Boott Crest Gates

It provides dam operators with the ability to raise and lower the inflatable gate depending on river flows. For example, during periods of high river flows, the crest gate can be lowered to allow water to flow over the dam’s spillway, thus minimizing pressure on the dam. Similarly, following periods of high river flow, the crest gate can be raised to restore the level of water held above the dam (the headpond) to normal levels.

The proposed Crest Gate can only manage approximately 36,000 cubic feet per second (cfs) river flow or less. Any river flow above this quantity would trigger the Crest Gate to be completely lowered and would not have any influence on water levels, such as the Mother’s Day flood in May 2006 which was a historic flow event of 96,000 cfs.

The head pond is the portion of the Merrimack River located above the PawtucketDam, extending approximately 18 miles upstream into southern New Hampshire.

No, the dam operators only have control up to the height of the Crest Gate, which can be raised up to five feet above the top of the dam. The water levels upstream of the dam are still 100% dependent on the river flow.

Currently, the owners of the Pawtucket Dam install five-foot wooden boards (flashboards) supported by steel pins in the dam’s spillway. This method has been used since the late 1800’s. When fully installed, the boards allow additional water to be held above the Dam. When the volume of water in the river rises to a point that it can go over the flashboards, the steel pins are designed to bend, tipping the wooden boards to protect the Dam.

The Crest Gate system functions very much like the traditional wooden flashboards with one key exception:  the operators have no immediate control over the wooden flashboards.  The boards disengage, resulting in uneven sections along the Dam during high flow events. They can not be replaced until flows decrease to a level deemed safe for workers to physically go onto the dam to replace them.  In contrast, the inflatable Crest Gate system can be raised and lowered evenly or separately along the length of the Dam based on actual or anticipated river conditions, giving operators a level of control unattainable with wooden flashboards.

The installation of the Crest Gate is expected to protect the Dam. Once installed, the Crest Gate will likely put less wear on the Dam than the traditional flashboard system, which requires the insertion of approximately 575, 1 ¾ inch diameter steel pins several times a year into the Dam’s masonry.

For users of the river upstream of the Dam:
Those who enjoy and live along the Merrimack River upstream of the Dam can expect to see improved water level management after the Crest Gate is installed. Most notably, the equipment will allow the Dam to maintain elevations as river flows normalize after a high flow event, as opposed to the current situation where it can take weeks to months for river conditions to reach a level that operators can safely reinstall the wooden flashboards to bring back the impoundment elevation to five feet above the dam crest.

For migrating fish:
Each spring, thousands of diadromous fish (salmon, shad, etc.) begin their annual migration up the Merrimack River from the Atlantic Ocean to spawn. The Pawtucket Dam is the second dam they encounter on their journey, which is why the Project owners have installed state-of-the-art fish passage equipment, including a “fish elevator.” The fish are guided by instinct to swim against the current to reach their destination. However, water falling over a dam’s spillway, such as occurs when flashboards have tipped, can create a “false attraction” as fish are drawn to the movement of the falling water rather than to the attraction flow designed to draw them into the fish passage apparatus. Federal and state fish experts expect that the installation of a crest gate on the Dam will decrease the "false attraction" condition and improve the successful passage of fish.

For the environment:
In addition to enhancing the passage of migrating fish, the Crest Gate will eliminate the usage of and debris from the wood used in flashboards. It will also increase the electrical output of the hydroelectric plant, providing additional KWh of clean, non-polluting electricity to the local energy supply.

For the City of Lowell:
Aesthetically, the Crest Gate will be an improvement over the wooden flashboards. The uneven release of the wooden flashboard sections resulted in the backup of natural and man-made debris behind and between board sections. The Crest Gate will be a uniform elevation across the length of the dam and is expected to minimize unsightly debris patches along the Dam and Northern Gatehouse. This, in turn, will allow the Dam to be protected from harm. The installation of the modern Crest Gate also showcases the endurance and importance of the Pawtucket Dam.

For the hydroelectric project:
The Crest Gate will allow the hydroelectric plant to operate more efficiently, thus incrementally increasing its electricity generation. Harnessing the power of moving water is based on the waters flow or fall. The Crest Gate will allow the water to “fall” from a higher elevation (head) more consistently than under the current flashboard system. In addition, the Crest Gate will eliminate the need to place project personnel on the Dam spillway to replace flashboards – a dangerous task at any time of the year.

The Crest Gate can not predict nor control high water events. During extreme high flow events, above 36,000 cfs, the Crest Gate can do very little to control the water level. However, with the ability to control the height of the Crest Gate – even during moderate high flows – the operators will be able to restore the headpond quickly following a high flow event.

An excellent summary of the Merrimack Watershed is available from the Merrimack River Watershed Council [Link: http://www.merrimack.org/].

Conditions during construction will be similar to what is currently experienced during routine flashboard replacement. Specifically, a portion of the riverbank will be exposed as water levels are lowered by approximately 5 feet. This condition may expose natural and man-made objects imbedded in the River’s streambed. It is possible that as the water surface elevation is lowered, parts of the River that typically store water will take on more riverine characteristics, as opposed to acting like a reservoir.

There will be little to no noticeable impact to the river flow downstream of the Dam.

The summer months are historically the periods of lowest flow in the Merrimack River, which makes it possible to lower the river to the level necessary to safely undertake the construction activity. River levels can not typically be lowered prior to July or after October due to typical annual flow patterns.

The owners would obtain the necessary permits and consultations with agencies and stakeholders. Below is a list of the current permit status:

At the National Level:

• An amendment from the Federal Energy Regulatory Commission (FERC), as required of any federally-licensed hydropower project;
• A permit from the Army Corps of Engineers, as required of modifications to any dam on a navigable waterway.

At the State Level:

• The Massachusetts Historical Commission would have to determine that the Project is “unlikely to affect significant historic or archeological resources”;
• The Massachusetts Executive Office of Environmental Affairs will need to issue a certificate which outlines the results of their findings under the Massachusetts Environmental Protection Act (MEPA). These findings are available for consideration by other Massachusetts permitting agencies.
• The Natural Heritage and Endangered Species Program of the Massachusetts Division of Fisheries and Wildlife will need to issue a “No Adverse Effect” determination;
• A Water Quality Certificate would need to be issued by the Massachusetts Department of Environmental Protection (DEP).

At the Local Level:

• Lowell Conservation Commission would review and issue an Order of Conditions.