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Decentralized Flood Control - Recharging Groundwater and Reducing Erosion

Chris is a registered professional civil engineer and has worked on a more than 300 public and private projects over the last decade.

Stock ponds/tanks in the Arizona desert.

Stock ponds/tanks in the Arizona desert.

In arid climates such as the American Southwest or in the Middle East, water is a precious commodity. Anyone who lives in these areas knows that we often face an interesting problem. Mother Nature either gives us too much water in the form of flooding or not enough water in the form of drought. Whether you believe in climate change or not, the evidence shows an increasing trend away from the average and towards these two extremes.

This dichotomy presents a unique opportunity. Is there a way to capture and store flood waters so that they can be used during times of drought? This is precisely the challenge that civil engineers, urban planners, hydrologists, and government officials face in arid climates. Where there is a will, there is a way. Human ingenuity and innovation have a solution to the problem.

At first, you might be thinking about constructing mega projects like large dams and reservoirs. However, these were the techniques that governments and engineers used decades ago. While these types of infrastructure do have their benefits, they are not without their downsides. Big projects are costly, risky, and often have detrimental effects on the environment. Furthermore, it has become increasingly difficult to design and build large-scale, regional projects for flood control or groundwater recharge.

This dam in central Arizona was build for Flood Control purposes. Despite the benefits it provides, it's construction and operation comes with many risks.

This dam in central Arizona was build for Flood Control purposes. Despite the benefits it provides, it's construction and operation comes with many risks.

A Paradigm Shift

In recent years there has been a paradigm shift in the industry to move away from building large scale projects for flood control, such as dams. One colleague of mine said, “If you can't build bigger then perhaps it's time to start building smaller.” Engineers and planners are now looking at designing decentralized flood control facilities at much smaller scales to accomplish the same goals. What this entails is designing multiple small structures that can be strategically located throughout a community or watershed, both urban and rural, that would have the same net benefits of a large structure. The advantage of this approach is that environmental impacts and risks are minimized with this approach.

For example, instead of building a dam to old 10 acre-feet of water, you could build 100 small catchments that each hold 0.1 acre-feet of water. The latter option would be easier to design and implement as there are less regulatory hurdles to overcome and less environmental impacts.

If you think about the hydrologic cycle and how Mother Nature works, a decentralized approach is much closer to what nature does then what you get with a mega project. Intuitively, this makes sense because in a natural environment a significant percentage of rainfall is absorbed into the soil at the point a contact with the ground. Infiltration occurs until the ground becomes saturated; at this point runoff begins to accumulate. Constructing multiple flood control facilities will spread out the water over a greater surface area which more closely mimics this natural phenomenon. Furthermore, huge swaths of land are not flooded which is what happens with large structures such as dams.

Types of Decentralized Flood Control Facilities

There are actually several varieties of decentralized flood control structures that can be constructed depending on the specific situation that you were dealing with. Some examples include micro-basins, rock check dams, bio swales, and stock tanks/ponds. Some techniques are better for mountainous terrain, whereas others are better in flat areas.

A micro-basin is a small earthen basin or depression in the soil with a nearly flat bottom surrounded by low berms. The berms serve to hold runoff and prevent it from flowing downstream. Micro-basins are a commonly tool in farming on terraces and flat lands but also have their purpose for stormwater control and erosion prevention. Trees and shrubs can also be grown in micro-basins to help hold the soil in place.

A rock check dam is a small dam, typically less than 1-2ft high, that is constructed within a natural low-flow channel or stream. The rocks serve to slow the water down, reduce erosion, and allow for the accumulation of sediments within and upstream of the dam. These create great habitats for wildlife and can even help to restore a watershed after wildfire.

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A bio-swale is a channel or low-flow feature designed to slow stormwater runoff down and get as much of it as possible to infiltrate into the ground. Usually these structures are lined with thick native grasses and shrubs to maximize the retention of runoff. More information on bio-swales can be found in an article on the subject by the United States Natural Resources Conservation Service.

A stock tank/pond is a structure that is similar to a micro-basin, but larger. A stock tank/ pond may measure anywhere from 5-10ft in diameter to up to 50ft across. Soil is removed from the central structure below grade and used to build up berms around its perimeter, except for an opening to allow the inflow storm water runoff. These structures are often constructed on or near small streams in order to maximize the amount of water captured. Stock tanks/ponds can often become sources of drinking water for animals and work well in creating new habitat, especially in the arid climates.

Historical Perspective

The idea of decentralized flood control is not actually a new concept. For example, in the 1930’s the Civilian Conservation Corp (CCC) built many bio-swales and stock ponds in Southern Arizona between the City’s of Eloy and Tucson (as well as other parts of the Country). In many cases, these structures are still actively working today to stop erosion, increase runoff infiltration, and promote habitat growth. It’s also interesting to note that these structures have had almost no maintenance over the past 80 years!

CCC Swale on Google Earth

For more information the Civilian Conservation Corps’ legacy, please refer to their legacy website: https://ccclegacy.org/

Video Highlighting an Example Project

Case Studies

Both the US government as well as many universities have invested resources into studying decentralized flood control and erosion control techniques.

The US Bureau of Reclamation (USBR) completed a study that monitored the effectiveness of check dams placed in a watershed on the north side of South Mountain in phoenix. This study was completed in partnership with Arizona State University. You can download the report here: https://www.usbr.gov/research/projects/detail.cfm?id=1751

The US Department of Agriculture’s Southwest Watershed Research Center has also completed a case study on the Santa Rita Range. According to their study:

“Today, rock check dams are being used in rangeland restoration projects throughout the southwestern US. These low-tech, non-engineered structures are commonly constructed in eroding channels where they cause sediment to deposit. The sediment stores moisture and creates conditions for the germination and growth of vegetation. Vegetation then causes more sediment to deposit and a feedback loop is created whereby channels are healed and vegetation is restored.”

— USDA's Southwest Watershed Research Center

Their study showed significant benefits to the construction of in-stream, check dams over a 10 year period of time. Not only did the work increase sediment deposition in the channel, new vegetation was shown to be growing in and around the structures. The full online Story Map, including before and after photos, can be read here: https://storymaps.arcgis.com/stories/0a85c57365ba493391257f6eb18dd2be

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2022 Christopher Wanamaker

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