Smith and Bybee Natural Area Floodplain Management Portland, Oregon

An evaluation of options to manage hydrology in the Smith and Bybee Lakes complex led to the restoration of its connecting channel and subsequent mitigation of seepage found in an existing levee. The design adds capacity to the connection channel through excavation, allowing Metro to better manage water levels in the lakes. Otak led the hydraulic modeling of the system to compare the benefits derived from varying levels of channel excavation for the resulting restoration design as well as alternatives analysis of proposed remedies for the levee hazard mitigation design.

Levee Seepage Mitigation and a Restored Channel for Improved Hydrology Management

From the simulation of spring freshet floodplain functions to control of invasive species and habitat for migratory waterfowl, Metro manages water levels across the seasons in Smith and Bybee Lakes for a variety of objectives. A control structure was constructed in 2003 to aid in this management. However, the water levels in Smith Lake were not responding to control structure manipulations due to sedimentation and partial obstruction in the channel connecting Smith Lake to Bybee Lake. After comparing options, a design concept to excavate the channel and restore capacity was developed along with the construction documents. After construction, an inspection revealed seepage in the levee near the control structure and also identified burrowing animals which further increased a risk of levee failure. An alternatives analysis to resolve the issue led to a 200-foot-long sheet pile cutoff wall with segments on either side of the water control structure. With the piles driven 20-feet below ground surface, the design increases the sub-surface flow path to reduce seepage to minimal levels while providing a physical barrier that prevents animals from burrowing.