Sensitivity Analysis of Hyporheic Exchange to Small Scale Changes In Gravel-Sand Flumebed Using A Coupled Groundwater-Surface Water Model
Authors: Mehedi Md Abdullah Al , Reichert Nora , Molkenthin Frank 
Published: March 2020

&lt;p&gt;Distribution of the hyporheic streamlines and residence time (HRT) is a crucial factor under streambed to understand the transport phenomena of environmental toxins, sediment metabolic rates in fluvial ecology as well as hydrological water budget. To quantify HRT, both the laboratory and numerical approach could serve as discerning tools. However, due to high heterogeneity in natural streambed sediment and topography, an efficient numerical model setup can prove to be pragmatic in comparison to tedious laboratory experiments for tracing streamlines. Moreover, repeatability of results, high amount of variation in the laboratory flumebed setup, greater insight into the 3D flow system and investigation possibilities with regard to individual streamlines or particular areas of HRT distribution cannot be well executed in laboratory. On the other hand, an automated generation of hyporheic streamlines with a range of various flumebed setups could propel a better understanding of the process and behavior of hyporheic streamlines and HRT distribution. Therefore, a robust numerical method could bestow to trace a large number of particles from various seeding locations at the flumebed. All of these facts enforce the necessity of numerical modeling of flume experiments to perceive the hyporheic exchange mechanisms at fieldwork and research, which are difficult to segregate under natural in-stream conditions. Keeping these issues in mind, we developed an automated numerical&amp;#160; method for quantifying the hyporheic exchange, where the surface water modeling software, HEC-RAS 5.0.5 and the subsurface flow and reactive transport code, MIN3P are coupled. A channel segment with a longitudinal dimension of 1 m and water surface elevation of 0.02 m is used for generating the hydraulic head distribution over the flumebed. A groundwater model domain of the dimensions of x:y:z = 1m:0.1m:0.1m is considered for the investigation of hyporheic exchange. A simple code for computing streamlines based on 4th order Runge-Kutta technique with the adaptive time integration method is developed using Matlab. Sensitivity analysis of streamline distribution and HRT to small scale changes (e.g. changes in dimension, distribution, and shape of the flumebed material) was performed, assuming a sand-gravel material mix. Various geometric shapes of gravel pieces (e.g. triangle, rectangle, trapezoid, and sphere) were used to vary the elevation of flumebed on a 1 mm scale. The results of the automated process show that the size, shape and distribution of trapezoidal gravel and sand portion in the streambed have a significant impact over hyporheic streamlines and HRT. High number and length of streamlines thus high HRT are found in case of the higher length of ridges created by the elevated portion of gravel pieces. In case of the increase of the length of gravel pieces along the longitudinal direction of flumebed, the length of streamlines and HRT decrease whereas the number of streamlines increase. Small scale hyporheic exchanges are found in case of increasing length of gravel pieces. Similar outcomes are also found for triangular and spherical gravel pieces. Both the number and length of streamlines are significantly reduced in case of the high number of gravel and sand portion on the streambed.&lt;/p&gt;