Left Hand Sediment Study
Left Hand Creek flood control improvements constructed within the City of Longmont in 2012 removed 172 of 197 structures that were previously located within the 100-year floodplain. Just nine months after construction, the project was tested during the September 2013 flood which has been estimated to be a 100-year event with a peak discharge of 6,000 cfs. From a flood control perspective, channel improvements were extremely successful at limiting damages to structures that most certainly would have been inundated without the project. Damages sustained during the September 2013 flood, including bank erosion and large quantities of sediment deposition, were repaired during the winter of 2014.
After 2013 flood damage repairs were completed, the occurrence of a heavy spring runoff in 2015 caused additional damage and sediment deposition within the City of Longmont. Damages sustained during the 2015 spring runoff required: a) channel bottom sediment removal to restore channel capacity required for flood protection and b) repairs of eroded and scalloped channel banks. The City also identified the need for a detailed evaluation of geomorphic and sediment transport conditions in Left Hand Creek to gain a better understanding of future creek response during recovery and inform development of sedimentation mitigation alternatives.
- Client/Owner: City of Longmont
- Project Location: Longmont, Colorado
- Approximate Project Cost: $340,000
- Project Completion Date: August 2019
- Project Components:
- Sediment impact analysis – evaluation and identification of impacts of sediment deposition on flood protection and river function.
- Post-2015 runoff channel repairs – this included design and permitting (environmental and local floodplain use) of channel bank repairs and sediment removal required to restore flood protection through the City of Longmont.
- Sediment source assessment – potential sediment sources between the Diagonal Highway and Foothills Parkway, upstream of the City, were evaluated to provide insight into future sediment delivery.
- Geomorphic and sediment transport evaluation – hydraulic and mobile bed sediment transport analyses were conducted to gain an understanding of existing river functions contributing to sediment deposition and loss of flood conveyance.
- Sediment mitigation alternative development – results of the sediment source assessment and sediment transport evaluations were used to develop a range of mitigation alternatives. Mitigation alternatives were developed to provide sustainable solutions that balance flood protection, sediment passage, and river health.
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