Bannack State Park Hangman’s Gulch Detention Basin

The Bannack State Park is a National Historic Landmark and the location of Montana’s first major gold strike. The former town is composed of over 50 log and frame structures dating back to 1863. On July 17, 2013, 2.2 inches of rain fell in 30 minutes overwhelming the existing Bannack State Park storm water drainage system and significantly damaging the historically significant town site. Pioneer was selected to support the Bannack State Park emergency response activities, perform a feasibility level alternatives analysis, provide engineering design, manage contractor procurement activities, and construction oversight services.

For Bannack State Park (for Belfor and Montana Fish Wildlife & Parks), Pioneer designed and managed construction of a detention basin to avoid devastating floods in the future.

The project involved designing and building an engineered storm water management system that was both cost effective and preserved the Historic Landmark’s aesthetic character. Initially several preliminary storm water management alternatives were evaluated to route or control Hangman’s Gulch runoff around or through the historic Bannack town site. A detention basin in upper Hangman’s Gulch was selected as the preferred alternative to detain and attenuate a 100-year 1-hour rainfall event. To complete this alternative, Pioneer completed a geotechnical site investigation and site topographic survey, and provided detention basin engineering and designs.

The final design incorporates a small, earthen embankment and concrete drop inlet structure (Hydraulic Structure). To design and construct the detention basin, we considered structural fill sources, embankment foundation design, and slope stability of both the embankment and abutments. Project construction took place in the summer of 2017. We provided construction oversight and will submit a final report.

The designs and calculations for the project included completing these tasks:

  • Hydrologic analysis to estimate the peak runoff flow and rainfall depth that occurred during the 2013 event to support conceptual storm water management designs and costs.
  • High water mark survey.
  • Open channel flow calculations to estimate a peak flow.
  • Rainfall-runoff modeling (HEC-HMS) to back-calculate the rainfall depth. A peak flow of 1,450 cubic feet per second resulting from a 1-hour rainfall depth of 2.2 inches was the final estimate.
  • Rainfall event frequency determination (recurrence interval significantly greater than a 100-year event).


Additional videos and updates

Bannack State Park won’t be taken by another flood anytime soon – 

Montana Fish, Wildlife and Parks’ Matt Marcinek joins Chet Layman of MTN News to talk about the construction underway for flood control at Bannack State Park. Pioneer designed the flood mitigation elements. See the video here.

Mitigation project underway


Outlet of Hangman’s Gulch and Hotel Meade during Hydrologic Site Investigation
Outlet of Hangman’s Gulch and Hotel Meade during Hydrologic Site Investigation