Upper Cooper Slough Basin Stormwater Master Planning#
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A hydrologic study of the basin was prepared as part of the Boxelder Creek/Cooper Slough watershed by the City of Fort Collins and Larimer County in 1981 and 2002. Additionally, a drainage master plan was prepared for the portion of the basin owned by Anheuser-Busch, Inc. in 1984 in conjunction with development of the brewery site.
In 2003, the City updated all stormwater basin master plans and adopted improvements for the Lower Cooper Slough Basin and identified the need for the Upper Cooper Slough as an area to be further studied.
In 2006, a master plan was completed for the Upper Cooper Slough Basin. During the same time period, the Boxelder Alliance Authority prepared a regional master plan for Boxelder and Lower Cooper Slough.
In 2014, as part of the Boxelder 6 project, the hydrology for the Boxelder/Cooper Slough basins was updated to EPA-SWMM from MODSWMM. The City is currently updating the Selected Plan for the Upper Cooper Slough basin.
In 2016, the City used the updated hydrology to evaluate alternatives for drainage components with the basin. In 2017, the City was preparing a draft of the Selected Plan when the developer of Montava came to the City and asked to re-evaluate those stormwater improvements to incorporate their development.
In early 2020, the City worked with the key stakeholders in the basin, including the Larimer and Weld Irrigation Company, and created a new updated Selected Plan of Improvements. The Selected Plan was completed in May 2021.
Problem Areas#
This Selected Plan identified flood hazards related to the overtopping of roadways and the Colorado and Southern Railroad, as well as spills out of the No. 8 Ditch and the Larimer and Weld Canal. During a 100-year storm, the railroad embankment impounds 250 acre-feet of stormwater and overtops by 900 cubic feet per second. If nothing is done to address the overtopping of the embankment, there is the potential the embankment could fail and cause damage downstream.
While overtopping of roadways and railroads is a significant hazard, there are numerous new developments proposed in the lower portion of the basin, expediting the need to update the selected plan for this basin.
Currently, the basin has no dedicated outfall channel to convey runoff from developed lands in the basin to existing waterways. The Larimer and Weld Irrigation Company has expressed concerns about allowing new development to use the No. 8 Ditch for stormwater conveyance without a coordinated plan from the City. No defined channel exists to convey drainage from the north of County Road 56 through future developments.
Recommended Solutions#
The recommended Selected Plan of improvements was developed to provide cost-effective solutions to mitigate existing damages and to mitigate the potential for future damages caused by new development. All projects in the basin were sized assuming developed land use within the Fort Collins Growth Management Area. The improvements include:
- North Poudre #6 spillway modification
- Sod Farm detention storage
- 8 Ditch improvements
- Colorado and Southern Railroad diversion structure
- Crumb Regional Detention Pond
- Anheuser-Busch Regional Detention Pond modification
- Cooper Slough Stream Rehabilitation
- Culverts at Vine Drive and Mulberry Street
- Stormwater Channel to Boxelder
In coordination with projects in Lower Cooper Slough at State Highway 14, these projects would prevent road and railroad overtopping, eliminate spills from the irrigation canals, provide an outfall for Upper Cooper Slough, limit the impact of off-site runoff, and mitigate the potential for future flood hazards from new development. The total cost of the improvements is $40 million.
Stream Stability and Habitat#
The Selected Plan of Improvements proposes to convey runoff above the Larimer and Weld Canal directly to Lower Cooper Slough. The additional volume of runoff into the slough could degrade the stability and habitat quality of the slough. Therefore, a series of stabilization and enhancement measures to Lower Cooper Slough are included to mitigate the change in hydrology.
New Development Guidance#
The implementation of the selected plan of improvements is critical to help mitigate the potential for flood damage related to new development. In addition to the improvements, new development is required to detain developed runoff at the 2-year historic release rate.
Did You Know?
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