February 2nd, 2009 Posted in Uncategorized | No Comments »
Spillway Diagram
The main dam on the Togus system is located at the south end of Lower Togus pond, just north of the intersection of of routes 17 and 226 in Chelsea. It is owned by WLA and is the only gated or controlled outlet on the ponds, with a maximum discharge reported at 390 cubic feet per second. The main gateway/spillway is a 25′ long concrete barrier with an upper surface that is about 7′ above the level of the average downstream tailwaters; this spillway represents the lowest desireable pond water level in most situations. The upper surface of a 9″ thick fixed concrete walkway is 32″ above the spillway. This walkway surface represents the “flood” stage of the pond, and serves as our reference “datum”; all water level measurements are given as a distance below the top of the walkway. The free space between these two fixed concrete barriers allows placement of 12″ boards to retain water during dry times of the year. These boards are the major means of water level control on the ponds, allowing a 12″ adjustment to what would otherwise be an unmanaged water level.
A secondary gateway is located at a penstock several feet from the main spillway. This gateway utilizes a number of 6” X 6” stoplogs, each 6’ long, that allow a more precise flow of water to a conduit at the base of the dam, presumably for the original purpose of driving long-removed mill machinery. The stoplogs run in channels extending downward almost to the level of the tailwaters. The secondary gateway will not discharge as much water as the primary in most circumstances, but can be employed to draw down the ponds during low-flow periods for purposes of dam maintenance, etc.; it was a failure at this secondary gateway that resulted in a drop in water level of several feet in 2000. Because there is always a small amount of leakage at these stoplogs some downstream flow is maintained even when pond water levels fall below the spillway level during prolonged dry periods.
The “back dam” is an uncontrolled (ungated) rock/earthen dam located on an undeveloped lobe on the southeast end of Lower Togus Pond. It is about 100’ long and 5’ high. This dam is currently owned by a local construction company. Unlike the primary dam the trees and bushes that have taken root on the back dam are probably beneficial, serving to hold it together. It is a leaky dam, forming a small stream that flows southward before joining Togus Stream below Route 17. Since it’s crest is near datum on the main dam the back dam would serve as a safety spillway during extreme flood events. Damage to or loss of the back dam would lower the pond water levels by up to five feet.
Spillway viewed from downstream
State and municipal laws can be utilized to address individual lake/pond water levels, but in most cases control of the water levels is left to the owner/operator of the dam. DEP provides guidance on water levels at www.maine/dep/blwq/doclake/waterlevel.htm , and will, upon petition, hold a public hearing and ultimately issue an order establishing a water level regime based on potential impacts to safety, access, fish and wildlife habitat, erosion, and water supplies.
On the Togus ponds, WLA management goals have targeted the highest water levels of the year for the period following spring runoff; water levels at this time may approach datum (i.e. “0″ inches) though we try to stay a few inches below this. While the water levels have risen ABOVE datum in recent memory during extreme flood events (e.g. the flood of ‘87) this presents an undesireable situation to the integrity of the dam and also floods a few backyards and low cellars on the ponds. During an average year the water level would slowly drop from a high in March-April of 8″-or-so below datum through the summer. The three main spillway boards would be placed at some time during the summer, though one or all might be removed temporarily in anticipation of heavy rain. Our goal during the spring/summer/early-fall is to maintain a fairly steady, slowly dropping water level throughout these seasons, but without constantly removing-replacing boards every time rain is forecast. Finally, sometime in October all three of the spillway boards are removed to allow the water levels to drop to what should be the lowest levels of the year prior to ice forming; this helps to prevent shoreline ice damage as well as provide for additional storage before the next years’ spring runoff. This 32″ maximum range is what we currently work with, and in normal years the working range is closer to 24″ between seasonal high/low waters. A typical season, then, might see water levels within a few inches either side of 8″ below datum in March-April, dropping to 18″ b.d. in June-July and 28″ b.d. through the fall.
We suspect that water levels measured at the dam are USUALLY lower than the static water level on Togus Pond (upper), sometimes by several inches, because of the lag time involved as the flow finds its way to the outlet. Due to the nature of the drainage, during extremely heavy rain events the flow reverses and water floods northward from Lower Togus Pond into Togus - - in this situation water level measured at the dam would be expected to be slightly higher than at Togus. We haven’t as yet established a water level measuring location on Togus Lake that is correlated to the dam and also accessible to anyone that is interested in recording the level at any given time; perhaps a metering stick can be placed at the WLA boat launch or near Route 105 at some point.
Though not without occasional management gliches and hiccups the Togus ponds have so far avoided the water level controversies that have plagued some lakes and ponds in Maine. Water level maintenance can become a pretty contentious issue. Anyone who has concerns that the water level is either too high or low at any particular time should first confirm whether it is within the target range given above; on rare occasion we may lower the water levels a little more or a little earlier than usual for road or dam maintenance purposes, and hope that folks will understand our reasoning on this. We understand that some properties are low, and so their owners would prefer a lower average water level; other property owners lament the weedy mudflats that stand between their dock and the deep beyond, and so prefer higher levels. We can’t please everyone all the time, but we’ll try to keep the discontent to a minimum as best we can. While the Dam Comittee chairman - - or any of the Association officers - - can be contacted with concerns or comments regarding water levels, the WLA web-site “forum” might also serve as a useful place to float ideas or to vent.
Jon Andrews
This piece will also be archived under “Lake Info”
