Environmental Risk at Dams

The potential toxicity of sediment impounded behind dams indicates a real need for the collection of current and credible data on the condition of dams, guidance on the sampling assessment, and management of sediment impounded behind dams. The Massachusetts Riverways Program is developing a GIS-based analytical framework, the Index of Environmental Risk, to:

(1) assess the environmental impact of dams on aquatic resources from their contributions to environmental degradation, and

(2) characterize the environmental hazard to aquatic and other public resources from the uncontrolled breach or catastrophic failure of dams.

A first-in-the-nation, this process helps focus scarce public dollars on necessary and environmentally appropriate dam repair and dam removal projects. It will also provide the baseline information necessary for dam owners, municipalities, and grassroots conservation organizations to address dams in their watershed communities

Partners
Massachusetts Environmental Trust
Federal and state agencies
USGS

Opportunities for Volunteers
Volunteers can help collect data to field assess the environmental impact of dams. Click here for more information.

Program Description

The Index of Environmental Risk will serve as a companion to the Massachusetts Office of Dam Safety’s current ‘life and property’ hazard rating system and enable the Commonwealth’s environmental agencies to proactively manage dams and associated water resources to prevent risk to life, property, and the environment. In order to evaluate the relative environmental impact of dams, we have proposed using three metrics that are based on data and attributes that are comparable across watersheds: fragmentation (physical barriers), inundation (acreage and length of habitat transformed from riverine to lacustrine condition), and degradation (hydrologic residence time as a surrogate for relative water quality). In order to evaluate the relative environmental hazard of dams, Riverways is working in collaboration with the USGS to calibrate and validate a model to predict the likelihood of contamination in sediment impounded behind dams, called the Regional Impounded Sediment-Quality Assessment (RISQA).

Of the 2,964 dams in the state’s database, over half are privately owned and nearly a third are municipally owned. The majority of dams in the State (61%) are classified as “low hazard” by the Massachusetts Office of Dam Safety, meaning that they are located where failure or misoperation will cause no loss of life and may cause minimal property damage downstream. Regardless of size, low hazard dams currently require inspection of their condition only once every 10 years. Only 20% of low hazard dams have a reported condition in the state’s database, leaving 1,449 dams where the condition is unknown.

The Index is intended for use as a planning tool for restoring aquatic habitats and preventing environmental damage. The Index will help focus scarce public dollars on necessary and environmentally appropriate dam repair and removal projects and will provide the baseline information necessary for individual dam owners, municipalities, and grassroots conservation organizations to address the future status of dams in their watershed communities.

Development of the Index is the first effort of its kind in the nation to integrate the planning and management of public and environmental safety at dams. Riverways continues to coordinate through the New England Stream Barrier Task Force, which is facilitated by American Rivers, to explore its applicability as a model for other states in the region. It fulfills a “Recommended Action” by The Aspen Institute’s Dam Removal: A New Option for a New Century, a report written in collaboration with experts from around the country, including Massachusetts Riverways and Office of Dam Safety.

INDEX OF ENVIRONMENTAL RISK: BACKGROUND

In Massachusetts, the Office of Dam Safety’s legislative mandate is to assess the probability of hazard based on “loss or damage to life and property” associated with dam failure, and they acknowledge that this restricts their ability to adequately prevent impacts to natural resources. Inspection and evaluation of dams in Massachusetts entails characterizing the structural condition of the dam (good, poor, unsafe, failing), the hydraulic adequacy of the dam (its ability to pass storm flows of various magnitudes), and location of people and structures downstream of the dam (high, significant, low hazard to life and property). Unfortunately, ensuring the safety of people and property downstream of dams does not eliminate all risks to the public and public resources.

For an example of the environmental impact of dams, the “Billington Street” Dam on Town Brook in Plymouth was rated by the Office of Dam Safety as a low hazard structure. However, the partial collapse of the culvert that carried the brook’s flow through this earthen dam rendered its fishway impassable, limiting access to several hundred acres of alewife spawning habitat upstream. Therefore, this dam could have been considered a high environmental impact. As the coastal pilot project of River Restore, federal and state agencies, private foundations, non-profits and corporate partners provided technical assistance and funding to the Town of Plymouth to restore the free-flowing brook. Dam removal was completed in the fall of 2002 through a training exercise by the U.S. Army Reserves under the Coastal America umbrella.

