Flood District Outreach Project
February 2022 Update
The development of the design continues to progress and Staff anticipates having the preliminary set of construction drawings in Spring 2022. The gathering of field data, including Survey and Subsurface Utility Engineering (SUE), which is necessary to the design is complete. Staff will continue to coordinate with the Mile High Flood District (MHFD), internal stakeholders, including Parks & Recreation and Public Works, as well as with the Broomfield Town Square development.
To improve the habitat of the channel as well as to reduce maintenance costs, the project will implement the latest guidance in stream design using riffles and pools for a High Functioning Low Maintenance Stream (HFLMS). The existing concrete bottom boulder lined channel will be converted to a more natural channel with sloped banks and vegetation, from US287 on the west to Main Street on the east. In addition, and to improve overall stream health, the project will introduce sinuosity throughout the entire reach which will change the alignment of the existing channel. The changes to the alignment to the existing channel vary throughout the reach and are more significant in some areas.
The proposed improvements also include updates to the existing pond in Midway Park, including its outfall structure, updates to the crossing at Emerald Street to increase flood conveyance and the installation of pedestrian underpass at Main Street to increase flood conveyance as well as to provide enhanced pedestrian/bicycle connectivity to the future Broomfield Town Square.
Typical Channel Profile showing Riffles and Pools
Introducing Sinuosity to the Channel
The City Park Channel - Midway Park Modifications project budget was adopted by City Council in 2019.
The upstream limit of City Park Channel is between Hoyt Street and Burbank, just west of Aggregate Industries. The downstream limit "channel" leaves Broomfield in a box culvert just west of Chase St. and flows along the south side of W. 120th Ave to Big Dry Creek (in Westminster).
FIG 1: City Park Channel Extents in Broomfield County
This project will span from HWY 287 to Main Street and is intended as a maintenance project and will leverage maintenance funds collected by the Mile High Flood District (MHFD) through mills collected from property taxes. The project will implement the latest guidance in stream design using riffles and pools for a High Functioning Low Maintenance Stream (HFLMS) corridor. HFLMS design incorporates softer elements (vegetation), as opposed to the boulder lined section existing today, to improve habitat and decrease maintenance costs. The design will include increased meanders and sinuosity to control velocity and manage sediment deposition locations.
FIG 2: Project Area
City Park Channel is currently defined by three sections, called reaches. These reaches are defined by roadway crossings. Reach 1 - Main Street to Emerald Street, Reach 2 - Emerald Street to Midway Blvd. and Reach 3 - Midway Blvd. to US 287. Construction will begin at the downstream reach (Reach 1) of the project and work upstream (west).
The existing stream corridor is in need of maintenance in multiple areas. The grey boulder lined section immediately west of Main Street is experiencing two significant issues: 1) water (from the creek or from irrigation) is running behind the boulders and displacing the fine material (sands and grout) anchoring the boulders in place. Some boulders have become completely dislodged and are tipped into or lying completing in the channel bottom. 2) the displacement of material by running water (sometimes called piping), is also creating sinkholes in the turf adjacent to the channel. These sinkholes may become a hazard for users of the park. The section of the creek not lined by boulders, between Nativity and Emerald schools is experiencing sloughing and erosion of the banks. Additionally, a large outfall pipe in this section is exposed for several feet. The most upstream section (from US 287 to Midway Blvd.) has similar issues with dislodged boulders.
The Mile High Flood District (MHFD), formerly known as Urban Drainage and Flood Control District (UDFCD) is a quasi-governmental body funded by mills collected from property taxes. The MHFD mission is to protect people, property, and our environment through preservation, mitigation, and education. To support this mission, the MHFD partners with local jurisdiction to construct, study and plan drainage and flood mitigation projects throughout the seven county Denver Metro area.
Broomfield will co-manage the project with MHFD project engineer and construction management staff. Broomfield will assist with outreach and updates and compliance with our Standards and Specifications. MHFD staff will manage the contractor, federal permitting (404 and environmental) and ensure compliance with the MHFD drainage criteria. The consultant and contractor are selected through the MHFD prequalified list. This ensures experience working in a creek corridor setting and understanding of the drainage and flood control impacts.
The creek corridor will certainly look different with multiple meanders and no boulder lined sections. The bank slopes will be flatter and there will be more vegetation. The intent is to keep the creek crossings in generally the same locations, though they may shift slightly. During construction, portions of the creek will be fenced off and not accessible. Post-construction a floodplain study will be submitted to FEMA to accurately delineate the floodplain based on the improvements.
