Dams in the Mohawk Watershed
Benefits with a hazard potential
There is a constellation of hundreds of dams in the Mohawk watershed, and this constellation includes over 30 High Hazard dams and over 50 Intermediate Hazard dams. All of the dams in the watershed were designed and built for water storage, hydropower, navigation, and/or water supply. While almost all are actively maintained and serve an important role, some are deadbeats that are in disrepair – these should be removed because they may pose a safety hazard. Of importance for flooding issues are mainly those dams that are High Hazard and Intermediate hazard, because failure of these dams can have economic consequences and may result in loss of life.
Dams are required to follow either NYS Dam Safety regulations that are enforced through the NY State Department of Environmental Conservation (NYS DEC) or the Federal Energy Regulatory Commission. FERC regulates the transmission and wholesale sale of electricity and natural gas in interstate commerce and they also license and regulate non-federal hydropower projects. In NYS, FERC rules supersede NYSDEC rules.
There is common confusion about the hazard classification of Dams in NYS: it is not based on the soundness or disrepair of a dam, but rather on consequences of failure. There are four classes of dams in NYS: High Hazard (Class C), Intermediate Hazard (Class B), Low hazard (Class A) and No Hazard (Class D). Dams are assigned a hazard classification by the DEC based on the consequences of dam failure. The NYSDEC has four classifications of dams in New York (here paraphrased from the NYS DEC Dam Safety page), they have an awkward letter scheme:
Class "D" or "Negligible or No Hazard" dam. A dam that has been breached or removed, or has failed or otherwise no longer impounds waters.
Class "A" or "Low Hazard" dam. Failure is unlikely to result in damage to anything more than isolated buildings, minor roads, and no significant utilities.
Class "B" or "Intermediate Hazard" dam. Failure may result in damage to isolated homes, main highways and railroads. Failure may result in the interruption of utilities. It may pose the threat of personal injury and/or substantial economic loss or substantial environmental damage. Loss of human life is not expected.
Class "C" or "High Hazard" dam. Failure may result in widespread or serious damage to home, highways, commercial buildings, railroads, and/or important utilities. It may result in substantial environmental damage, such that the loss of human life or widespread substantial economic loss is likely.
Do we need to worry about breach or failure of dams in the Watershed? Absolutely. During flooding from Hurricane Irene (August 2011) and then Tropical storm Lee (Sept 2011) we had nearly ten dams that either breached or had their EAP activated, and thus there were serious issues with flooding and impairment of local infrastructure. The EAP is the Emergency Action Protocol, and this is the plan on file for the actions to be taken if the dam is threatened, breached, or has failed. A primary point of contact when an EAP is activated is the county Emergency Manager.
Breach of Lock E10 during the Irene/Lee event. This is a removable dam on the Mohawk near Amsterdam NY that is a Class B - Intermediate hazard dam (photo JI Garver).
Part of the requirement of licensing for Class C and Class B dams is that a dam break assessment is done to determine inundation and the potential downstream damage that may occur – this generally includes inundation maps of areas that will be flooded. Dam failure analyses are done for Sunny Day conditions and Rainy Day conditions. In the last decade, it has been Rainy Day (or full flood conditions) that have been the most significant in the Mohawk Watershed for dam failure. The issue is what happens during failure when the dam fails during flood conditions – in this scenario there are the flood waters, and then potentially waters released from impoundment. In this type of analysis the flood situation goes from bad to worse.
Many communities in the Watershed live downstream Class B and Class C dams, and in virtually all cases we don’t need to worry about these dams because we have strong regulations in place. However, problems do occur especially when extreme precipitation events (> 2 inches of liquid equivalent, rain in most cases) result in flood conditions. These extreme events are not uncommon, especially here in the Northeast, and thus it seems prudent to evaluate were we are with dam safety.
Perhaps the most significant recent dam concern in the watershed was the Gilboa Dam, a Class C dam on the upper parts of Schoharie Creek, which was found to be deficient in 2005. This finding raised concerns with residents in the Schoharie Valley, and ultimately the Dam Concerned Citizens group was formed to advocate for dam rehabilitation and dam safety. Ultimately NYC DEP spend hundreds of millions of dollars on a full restoration of the dam that was largely completed within the last year.
There are several steps to get started on understanding dam safety. The first is to evaluate the local All Hazard Mitigation Plan (AHMP — here is an example for Schenectady County). Every county in NYS has an AHMP, which is required by Federal Emergency Management Agency (FEMA) if a county seeks funds for hazard mitigation. These county plans have a detailed analysis of floods and flooding, and in the flood section should be a section on Dam Failure (specific to each County, but some dam failure scenarios cross county boundaries). New York State has a State Hazard Mitigation Plan, which is also a good starting point.
In 2013 FEMA put out a publication entitled Living With Dams: Know Your Risks and this PDF describes in a general way the associated risks of dams and guidance for those living near dams. Finally, note that the Dam Concerned Citizens website is a local resource for dam safety information.
This and other Notes from a Watershed are available at: https://mohawk.substack.com/