For an example of the environmental hazard of dams, Perryville Pond Dam on the French River is not rated as a high hazard structure by the Office of Dam Safety. However, it is known that the sediment that has accumulated in this former millpond is contaminated with heavy metals and an uncontrolled breach of this dam would release these contaminants into the river downstream into waters under the State of Connecticut’s jurisdiction. Therefore, this dam may be considered a high environmental hazard. Fortunately, a temporary repair was made to the dam in the fall of 2001, in effect “buying time” for a long-term solution to be identified and implemented.

It is difficult to determine the extent to which the Perryville Pond and Town Brook situations are illustrative of Massachusetts dams and millponds without further investigation of existing and potential contaminants in impounded sediment and of biota within the impoundment and in the river downstream. Therefore, Riverways has developed a framework to compile an Index of Environmental Risk to identify environmental hazards, where an uncontrolled breach of a dam could put public environmental resources at risk, and environmental impacts, where dams are contributing to environmental degradation.

Index of Environmental Risk: Summary of Components

Environmental Impact

Dams act as barriers to the movement of resident fish species and migration of anadromous fish species to spawning, nursery, or refuge habitat. Dams also transform free-flowing riverine habitat into slow-moving ponded habitat, decreasing fluvial species and increasing lacustrine species. Dams alter the natural sediment transport regime in rivers, trapping sediment in the impoundment and preventing its continued movement downstream. Impounded waters are characterized by lower oxygen levels and higher water temperatures than free-flowing river reaches.

The environmental impact classification is not always causal to dams, but may be related to the presence of one or more dams. An example of a causal impact would be where the dam presents a physical barrier to fish movement and migration. An example of a related impact would be changes in fish community from fluvial to lacustrine fishes. Water quality changes (temperature, dissolved oxygen, and nutrients) may be causal or related.

Comprehensive decision-making relative to the causal and related impacts of dams would include: 1) landscape-level screening, 2) watershed evaluation and prediction, and 3) site-specific field data. Development of the framework of the Index of Environmental Risk has focused on landscape-level screening, performed at the scale of watersheds. We have proposed using three metrics that are based on data and attributes that are comparable across watersheds: fragmentation (physical barriers), inundation (acreage and length of habitat transformed from riverine to lacustrine, or ponded, condition), and degradation (hydrologic residence time as a surrogate for relative water quality).

Environmental Hazard

When a dam fails and breaches (partially or catastrophically), there is potential for water and sediment impounded behind the structure to move downstream. Potential environmental hazards associated with dam failure can include physical disruption or damage to sensitive aquatic species or aquatic habitats downstream from flood waters (scour) or sediment (deposits of silt or sand). In addition, water quality impacts may result as contaminants are mobilized from impounded sediment and/or downstream land uses that become flooded (industrial sites, wastewater treatment plants, animal waste storage facilities, etc.).

Due to the paucity of modeled and mapped inundation zones likely resulting from dam failure, we have proposed integrating consideration of environmental hazard in local, regional, and statewide plans for hazard mitigation which are currently in the process of being updated. Due to the paucity of site-specific sediment quality data, we have initiated the development of a predictive model to rank the likelihood of contamination in impounded sediment. The model will integrate current and past contaminant loading, fine-grained sediment loading, and the sediment-trapping efficiency of the dam, and assess the degree of influence of various sediment and contaminant sources (both point and non-point) depending on their hydrologic proximity to the dam and its impoundment.

Riverways is working with USGS on the Regional Impounded Sediment-Quality Assessment (RISQA) to test the proposed model against sites of known sediment quality and quantity, in addition to sites representing a range of size and expected contaminant level where sediment quantity and quality assessments will be completed.

The range of expected contaminant severity will be integrated with appropriate aquatic thresholds to generate relative ratings of high, significant, and low environmental hazard of dams in select watersheds. Once confirmed, these ratings can be used to prioritize those locations that are likely to pose a threat to public safety and the environment. More detailed studies of sediment-quality, sediment-transport, and risk assessment can be performed at dams where impounded sediment is likely to pose a high environmental hazard. These data, in turn, can then be used to make informed management decisions concerning impounded sediment.

Reports (For copies please contact Tim Purinton at 617-626-1542):

“Watershed-Scale Assessment of Environmental Impacts and Hazards of Dams:  Opportunities to Integrate the Environmental Hazards of Sediment Impounded Behind Dams into Natural Resource Decisionmaking.” prepared for Riverways by Fuss & O'Neill, Inc.