Tom Ryan on the Tucson Flood

August 14, 2006 |

On Monday we experienced a substantial flood event on the Rillito River which runs east-west through the city here. Measurements of the peak flows indicate the largest ever measured for the Rillito, and the second largest based on models (the models of the 1913 flood indicate slightly higher flows). Now that the water has receded, it is clear that the end result is:U.S. Army Corps of Engineers 1, Mother Nature 0. The river did not jump the flood protection structures on the banks, and very little damage resulted, compared to the last two floods in 1983 and 1993. As a result of those two floods the feds and the county engineered the Rillito Riverbed in the 1990s, widening in places, raising banks, and soil cementing twelve feet thick on all banks, in effect ‘channelizing’ the river. Well it worked although the high water came within a few feet of topping throughout the course of the downstream section of the river and topping the nine bridges across the river (two of which were damaged enough to be closed).

There are some other interesting aspects to consider:

Leverage. Although the volume of the flow appears to be in the 50-100 year return period category, the precipitation that created the flow was in the 5-10 year category (2-3 inches in 12 hours).

Pre-conditions. The large flows resulted from a relatively small amount of rain due to the ground already being saturated by previous days of rain.

Efficiency. Rainfall was concentrated in the foothills of the Catalina Mountains, rather than the valley or high up in the range. This led to a rapid concentration, high water mark effect.

Unpredictable Consequences. The worst damage to the banks from scour and to the bridge piers is from objects (tree trunks, propane tank, Honda Civic are just a few of the items I witnessed) entrained in the flows hitting and snagging on the bridges and structures.

Market analogies left to the reader!


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