riprap

Riprap being extended on Rocky Branch. (STEVE ANGERS PHOTO)

During the 2020 fishing season, I did quite a bit of fishing in the Saco River Watershed — the upper Saco from Saco Lake to the River Street Bridge in Bartlett, and the tributary waters from Willey Brook to the Rocky Branch (near and dear to my heart). It is a marvelous watershed for fishing and recreating.

As we all know, Hurricane Irene took a terrible toll on the health of all rivers in the Saco River Drainage.

Roads washed out and homes were flooded. FEMA, the Federal Emergency Management Agency, provided millions of dollars in relief. One of the areas where funds were expended was to do bank stabilization on rivers that had blown out and created issues with landowners. The method of choice for the bank stabilization was the use of riprap revetments.

As I fished the Saco River Watershed this summer, I noticed that many of the revetments were failing.

This is not surprising, as most riprap has a life expectancy of 10 years. With each high-water-flow event, the water power gets pushed toward the stream bottom and scours the bottom of the river. As the river channel deepens, rock from the riprap collapses into the void. The entire riprap begins to sag. One can look at First Bridge Park to see all this unfolding.

In the FEMA document “Engineering With Nature — Alternative Techniques To Rip Rap Bank Stabilization,” FEMA had this to say about riprap:

”Riprap impedes the natural functions of a riverbank or shoreline, as it interrupts the establishment of the riparian zone, or the point of interface between land and flowing water.

"A properly functioning riparian zone is important for a number of reasons; it can reduce stream energy and minimize erosion; filter pollutants from surface runoff via biofiltration; trap and hold sediments and woody debris, which assists in replenishing soils and actually rebuilding banks and shorelines; and it provides habitat diversity and an important source of aquatic nutrients. Not to mention, a naturally functioning riparian zone simply looks better.”

The report goes on to say, “Problems arise because the effects of riprap do not stop at the point of installation. When positioned along a section of riverbank, for example, riprap has a number of negative impacts on the surrounding environment. Riprap tends to increase the speed of water flow along an armored reach, as the water has no points of friction to come up against and nothing to slow it down.

"This additional strength of flow presents issues further downstream from a riprap protected bank, as water is deflected off the riprap and directed at other points of riverbank. The increased strength and speed of the water only increases erosion suffered at these new locations, the typical result of which is the necessity of installing additional armoring, which merely moves the problem further down the stream.

“Another aspect of riprap is its considerable effect on wildlife, specifically fish that live in and utilize streams and rivers where eroding banks have undergone armoring. While erosion can cause potential problems for fish, especially in high-silt locations, the installation of riprap leads to other, more significant, issues.

"When riprap is the primary or only form of riverbank stabilization measure, the end result is typically a uniform, smooth channel, with no complexity. This means that there are no areas of vegetation either in or overhanging the water, leaving fish at risk from predation. In addition, a lack of riverbank diversity denies fish a place to seek refuge during periods of high-water, which often results in their being washed out of a fast moving system during flooding.”

The report goes on to outline viable alternatives to riprap.

With many riprap projects on the horizon, we need to take a different look at this challenging issue.

Tip Of The Week

Read the entire FEMA report at tinyurl.com/femareportriprap.

Steve Angers, a native to the Conway area, is the author of the book “Fly Fishing New Hampshire’s Secret Waters” and operates the North Country Angler.

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