With all the recent flooding happening both in the Midwest and regionally (due to recent wildfires), now seems like a good time to talk about the flood potential of where you live in the desert southwest. When you live in the desert, most people don’t think much about flood preparation. However, there are times when it can flood severely, typically in a floodplain like anywhere else. There is a second type of flooding that can affect more people – that of sheet flooding. Sheet flooding is when enough rain falls on a gentle slope that it no longer sinks into the soil, but moves downhill. This type of flooding can still build up a decent depth, and I am planning for six inches max on my property near the house(I do like to over prepare).
When determining what potential flooding may happen on your land, there are several factors you must consider.
1. What is the lay of the land? Is it flat, in a depression or hill top, or on a slope? Check your local city or county maps or codes to determine if you are in a known floodplain. If you find you are outside of a floodplain, you may still want to look at your topography to determine if sheet flooding or large ponding may affect you.
2. What is your soil like? Sandy soil will drain much faster than clay soil. This difference can easily determine the flood potential of your land.
3. What are the average rainfall and the historic greatest rainfall, especially in a shorter period of time, for your area? I am rural, and I don’t have exact numbers for rainfall in my area. I know Tucson averages around 12 inches a year, and that is a good start, but there are other ways of figuring my area out, as I will show you.
Ok, so you have these questions. How do you find the answers? As I stated earlier, you might find some relevant information from your city, town, or county land use maps and other related local websites and agencies. However, if these are not available to you, than there are other governmental entities that can help. For topography, you can explore your area or get a topo map from the USGS. Getting a general lay of the land will help you understand the potential of water entering your site from the surrounding land. To get a better understanding of all three questions, a great website is the NRCS website at http://soils.usda.gov/. Click on the soil survey tab at the top and then the web soil survey link. This site is a bit technical and difficult to run on some computers, but it will give you some of the best information out there. If you are unable to use this site, think creatively. Yes you can hire a firm to get you all the information you want, or you can save some money and hire a student in the geology, landscape architecture, or planning departments at a local college to look up this website for you. Another website I use and is good for southwestern climate is from the Western Regional Climate Center at http://www.wrcc.dri.edu/ ; look at the historical information.
All of this information so far can only help you so much. If you are like most people, a good example will go far in helping you understand things. To this end, and because this blog is about where I live, I will use my property as an example.
Lay of the land. From looking out my window and from exploring the area, I can tell there is a general slope from east to west (Fig. 1). I also know that immediately around me as well as at the larger scale, the topography is gently rolling with braided washes of various widths and depths weaving their way downslope. I also know from my observations and from Google Earth that there is a slight hill to my east south east. The hill blocks views of houses I know are there in the distance and the earth image confirms it with the darker lines created by denser vegetation at washes leading away from a central area (Fig. 2). On the property, I have watched how water runs during rain to get a good idea of how the water flows across my landscape. This information I than transferred to a drawing; (Fig. 3) a computer drawing in my case, but it could be hand drawn just as easily.
So now I have the general lay of the land, let’s move on to the soils. Again I know from experience through digging that I generally have loamy soils with lots of rocks. Sandy areas exist in the washes and there are some areas of clay. But more information would make me feel better, especially as the NRCS analysis will tell me of the greater picture surrounding me. So what does this analysis tell me? It tells me I live in the Bucklebar-Hayhook complex (a complex is two or more soils intricately mixed) with 1% to 10% slopes. In general, these soil types are found from 2090 to 2990 foot elevation with an annual precipitation of 10 to 12 inches of rain. They are formed on fan terraces and consist of mixed alluvium soil that is mainly sandy loam with some clay loam and are well draining. The analysis also tells me that the depth to the water table is greater than 80 inches. This confirms my analysis and expands it to the larger area. However, I learned more about the rainfall and was reassured that long lasting flooding would not likely happen due to the well draining soil and deep groundwater table.
Finally, the rainfall. We just learned that there is an average rainfall of 10 to 12 inches for this area. We also know Tucson averages just over 12 inches. But I want to know more. I have kept inconsistent rainfall data in the 12 years I have lived here and found that I average around 10.75 inches of rain. But if you do not have your own data, you may want to ask your neighbors if anyone has collected information over the years. Also, if you are rural like I am, I live approximately 25 miles from Tucson; you can look for closer known data. For me, it is the town of Red Rock found on the Western Regional Climate Center website. Red Rock averages 9.78 Inches of precipitation a year. I am upslope and closer to some mountains so I should receive more which aligns with my own tally.
Hopefully this will get you started on informing yourself of the flood potential you have. Next time, we will look at the strategies I am employing to mitigate my flood potentials.
Fig. 1 North out the kitchen window
The slope flows from east to west and is between 3-7%. This area is half way down a bajada or alluvial fan slope of the northwest part of the Tortolita Mountains.
Fig. 2 Graphic using Google Earth and Photoshop showing the nearby hill and large scale water flow
You can determine high points on a topographical map or aerial by noting the layout of washes, highlighted here in grey. The property is the dot on the left and the oval denotes the hill.
Fig. 3 Medium scale of water flow on the property using Google Earth and Photoshop
The larger blue lines show the locations of the current main water channels that flow only during heavier rainfall. The blue swath is the potential flooded areas associated with the main wash during the most extreme events. Finally, the smaller arrows show sheet flow with the only threat to the house coming from the drive parking area to the east of the house.
