Site Evaluation and Classification
In June 1999, the utility company's consultant, Simons & Associates, Inc., presented an Erosion Control Plan (ECP) which described a methodology for classifying and prioritizing sections of river bank for repair (link to report
).This methodology is based on the Rosgen Method (Rosgen, D.L. and H.L. Silvey. 1996. (Applied River Morphology
) to quantify river bank erosion. Simons & Associates’ modified Bank Erosion Hazard Index (BEHI) approach applied a quantitative ranking system based on the river bank characteristics observed during the classification process. This methodology assumes that specific characteristics of a river bank can be inventoried and collectively assessed to determine the bank's susceptibility to erosive forces. These characteristics include:
Bank Material -
The soil/sediment type and characteristics control a significant portion of the forces resisting erosion. For banks that are dominated by non-cohesive sediment particles (sand, gravel, cobbles, boulders), the weight of the individual sediment particles provide the resistive force. For cohesive sediment (clay and silt, to some degree), electrostatic bonds provide cohesion and make these sediments more resistant to erosion than non-cohesive sediments.
Bank Geometry -
The key factors are bank slope, bank height and the presence or absence of a beach below the bank. Beaches provide a buffer zone that dissipates the erosive forces of wave action. Steep banks and those with greater heights are less resistant to erosion than flatter, shorter banks.
In general, river bank vegetation increases bank stability because the root structure of the vegetation binds the soils together. However, in some cases, if a bank is undercut, vegetation, especially trees, can topple over and release large amounts of sediment to the river. Vegetation can help to dissipate the energy of flowing water.
Erosion/Stability Characteristics -
Factors indicative of susceptibility to erosion include overhanging upper banks, teetering or leaning trees, traces of continual sliding of bank material, mass wasting, undercutting of lower banks, and any other noted characteristics.
To document existing river bank conditions and provide data for ranking the erosion sites, Simons & Associates compiled a continuous digital image log of the river bank at near-bank scale using a digital video camera and Global Positioning System (GPS) technology. Observations of river bank conditions were recorded in real-time on one of the audio tracks of the video image. These observations included comments on the four characteristics described above. The equipment used for digital image logging and recording of observations consisted of the following: a boat (provided the platform for the equipment); an integrated sub-meter Differential GPS unit (Trimble ProXRS); a digital video camera linked directly to GPS; and a hardware item linking the GPS and digital video image which allows recording of comments on one audio track of the videotape while the continuous GPS data stream is recorded on the other available audio track.
Maps were then prepared to illustrate the distribution of risk features by category (bank material, bank height, bank slope, degree of vegetation, and other erosion/stability characteristics). A composite map presented the total number of risk features per 100 feet of river bank and indicated relative levels of bank instability.
Using a similar methodology, New England Environmental, Inc. (NEE)
, conducted a full river reconnaissance survey in July 2001 and November 2004(link to report
).At the request of the CRSEC, the November 2004 survey was conducted during leaf-off conditions. Simons & Associates conducted a survey in 2008 (link to report
Site Prioritization Methodology
In general, sites with the highest score indicate either recent or on-going erosion or the potential for continued erosion and, therefore, have the highest priority for repair. The ECP provided a list of 20 of the highest ranked sites which included a total of approximately 15,000 linear feet of eroding river bank. The list of the 20 highest ranked sites has been used by the CRSEC as the basis of the site selection process for bank stabilization projects.
The 1999 ECP established two criteria for prioritizing erosion sites in the Turners Falls Power Pool:
1) the potential and imminent threat to structures posed by an eroding river bank; and
2) the quantity of sediment contributed to the river by an eroding river bank.
In addition, the CRSEC established other criteria which they deemed important in the site selection process, including:
1) protection of agricultural land;
2) conservation of archaeological resources;
3) enhancement of wildlife habitat diversity; and
Committee members also reprioritized sites based on observations of river bank conditions during boat tours and the following criteria,
including: proximity of sites to each other for efficiencies of cost and equipment mobilization/demobilization,
potential access difficulties for heavy equipment, and bank height.