By Marc Tardif, 7/3/2013
Last year, the TPA newsletter included an article by Dana Little and Susan Trask summarizing some of the many considerations associated with water level control. The article was largely in response to inquiries the board received from the general membership concerning the extensive flooding we experienced in June of last year. To further address membership concerns, the board established a water level committee with the task of identifying the natural and manmade influences having the biggest impact on water levels and flooding. The ultimate goal of the committee is to determine if viable opportunities exist to reduce the extent and duration of flood events. The board does not endorse control of normal water levels on Taylor Pond, and the water level committee is not engaged with any activity in that regard.
Over the past year, the water level committee has been very active with field surveys and meetings with professionals knowledgeable in hydrology and local conditions. The committee would like to acknowledge and thank the following organizations for their contributions of time and expertise which has led to the preliminary conclusions contained in this report: Stony Brook Land Use Consultants; Jones Associates Land Surveyors; John Field Geology Services; Auburn City Engineers office, Auburn Water and Sewer District, and the Auburn Public Works Department. A substantial amount of information has been provided by these sources and will be made available to view on the TPA website.
Flooding is a function of the broad and complex subject of hydrology. There are three primary factors that affect the extent and duration of a flood event. 1) The amount and rate that water is introduced to the watershed. 2) Storage capacity of the watershed at the onset of precipitation. Before flooding occurs, features in the watershed that are capable of holding water need to fill and overflow. This includes depressions in the land, soil saturation, dams, and the pond itself. 3) The rate at which water is allowed to exit.
Taylor Brook is the primary outlet for water exiting the pond in both normal and flood water conditions. Six features of the brook have been identified from the pond outlet to the Kendall Dam 1.5 miles downstream that affect both conditions in and around Taylor Pond. The brook elevation drops dramatically immediately after the dam, so there is no impact on the speed of pond water level recession from conditions located further downstream.
The first feature effecting the time it takes for water levels to recede is the fact that there are only two feet of elevation drop over the 1.5 mile stretch. The very gradual slope provides minimal energy to move water downstream and away from the pond. Thick vegetation throughout the stream course further reduces flow rates and results in what can be described as a very sluggish waterway.
The second feature of interest can be found a few hundred feet downstream from the pond outlet. Here we find a heavily vegetative area rooted in silt deposits that have raised the bottom of the stream channel. This raised area is referred to as a berm and extends the full width of the brook. The bottom of the channel in the berm area is higher than any other point along the 1.5 mile course. The significance of this naturally created feature is that this is the point where water would stop flowing from the pond and into the brook under receding low water conditions. Water levels below this elevation would be the result of water exiting by ground infiltration, evaporation, and transpiration. The berm has little or no significance relative to flood water dynamics.
The third significant feature is located just downstream of the berm where two culverts are installed at the point that the brook passes under Hotel Rd. Unlike the berm, this feature has no effect on normal water levels. However, under flood water conditions, this feature acts as a dam of sorts that limits pond discharge to the maximum flow capacity of the culverts. Another negative characteristic associated with this feature under flood conditions is that large amounts of water accumulating from the downstream Taylor Brook watershed backs up against the culverts further reducing water discharge rates from the pond.
The fourth feature encountered traveling downstream from Hotel road is a large beaver dam located adjacent to the Granite Mills Estates development. The dam traverses the entire width of the brook, and water elevation drops one foot between the upper and lower sides of the dam. This feature doesn’t have much if any effect on normal water level since its elevation is slightly below the height of the berm. The dam does have some negative impact on mitigating a flood event in that the water volume retained by the dam is volume that is not available for storage of storm water accumulations.
The fifth feature of interest is the slab bridge located on the driveway to the Kendall property. This is probably the most significant manmade influence affecting the time it takes for flood water levels to recede. The bridge acts in the same manner as the Hotel Road culverts by restricting flow rates. The restricted flow at this point exaggerates the backed up water condition at the Hotel Road culverts. The only impact this feature might have on normal water levels in the pond would be the slight increase in the time it takes for water levels to recede.
The sixth and last feature to discuss is the Kendall Dam which is located just below the Kendall driveway bridge. The dam has a higher flow capacity than the bridge, and is equipped with a currently inoperable sluice gate which might be used to further increase flow in a flood event. Flow restriction over the dam is somewhat moot at this time since the upstream bridge is more restrictive than the dam. The dam has little or no effect on normal water levels in Taylor Pond since the elevation of the dam’s spillway is below the berm elevation. The Kendall dam has the same effect as the beaver dam under flood conditions in that the volume of water retained by the dam is volume not available for storage of storm water accumulations.
The information used to prepare this report is reliable and adequately detailed to support the conclusions expressed above. Given the heightened level of understanding we now have, several options to reduce the extent and duration of flood events have been suggested. The most promising options entail methods to increase the flow capacities of the Kendall Road Bridge and Hotel Road culverts. Unfortunately, the existing data we have is not adequate for the purpose of quantifying the extent that any one feature contributes to the overall problem of flooding. If undertaken, the next step in this process would involve an expert analysis to determine benefits which would be realized by modifying existing features. The value of any proposed benefit would need to be weighed against the cost to implement modifications. To be viable, several state and local authorities having jurisdiction would need to be on board with the process. The concerns articulated by Susan and Dana in the 2012 newsletter remain pertinent and should be revisited before additional action is taken.