Possible solutions and alternatives for the future of existing dams
Scenarios
Dam Removal
Dam removal provides full habitat connectivity and fish passage up and downstream. It eliminates the risk of dam failure and avoids long-term maintenance and repair costs. It represents a “one and done” solution to the many aging dams in the region. Removal also allows canoes and kayaks unobstructed passage downstream without the need for long and sometimes dangerous portages (carrying a boat around the dam).
Dam removal requires careful study and engineering to assure that neighboring infrastructure, such as bridges and roadways are not damaged by changes in the river channel. Studies are also needed to understand the impact of lowering the upstream water elevations on wells, upstream wetlands, recreation and private properties along the reservoir.
While dam removal can be expensive up-front, there is no long-term cost or maintenance required once the dam is removed.
Case Studies
Conventional Fishway
A conventional fishway, commonly nicknamed “fish ladder,” is a structure that is built alongside an existing dam that is intended to provide a corridor for aquatic species to swim up and over the dam barrier, then back down. Conventional fishways may be an option where there is a desire to provide some fish passage without removing the dam. Selecting the most appropriate conventional fishway for a given project will depend upon the slope, budget, flow conditions, and target species.
Those unfamiliar with fish ladders may be surprised at the complexities and challenges that must be overcome for these to function well with a variety of species (e.g., river herring, shad, eels) and life stages (juvenile vs. adult). The iconic image of salmon fighting through rapids and jumping over waterfalls does not capture the realities of most migrating fish. Fish vary dramatically in their swimming speed, stamina and leaping ability. The slope, turbulence, placement of resting areas and other design features are essential elements. Unfortunately, a “one-size-fits- all” approach with fishways rarely works. Some fish such as River herring are not capable of leaping over obstacles. American shad can surmount maximum velocities of a fish ladder that are 40% greater than what River herring (blue-back and alewifes) can handle and 8 fold greater than American eel.
A second complication concerns how to attract the migrating fish into a fish ladder. Fish seek particular velocities and pathways through a river channel as they move upstream. If the entrance placement and water flow out of a fish ladder does not match preferred conditions, migrating fish will not use the ladder and can mass in huge numbers at the base of a dam – negating the entire fish ladder. The design of fish ladders that will accommodate multiple fish species and life stages is not yet a mature science and many fish ladders require costly alterations after construction.
While a conventional fishway can provide some fish passage, they do little to address the other impacts of dams. In this scenario, the dam as well as the fish ladder will be visible. If a dam has received a letter of deficiency, the dam would need to be repaired prior to constructing the fishway. In addition to this upfront cost, this scenario requires long-term maintenance of the dam and the fishway. Conventional fishways vary in cost, aesthetics, and performance depending on the design and dam structure. They are not inexpensive and frequently cost upwards of half a million dollars.
There are 3 general types of conventional fishways found within Narragansett Bay Watershed:
Denil fishway – Denil Fishways are typically designed as a switchback with a series of baffles (notched boards) followed by pools to reduce water velocity and provide fish resting areas. The fish swim up each successive baffle to gain height and rest in the plunge pools along the way to conserve energy. The entrance needs a strong flow to attract the fish species to use the ladder.
Alaskan steeppass fishway – The Alaskan steeppass is a prefabricated, modular style of Denil fishway made of aluminum alloy. They are inexpensive to construct and maintain, effective with steep slopes, and easily retrofitted to address fishways that are not working. However, Alaskan steeppass fishways only work with dams that have a minimal drainage area and relatively stable flows.
Weir and Pool fishways – This fishway consists of a narrow artificial channel with an entrance along the river’s edge. It is a series of small overflow weirs (low dams) and pools of regular length. The pools are constructed in the form of steps and these pools are divided by overflow weirs. Fish need to jump from one pool to another to migrate upstream, which can be a challenge for some species. Pool and weir fishways are suitable for many situations, however, they require a large space to construct.
