MODELING THE INFLUENCE OF HETEROGENEITY ON HYPORHEIC-ZONE PROCESSES

It is important to understand hyporheic zone processes because they control the mixing, transport, and distribution of dissolved oxygen, nutrients and contaminants, and they influence riverbed habitat characteristics such as water temperature. We created a three-dimensional finite-difference numerical flow model to represent a fluvial system dominated by sandy gravel (SG) with a single meandering channel. Stratasets of open-framework gravel (OFG) often occur in the riverbed deposits and we seek to understand the influence of OFG strata, because they have much higher permeability than SG and thus provide preferential pathways for solute and nutrient exchange in the hyporheic zone. To create one realization of heterogeneity, we used a Markov Chain approach for the stochastic simulation of percolating OFG strata within SG. OFG has a mean permeability of 2×10⁴ Darcys, SG has a mean permeability of 1×10² Darcys, and the proportion of OFG within SG is 0.28. The OFG stratasets can be decimeters thick and meters to tens of meters in lateral extent. A surface water flow model by Stonedahl (2010) was used to provide starting head values for the groundwater flow simulation. This hyporheic zone model is being used for ongoing computational experiments to quantify flux rates and residence time distributions.