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Application of a 2-dimensional hydrodynamic model for the assessment and design of instream channel restoration works Lacey, Ralph William Jay

Abstract

Currently, stream restoration activities in British Columbia emphasise the use of large woody debris (LWD), boulder, and other habitat structures (Slaney and Zaldokas 1997). The purpose of these works is to create locally varied hydraulic conditions; promote scour and pool formation; and create low velocity refuges. Little work, however, has been done to characterize the changes to local flow hydraulics, morphology and in stream habitat that results from these efforts. This study provides a methodology, through the use of field surveys and 2-dimensional hydrodynamic modelling, to assess morphological and hydraulic effects of instream LWD and boulder structures. Changes in available fish habitat were quantified through the use of Bovee (1978) probability-of-use curves. The selected field study site is a side channel of the Chilliwack River. Instream structures, installed in the summer of 1999, were subjected to a bankfull flow event that caused significant scour and bed morphology change. Results indicate that pool area increased by 50% due to the hydraulic effects of the instream structures. Two-dimensional flow model velocity and depth predictions compare favourably to recorded field values, while the predicted shear stresses, derived from the model's output, coincide reasonably with the newly formed pool locations. Pre- and post-restoration differences in fish habitat were quantified for a range of discharges using the weighted usable area (WUA) method (Bovee 1982). This analysis determined that the greatest benefit offered by the instream restoration structures was in supplying low velocity refuge areas during high flow events.

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