Nature-Like Fishway
A nature-like fishway resembles a natural river with a series of pools and riffles. It consists of a wide, low-gradient channel that is constructed with rocks and boulders that are gradually terraced to make up the height difference between the below-dam and above-dam elevation. Boulders in the channel create multiple pathways that vary in length and velocity to allow multiple fish species to swim upstream. Because nature-like fishways are wide and gradual channels, they also provide improved fish habitat connectivity up and downstream. Nature-like fishways may be suitable for low height obstructions, where upstream water level control is not essential, and if there is a need or desire to preserve the upstream impoundment due to recreation, well water, contaminants or habitat. Nature-like fishways vary in cost, aesthetics, and performance depending on the design and dam structure.
River-Wide Nature Like Fishways – In this scenario, the river downstream of the dam is gradually raised to the elevation of the dam across the whole river channel through a series of rock/pools. Because the fishway spans the whole width of the river channel, the dam is no longer visible. In this scenario, there needs to be sufficient space downstream to build up the channel gradually to the height of the dam. Frequently, the dam is partially lowered to reduce the elevation the downstream channel has to be raised. In addition to full width nature-like fishways, there is an option for partial width fishways that only extend across a portion of the river width.
Bypass Nature-Like Fishways – In this scenario, a new channel is constructed to bypass the dam and connect the river upstream of the dam to the river downstream of the dam. This alternative requires that there is land adjacent to the dam where a channel can be excavated and graded using the rock/pool strategy. It is a good alternative to consider if the dam is a historic structure or there is the desire to protect views of the dam structure itself. If a dam has received a letter of deficiency, the dam would need to be repaired prior to constructing a by-pass channel. In addition to this upfront cost, this alternative requires long-term maintenance.
Keeping and Repairing the Dam
For social, economic, and/or environmental reasons, keeping and repairing a dam is sometimes the option selected for a dam and its community. Some reasons a dam may be preserved include: the presence of contaminated sediment behind a dam; the dam is listed on the national registry of historic places; rare or endangered species are found in the upstream reservoir; and there are multiple dams downstream that block fish passage. Depending on the location of the dam within the watershed and the importance of fish passage, repairs to the dam can be combined with other fish passage structures. For example, if a dam is located in the headwaters (upstream) where anadromous fish passage is not a priority and where the cost of providing fish passage is very expensive, the decision may be made to just repair the dam. However, if the dam is located close to where the river meets the ocean and providing fish passage is a priority but removal is not possible, the dam may be repaired and a fish passage structure may be added. If the decision is made to keep a dam, structural deficiencies must be addressed through repairs and future inspection and maintenance needs to be factored into cost estimates.
With dam repair, the structure of the dam would remain and the existing surrounding ecology would be unaffected. However, unless fish passage structures are added, there would be no fish passage or habitat connectivity above this dam. And while this may be one of the least expensive alternatives in the short term, this alternative requires long-term maintenance so cost over time needs to be considered.
Within the Narragansett Bay and coastal watersheds, there is a need to address aging dams that are in poor condition and in need of repair. Each dam is unique and has different ecological, social, physical and economic factors that need to be considered when exploring solutions. Given that most dams within New England are small and obsolete, there are often a range of alternatives that can achieve multiple objectives. Dam removal is frequently the most cost-effective way to manage aging dams. Removal will restore most natural river functions and ecological connectivity, eliminate future risks of failure, and avoid long term maintenance and repair costs. However, the social, physical and economic aspects of the local community often warrant consideration of alternatives. Conventional fishways or nature-like fishways are often used in combination with either no or partial lowering of the water levels upstream of the dam. Where dams are not removed, repair and long-term maintenance costs and the potential consequences of dam failure to property, infrastructure and livelihoods need to be identified through engineering studies. The future of any particular dam may warrant the exploration of other options that move beyond what is often perceived as just two options of either keeping or removing the dam.
Dams are constructed landscapes that can require creative thinking to address the often competing trade-offs of a decision. Below are a number of future scenarios that have been used within the region to address aging dams. See the “Decision-Making Tools” section to explore how to engage in conversations about the range of alternatives and to explore how they meet the project objectives.