Boxelder Creek Improvement Projects
Completed Summer 2016
Early in 2005 stormwater planners in northern Larimer County took a holistic approach to addressing flood hazard and stormwater drainage problems in the Boxelder Creek watershed. Because the Boxelder Creek floodplain affects many property owners and several local governments, intensive efforts were made to develop a regional flood hazard mitigation plan. An Alliance was formed in early 2005 to develop a "regional" solution that was more efficient, wide ranging and cost effective than the entities could develop independently. This Alliance resulted in the creation of a Boxelder Creek Regional Stormwater Master Plan, completed in October 2016.
This Master Plan outlines numerous projects all designed to address flooding hazards in the drainage basin. Anderson Consulting Engineers was hired to design and oversee the construction of several projects related to the Master Plan.
Background:
The Boxelder drainage basin encompasses over 265 square miles, extending from just north of the Wyoming border to the Poudre River in northern Colorado. The Master Plan described above outlined numerous projects to reduce flooding in the lower portion of the basin where life and property damages would be the greatest. Anderson Consulting was hired to design and oversee the construction of five projects related to the Master Plan. The primary stakeholders and their desired overall goals for the projects Anderson Consulting was involved with are listed below:
- City of Fort Collins- Primary goal is to reduce flooding potential and road over-topping in project area.
- Town of Timnath- Goal is to reduce flooding potential in the Town of Timnath
- Larimer County- Reduce road over-topping in the project area.
- Lake Canal Irrigation Company- Primary goal is to improve the efficiency of their water delivery system.
The primary goal of the five interrelated projects Anderson Consulting was involved in was to reduce flooding, road over topping, and improve irrigation delivery systems. The locations are depicted in the map on the right and detailed below
Boxelder Creek Channel Clean Out:
This project involved cleaning debris and overgrowth out of the Boxelder Creek channel from downstream of Mulberry Street to just upstream of Interstate 25. The channel was widened in order to increase flow capacity.
The Boxelder Creek channel had become overgrown with non-native trees and vegetation and flow capacity was diminished by sediment accumulation and encroaching banks. The clean out began just downstream of Mulberry Street and progressed downstream towards Interstate 25. Special tracked carriers were used to remove the debris in order to lessen damage to the job site.
The end result was a wider and deeper channel that flowed without obstruction, allowing the Lake Canal Irrigation Company to more efficiently move their irrigation water to their diversion structure downstream. By August of 2015 the cover crop had fully come in and the site was on it's way to becoming fully re-vegetated.
Concrete Panel Removal:
Two of the culverts originally constructed by CDOT underneath Interstate 25 were plugged with concrete panels at some point in time. The overflow from this blockage of the culverts resulted in a FEMA 100yr floodplain that extended south along the east side of I-25, flooding large sections of Timnath. The opening of these plugged culverts allows more water to stay in the Boxelder Creek channel and reduces the floodplain near Timnath.
The culverts were plugged with concrete panels on both the upstream and downstream sides. Over the years sediment had accumulated burying a good portion of the panel blocking the culverts.
The sediment and concrete panels were removed and the site was re-graded. Un-blocking the two culverts increases the flow capacity of Boxelder Creek at this location which was previously a bottleneck during high flows that caused flooding.
Lake Canal Siphon
Anderson Consulting Engineers was responsible for the design and construction oversight related to the new Lake Canal Siphon.
Originally the Lake Canal was conveyed over Boxelder Creek in an aging concrete flume.
As constructed the flume over the creek restricted flows, backing up water to the north east creating dangerous flooding potential. The flume was also not large enough to carry the ditch company's full appropriation of water.
Anderson Consulting Engineers designed an inverted siphon for this crossing that consisted of the following:
- 145 foot, 96" diameter concrete siphon under Boxelder Creek
- Flow wasteway/cleanout structure
- Flow measurement structure utilizing a Parshall Flume
- Ditch bank emergency lateral weir
Siphon construction began in January of 2016. The 96" diameter pipe was placed in location and forms were constructed for the siphon headwalls.
Concrete pours continued through February and March of 2016.
The siphon was completed by April and ready for the 2016 irrigation season. Lake Canal could now convey their full appropriation of water underneath Boxelder Creek and flooding issues upstream of the crossing had been mitigated.
Fully Functioning siphon Summer 2020
Siphon and Boxelder Creek Summer 2020 (view to the east)
Siphon and Boxelder Creek Summer 2020 (view to the west)
Prospect Bridge Replacement:
With a primary goal of reducing the over topping of Prospect Road during elevated storm events, Anderson Consulting designed and oversaw the construction of the new Prospect Road and Boxelder Creek crossing.
The original Boxelder Creek crossing under Prospect Road was under-sized and did not have the conveyance capacity for larger storm events. Therefore during larger storm events Prospect Road, a major transportation artery in Fort Collins, would overtop and become a public health and safety issue. The under-sized crossing was removed and the site was prepared for the installation of the new rectangular concrete box culverts (RCBC's)
Two 14 foot wide, 5 foot high and 135 foot long RCBC's were placed in position and the forms for the apron and wingwalls were constructed.
By September of 2016 Prospect Road was reopened and the Boxelder Creek crossing was in the final stages of completion. The box culverts provide increased flow capacity for Boxelder Creek underneath Prospect Road. This increased capacity reduces road over topping and improves the public health and safety of a major transportation artery into and out of Fort Collins, CO.
Prospect bridge Summer 2020
Boxelder Creek Overflow Channel:
The final step in this series of projects was the Boxelder Creek Overflow Channel. The overflow channel was designed and constructed to carry approximately 3,400 cfs during high flow storm events and divert these flows to the Poudre River floodplain. The overflow channel option was selected because the Boxelder Creek main channel capacity is limited due to the new Colorado Department of Transportation (CDOT) bridge downstream of Prospect Road and the fact that conveyance of large storm flows in the main channel would inundate the Boxelder Sanitation District facility near Boxelder Creek's confluence with the Poudre River. The overflow channel was a more cost effective approach developed by Anderson Consulting to avoid replacing the CDOT bridge and mitigating the flooding potential near the Boxelder Sanitation District facility.
Construction of the overflow channel began in early June of 2016. A large section of Prospect Road was removed and the ground was prepared for the rectangular concrete box culverts (RCBC) that would convey storm flow underneath Prospect Road. Excavation of the 2,300 foot overflow channel to divert storm flows away from Boxelder Creek began at the same time.
The installation of the six 12 foot wide, 4 foot high and 112 foot long RCBC's continued through July and August of 2016.
By early September 2016, the grading of the channel upstream and downstream of Prospect Road had been completed and box culverts were in place. Prospect Road was reopened to through traffic. Revisiting the site in June of 2017 showed re-vegetation of the site was progressing as planned. The overflow channel provides vital channel capacity during elevated storm events that improves public safety (through reducing road over topping) and reduces flooding potential of the Boxelder Sanitation District facility and Colorado State Welcome Center both downstream of Prospect Road.
Boxelder Creek overflow channel Summer 2020
Results:
These interrelated projects achieved a variety of goals for a variety of stakeholders. The projects:
- Completely eliminated the Timnath overflow path on the east side of Interstate 25, reducing dangerous flooding potential.
- Improved the ability of the Lake Canal Irrigation Company to deliver water through Boxelder Creek to the Lake Canal and also increased their capacity to deliver their full appropriation of water to their users via their ditch.
- Reduced road over topping of Prospect Road, improving public health and safety.
- Mitigated flooding threats downstream of Prospect Road related to the Colorado State Welcome Center and the Boxelder Sanitation District facility.
