Effect of Combined Subsurface Structures and Steps on Hyporheic Exchange

The deployment of artificial structures in streambeds has been proposed as a way to enhance 21 hyporheic exchange, and numerical models can be used to quantify their effects. In this study, combinations of different structures-that is, boxes, steps and a new type of subsurface structure (L-shaped structure)-were considered to evaluate their potential applicability on river restoration. Flow-3D and COMSOL were applied to simulate surface and subsurface flow, respectively. The performance of the structures was evaluated on the basis of hyporheic flow and residence time distributions. For the structure sizes here considered, results showed for steps (single step, combination of two steps) and L-shaped structures (single L-shaped structure, combination of two L-shaped structures) most hyporheic flowpaths return to the stream after 5 and 2.5 hr, respectively. Instead, shorter residence times (<0.25 hr) were found for boxes (single box, combination of two boxes). For combinations of steps and permeable boxes, the values of hyporheic flow per unit width are higher (0.35 and 0.3 m(2)/hr, respectively) than for the combination of L-shaped (0.06 m(2)/hr). As a result, the combinations of steps and boxes are more effective in increasing hyporheic flow. However, when subsurface structures are combined with steps the resulting hyporheic exchange is dominated by the steps. Therefore, the combined use of in-stream and subsurface structures separately may increase their benefits for hyporheic exchange, but when steps are the other subsurface structures provide minor advantages.