2D Effects of Geomorphology and Discharge on Hyporheic Exchange—a HEC-RAS Modelling Study

Abstract: Hyporheic exchange is an ecologically and biogeochemically essential function of rivers and streams. One important driver of hydrostatic (hyporheic) exchange is gra- dients in the hydrostatic hydraulic head at the streambed. This thesis investigates the impact of discharge on hydrostatic exchange in two stream reaches in Uppland, Sweden, with different geomorphological characteristics. By comparing 1D approx- imations of hydrostatic head variations along different longitudinal profiles across the streams, the use of a 2D hydraulic model for defining such variations is evaluated. Channel topography and discharge data have been obtained through field surveys in the two streams and form the basis for the setup of two HEC-RAS 2D models. The models have been calibrated against stream-depth measurements, validated against stream depth and stream velocity, and used for simulation of a range of discharges in both reaches. Water surface elevations, obtained for the different discharges in three profiles along each reach, have been used as input in a spectral model evaluating flow across streambed area; average hyporheic exchange velocity W. The results show that W , and thereby the hydrostatic exchange, decreases with increasing dis- charge and varies between different longitudinal profiles in the reach with the most complex geomorphology. For the reach with simpler geomorphology, the effects of discharge, as well as variations across the streams, are negligible. This implies that a 1D approximation of the hydrostatic head variations at the streambed can be sat- isfactory for a stream with simple geomorphology, whereas a 2D evaluation is more accurate for a stream with a complex geomorphology.