Dam Break

Tempe Town Lake is a recreational lake that occupies the Salt River channel in Tempe, Arizona. Humans de-watered the naturally perennial Salt River in the mid 20th century due to diversion and groundwater pumping. Tempe sought to return the ugly and artificially dry river bed into something more attractive, and decided to restore water to the channel by creating a lake.

The lake’s dam is a series of air-inflated rubber bladders on both the upstream and downstream ends of the lake. Deflating the bladders allows the passage of natural floodwater and sediment, and are re-inflated to impound water and re-establish the lake. Tempe claims the rubber dams are less expensive, and ideal for an urban recreational lake.

On the night of July 20, 2010, one of the inflatable dam sections catastrophically failed, sending the entire contents of the lake downstream. Fortunately, no property damage or injury occurred as a result of this failure. Had a daytime failure transpired while boaters were on the lake, a less-fortunate outcome would be expected.

Having a once full lake catastrophically drain is a unique occurrence, so last week I took the opportunity to see the effects of this dam rupture and make general observations about the empty lake.

A few puddles remain in low areas of the channel, but in most places the river bed is dry. The river bed is coarse, with gravel and cobbles comprising most of the channel. In a few areas where the sediment is finer, mud cracks formed as the lake bed dried. While I could not get down in the channel and measure, the largest cracks are about 3-5cm wide and 10-15 cm deep.

I noticed this formerly submerged feature in the river bed. From my distant vantage point, accurately discerning this feature is impossible. It could be the remnants of some concrete structure, or could be a small outcrop of bedrock. This portion of the river lies between two sedimentary buttes, so possibly this is a small portion of exposed sandstone or conglomerate. Without closer inspection, I can’t say for sure.

The lake is basically a bent rectangle with a flat bottom and almost zero natural cover for young fish. Manmade structures from old tires, pallets, or plastic tubing provide the only protection for young fish. The fish are recreational imports only – as the destruction of the native Salt River habitat occurred long ago.

New vegetation already grows in the dry lake bed – at this point only grass and weeds.

Constuction of a new dam of the same design is underway. 🙂 Tempe anticipates the new dam will not fail, as the previous dam (of the exact same design) did 🙂

I hoped that the flood event created some interesting bed scour, but unfortunately I was too far away to notice any. The construction vehicles likely obliterated any erosional evidence of the flood. The small amount of water flowing through the river is discharge from a wastewater treatment plant upstream, and helps support some young riparian vegetation both upstream and downstream of the lake.

Tempe has a warning siren near the lake. The siren and lights intend to warn lake visitors of emergencies, but ironically a dam failure is not one of them.

I am slightly disappointed that I did not see anything truly unique such meter-deep mud cracks or flood erosion and destruction, but at least the bike ride down there was relaxing.

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