transmissivity for stream aquifer interaction

Abstract

A river comprising boundary of flow domain is often encountered in regional groundwater flow modelling. When a river fully penetrates an aquifer and has considerable stream discharge, the river is to be treated as a boundary of prescribed head. In such a case, the region on each side of the river behaves independently. However, a situation is rarely seen where a river completely penetrates an aquifer. In case of partially penetrating river with considerable stream discharge in comparison to the seepage losses besides treating the river as specific head boundary, the exchange of flow between the river and the aquifer has to be introduced through boundary while modelling the groundwater flow. The recharge from a river to an aquifer is proportional to a difference in the level of water in the river and in the aquifer in the vicinity of river. The coefficient of proportionality is recognised as reach transmissivity, which depends on the stream-bed characteristics and shape of stream cross section besides the aquifer parameters.

In this report the reach transmissivity has been determined for a river with large width. The recharge from a river with large width to an aquifer, where water table exists at shallow depth, has been quantified using conformal mapping technique. The recharges occurring through bed and side of the river have been estimated separately. The suitable position of observation well to monitor ground water table, which can be used in assessing river recharge to the aquifer, has been identified. The influence of river stage and position of water table on reach transmissivity has been analyzed. It is found that, when the observation well is located at a distance more than 0. 5 Di, where D. is the depth to impervious stratum below the river bed, 1 the corresponding reach transmissivity is independent of the water table position at the observation well. The reach transmissivity depends on the river stage.