By Warren Tenney
The City of Tempe is finishing the largest public works project in its history. Tempe Town Lake has a new $47 million steel dam 900 feet long and, in some places, sunk 80 feet into bedrock. The new dam is the nation’s largest hydraulically operated steel gate dam system and is designed to last more than 50 years.
Tempe Town Lake is a manmade lake carved out of the Salt Riverbed. It is a 17- year-old flood control project turned into a recreational amenity and a draw for commercial and residential development. The lake now attracts 2.4 million visitors a year and hosts 45 annual events. The city estimates its economic impact at $1.5 billion. With its new dam, Tempe Town Lake is guaranteed to be a landmark in Tempe for decades.
The new Tempe Town Lake dam has eight steel gates that are 17 feet high and 106 feet wide. Each gate weighs about 260,000 pounds or 130 tons. Two hydraulic pistons raise and lower each of the gates in and out of slots in the foundation. These pistons are set to automatically adjust to water levels within the lake.
Like any public works project, the new dam had its challenges. The groundwater is close to the surface in the Salt River bed. Twenty dewatering wells had to be built to remove water from the foundation site, move it to a pond and, eventually, feed it to the riparian habitat on the west side of Tempe Town Lake. A record rain in 2014 dumped 5.6 inches on some parts of Tempe and runoff from the storm wiped out the initial excavation work for the dam’s foundation. Granite was closer to the surface than expected at two points in the dam site making building the foundation more time consuming than expected.
The original inflatable rubber dam was installed in 1999 and it had to be replaced. Tempe considered several proposals to remove the old rubber dam, including using divers. Draining the lake was the least dangerous and most cost-effective option. The city drained Tempe Town Lake in February. As much water as possible was pumped into the Salt River Project canal system and put to use. The rest was sent downstream to benefit riparian areas.
Tempe began refilling the lake on April 12. It takes two weeks to restore water to the lake. The new dam is set back about 100 feet from the old inflatable rubber dam, making Tempe Town Lake just a little bigger. The lake is from 5 feet to 18 feet deep and mostly filled with water from the Upper Salt River and Colorado River. The lake also captures water from storm drains and rainwater runoff that rushes through Indian Bend Wash.
Thousands of fish rescued from the lake and temporarily held at the Marina were the first to return to their habitat. Water activities for everyone else resumed this weekend. The City of Tempe is planning a dedication of the new dam at 9 a.m. on Saturday, May 14 at Tempe Center for the Arts, 700 W. Rio Salado Parkway. Construction, engineering and water experts will give informational talks about how Tempe Town Lake works.
For 46 years, Arizona Municipal Water Users Association has worked to protect our member cities’ ability to provide assured, safe and sustainable water supplies to their communities. For more water information visit www.amwua.org.