ESTIMATION OF LOST DISCHARGE RATE IN AN ALLUVIAL FAN BASED ON THE OBSERVATION

Abstract

It is clear that a part of surface water discharge is lost as unconfined groundwater discharge around the alluvial fan apex. The average lost discharge rate was about 20 % by ADCP survey in a previous study on the alluvial fan of the Nagara River. However, the variation of lost discharge rate with changing of water surface elevation is unknown. Then we set a survey site at a point bar where there was a distributary point of two palaeochannel. Under the different surface discharge condition, we manually observed groundwater elevation at 19 wells and automatically observed one at 4 wells in order to obtain the shape of groundwater table. Additionally we observed surface water discharge again by ADCP and the average of lost discharge rate was 18 %. By not using Dupuit’s assumption but using the depth of the unconfined aquifer which has quite curved streamlines, we consequently derived macroscopic parameters of exchange between surface water and groundwater on an alluvial fan. The estimated transmissivity T was between 5 m²/s under normal stage of water and 30 m²/s under flood stage. The permeability k was estimated as 0.04 m/s at this time.