What is Natural Channel Design?

While many methods of stream restoration exist, the San Antonio River Authority's (SARA) stream restoration projects will focus on a particular restoration technique known as Natural Channel Design (NCD). 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 particular stream can include the interactions of climate, geology, topography, vegetation and land use in its watershed.

The end result of the NCD method develops a functional, self-sustaining stream system that provides valuable hydraulic (water transport), geomorphic (sediment erosion and transport) and ecological functions. By improving hydraulic and geomorphic functions, NCD creates a stable stream channel that can move water and sediment generated by its watershed while maintaining channel structural characteristics (such as dimension, pattern and profile) so that over time, the channel does not fill with sediment or erode continually downward.

The NCD approach uses more natural reference streams to compare the degraded stream functions with a functioning stream, provide a "blue print" for the stream restoration design and establish performance standards during post construction monitoring. In addition, establishing and maintaining good communication with the project stakeholders and using technical judgment on what is practical for each project site is a critical element to a technically successful and cost effective project. Further detail can be found in the Rosgen Stream Classification Technique section of the Natural Resource Conservation Service (NRCS) Engineering Manual.

Changing the Channel Shape

As a part of NCD stream restoration projects, the relationship between the stream and its floodplain is returned to a condition that maximizes stream function. In a natural stream system, a stream will overflow its channel on average every 1.5 years, which 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.

In a degraded stream system, channels can erode downward so deeply that it is no longer possible for water to come out of the channel during anything but an extremely large flood event, and few stream functions can occur. There are many methods of stream restoration available using NCD techniques to correct this problem, depending on the availability of space adjacent to the channel for increased water on the floodplain without damaging nearby infrastructure.

A more challenging situation is when streams are separated from their floodplains as a result of channelization projects that are used to protect infrastructure next to the stream. During channelization, the stream channel and its associated flood control channel (shown in red) is lowered below the original floodplain so much that only an extremely large flood event will bring waters in contact with its original floodplain. Channelization is an effective flood control method, but does little to allow the stream to function in a natural way and provide other benefits. To address this loss of function, an NCD Three-Stage Channel can be constructed within the flood control channel. This essentially allows a floodplain to be constructed within the flood control channel that is lower than the original floodplain. This allows for a more stable stream that performs more of the functions of a natural stream, without endangering the nearby infrastructure.

East Salitrillo Creek Demonstration Project

On January 10, 2011, SARA's Stream Team began construction on a stream restoration pilot project on East Salitrillo Creek in the City of Converse. The segment of the stream to be restored is a quarter-mile long and flows through Judson's High School campus near Schaeffer Road. Prior to restoration, this segment of the stream was very unstable due to increased stormwater runoff from upstream development and the removal of deep rooted native grasses. The large amounts of stream erosion and lateral movement of the channel was endangering nearby infrastructure of utilities, roads and athletic fields and created sediment maintenance issues downstream. Above Image: Example of the downward erosion of East Salitrillo Creek and its instability as it flows through the Judson High School campus.

Using natural channel design (NCD) principals, SARA intends to restore the stream channel and riparian areas to a more natural condition using native materials and minimal structures to reduce existing maintenance costs for removing downstream sediment and repairing infrastructure. The ability of streams to transport sediment creates the stream form. The key to this design method is to balance sediment transport while enhancing natural stream functions. Some of the project design features include:

Stabilization: By partially realigning the stream and incorporating natural hardened structures into the stream at key locations, erosion of the channel can reduced significantly. Root wads (tree stumps with root mass attached) and vanes made of logs or boulders are placed on the outside bends to protect the bank soil. Structures made from boulders and logs are periodically being placed in the stream to dissipate energy and prevent erosion downward. Plunge pools lined with rock will dissipate the energy of stormwater as it enters the stream at two major tributaries.

Water Quality Treatment: Two bioswales will be constructed on the west side of the creek to treat stormwater inputs from Judson High School before it enters the stream. The bioswales will consist of littoral shelves (flat pools for water storage) separated by a boulder cascade. By storing water longer, plants and the bacteria that live in soil have more time to process pollutants that might otherwise go directly into the stream.