Alternate: Fitzsimmons-Peoria Stormwater Outfall: Reducing Flood Risk in a Highly Urbanized Area

A high density area undergoing rapid redevelopment in the City of Aurora (City) experienced historical flooding due to undersized stormwater infrastructure constructed in the 1970s. As a result, the City prioritized implementation of the Fitzsimmons-Peoria Stormwater Outfall Project to comply with current stormwater standards and provide much needed flood mitigation. The project was a large-scale effort that replaced approximately 15,000 feet of aging, 24- to 96-inch storm drain through a highly urbanized area that is undergoing rapid redevelopment. The project improved flood protection for approximately 230 residences and 40 businesses. Following preliminary design, Carollo assisted the City in contracting a Construction Manager at Risk (CMAR) contractor, BT Construction. Final design efforts were completed as a collaborative team consisting of the City, the CMAR contractor, and Carollo. Together, the team evaluated alternatives to mitigate site constraints throughout the project while minimizing public disruption and project cost. Carollo constructed a computer model using InfoWorks ICM with 1-D and 2-D components. Pipeline alignments were evaluated for hydraulic capacity and selected with consideration to construction sequencing, construction duration, protection of existing utilities, minimizing disruptions to traffic, and project cost. Carollo also performed HEC-RAS modeling of Sand Creek utilizing the existing published FEMA FIRM maps and base flood elevations with new flow rate inputs for the project basin. While the discharge flow rate increased significantly, the discharge time was well ahead of the peak attenuation of base flood elevations at the discharge location. Therefore, our team was able to prove a no-rise condition and avoid the need to submit a LOMR. Most of the pipeline was constructed using open trench construction with passive shoring (trench boxes). Four major arterial roadways required large diameter trenchless crossings. The variability of ground conditions required a combination of open face tunnel boring machines (TBM) for two of the tunnels and closed face microtunnel boring machine (MTBM) for the other two crossings. The project team evaluated each construction technique with consideration to cost, feasibility, safety, and minimizing public impact. The project also required large manholes and junction structures which were designed as a combination of both cast-in-place and pre-cast structures, with a preference for pre-cast to minimize construction duration where feasible. Now fully complete, the new storm drain system has the capacity to collect and convey the 100-year storm event to the discharge at Sand Creek. Using the CMAR delivery method, the City was able to develop alternatives during the design phase that improved basin stormwater management, mitigated risk, minimized public impacts, and totaled an overall cost within the Contractor's $31M Guaranteed Maximum Price.