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Brantner Gulch at Holly Street

Thornton, Colorado

Pedestrian/Stream Underpass


Urban Drainage and Flood Control District, City of Thornton


Civil : ICON Engineering, Inc. Structural: LONCO, Inc.


T. Lowell Construction, Inc.


June 2012

Technical Description:

  • Span: 24 ft
  • Rise: 11 ft
  • Length: 90 ft

The area surrounding Brantner Gulch is developed primarily with single family residential subdivisions, schools and the City of Thornton’s Trailwinds Park. Holly Street is located adjacent to the Gulch and provides residents with access to local highways. In addition, a regional trail owned and operated by the City of Thornton traverses along the Brantner Gulch open space but without a safe trail crossing at busy Holly Street.

In August 2008, several flooding events in a row caused Brantner Gulch to overtop Holly Street and wash out the eastern half of the roadway. Home video filmed during the storm showed automobiles traveling dangerously through the floodplain. Within a few days, the street was restored to traffic, but the flood conveyance under the street was still quite limited. That fall, City of Thornton and Urban Drainage and Flood Control District (UDFCD) officials met on site and determined that a joint project to improve the drainage conveyance under Holly Street would focus on the following issues:

1. Evaluate the effects of existing developer-constructed detention ponds on the 100-year peak discharges and evaluate the necessity of the proposed regional detention pond,

2. Minimize changes to the Base Flood Elevation (BFE) upstream of Holly Street and to the roadway profile,

3. Provide a crossing along Brantner Gulch capable of conveying the 100-year storm,

4. Provide a pedestrian underpass to allow safe travel along the Brantner Gulch Regional Trail,

5. Design improvements to fit within the available budget while allowing for future roadway expansion,

6. Determine a construction sequence to allow for construction over the summer while school was closed.

Based on previous Major Drainageway Planning (MDP) studies completed by the UDFCD, a regional detention pond was proposed to be constructed on Brantner Gulch just upstream of Holly Street. The MDP called for raising the street profile over 7-feet, with the roadway embankment serving as the “dam” for the detention. This option would have raised the BFE upstream of Holly Street by several feet. Holly Street at the Brantner Gulch crossing was a two lane road, but the City’s masterplans called for it to eventually be a 5-lane arterial.

Raising the embankment 7 feet and widening for the ultimate roadway would have caused significant impacts to upstream wetlands. The City did not have sufficient funds to construct the ultimate roadway cross section, but widening the embankment in the future was not ideal either. Based on the findings of the MDP hydrology review, it was determined that regional detention was not required along Brantner Gulch at Holly Street.

Upstream of Holly Street, the floodplain is shallow and widespread making it challenging to collect and convey stormwater under Holly Street while also providing a safe pedestrian underpass. To remedy issues 2-6 above, a 24’ x 11’ x 90’ CON/SPAN ® B-series Bridge System was chosen as the solution. During preliminary design, future roadway improvements were taken into consideration and planned for. The tiered drop structure was constructed upstream of Holly Street with a meandering crest providing sufficient crest length to collect and convey runoff produced by minor storms without flooding the trail. The structure was also designed to provide erosion protection during major storm events. As requested, construction occurred over summer break.

To accomplish this considerable amount of construction in such a short time period, existing overhead electric lines were placed underground and relocated prior to construction. The 12-inch gas line was temporarily relocated with a smaller diameter line during the low use summer period placing it out of the Contractor’s work area. In addition, T. Lowell Construction devised a 25 ft tall shoring built from the top down along Holly Street to allow the eastern side of the arch culvert to be constructed prior to closing the roadway.

During design, an unusually large number of utilities — with seven different owners! — were discovered within the Holly Street corridor. This included two large diameter waterlines; one small diameter waterline; a City sanitary sewer; a Metro Wastewater sanitary interceptor paralleling Brantner Gulch; one gas line; two electric lines; and two communication lines. During construction, the contractor, utility companies and project sponsors worked cooperatively resulting in the development of construction phasing alternatives and techniques that helped make the project successful. For example, the contractor and gas company worked together to temporarily relocate the gas line thus allowing the contractor to move directly into phase two of the bridge installation without delay.

The trail system upgrades, along with the new underpass, have greatly improved user circulation and provide a safe crossing for pedestrians at Holly Street. In fact, the trail improvements have been so successful that a nearby residential community is considering the addition of a trail in order to connect their community to the recently completed improvements.

“The CON/SPAN team was very helpful,” stated Cindy Rose, P.E., Project Manager with ICON Engineering, Inc. ”They provided us with what we needed to complete our design, construction plans and bid documents in a very timely manner.”

The project was constructed on-time and within budget and is also the winner of the American Public Works Association (APWA) Award in the Drainage & Flood Control - Large Community category.

Technical Description:

  • Span: 24 ft
  • Rise: 11 ft
  • Length: 90 ft

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