flood damage threatens local dam & spillway
summit lake flooding causes significant damages and is in need of repairs
originally built in the late 19th century, summit lake spans 250 acres across the west side of creston, iowa, and is a popular destination area for fishing and recreational use. for many years, the lake was privately owned by the municipal taxpayers water company until the city of creston purchased it in the mid-1930s. almost immediately after purchasing, the city raised the earthfill dam and spillway to almost double the capacity of the reservoir. this arrangement lasted until the historic floods of 2008 caused significant deterioration.
creston water works partnered with snyder & associates to inspect the flood damage and develop a plan for making any needed repairs. because repair work would likely require lowering the water level in the lake, the decision was made to seek out funding assistance and address several other lake issues at the same time. these include making improvements for better recreational access to the lake and stabilizing the erosion-damaged shoreline.
spillway repair & reconstruction to keep dam stable
the snyder & associates team evaluated the dam and spillway and provided an extensive engineering report detailing our findings. after conducting a multitude of tests, it was clear damage was evident and a geotechnical exploration was undertaken to determine the dam’s underlying stability. after the evaluation, it was determined that the earthen section of the dam was in stable condition.
however, the spillway section had developed hollow pockets underneath the concrete. this was allowing lake water to seep beneath the spillway putting the entire dam at risk. since this structure was built nearly 100 years ago, very little design information existed. our design team utilized their years of expertise with dam removal and modification to determine hazard risks and formulate a cost-effective repair plan.
with the scope of the damage known, we designed a modern spillway replacement that would enable the city to maintain the lake in a safe and usable condition. our team began by creating a dam breach analysis report to determine hazard classifications and
the old spillway section had developed hollow pockets underneath the concrete.
assure downstream safety measures were adequate before the reconstruction was permitted by the iowa department of natural resources (idnr).
throughout the entire design and analysis process, the spillway continued to have a constant stream of water flowing below the concrete, decreasing its ability to withstand large flows of water. working with the city, permitting agencies, and stakeholders, our team developed a plan to substantially drain the lake to allow construction efforts and increase safety measures.
the new concrete spillway section was designed to safely accommodate the expected flow rates established by the breach analysis. this design included a new energy-dissipating outlet structure, stilling basin, and downstream creek stabilization efforts to eliminate erosion on the new structure. the new spillway section was also constructed vertically, as opposed to sloped, to comply with idnr regulations concerning the control of undesirable fish migration into the lake. additionally, our team assisted the creston water works in securing funding from fema and the iowa department of economic development (ided) to help make the needed improvements economically feasible.
