North Creek Levee Reach Hydraulic Monitoring

A 1 ½ mile reach of North Creek in Bothell, Washington is confined between earthen flood control levees. Over the past decade, capacity of the levee reach has decreased due to vegetation growth, sediment deposition, and increased flood discharges resulting from upstream development. These factors lead FEMA to conclude that the levees did not meet their requirements for flood protection. Beginning in 2009, the levees were upgraded to meet FEMA requirements. As part of the long-term monitoring plan, WSE has been retained to monitor the influence that vegetation and sediment are having on flood levels within the project reach.

Role/Services: WSE is monitoring flood levels by maintaining a network of 10 crest stage and pressure transducer gages within the project reach. WSE is also collecting discharge measurements to develop and maintain a stage-discharge rating curve for the reach. This information, combined with periodic channel cross section surveys to be collected by others, is used to maintain a calibrated HEC-RAS numerical model of the project reach. The model helps to monitor channel capacity in-order to determine if or when vegetation and sediment management activities will be required.

Outcome: The gage network was installed in 2009 and annual winter monitoring began in 2011. Monitoring captured a flood event in March 2012 that was large enough to allow model recalibration.  The reach appears to have maintained capacity. WSE continues to monitor flood levels for this reach.