Final Report

Kinkade Island Dike removal

IAC Project Number 99-1304D

Joel Freudenthal

Clallam County Department of Community Development

The Kinkade Island Dike Removal Project was completed on September 15, 2000. Attached to this report please find a CD which includes the engineering analysis performed by West Consultants for the project, as well as digital photos of the completed project.

The engineer’s report recommended that the project be scaled back from the original goal of complete removal of the dike. The permission of the adjacent landowners whose homes were protected by the dike would not have been received without certain conditions. Primary among these conditions would have been the replacement of two wells which exist in the floodplain immediately behind the dike. While replacement of these wells was possible from a practical standpoint, from a policy standpoint it would have been an inappropriate use of public funds. As the engineering report states, the existing conditions on Kinkade Island result in a very high degree of flood hazard to the residents on the island. It is the County’s and the City of Sequim’s long-term objective to reduce this flood hazard through fee simple purchase of the 7 homes which exist on the island. To install new wells for two of these homes would have simply meant that the appraised value of these residences would increase, and result in further expenditure of limited public funds to acquire these properties.

The conditions on the island did allow for removal of a portion of the dike however. There was an existing location approximately mid-way along the dike where the dike had already partially eroded. During two-year and higher magnitude flood events, this portion of the dike was overtopped, and water entered the floodplain behind the dike. This periodic overtopping created a more-or-less predeveloped side channel which had been naturally excavated by floodwaters. The lower portion of this channel (approximately the lower 400 feet) was spring-fed year-round, and the upper portion (700 plus feet in multiple channels) remained dry except hi flood events. Even this lower portion was not accessible to fry or adults due to several pieces of LWD which blocked the channel, and which the channel did not have enough sustained energy to scour or deform the bed to provide unrestricted access. Removal of the dike at the upper end of this channel to allow water from the Dungeness River to flow through this channel for most of the year would result in creation of a large side-channel off-channel area with minimal disturbance to the mature riparian/floodplain vegetation which exists on the site. Similar channels (i.e. with access and flow from both ends of the channel) have been shown to be both a limiting and preferred habitat type for rearing of the Spring Chinook in the Dungeness River.

This is what was implemented on the island. The engineers report recommended that the inlet be armored with LWD and boulders, and the supplied a specific inlet elevation for the new channel. On site, the channel configuration was changed somewhat due to the configuration of the dike and existing stable accumulations of LWD and the dike. The inlet was constructed at a lower elevation than recommended by the engineering report to allow for a greater period of time during the year (approximately 10 months) when the flows in the river are sufficient to water the channel. The obvious benefits to increasing the flows within the channel had to be weighed against an increase in flood hazard or stability of the inlet channel. The inlet itself was constructed in such a way that river flows would backwater into the channel, as opposed to being directly diverted from the river channel. Secondly, conditions at the inlet location arc extremely stable, ample LWD was available on the site to armor the channel, and the bed materials of the inlet itself is a large cobble material which should inhibit downcutting.

As of this writing, the new channel has had water flowing through it for only less than a portion of a day. Flow conditions in the river have been extremely low, at approximately the 80 to 90 percent exeedance level, since the project was installed. I have attached a print out of today’s flow from the USGS gage available on the internet. Note that today's flow is 138 cfs, and the 80 percent exceedance level is 132 cfs. Monitoring of the site during several small freshets indicates that the channel is watered at about 320 cfs, and will have flow for approximately 10 months out of the year. These months correspond with the rearing life-history stage of juvenile chinook, which exit the river in late July. The lower portion of the channel should provide rearing habitat for coho and steelhead for the remainder of the year. Historical use of other side channels in the area by pink and chum indicate that if this new channel is used for spawning in the upper portion (and the substrate and cover should provide excellent spawning habitat) the channel will remain watered well beyond emergence in most years.

We are proceeding with the monitoring of the site, and will have cross-sections and pebble counts from the river, the new side channel, and Kinkade Creek within the next several weeks, we will repeat these measurements for the next five years. We will also monitor the new side-channel for use over the winter and incorporate the side channel into the ongoing life-history/abundance studies of chinook juveniles which is associated with the captive broodstock program for the Dungeness Spring Chinook.


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