Incorporating macroinvertebrate biological structures into gravel-bedded stream fluid dynamics using 3D CFD modelling

In gravel-bedded streams and rivers, silk nets of hydropsychid caddisfly larvae (Trichoptera:Hydropsychidae) can increase the critical shear stress, τ∗c, required for sediment particle motion, reduce bedload flux, and provide a low flow refuge for other organisms. However, there is still a limited understanding of the mechanisms that regulate how biological silk structures might influence near-bed fluid dynamics. Part of the challenge in identifying both the per capita and population level influences of caddisfly silk nets lies in the difficulty of modelling such fine-scale and detailed architecture of the structures. Here, we use a 3D computational fluid dynamics (CFD) model to evaluate the impact of silk nets on shear stress and pore space velocity by varying silk net aerial density within a modelled gravel bedded stream where the silk nets are simulated as small rigid bodies of realistic distribution, geometry, and orientation. The model revealed that silk net densities of 330 and 735 nets/m2 reduce the average shear stresses by an average 6.0% and 11.6%, respectively, over a range of inflow velocities. Silk nets also result in near-bed flows close to 0 m/s by reducing velocity by up to 60%. Our research sheds light on the important influence of small but numerous aquatic animals on water flow paths and highlights the significance of adding biological complexity into water flow models in lotic ecosystems to improve erosion models that may be used in river channel and riparian habitat design.