Effectiveness of 1D and 2D hydraulic models for instream habitat analysis in a braided river

There is considerable interest in the use of 2D hydraulic models for the prediction of instream habitat especially for complex hydraulic situations such as those found in braided rivers. The general assumption is that the greater spatial resolution of 2D models and their hydraulic modeling will give better predictions of instream habitat. We apply a 1D model and two 2D models to a section of braided river and compare measured and predicted water depths and velocities at two flows, as well as habitat predictions over a range of flows. The correlation between predicted and measured depths and velocities was higher for the 1D than for the 2D models. Practical limitations on topographic definition and the subjectivity associated with 2D calibration resulted in errors in predicted water levels that could cause braids to either flow or stop flowing. All three models generally predicted similar trends in habitat (weighted usable area) variation with flow, although there were differences in the magnitudes, location of maxima and changes in gradient. The differences between the 1D and 2D model predictions could not be attributed to the greater spatial resolution of 2D models, because there was as much difference between 1D and 2D habitat–flow relationships as between the two 2D models. The difficulty in acquiring sufficient and accurate bed topography and the skill required in calibrating 2D models is a practical limitation to their utility, and it cannot be assumed that they are better simply because they require more data and the time and effort required to develop a good 2D model is not warranted in many situations. The main advantage of 2D models over 1D models is that they should provide more accurate predictions outside the calibration range of 1D models, especially at high flows in braided rivers, but improved calibration and validation techniques are required.