   Flood Mitigation and Recurrence Intervals - January 19, 2005 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County When we are in the midst of drought we forget quickly what rain looks like and what it can do…until the flood. It’s much like how our memory treats a devastating wildfire or other calamitous situations. For the next six months, we are very aware of what happened. After a year, it starts to fade. In two years time, we forget about the potential risks and become complacent. This is probably some sort of inherent protective mechanism that is part of our design to help us deal with suffering and loss. However, we should try to be prepared when fires, floods, earthquakes, and other emergency situations arise. And there are things we can do to be prepared. I don’t live along Oak Creek, but I’ve seen the photos of the December 29, 2004 flood. It was an impressive display of the power of nature and many people sustained property losses. My home is along Miller Creek in Prescott where the flood took away lots of soil, eroded away a large portion of my concrete low water crossing, and created some escape routes in the fence for our dog. Further downstream from my house, two young men lost their lives as they tried to navigate Granite Creek in a canoe. There are certainly risks associated with living along north-central Arizona’s stream courses. Streamside (and arroyo-side) property owners can take steps to minimize soil losses and increase bank stability. Grasses, sedges, and rushes have fibrous root system to hold the soil and lie flat when water flows over them. These are the best plant species to hold the soil. Trees and shrubs slow the flow of water, but can also create debris dams that can alter streamflow patterns. Allowing streams to meander dissipates energy and slows soil losses from banks. Rocks, from cobbles to boulders, can also dissipate the energy of flowing water. Artificial structures are sometimes used to slow streamside erosion and prevent gully formation. These structures include fabrics, sandbags, check dams, straw bales and wattles, etc. Culverts, concrete, and channelization all increase the velocity and concentrate the energy of flowing water. What about flood characterization? We’ve all heard reporter or expert talk about the 50 or 100 year flood. These are measurements of flood probability, or more technically stated, flood recurrence interval. It requires several years of data and relies on the peak discharge (maximum flow) for each year. The estimate becomes increasingly reliable with longer periods of record. No, I won’t bore readers with the details of the calculation (even though it is really tempting). However, I think flood recurrence interval is poorly understood by many people have included an example from recent storms to clarify general misconceptions. Using USGS data from the Oak Creek Gauging Station near Cornville, I calculated flood recurrence interval for the 63 years of record available at that site. The peak flow measured on December 29, 2004 was 20,300 cfs (cubic feet/second) and the recurrence interval was 11 years. The highest peak flow recorded for that station was February 19, 1980 at 26,400 cfs and had a recurrence interval of 64 years (because we only have 63 years on record). The peak flow from February 20, 1993 was 26,000 cfs, was the second highest on record for this station and had a recurrence interval of 32 years. Interpreting this data, it says that, on the average, we could expect a flood of the magnitude seen on December 29, 2004 every 11 years. If we had 99 years of records, the highest peak flow during that period becomes the discharge rate of a “100 year flood”. Like many statistics, these data can easily be misused or wrongly interpreted. For instance, a 100 year flood may not occur only once every 100 years. Correctly stated: there is a 1% chance that a 100 year flood could occur in any given year. Keep in mind that flood recurrence interval is based on probability. It's just like gambling. The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |