Bluff erosion along the dynamic lower 2 miles of the Mad River was threatening multiple land uses including a dairy pasture, treated wastewater disposal, and residential development after years of both natural processes and management activities. Stillwater Sciences provided technical assistance to the County of Humboldt by evaluating geomorphic conditions and conducting hydraulic modeling to assist in bank protection design. In addition, Stillwater Sciences supported the County in discussions with regulatory agencies regarding potential impacts of bank protection measures on protected species and critical habitat.
Historical aerial photograph interpretation, review of old cross-section surveys, and targeted field topographic surveys were used to evaluate historic conditions, patterns of channel migration and erosion, and natural and anthropogenic factors affecting the project reach. Sequential annual channel planform and flow pattern maps were generated in a Geographic Information System (GIS) to evaluate channel dynamics in the lower two miles of the Mad River. These analyses were used by County engineers to incorporate potential future channel migration and streambed changes into the project design.
Stillwater also used the River 2D hydraulic model to assist the County in assessing potential effects of the project within the context of the dynamic geomorphic setting. The results of the model and their interpretation allowed the County to adjust the project design to avoid or minimize adverse effects to the river channel while still providing the necessary bank stabilization.
A number of sensitive fish species (Chinook and coho salmon, steelhead trout, and green sturgeon) inhabit the lower Mad River and project area. Because of this, Stillwater also provided technical support regarding instream habitat effects, fish-friendly design elements, construction impact mitigation, and regulatory assistance.