Coal Creek Flood Mitigation Project
The completion of the Coal Creek Flood Mitigation Project with the support of a FEMA Pre-Disaster Mitigation Grant has taken much of the Town of Wellington out of the 100 year floodplain.
The Town of Wellington had close to 200 homes, dozens of businesses, and two schools mapped in the Coal Creek floodplain for more than 50 years. It is within the Boxelder Creek Watershed in northern Colorado. Kevin Houck of the Colorado Water Conservation Board noted that prior to mitigation, the regulatory floodplain associated with this watershed contained an extraordinary number of properties. As a result this watershed was one of the more difficult to regulate as well as one of the more potentially dangerous watersheds in Colorado from a flood hazard perspective. The Boxelder Basin has a massive size of more than 265 square miles. Just from the Coal Creek tributary of the Boxelder Basin, 100-year flows of 1,700 cubic feet per second (cfs) were expected through the Town.
A number of years ago, a multi-jurisdictional stormwater master plan was completed to address the risk. It identified three regionally significant projects to mitigate the stormwater needs, the first of which was the Coal Creek Flood Mitigation Project to address flooding through the Town of Wellington.
The Coal Creek Flood Mitigation Project was designed to intercept flows north of the Town, convey them to the existing Clark Irrigation Storage Reservoir, and temporarily detain a volume of up to 540 acre-feet of floodwater. This required expansion of the Clark Reservoir inlet canal and the addition of flood storage within the reservoir. The original estimated project budget in the master plan was $6 million to $8 million.
In order to make the project feasible from both an engineering and financial perspective, the team developed an innovative and multi-pronged approach to create the necessary storage in Clark Reservoir. This included raising the emergency spillway to add 175 acre-feet, dredging to add 100 acre-feet, and developing a “shared storage” agreement with the owner of the reservoir, North Poudre Irrigation Company, for the remaining 265 acre-feet. The shared storage concept involves the irrigation company limiting the amount of irrigation water it stores during the May-through-September flood season such that 265 acre-feet of storage is available for stormwater. This innovative approach also avoided potential impacts to more than 12 acres of wetlands. The Federal Emergency Management Agency (FEMA) agreed to the concept as long as it was included in an Operations and Maintenance Agreement between the parties.
The construction of the project included dredging 170,000 cubic yards of sediment from Clark Reservoir. The material was dredged and pumped about a mile to the north and dewatered in a series of dewatering cells, with the return flow brought back into the Clark Reservoir.
The final project cost was $5.1 million. A FEMA pre-disaster flood mitigation (PDM) grant administered through the Colorado Office of Emergency Management provided $2.9 million in funding for the Coal Creek project. The remaining $2.2 million was paid for with matching funds from stormwater fees collected by participating jurisdictions Wellington, Larimer County and the City of Fort Collins. Larimer County managed the project with consultant Ayres Associates providing engineering services. The project was completed within one month of the three-year PDM grant period. The Boxelder Basin Regional Stormwater Authority will operate and maintain this component of the flood control project.
The revised floodplain mapping became effective February 4, 2013. The Coal Creek Flood Mitigation Project practically and economically accomplished the goal of benefiting the occupants of nearly 200 homes, two schools, and several dozen businesses in the Town of Wellington by protecting them from the 100-year Coal Creek flood flows.
|Inlet Canal During Contruction
Larimer County Engineering Department (Mark Peterson, PE), 970.498.5714, MPeterson@larimer.org
Ayres Associates (Andrea Faucett, PE), 970.223.5556, FaucettA@AyresAssociates.com
Completed On Behalf Of: Boxelder Basin Regional Stormwater Authority (Stan Myers, PE), 970.669.3611 firstname.lastname@example.org