Natural Processes Affecting Stream Bank Erosion:
An Update to "Engineered Solutions to Stream Bank Erosion"

In February of 2001, we reported on stream meanders and processes involved in the formation of point bars and cut banks (See Engineered Solutions to Stream Bank Erosion: A Case Study. The case study examined an area along a reach of the Sandy River in Avon, Maine (Refer to Figure 1 for location maps). At the time of this earlier website posting, we noted that there was concern that erosion along the outside of a meander or river bend appeared to be threatening a nearby home. A proposal submitted to the Maine Department of Environmental Protection (MDEP) included the excavation of a number of gravel bars upstream, at the point bar itself, and downstream of the meander to the low water level. The idea behind this approach was to lessen erosion by increasing the cross-sectional area of the channel and to decrease the amplitude of meandering.

Figure 1

Out of concern that the bars would build back in a short period of time and require ongoing excavation, staff at the Maine Geological Survey (MGS) examined aerial photographs and maps and determined that an alternative approach might be to truncate the point bar by dredging a former channel. We believed that this approach would be more in keeping with natural stream processes, especially since the river had occupied the very channel we proposed to be dredged at some time in the not too distant past. We further believed that the opening of the channel would occur anyway and this action would just be an acceleration of the natural process. The expected result of cutting the point bar with a dredged channel would be the formation of an oxbow in front of the property and much less active erosion along the cut bank.

Although the MDEP was willing to approve our alternative to the original proposal, the applicant ultimately chose not to pursue the project further. A possible explanation for abandonment of this project was that there would have been very little economic incentive for an excavation contractor to participate, especially if their only compensation would be the reduced volume of gravel to be removed from the site. Our proposal would have accomplished the goal of protecting the house and at the same time, remove much less material from the river system than originally proposed.

Little did we realize in 2001 how soon our prediction would become reality. In the spring of 2005, the Sandy River took back the old river channel it once occupied and very effectively bypassed the meander! In effect, the river did exactly what we had proposed. The erosional and depositional rates at this site, identified as the "Voter Site," have been studied in some detail by the Department of Natural Sciences at the University of Maine at Farmington (UMF) and their students since before 2001. Methods of documenting such changes in the river have included aerial and land-based photography and various surveying techniques. A series of aerial photographs compiled by UMF for the years 1951, 1966, 1990, and 2002 clearly illustrate how this reach of the Sandy River has evolved over a period of 41 years (Figures 2-6). Although the river continued to occupy the meander channel in 2002, a small channel cutting across the meander can be readily seen (Figure 6).

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Additional land based photographs taken from 2002 through 2005 also illustrate the dramatic changes occurring with the river (Figure 7). Cross-sections A-A', C-C', and E-E' developed by UMF and their students from 2002 through 2005 clearly illustrate the shift in the thalweg (deepest part of the river channel) to the north over time (Figure 8).

Figure 7

Figure 8

In summary, dynamic rivers and streams, such as the Sandy River which is constantly changing course, provide an excellent opportunity for everyone to actually observe erosion and deposition. Sometimes readily recognizable changes occur on a decade scale. On the other hand, as with the case with the "Voter Site," significant changes in river course can occur much more quickly.

The Maine Geological Survey wishes to acknowledge and thank the Natural Sciences Department at the University of Maine at Farmington for permission to use the various photographs and other information pertaining to this site. Refer to the following link for a brief introduction to some of UMF's research activities concerning erosion and sedimentation processes: Erosion and Sedimentation Dynamics.

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Reference

Shanti, Romany, 2005, Sandy River Point Bar Project 2002-2005, University of Maine at Farmington Powerpoint presentation (unpublished).

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Originally published on the web as the April 2006 Site of the Month.

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Last updated on April 25, 2012

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