If you’re familiar with the San Antonio River, you may have heard the surrounding area described as “Flash Flood Alley.” The area’s terrain and soil types, coupled with its propensity for frequent downpours, have made it the focus of numerous flood control projects over the years – some more successful than others. While past methods exacerbated issues, the San Antonio River Authority (SARA) eventually began seeing success with a restoration technique known as Natural Channel Design (NCD). In this article, we’ll look at how not to approach a stream restoration project and how NCD can aid in successfully restoring a stream to its floodplain.
Need for Flood Control Projects San Antonio has experienced many significant floods dating back to the early 20th century. One particular flood inundated downtown San Antonio with 10+ feet of standing water that did not dissipate for many days. This was the great flood that began a process of misguided flood control practices.
Harmful Flood Control Practices The traditional approach to flood control was to reshape the main channel with a trapezoidal cross section which presents the optimal configuration for the conveyance of flood waters. Since rivers are living things, the “improved” channels would meander over time, requiring the government to come in and harden the affected creek sections with expanses of concrete rip rap.
The cumulative impacts of these myriad improvements were problematic. The improved conveyance meant the flood waters would move downstream with increased energy, becoming even more damaging than before. Furthermore, trying to lock in and control the forces of nature by hardening the slopes and dredging the channels proved to be pure folly on the part of us mere mortals. The improved channels deprived the creeks of their natural cleansing processes. The last major issue is that the river channels optimized for drainage and runoff no longer served as suitable habitat for the natural plants and animals that are part of the diversity of our ecosystem and genome. A better technique was needed.
Natural Channel Design A natural stream will overflow its channel an average of once every 1.5 years. This allows pollutants to be processed on the floodplain, wetland-adapted plants to flourish in these areas, and most stream functions of value to humans to be performed. However, the relationship between the stream channel and its floodplain can change under two main scenarios: stream degradation and intentional human alteration. As a result, flood control projects are necessary to restore the stream to its original floodplain.
With that said, let’s talk about natural channel design, the technique typically used by SARA. The NCD approach relies on the principles of fluvial geomorphology to evaluate the current state of the degraded stream and that stream’s potential for restoring its historic functions. The fluvial geomorphology of a stream can include the interactions of climate, geology, topography, vegetation, and land use in its watershed. As part of an NCD stream restoration project, the relationship between the stream and its floodplain is returned to a condition that maximizes stream function.
Three-Stage Channels One way the natural channel design method accomplishes stream restoration is by using a three-stage channel. When streams are separated from their floodplains due to typical channelization projects, it often lowers the stream channel and associated flood control channel below the original flood plain so much that only a rare flood event will bring the waters in contact again. The result is the loss of the natural stream benefits. To address this loss of function, an NCD three-stage channel can be constructed within the flood control channel which essentially creates a floodplain within the flood control channel that now connects to the stream. The result is a more stable stream that acts as a natural stream without endangering nearby infrastructure.
SARA has also collaborated with the City of San Antonio and Bexar County to incorporate linear park elements with the NCD projects along the San Antonio River and San Pedro Creek. These parks have been very successful endeavors with people using them as hiking and biking trails. Many include workout stations in the parks. So, the NCD projects serve several functions to restore habitat, purify the runoff water, protect buildings and infrastructure, and improve the aesthetic appeal through thoughtful planting and preservation of trees and other plants. All these aspects provide enhanced safety, recreational, and wellness experiences for the community.
Every stream receiving NCD treatments should be evaluated and assessed in terms of its context and individual needs with respect to its place in the broader ecosystem. There is no “one-size-fits-all” approach. Rather, each stream should be assessed for a targeted approach. Click here for a quick list of some of the various tools in the toolbox that can be implemented to achieve the desired outcomes.
Civil engineering is a science, and flood control methods of the past haven’t always gotten it right. But through seeing their detrimental effects, the industry adapted by developing the more environmentally friendly natural channel design methods. When a significant number of NCD projects are effectively applied in a watershed, restoration can result in several benefits to the related ecosystem. Whether it’s additional storage of flood waters, removal of pollutants by plant uptake and cohesion to the soil, or additional recharge of groundwater, NCD methods offer all the stream restoration benefits without the negative downstream effects of more traditional flood control methods.
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