Creston Water Works initially retained Snyder & Associates to evaluate the condition of the concrete spillway section of the existing earthen dam. The engineering report showed evidence that flooding had damaged the existing dam and concrete spillway. Snyder & Associates was then asked to design an extensive spillway replacement to enable the owner to maintain the lake in a safe, usable condition long-term. Very little original design information existed; therefore, experience and inference of the original design became critical to determine existing hazard risks and formulate a cost-effective repair. The lake had previously been used for water supply, and the need to possibly return the lake to that status was considered during reconstruction design. The team assisted Creston Water Works in securing FEMA and Iowa Department of Economic Development (IDED) funding through HUD to finance the project. In addition to dam repair, funding provided by the Iowa Watershed Improvement Board (WIRB) was used to stabilize an extensive amount of shoreline and preserve the condition of the lake.
A Dam Breach Analysis report was also completed to determine hazard classification and assure downstream safety prior to the Iowa Department of Natural Resources (IDNR) permitting reconstruction. During the time required for design and permitting, the existing structure became less capable of withstanding large flows. Working with the owner, permitting agencies, and stakeholders, Snyder & Associates supervised draining of the lake, which simplified construction improvements and provided a safer work site.
A new concrete spillway section was designed to safely accommodate expected flow rates established by the breach analysis. A new energy-dissipating outlet structure, stilling basin, and downstream creek stabilization were constructed as part of the project. The new spillway section was designed to comply with IDNR regulations as well as desires to control undesirable fish migration into the lake.