Emergency Stream Intervention

Digger performing emergency stream repair

After several devastating storms, including Hurricane Irene and Tropical Storm Lee, many streams and creeks in the Catskills were left unstable and damaged. In response, state and local agencies worked together to restore these damaged waterways in Delaware County. These agencies included the Delaware County Soil and Water Conservation District (SWCD), New York State Department of Conservation (NYSDEC), Delaware County Planning Department, and New York City Department of Environmental Protection (NYCDEP). From this collaboration, came the Post-Flood Emergency Stream Intervention Program, which has been embraced and expanded to apply to the entire state by the NYSDEC.

Why restore a stream?

While a damaged stream may naturally regenerate over time, it is a slow process and the affected area is vulnerable to further damage by subsequent storm events. Natural restoration may involve the stream bed moving which may, in turn, negatively impact human developments along the water way. Stream restoration combines the resilience of a “natural” repair while ensuring that roads and other developments are not damaged.

West Kill stream restoration-before

West Kill stream restoration-before

Restoring a stream allows water flow to control and stabilize the stream bed and banks thereby protecting the aquatic habitat. Reducing erosion and sediment transfer are part of this process. With the stream stabilized, the return of a “normal” aquatic habitat is encouraged. Once habitat is reestablished, vegetation and animal life return, further improving both water flow and quality, and bed and bank stabilization—yielding an overall healthier environment.

West Kill stream restoration-after

West Kill stream restoration-after

Often, in large events, damage is not limited to a single location and a strategy needs to be developed to address competing issues. Identifying critical areas, where more extensive work is required, and phasing this work so that critical areas can receive additional work at a later time, are important steps in making any stream repair. This phased-in stream restoration is known as Emergency Stream Intervention.

Emergency Stream Intervention

Typically, emergency stream intervention work is required after large rainfall events where heavy runoff has caused extensive erosion and streambank damage—sometimes rerouting the flow path of the stream in the process. The stream intervention process includes evaluating undamaged sections of the stream above and below the damaged area to gather data on the natural physical dimensions of the stream. These data, combined with hydraulic information, are used to determine the necessary stream dimensions for the damaged area that will allow for a steady base flow and provide excess area adjacent to the stream for flood flow.

Physical dimensions gathered from an undamaged section include:

Once this information is known, it can be compared to Regional Bank Full Hydraulic Geometry Tables, developed by the NYSDEC using information gathered at United States Geological Survey (USGS) gaging stations. Eight geographic regions have been produced for New York State based on the physiographic and geologic characteristics that affect stream flow. These tables should be used for emergencies only.

MAp showing Eight geographic regions that have been produced for New York State based on the physiographic and geologic characteristics that affect stream flow

Stream Bed Restoration

Exactly how a stream bed is to be restored depends upon the slope of the stream. Depending on the slope of the flow path, stream bed and banks would be designed to promote a meandering riffle-pool sequence on slopes less than 2 percent, or a step-pool sequence on slopes greater than 4 percent. The design on slopes between 2 and 4 percent will depend on the existing stream channel characteristics.

Riffle-pool

Photo of a riffle pool

In the riffle-pool sequence, the intent is to develop and maintain a balance in the ratio of the length of riffles to the length of the pools. This helps regulate the higher velocity in the riffles and the slower velocity in the pools where the flow energy dissipates.

Diagram of a riffle pool

Diagram via the National Park Service

Cross section field dimensions for a stable riffle section are shown in the diagram to the left. These dimensions, horizontal and vertical, are important when developing a cross section of the damaged area.

Step-pool

Photo of a step pool

In the step-pool sequence, the flow velocity, and therefore its energy, is dissipated through a series of step pools, similar to a check dam or speed hump in slowing down flow velocities. Step-pool sequences are typically found in areas of steeper slopes like headwaters and narrow valleys.

Diagram of a step pool

Diagram via the National Park Service

Often emergency repairs will not provide adequate time for the development and construction of step-pool sequences. These areas require additional structures and careful monitoring to be successful. Working with the NYSDEC and the SWCD is strongly recommended when developing a step-pool sequence.

In both cases, when redeveloping damaged areas, attempts should be made to utilize on-site debris, such as tree root wads, logs, and brush. Careful planning can also make valuable use of the existing stream bed and flow deposit material.

Dredging

Historically, fixing a stream included widening and dredging to move the water through effectively, and is often the immediate approach to increasing stream capacity. Three things typically occur when a stream is dredged:

  1. A drop is created at some point in the bed in the transition from the natural-bed elevation and the dredged-bed elevation.
  2. As the stream flows over this drop, the upstream edge of the dredged area erodes the bed and the banks of the stream, and migrates upward against the stream flow, known as head cut.
  3. Head-cut erosion results in deposits at the lower end of the dredged area and further downstream. This head cut will continue until the stream finds stability and equilibrium in the new bottom-bed elevation.

diagram of effects of dredging

Diagram via the NYSDEC

The rest of the solution

Any work associated with the stream intervention process will require multiple environmental permits. Having a prepared emergency action plan will facilitate a successful response during flooding events by having pre-established protocols and identifying critical immediate response elements of the area. A documented plan will further facilitate the approval process from lead agencies, potential funding reimbursement after an event, and the coordination of materials, equipment, and labor in a time of emergency.

Emergency Stream Intervention is part of an overall flood response strategy focused on obtaining the best results with minimal negative impacts. Typical protocol for flood response would be developed prior to any flood emergencies as part of the region’s emergency response plan, and would include two levels of priority, immediate and high. Immediate priority are actions required during or immediately after the flood and include the opening of clogged bridges and culverts, opening closed roads, and maintaining the operation of critical resources such as fire stations, power plants, rescue stations, water supplies, sewage treatment systems, and hospitals. Immediate actions address the maintenance of critical functioning services that promote recovery and protect human life. High priority actions are the first step in the post clean-up process and include opening and re-establishing stream channels to prevent additional flooding.

Once critical areas and priorities have been established, further investigation is necessary to determine what can be done. As with any project, documentation is critical for financial reimbursement and future planning and evaluation. Documentation would include sketches, photos, and records of equipment, material and labor and dates and times of any work. Future work or evaluation should be noted in the documentation.

Learn more

If you are interested in learning more on the subject, we recommend reviewing the NYSDEC web site and participating in NYSDEC supported Stream Intervention trainings. The website provides excellent guidance and information associated with planning for and repairing damaged streams. If you do plan on any stream repairs, you should contact your local Soil and Water Conservation District and/or the NYSDEC. They can assist you in multiple ways, including the evaluation, planning, implementation and the monitoring of a project site. No work should be done within a stream without initially determining which permits are required and from whom. Almost all work in a stream will require an approval and/or permit.

References

New York State Department of Environmental Conservation

Post-Flood Stream Intervention Training Manual

Lord, M. et al. 2009. Fluvial geomorphology: Monitoring stream systems in response to a changing environment. The Geological Society of America via the National Park Service

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