The Effects of Mesh Resolution On Two Dimensional Hydraulic Model Results and Habitat Estimates

Two-dimensional hydraulic models are commonly used to estimate the quantity and quality of available aquatic habitat. The hydraulic conditions simulated by a 2D hydraulic model are averaged over the area of the mesh cell and vertically through the water column. Because of this averaging and other simplifications, the model results do not represent the full degree of hydraulic variability present in a real river. Reclamation conducted a systematic investigation of the effect of model resolution (cell size) on model hydraulics and habitat predictions.

Reclamation developed hydraulic models of varying cell size at three sites on the Klamath River using Reclamation’s Sedimentation and River Hydraulics two-dimensional model, SRH-2D. The reference resolution (denoted as 1x) is the same as that of Reclamation’s Iron Gate Dam to the Pacific Ocean Klamath River model. The other meshes were created by doubling the number of mesh nodes in both the streamwise and stream normal direction (2x), doubling again (4x), and doubling one final time (8x). The resulting meshes have channel elements that are approximately 1/4, 1/16th, and 1/64th the area of the 1x mesh.

The primary difference between a coarse model mesh with large cells and a higher resolution mesh with smaller cells is how well the mesh represents the river bathymetry. This study examined the effects of varying mesh resolution on channel cross sectional area, water surface elevation, bed elevation, model wetted area, model water depth and velocity, and juvenile salmonid habitat estimated using a habitat suitability index developed for the Trinity River, the largest tributary of the Klamath River.

Cross-sectional area tended to increase, and the bed elevation tended to decrease with increasing mesh resolution, which lowered water surface elevation. The increase in cross-sectional resulted in a slight increase in total wetted because shallow areas were better represented by the higher resolution meshes. The amount of slow, shallow water favored by juvenile salmonids increased with increasing mesh resolution. This increase in the amount of slow, shallow water in combination with the increase in wetted area, resulted in an increase in estimated juvenile salmonid habitat with increasing mesh resolution. For the highest resolution meshes, there was a 10-44% increase in estimated habitat relative to the lowest resolution mesh, depending on site and fish life stage.