Erosion is a natural process that, in free-flowing rivers,
reaches a state of equilibrium: the amount of sediment lost from one area
equals the amount of soil deposited in another area. This natural cycle
of erosion and sedimentation is beneficial to the aquatic ecosystem. Gravel
beds are created for fish spawning and a series of pools, riffles and
runs also provide habitat. Human use of the river and land within its
watershed that increases the rate of erosion and sedimentation can result
in the loss of aquatic habitat, valuable agricultural soils, damage to
roads, buildings and other structures, and pollutant-laden sediment.
The causes of bank erosion are complex and interrelated
and include: river flow velocities, water level fluctuations, bed and
bank material, pressure imbalances at the bank face, absence of bank vegetation,
obstacles in the stream, waves and boat wakes, freeze-thaw and wet-dry
cycles, and ice and debris.
Banks comprised of sands and silt, like those in the
Turners Falls Power Pool, are easily eroded while cobbles, which are heavier,
and clay, which is more cohesive, are more difficult to dislodge. Pressure
imbalances along the bank face can be the result of groundwater seepage
or the rapid drawdown of water. For example, when the water level of a
river is high for a period of time, water tends to seep into the bank.
When the water level drops rapidly, the bank materials can not drain as
fast, and the remaining water in the bank face increases the outward pressure
on the soil and reduces its stability. Riparian vegetation, such as trees,
shrubs, grasses and herbaceous plants, reduces the velocity of surface
water runoff, helps to filter pollutants, and provides resistance to erosion.
Unvegetated banks and those comprised of sands and silts are also susceptible
to undercutting by water level fluctuations, waves, and boat wakes.
Bank Stabilization Projects in the Turners Falls
The banks of non-cohesive, alluvial sand and silt which dominate the Turners
Falls Power Pool section of the Connecticut River typically exceed
twenty (20) feet in height. Erosive forces have destabilized many
sections of bank resulting in slumping and mass wasting of large
sections of bank and the loss of trees and other riparian vegetation
on the top of the banks. Over the years, various efforts have been
undertaken to reduce the loss of agricultural land, riparian buffer
zones, archaeological resources and private property. In the mid-1970's,
following construction of the Northfield Mountain Pumped Storage
Facility, Northeast Utilities made efforts to stabilize major segments
of the river bank using tree clearing and hydroseeding and some
rip-rap revetment. The Army Corps of Engineers (ACOE) used several
techniques, which were considered innovative at the time, including:
precast cellular concrete block mattress, used auto tire wall and
used auto tire mattress. All three treatments included vegetative
protection on the upper bank.
The license to operate the Northfield Mountain Pumped Storage Facility is issued by the Federal Energy Regulatory Commission (FERC). The language in the license, specifically Article 20, makes the operator of the facility responsible for any soil erosion and siltation that occurs in the Power Pool as a result of construction and operation of the facility. In an effort to comply with their license requirements, NU hired Northrop, Devine & Tarbell to prepare a Master Plan that would inventory and assess riverbank conditions in the Power Pool and recommend a plan of action for mitigating erosion. The draft of this report was issued in June 1991 and was deemed unsatisfactory by many of the stakeholders. There were strong objections to the proposal to stabilize eroding bank with rip-rap and other "hard" engineering techniques because of concerns about the aesthetics, the lack of native vegetation, and the loss of wildlife habitat if these techniques were used. The proposed work was ultimately abandoned when state regulators refused to issue necessary permits for the work.