Capitol Avenue Outfall
Bismarck, North Dakota

Project Details:
Urban storm water runoff from intense rainstorms has caused serious erosion along the Capitol Avenue Outfall channel since development began in the 1970s. This erosion resulted in significant damages to the channel and also exposed an adjacent 12-inch vitrified clay pipe sanitary sewer. Early attempts to protect the sanitary sewer with riprap failed when the rock washed downstream during subsequent rainstorms. The erosion extended from the outfall to the confluence with Hay Creek (approximately 1,600 feet) and ranged from 10 feet wide and several feet deep to nearly 20 feet wide and over 12 feet deep.

The city of Bismarck, Bismarck Parks and Recreation District, and the Burleigh County Water Resource District received an EPA Section 319 Grant for completing a water quality and quantity assessment of the Hay Creek Watershed. This grant also provided assistance to develop and construct Best Management Practices to address existing water quality issues and concerns. Houston Engineering assisted the city of Bismarck in identifying an acceptable and economical solution for the Capitol Avenue Outfall by completing an alternatives analysis, and then preparing the final plans, specifications, and bid documents for the selected alternative.

Houston Engineering prepared a detailed alternative analysis for reconstructing the Capitol Avenue Outfall, which defined the problems, project goals, alternative solutions, and included a value engineering approach. The selected alternative defined multiple structural measures designed for the 100-year flood peak including:

• 17-foot vertical cast in-place drop structure
   
• 450 lineal feet of 96-inch reinforced concrete pipe
   
• 120 lineal feet of a 10-foot rise by 12-foot span reinforced concrete box culvert with energy dissipation blocks
   
• Rock riprap stilling basin
   
• 1,200 lineal feet of reconstructed channel that included an armored low flow section and a designed floodplain

In addition, Houston prepared plans, specifications, and cost estimates for a 2.2 acre wetland treatment system constructed to improve the water quality within Hay Creek. The wetland system will reduce the inflow of sediments to the Creek, reduce peak discharges, and will also provide wildlife habitat and aesthetics for those enjoying the adjacent walking trail.

Project Benefits:

• Aesthetically pleasing construction design that used a combination of traditional and nontraditional methods
   
• Prevention of future erosion along the outfall
   
• Measurable reduction of sedimentation into Hay Creek
   
• Significant improvement in the quality of the discharge waters

Project Award:
2004 Engineering Excellence Honor Award, Category G
(Water Resources) from the ACEC North Dakota