Yuma Ag and You – Land Development, Look at the Soil First!

by Bobbi Stevenson-McDermott                   Aug. 2, 2015


There have been numerous articles and letters to the editor recently about the potential urbanization of more agricultural lands as well as inappropriate land uses, apartments in an area of single family homes; 4 to 6 lot per acre homesites on active farmland and other conflicting land uses. What most people do not realize is the collateral effects of these developments.

Looking at the site west of Avenue D off 8th Street, the soil is a Holtville clay. This soil is rated in the USDA Soil Survey of the Yuma-Wellton area as having severe hazards for buildings with out basements. One of the features of the soil is severe shrink-swell characteristics, meaning when dry, it is like your dry kitchen sponge, however when wet, it swells just like a sponge. Then when the soil dries, it does so unevenly so the east, west and north sides of a structure may sink as the soil dries, while the north side, doesn’t move. This can lead to cracking floors and walls or shifting of the homes foundation. Without proper footings, this also causes block walls and fences to move, creating cracks and possible hazards of collapse. Since the Holtville soil will be also under the roads in the subdivision, they will be subject to shifting and cracking, as well. Holtville is rated as having low strength for foundations.

Let’s look at the Holtville clay’s ability to absorb rain or irrigation water. Water enters this soil at less than one-tenth of an inch per hour. This means, it would take 10 or more hours for an inch of water to penetrate the soil. In addition, after it is below the soil surface, the water moves very slowly downward, again due to the shrink-swell properties. As the lower soils get wet, they swell and fill the root holes and spaces between the soil particles, keeping the area soggy for days at a time.

Development creates many flat impermeable surfaces, roads, roofs, driveways and sidewalks. Ideally all the water falling on a home or lot should be retained on the lot with no runoff into the street or on neighboring lands. This water on Holtville clay retention areas will possible stand for days at a time. In summer this leads to mosquito problems and other insect pests.

It is possible to fix many of these problems but it takes a lot of money. Foundations can be excavated to a 3 foot depth and refilled with sand. The sand does not react to water and will buffer the foundation from the movement of the soil underneath it. Water and sewer lines should also be buffered with plenty of sand because the shrink-swell feature will also crack buried pipelines, corrode many types of metal and create problems with landscaping and irrigation systems. Swimming pools are very susceptible to damage in these very heavy clay soils.

The second rezoning causing great discussion is the very sandy, steep hillside on Avenue 9E and 24th Street. This soil is a Rositas sand. The problems here are first, the proposed density of residents in an area that is not designed for them.

Even more important is the type of soil that is proposed for development. In contrast to the Holtville clay, this soil is extremely coarse, with water intake of 3 to 15 inches per hour. However, this soil has severe limitations for shallow excavations due to cutbanks caving, moderate limitations because of the slope of the site.

With the slope on the site, it would probably be leveled in stair steps to accommodate as many units as possible. Rositas sand has hazards of seepage and piping soils, this means that the soil will move with wind and water. Driving through the subdivisions existing along the edge of the Yuma Mesa from Yuma to the foothills, you will observe the attempts of folks to keep the soil where it belongs. All types of retention walls and barriers exist, but the sand still moves because it is round and will not stack in neat piles. The maximum slope that can be reasonable maintained is 5 or 6 to 1, meaning for each vertical foot, the slope needs to reach out 5 or 6 feet. This is not what developers want, since it greatly reduces the developable acreage.

This site also sits above prime farmlands, and above a major irrigation main canal. All the impermeable surfaces of the development will shed water which can cause great damage to anything below it. A couple of examples are the canal below the swim park. Water from all the paved area washed over the edge of the mesa and into the canal, causing expensive clean-up and service interruption. Above another subdivision, the installation of a solar field proved damaging to homes below the panels when a monsoon rain dumped huge amounts of water on a small area, and with the soil shielded by the solar panels, the water accumulated and inundated part of the housing area.

Contractors building on sand often assume that because it is so porous they do not need to really worry about runoff. Compaction of the soil, surrounding properties that contribute to the runoff draining their properties onto neighbors or undeveloped lots all must be planned for to have a successful development.

Planning and zoning regulations clearly state how water must be handled and retained, but do not say anything about the soils on the site, information that is free and available to all. It is a matter of paying now for proper developments or paying later for damaged homes and infrastructure. As my husband says, put the round pegs in the round holes and the square pegs in the square holes!

Bobbi Stevenson-McDermott is a soil and water conservationist. She can be reached at rjsm09@msn.com.