Over the last 30 years, the City of Seattle’s Madison Valley area has experienced wastewater backups and flooding an average of every 4 years. In 2004, stormwater again overwhelmed the combined sewer and backed up into local residences.
RH2 was selected by Seattle Public Utilities to work with a team of consultants and SPU staff to find solutions to the flooding problem. RH2’s primary role was to coordinate with rainfall and modeling consultants to evaluate the existing system, characterize the problem, and develop alternatives to eliminate sewer backups and reduce flooding. The analysis resulted in one interim solution that could provide a significant amount of storm protection and three long-term solutions that required significantly more funding and further consideration.
The city elected to construct the interim solution as soon as possible, and it was designed, approved, bid, awarded and under construction in approximately 6 months.
The interim solution was within weeks of completion in December 2006 when another large storm overwhelmed the system. RH2, acting as the lead consultant, worked with SPU and other consultants to evaluate the system impacts and further develop alternatives for a long-term solution. The long range project was further developed and analyzed through the use of both physical and numerical models developed by others. A variety of alternatives were developed and evaluated with potential storage values of 1 to 5 million gallons and pipe sizes of 48 to 84 inches.