Interior Drainage Study Consolidated Diking Improvement District 1 (WA)
The Consolidated Diking Improvement District No. 1 (CDID1) oversees the operations and maintenance of numerous ditches, drains, and pump stations, all located within a series of exterior levees. WEST Consultants, Inc. (WEST) was contracted to provide the necessary interior drainage information required for certification of the levee system.

Hydrologic and hydraulic analyses were conducted to identify the sources of flooding, the extents of the flooded area, and the water surface elevations of the base flood. To facilitate the model development, a detailed topographic survey of the levee-protected area was completed by WEST. Due to the complexity of the flooding conditions within the CDID1 area of operations, a two-dimensional combined hydraulic and hydrologic model was used to calculate runoff volumes and rates, flow depths, and flooding extents. A FLO-2D model was created based on the topographic survey data, culverts, pump stations, interior levees, spatially variable hydraulic roughness, buildings, and spatially variable infiltration rates were some of many unique features that were incorporated into the comprehensive model.

The FLO-2D model was calibrated and verified against previously recorded events. The 1% annual chance event was then simulated to determine locations of flooding within the CDID1 area of operations during this event.