Measuring Effectiveness of Culvert Replacement: A Case Study Measuring Change in Structure and Function after Implementation of the Carpenter Creek Restoration Project, Kitsap County WA

In the 1950’s the natural tidal hydraulics of the Carpenter Creek watershed, a 30-acre high value salmon bearing Puget Sound Lowland Stream in Kitsap County Washington, was artificially controlled by a 6-foot box culvert on South Kingston Road and a 6-foot cylindrical pipe culvert under West Kingston Road, creating a high-velocity regime immediately above and below the culverts while slowing, and alternation of the timing of flushing of the lower Carpenter Creek. This manipulation of the tidal exchange created deep scour holes within feet of the culverts, trapped juvenile salmonids at low tide, and transformed the estuary area between the culverts into a low energy, depositional environment with an unnatural channel thalweg. To restore the natural hydraulics of this system, the Carpenter Creek Restoration Project was proposed by Stillwaters Environmental Education Center, an environmental education and restoration organization in Kingston, WA. Stillwaters recognized the ecological importance of improving this small stream and its physical relationship to Appletree Cove since it offered juvenile salmon feeding and rearing habitat as well as educational opportunities for the local and scientific community. They obtained agreement of the US Army Corps of Engineers to undertake a Feasibility Study for a Section 206 Restoration Project to replace both culverts with more appropriate structures. Due to federal budget uncertainties and lack of full funding for both projects, in 2010 Kitsap County agreed to undertake final design and construction of the South Kingston Road culvert replacement. One phase of the project was completed in February of 2012, replacing the 6-foot box culvert with a 50 foot bridge. As part of the restoration project a pre and post-construction habitat monitoring program is being undertaken by Stillwaters. One of the monitoring efforts is comparative sediment grain size analysis upstream and downstream of the new bridge. Sediment sampling and analysis was performed by volunteers using sampling and size fractionation protocols developed in consultation with local environmental scientists using simple equipment. A total of 66 samples were collected along 5 transects above and below the culvert, 33 in June 2011, prior to the culvert replacement and 33 in June 2012 five months after the project was complete. Transect locations were recorded and distances between quadrates were noted so that post-construction sampling could be performed at the same locations. A simple volumetric size fractionation analysis was performed on the samples by volunteers using readily available equipment with consistent training and oversight for quality control. Results indicated that at most locations, significantly difference in grain size distribution were observed after the culvert removal