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Lake Level Management

 

The following information has been made available courtesy of Steve Doyon with the NH DES.

 

DESCRIPTION OF THE WINNIPESAUKEE RIVER BASIN AND CURRENT MANAGEMENT PRACTICES

The Winnipesaukee River watershed, above the location of the streamflow measuring gage in Tilton, is 471 square miles in area. Of this total, 363 square miles (or 77%) lie above the Lakeport Dam.

The following table lists the impoundment and watershed sizes of selected sites above the Tilton gage:

LOCATION

SURFACE AREA OF IMPOUNDMENT

(square miles)

DRAINAGE AREA

(square miles)

Lakeport Dam

71.0

363

Avery Dam (Opechee Lake)

0.8

374

Lochmere Dam (Winnisquam)

6.8

425

Silver Lake

0.34

458

Tilton Streamflow Gage

NA

471

 

HYDROELECTRIC FACILITIES DOWNSTREAM OF LAKE WINNIPESAUKEE

There are 10 sites downstream of Lake Winnipesaukee that use the river flows to generate hydroelectric power, and NHDES has entered into water user contracts with the owners of each of these sites. Under the terms of the contracts, one of the objectives of NHDES is to provide water to these facilities in usable quantities, insofar as ongoing conditions will allow, to increase the generating potential of the hydro operations. It is understood by these water users, however, that NHDES also has obligations to reach and maintain certain target elevations for the purposes of promoting the reasonable use and enjoyment of the lakes and rivers by recreational users, and to minimize the risk and effects of damaging flooding.

In addition to the water user contracts, NHDES currently maintains a lease agreement with Algonquin Power Systems for the use of the Lakeport, Avery and Lochmere Dams consistent with the purpose of generating hydropower at these locations.

NHDES coordinates a shutdown of flows from the Lakeport Dam on an annual basis to allow the developers an opportunity to maintain the generating equipment and the dam structures. Throughout this shutdown period, which generally lasts about 2 weeks, flows from the Lakeport Dam are kept between 5 and 25 cfs, with corresponding flows in the Winnipesaukee River in Tilton of between 40 and 50 cfs.

Listed below are the hydroelectric generating sites currently under contract with the NHDES:

Winnipesaukee River:

Dam

Town

Owner

Turbine Capacity

(cfs)

Number

of

Units

Lakeport

Laconia

Algonquin Power Systems

1,000 – 1,050

3

Avery

Laconia

Algonquin Power Systems

700

2

Lochmere

Belmont

Algonquin Power Systems

1,150 – 1,200

4

Clement

Tilton

Algonquin Power Systems

1,250

1

Riverbend

Franklin

Algonquin Power Systems

1,200

1

Franklin Falls

Franklin

Franklin Falls Hydro

750 – 800

3

 

Merrimack River:

Dam

Town

Owner

Turbine

Capacity

(cfs)

Number

of

Units

Garvins Falls

Concord

PSNH

5,500

3

Hooksett Station

Hooksett

PSNH

1,850

1

Amoskeag

Manchester

PSNH

5,500

3

 

WATER CONTROL MANAGEMENT PROCEDURES

The Winnipesaukee River basin is managed, based upon years of data and experience, to balance the many and diverse interests within the basin. Day to day lake levels and discharges are coordinated to stay within an operating range that best serves these interests. In general terms, stored water is preserved during the summer recreational season and released in the fall to serve the needs of the hydroelectric interests along the basin and to enhance the lake’s ability to safely store flood waters during the typically high runoff months of March through May. During extreme events, the goal of NHDES is to strike a balance between high lake levels and high stream flows, both of which can be significantly damaging.

Listed below is a brief description of how the reservoirs are operated and managed from Lake Winnipesaukee to the stream gaging station site on the Winnipesaukee River in Tilton. All elevations referenced are based upon the NGVD datum.

Lake Winnipesaukee:

Highs:

June 1984 505.89

June 1998 505.53

October 2005 505.25

Lows:

Nov. 2001 501.86

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Water Level and Flow Constraints:

June 1 through Columbus Day (Summer Recreation Season)
To the extent that weather conditions allow, levels are managed between 504.32 and 502.80 to facilitate the use of the lake for recreational enjoyment. Depending upon events and forecasts lake levels are allowed to climb to 504.80 (about 6 inches above full) for short periods of time to allow discharges at Lakeport to be managed to alleviate or lessen downstream
flooding/flood peaks and to avoid exceeding the capacity of downstream hydropower plants. Currently, a considerable amount of judgment is applied when making decisions regarding flow release during these “high water” periods. Since the wasting of water in excess of the hydropower capacity at Lakeport Dam (1,050 cubic feet per second (cfs)) is of concern, the
decision to do it is based upon factors such as: rate of rise (inflow), soil moisture condition, basin vegetation status, precipitation forecast and estimated length of time to return levels to within the normal range. When levels reach 504.50+/-, the number of complaints regarding shoreline structure damage or usability increases. This 71-square-mile lake has hundreds of permanent and seasonal homes with varying degrees of shoreline development When the level rises much above the full pond level of 504.32, impacts begin to occur. There is at least one marina that experiences problems with bridge access (low clearance) when levels begin to climb above full.

