Water, water everywhere: past flooding teachers us how to better handle future floods.
Is New York equipped to deal with another Hurricane Irene or Superstorm Sandy? Is our current response after a storm the right approach to ensure the protection of our communities, residents and natural resources?
In planning the appropriate response to a flood, we need to consider stream dynamics. A stream that is in balance with its landscape is able to pass water and sediment during small and large flood events, and then regain its natural shape. Streams are said to be in equilibrium when the volume of water is enough to transport the available sediment without it building up in the channel (known as aggradation) or cutting down the stream channel bed (known as degradation). However, stream sections can become unstable when human activity upsets that equilibrium and alters the stream's ability to move its water and sediment effectively. When this happens, the stream can become a source of flooding for communities located along its banks.
During the floods of 1996 and 2006, the town of Middletown and village of Margaretville, Delaware County were besieged by unprecedented flooding. Margaretville was under several feet of water, and people were stranded in their houses as floodwaters rose. Emergency response personnel and their heavy machinery were immediately brought in to clear stream debris from the East Branch of the Delaware River. This work included wide-scale dredging and widening of the stream channel.
The floodwaters soon receded, and the flood recovery process immediately ensued. It wasn't until several weeks later that the same stream channels that had been cleared were once again filled. Also, the areas where the stream channel had been significantly widened was accumulating substantial amounts of sediment. The stream flow re-directed around these large gravel deposits, scouring the banks and causing further stream bank erosion just downstream.
As demonstrated in these floods, flood responders often go in and "clean out" the stream channel following a major storm. Oftentimes, they remove as much debris as possible so water moves downstream faster and no longer backs up. Many times, the clean-out includes increasing the size (i.e., width and depth) of the stream channel. The idea is that by increasing stream channel width and depth, the stream would no longer flood its banks because all the floodwater would be contained in the stream's channel. However, this traditional post-flood response frequently results in over-excavation, improper channel sizing, channel straightening and disconnection of the stream from its floodplain. Substantial environmental consequences, including lost or damaged aquatic habitat, stream bank failure, channel instability and increased sedimentation can result.
Fortunately, we now know a lot more about stream geomorphology than we did 15 or 20 years ago. While it may appear logical that dredging streams deeper and wider will alleviate flooding, we now know that this approach can actually increase flooding downstream. It can also dramatically affect the stability of the entire stream reach.
When you dredge a stream, a headcut (an abrupt steepening in the streambed) immediately forms on the upstream side of the dredging. As water flows over the area, the streambed erodes, and the headcut continues to move upstream. All the while, a huge amount of sediment from the streambed and banks is released and deposited downstream, altering the natural stream channel and flow. The stream tries to achieve equilibrium with the new stream bottom elevation and slope. The result is erosion downstream of the gravel deposit and outside the dredged area.
Experience with these consequences of post-flood stream work drove the creation of a new and innovative approach to address flooding. Developed by Delaware County Soil and Water Conservation District, and the Department of Environmental Conservation (DEC), with financial support from the New York City Department of Environmental Protection, "Post-Flood Emergency Stream Intervention Training" is a science-based program that incorporates stream dynamics, and hydrologic and hydraulic principles. The program is specifically tailored for people involved in post-flood work, including staff at local soil and water conservation districts, county and town departments of public works, New York State Department of Transportation, and local contractors. Flood responders are given tools, knowledge and practical skills to undertake emergency stream intervention work that won't make things worse, and that will be more likely to survive the next storm.
An important topic covered in the training is priority repairs when public health and safety are immediately threatened. This includes removing flood debris from plugged culvert pipes and bridges, opening up clogged stream channels, repairing or replacing critical infrastructure, and reopening roads. The training program teaches responders to recognize where to work and where not to work, which can help focus and expedite flood recovery efforts.
Training participants are also taught: how streams move water and sediment; the different types of streams and why they take the shapes they do; how floodplains work; and how to design the proper stream channel dimensions that should be re-established after the storm for the stream to be able to properly move its water and sediment (see Figure 1). These dimensions ensure that stream channels are not made too wide or too deep.
When Hurricane Irene and Tropical Storm Lee hit, this approach was immediately put to the test in Delaware County. Municipalities and contractors used their training to quickly identify 37 different sites along two-and-a-half miles of stream that needed intervention. These sites included stream channel avulsions (sudden displacement of a river channel), debris jams and gravel deposition sites that significantly compromised the stream's capacity, which in turn affected water quality, aquatic resources and public infrastructure. Stream channels were re-dimensioned scientifically and streams were reconnected to their floodplains. Today, the 37 sites have either stabilized or are in some form of natural stream adjustment not needing any further remediation--a true testament that this approach works.
Another topic covered in the training is the various structures (such as cross vanes, straight vanes, J-hooks and hardened riffles) that can be constructed to stabilize stream channels and improve water and sediment transport. Planting vegetation is a key strategy for protecting the stream banks. Roots hold the soil in place, and the foliage protects the banks from erosion caused by rain and wind. Building hardened barriers like retaining walls were the go-to solution in the past. However, these structures generally raise water levels and simply shift devastating floods from one part of a community to another.
After a major flood event, everything can't be done at once. The training program provides guidelines to help responders identify what should be done immediately versus what can safely be done later. Ensuring human safety is paramount, but it doesn't necessarily have to be at the expense of fish and wildlife populations. Following the guidelines and recommended procedures outlined in the training program can provide protection for both and eliminate the need for communities or agencies to go back and repair mistakes. This saves time, money and resources--all of which are in scarce supply after a flood.
So when, not if, the next major flood event hits New York, we will be ready. The experience and knowledge we've gained since weathering the effects of storms like Irene, Lee and Sandy have given us the skills to better address Mother Nature's wrath without putting our neighbors, communities and natural resources at further risk.
Measuring channel size and shape
Important stream measurements: bankfull width (red line); bankfull depth (white line); bottom width (yellow line); and floodplain width (green line). Designing proper stream channel dimensions is important in re-establishing water flow. Scientists use two different approaches to do so. The first (and preferred) option is to measure the width, depth and slope of an undisturbed area of the stream, and then duplicate the measurements at impacted sites. The second option is to use the U.S. Geological Survey's StreamStats program to calculate channel dimensions. (See DEC'S website at www.dec. ny.gov/lands/86450.html for more information.)
Thomas R. Snow Jr. is DEC's New York City Watershed and Hurricane Sandy Recovery Coordinator in Albany.
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|Author:||Snow, Thomas R., Jr.|
|Publication:||New York State Conservationist|
|Date:||Dec 1, 2014|
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