TWO-DIMENSIONAL HYDRAULIC SIMULATION OF PHYSICAL HABITAT CONDITIONS IN FLOWING STREAMS

Abstract

Instream flow needs (IFN) assessment studies are performed to provide guidelines for stream water management and to assess the impacts of different water projects such as weirs, dams and stream diversions on the available fish habitat. Many of the IFN assessment techniques require hydraulic parameters such as water depth, flow velocity, wetted perimeter or top width as input variables. The Physical Habitat Simulation System (PHABSIM), one of the most widely IFN assessment models used in North America, requires precise values of depth and velocity at numerous points within the study reach to produce relationships between streamflow and usable habitat area for different life stages of varying fish species. Numerical flow simulation is applied to obtain results for unmeasured flows. At present, the flow simulation techniques applied in most IFN assessment methods are rather simplistic. In PHABSIM, a one-dimensional approach, such as the application of HEC-2, is used to determine water surface elevations along the study reach. Then a technique based on a combination of Manning's equation and regression analysis is used to obtain the velocity distribution across the channel. As a typical length scale of physical habitat study sites is of the order of a few stream widths, the accuracy of the above approach is questionable, and a two-dimensional model may be more appropriate. The results of flow modelling of fish habitat based on one-dimensional and two-dimensional assumptions are compared. Firstly, the models are tested and compared for a hypothetical flow situation of flow over a side-bar, which shows a clear difference between the results of the one- and two-dimensional approaches. Then the two models are used to simulate the flow of water in a real fish habitat reach and the computed velocity results are compared with field velocity measurements. The results of the two-dimensional approach appear to be significantly better than the one-dimensional approach.