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Left Hand Creek Project Shrinks Floodplain

LONGMONT, CO - Only months before the flood of September 2013, the city of Longmont completed a mitigation project that kept a community of residents dry and resulted in a net savings of an estimated $4 million dollars. The Left Hand Creek Flood Control Project proved to be a smart move when Longmont became one of the hardest hit cities in the catastrophic flood.

Left Hand Creek meanders through Longmont and had a history of flooding in the city’s Southmoor Park neighborhood whenever rainfall was exceptionally heavy. Stormwater would overflow the channel and flood approximately 200 neighborhood homes. The creek channel wasn’t large enough to carry additional water and bridges were subject to being washed out.

The goal of the Left Hand Creek Project was to remove most of the homes in Southmoor Park from the mapped floodplain so they would not be affected in the event of another flood. Upgrading bridges and channel capacity would keep stormwater in the channel and away from the homes in the neighborhood.

The upgrade was multi-faceted and included increasing the conveyance capacity of the Left Hand Creek drainage system, replacing box culverts at the South Pratt Parkway crossing; installing additional box culverts at the Main Street crossing; removing and replacing riparian vegetation; lowering the channel profile and channel grading; grading and stabilizing portions of the banks; installing a concrete retaining wall; and modifications to the Left Hand Greenway Trail.

The new grading provided an additional flow area needed to ease the passing of stormwater.

This required removing existing trees and planting new trees, so new trees were planted outside the main channel flow area. As a result of the project, 169 homes were removed from the floodplain and others, although still in the floodplain, were at reduced risk.

Total cost of the project was $5,689,013. FEMA grants funded $3 million and the State of Colorado matched this with $2,689,013. If the project had not been undertaken, FEMA officials say the residential repair costs would have amounted to almost $7.47 million; more than $442,000 in loss of use; and $680,000 in public assistance for infrastructure, amounting to almost $8.6 million in repairs. The project took nearly two years to complete, wrapping up just in time to protect the area during September flooding.

“When the rains hit, we got 7½ inches of water in Longmont,” said Steve Ransweiler, engineering services project manager for Longmont Public Works. “It’s simply amazing that the project did just what it was designed to do. Rarely do we get to do a project like this and have it tested so quickly.” The September flooding left the South Parkway Bridge intact when water topped the bridge. A small pedestrian bridge spanning Left Hand Creek was spared. Water flowed quickly and debris deposits collected, as designed, between the Hartley pedestrian bridge and the Main Street Bridge. The floodwaters overtopped the Main Street Bridge, but did not cause damage to the bridge itself.

The bottom line: Water did not enter any of the 169 homes in the Southmoor Park subdivision that had been removed from the floodplain.

“Homes would have likely received 8-to-10 feet of water if it had not been for the flood control project,” said Ransweiler.

It took between two to four days and four crews to clean up the muddy streets, but residents who evacuated the neighborhood found their homes dry and intact when they were allowed to return.

For additional information visit: http://ci.longmont.co.us/lefthandcreek/

Last updated Jun 3, 2020