Harts Lake Loop Road South Horn Creek Culvert Replacement
Culverts
Herrera assisted Pierce County by preparing a Preliminary Hydraulic Design report for a culvert conveying Horn Creek, a tributary to the Nisqually River. The existing culvert creates a fish passage barrier through high velocity flows and is located near a sharp 90-degree roadway bend. Replacement of the culvert will provide fish passage and improve flow conveyance. Herrera evaluated stream conditions through field assessment and review of background materials. Collaborative discussions with the Puyallup Tribe and the Washington Department of Fish and Wildlife helped hone specific design goals and approaches, resulting in a focus on maintaining 2-year flow velocities less than 2 feet per second. Herrera developed base mapping and a hydrologic model of the watershed to support design development. A two-dimensional hydraulic model was developed to compare velocities under different design scenarios. Based on results from model iterations, Herrera identified the minimum, and therefore more cost-efficient, hydraulic opening for the proposed bridge span that will maintain velocities low enough for juvenile fish to pass. Herrera provided options to the County for the channel configuration along the roadway curve, and refined the design based on feedback from the County and permitting agencies. Large wood is included in the design to provide aquatic habitat complexity and refugia. Realignment of the stream through the center of the valley and along a straighter section of road improves geomorphic function and constructability, and controls costs.