Using two-dimensional hydrodynamic models at scales of ecological importance

Modeling of how features that are important in assessing stream habitat con ditions has been a long-standing interest of stream biologists. Recently, t hey have begun examining the usefulness of two-dimensional (2-D) hydrodynam ic models in attaining this objective. Current modeling practices consider relatively long channel sections with their bathymetry represented in terms of large, macro-scale, topographic features. Mese-scale topographic featur es, such as boulders, root-wads and other obstructions are typically not co nsidered in the modeling process, instead, the overall effects of these flo w obstructions are captured through increased values in the channel roughne ss parameters. Such an approach to 2-D modeling allows one to accurately pr edict average depth and velocity values; however, it is not capable of prov iding any information about the now patterns in the vicinity of these obstr uctions. Biologists though have known that such meso-scale features and the complex velocity patterns generated by their presence, play an important r ole in the ecology of streams, and thus cannot be ignored. It is therefore evident that there is a need to develop better tools, capable of modeling f low characteristics at scales of ecological importance. The purpose of this study is to expand the utility of 2-D hydraulic models to capture these ho w features that are critical for characterizing stream habitat conditions. There exists a paucity of research addressing what types of topographic fea tures should be included in 2-D model studies and to what extent a boulder or series of exposed boulders can influence predicted flow conditions and t raditional useable habitat computations. Moreover, little research has been performed to evaluate the impact mesh refinement has on model results in n atural streams. Numerical simulations, based on a natural river channel con taining several large boulders. indicate that explicitly modeling local obs tructions/boulders can significantly impact predicted how parameters. The p resence of these obstructions create velocity gradients, velocity shelters, transverse flows and other ecologically important flow features that are n ot reproduced when their geometry is not incorporated into the hydraulic mo del. Sensitivity analyses show that reducing element sizes in the vicinity of obstructions and banks is crucial in modeling the spatial now patterns c reated by meso-scale topographic features. This information, combined with similar data obtained in future studies, can provide guidelines for the pla cement of fishrocks and other structures often used in stream restoration p rojects as well as determining what types of mesoscale topographic features might need to be incorporated into habitat suitability studies. Such infor mation may also ultimately allow new spatial habitat metrics to be develope d. (C) 2000 Elsevier Science B.V. All rights reserved.