Issues associated with the lower end of the summer recreation range relate to hull/motor damage to boats and, like high water, to structure usability. Many abutters are accustomed to a certain range of water levels during the recreation season. However, when the lake is at the lower end of the range, docks and other structures may become less accessible (ex: hull draft is larger than depth at dock). The 250 cfs minimum outflow at Lakeport Dam can sometimes far outpace inflow during the summer recreation season and this, combined with normal summer evaporation, will cause the lake level to typically drop 12 to 15 inches over the course of the summer.

Columbus Day through December 31st
To the extent that weather conditions allow, the lake level is managed between elevations 503.00 and 503.50 during this period by releasing water at a rate that can be fully utilized by the downstream hydropower plants (250 to 1,050 cfs). This is done to facilitate shoreline property maintenance. If deemed appropriate, and based upon basin conditions and weather forecasts, levels will be allowed to climb above this range to prevent releases at Lakeport from exceeding 1,050 cfs. When the level begins to encroach on the full summer elevation (504.32), releases will be increased beyond the capacity of the hydropower plants to make lowering the lake level the first priority. DES also endeavors to reach a target level of approximately 502.80 on December 31st, which is approximately six inches above the full
drawdown depth of two feet. The water stored in this remaining six-inch band is release during the coldest months of January and February.

January 1st through May 31st
To the extent that weather conditions allow, water is released from Lakeport Dam to reach the full 2-foot drawdown (502.32) in the lake by about the end of the first week of March. The actual depth of the drawdown varies from year to year depending upon meteorological conditions; including prevalent base flow rates and snowpack. From March through May 31st the lake level
is allowed to climb as snowmelt, soil moisture and lake ice conditions allow. Discharges at Lakeport Dam are made based on both actual and predicted weather events. Ideally, flows will be maintained between 500 and 1,050 cfs throughout the period. During extremely cold periods, flows lower than 500 cfs cause frazzle ice to form along a steeply sloped reach upstream of
Franklin center, causing maintenance problems and reduced turbine efficiency at the local hydropower stations.

Summary of Operation Under Normal Conditions:
Lake Winnipesaukee is filled to between elevation 504.10 and 504.32 by June 1st. From June 1st to Columbus day, Lakeport Dam is operated as necessary, and in conjunction with other dams along the reach depending upon prevailing conditions, to maintain a minimum discharge of 250 cfs and to keep the lake from rising to more than 6 inches over full (504.80). Natural
meteorological conditions, coupled with the minimum discharge, typically cause the lake to gradually drop during the months of July through October. On or near Columbus Day, a two-week shutdown of flows at Lakeport Dam is initiated to facilitate maintenance in the river reach from the dam to the confluence of the Winnipesaukee and Pemigewasset rivers. After the
shutdown, discharges are returned to between 250 and 1,050 cfs, depending upon prevailing runoff and water level conditions within the basin, to facilitate the generation of hydropower. These increased flows are maintained until the lake level drops approximately 1.5 feet to elevation 503.00. Once this level is reached, the releases are managed so as to provide for a lake level at or slightly above this elevation on January 1st. Additionally, DES endeavors
to maintain a discharge between 500 to 750 cfs through the extremely cold months, as lower flows during these periods promote the formation of frazzle ice, which complicates the operation at some of the hydropower facilities. The objective is to achieve a maximum drawdown elevation of 502.32 in late February or early March, and then begin refilling the lake in mid to late March depending upon forecasts and the extent of the snowpack.

Summary of Operation Under Flood Conditions:
Releases from Lakeport Dam are kept to a maximum of 1,050 cfs (the maximum capacity of the hydropower plant at Lakeport), if possible. If the lake continues to rise or if more rain is forecast, releases at Lakeport Dam are generally increased in stages of 250 - 500 cfs per day, and flows are adjusted at the other dams along the river to keep pace with these releases and local inflow. Flows above 1,050 cfs are achieved by operating the hydro plant at maximum capacity and incrementally opening the dam’s 3 large floodgates. After the peak of the flood has passed, and after flows in the lower portion of the basin have had an opportunity to recede, flows at Lakeport Dam are increased and adjusted to keep the lake level receding
until it returns to the approximate level for that time of year. Any changes to the discharge at Lakeport Dam are made in consideration of the effects of those changes on the downstream reach.

The Weirs channel, which is relatively narrow and may be as little as 5 to 6 feet deep in some areas under full lake conditions, may act to back water up into Lake Winnipesaukee during high runoff events. In addition, the measuring flume just upstream of the Lakeport Dam may inhibit the ability to pass water downstream. However, the capacity of the flume exceeds other
constraints to flow downstream of Lakeport Dam. Therefore, the flume is not considered a limiting factor in flood operations.

Drawdown

Although Lake Winnipesaukee is operated within a 2-foot wide operating band, there isn't a traditional drawdown of this lake as experienced by many other smaller impoundments throughout the state. Rather, the traditional 9 to 12 inch drop of the lake level experienced through the summer is maintained through the Fall season. Then in January, flows are increased at Lakeport Dam to gradually lower the lake to about elevation 502.32 - or 2 feet below full lake. This level is usually reached in late February or early March.