City, county officials see S. Platte Master Plan

An alliance or coalition is needed to carry on with work started with the Lower South Platte River Master Plan.

That was the recommendation from CDM Smith, the consulting firm that did the year-long study of ways to mitigate flooding on the South Platte between the Morgan-Weld county line and the Nebraska state line.

Eli Gruber, an engineer with CDM Smith, presented an overview of the plan to Sterling city and Logan County officials Tuesday. The session was sponsored by the Logan County Water Conservancy District, which is dedicated to finding flood mitigation solutions for Logan County.

The study focuses on three areas of high risk for flooding and four other areas that are at a medium risk for flooding.

The high-risk areas are the Rainbow Bridge area at Fort Morgan, the Hillrose area, and Sterling.

Gruber said the Sterling reach presents the greatest challenge because of a combination of high population density in the flood plain and infrastructure that restricts river flow. The three-bridge combination over the South Platte, a railroad bridge less than a mile downstream, and an irrigation diversion structure all impede the flow of water and sediment in time of flood.

Gruber said the study managers looked at a levee system to protect Sterling but levees high enough to protect the city from a 100-year flood would stress the main bridge beyond its capacity.

“I doubt (Colorado Department of Transportation) would get on-board with that,” he said.

Rather, the study recommends using the entire width of the river’s historic course to carry the water past Sterling, including a secondary, “high-flow” channel designed to relieve stress on the main bridge. Gruber said a lower berm between the river and Sterling would be useful to prevent flooding from a once-in-25-years flood.

The master plan was financed by the Colorado Water Conservation Board and the Colorado Department of Local Affairs at the request of the Morgan County Commissioners. Commissioners in Logan, Washington and Sedgwick counties also signed on to host meetings and workshops during the study.

The plan breaks the 143 miles of river into 19 sections called reaches. Gruber said the reaches were defined according to how the river changes the land around it, political boundaries, and environmental aspects.

Because of the nature of the South Platte, Gruber said, many of the recommendations have less to do with preventing flooding than with mitigating the damage of a flood and then quickly recovering from it.

Shane Miller, executive director of the LCWCD, said his board hosted the meeting because the district is formulating its master plan and wanted to be able to incorporate parts of the Lower South Platte plan into the LDWCD plan.

Gruber gave a similar presentation to the board of directors of the Lower South Platte Water Conservancy District earlier Tuesday morning.

Jeff Rice: 970-526-9283, ricej@journal-advocate.com