Achieving contrapuntal balance at a lake outlet: Restoring salmon spawning habitat with gravel augmentation design

Abstract

The degradation of gravel bed rivers in logging and climate-impacted watersheds has led to an increase in salmonid habitat conservation and restoration efforts. Gravel augmentation is one such technique that seeks to restore altered aquatic habitat and sediment transport dynamics in sediment-starved streams by placing gravel pads in the river. The pads act as a sediment source that can be reworked by flow or fauna to suit their ecological needs, thereby allowing the river to “heal” itself without imposing static structural designs on a natural environment. Common points of failure for these projects include: (1) the immediate scour of the gravel pad which can wipe out buried eggs, (2) the infilling of the gravel interstices with fine particles which can limit oxygenated hyporheic flow and choke the buried eggs, and (3) low utilization by the target species due to unsuitable hydraulic conditions for spawning. The risk of the first two hazards can be limited by designing the gravel placements at downstream of lakes, as the upstream lake can buffer peak flows to its outlet stream and trap fine sediment. Much like composing counterpoint in music, a delicate balance is needed between multiple engineering criteria to identify the optimal area along the accelerating channel length where most criteria are fulfilled. Despite this understanding and the common use of lake outlets for these projects in industry, there are few design guidelines tailored for these environments. This project evaluates the design criteria of a gravel augmentation project for salmon spawning habitat restoration at a lake outlet. A calibrated two-dimensional (2D) hydrodynamic model was developed using TELEMAC-2D. The sediment transport predictions from the model were verified by tracking tracer stones equipped with Radio Frequency Identification (RFID) after one year. The model results were used to identify optimal placement areas. The 2D model was found to adequately capture the measured depth-averaged velocities when set with appropriate boundary conditions and calibrated with the roughness coefficient. However, the roughness-dependent shear velocity calculation in the model formulation results in a direct dependency of the model outputs on the sole calibration parameter. This relationship is highly sensitive and creates modeling artefacts when using roughness zones to calibrate the model. The tracer results indicate that the model appears to overpredict sediment transport due to the limitations of the deterministic approach of assessing grain mobility, which is inherently a stochastic process. Although there is likely error associated incorrect assumptions on critical mobility thresholds and representative grain sizes, the relative lack of tracer mobility during the study year limits the current ability to revise these assumptions. Nevertheless, the results show that the application of 2D hydrodynamic models to sediment transport predictions should be approached with caution and should be accompanied by field validation to ensure confidence in the model conclusions. Using the model as is, the optimization analysis results suggest that designing to prioritize longevity may require compromising the other design criteria. Conversely, optimizing the design to minimize the risk of fines infilling severely limits the available placement areas. Prioritizing stability may decrease the likelihood of immediate utilization but may be required due to the uncertain availability of funding for re-injecting gravel after the initial construction.