The
US Army Corps of Engineers and private consultants working for
the Riverways Program have conducted extensive hydraulic and
hydrologic evaluations of the Baker and T&H Dams to
evaluate flooding considerations. They have concluded that the
Baker Dam causes increased flooding and that the
T&H Dam serves no meaningful flood control purpose. They
have also concluded that removing the dams would reduce
flooding on the Lower Neponset.
The
Baker Dam is particularly problematic since floodwaters
back-up behind the dam, raising floodwater levels during major
storms and causing water to spill over onto the Neponset
Greenway bikepath, the MBTA tracks and during larger floods,
into the Milton Landing area, into the planned Baker Square
Condominium common area near the dam and, under certain
conditions, and into the Baker Square parking area. Making
matters worse, these floodwaters would likely be carrying a
load of toxic river-bottom sediments.
The
Department of Conservation and Recreation recently completed
dam safety engineering inspection reports on the Baker and
T&H Dams. These were the first formal dam safety
inspections completed on these dams in approximately 26 years.
Those inspection reports recommended increasing the flood
hazard classification for both dams to “significant”
because the failure of one or both of these dams would create
additional flooding and may contribute to the loss of life and
damage to property and infrastructure.
The
Baker Dam is more problematic in terms of its failure
potential because of its close proximity to the historic mill
buildings in the Lower Mills area, with the dam actually being
an integral part of the foundations of two of the historic
buildings.
Since
the dams were completed in the mid-1960s, there have only been
two significant flood events on the
Neponset
River
, in 1968 and 1998. Each of these storms was a relatively
modest flood event, at or below the 40-year flood level as
estimated by the US Geological Survey upstream. The Baker Dam
flooded the railroad tracks and bike path during both of these
events, with 1968 being a major overflow and 1999 a more minor
overflow. During 1968, flooding also inundated the Milton
Landing area.
The
recent dam safety inspection reports for the dams indicate
that the Department of Conservation and Recreation, which owns
and operates the dams, has no operations manual for the dams,
no operating records, no emergency operating plans, no
emergency action plan and no design plans or hydraulic data
for either of the dams.
Recently
updated bathymetry data show that the flood storage capacity
of the T&H Dam is only 60 acre-feet, even using the most
optimistic assumptions. Analysis of the hydrograph by Milone
and MacBroom Inc. estimates that, at best, the T&H Dam has
the capacity to store only 0.15% of the 100-year flood volume
and reduce the peak discharge rate by approximately 0.14%.
Expressed another way, if the T&H Dam were completely
drained before the commencement of the storm and the gates
were raised at just the right moment to shave the peak off the
“floodwave,” it would take less than 15 minutes for the
pond to refill.
June
2008
