Coastal waters are continually being impacted by pollution from a wide variety of sources. There is a clear need to understand processes operating in
our coastal waters, from basic sciences (chemistry, biology, geology and physics) through to economic factors and policy decisions. A physical understanding of estuarine dynamics provides the basis for
more complex biological-chemical-economic models used in policy decisions. An additional area of research involves computational
models of the hydrodynamics of estuaries and coastal waters.
As part of a larger project to assess nitrogen transport within
Connecticut's estuaries, we have developed a flow model
for the Housatonic River estuary, which drains a large portion
of northwestern Connecticut and feeds into Long Island Sound.
One goal of the physical modeling is to understand what factors control
residence time of water within the estuary, which in turn effects
water quality. We have added the capability to the flow model to
advect tracer particles within the water column. Particles are
introduced as "clouds" at the start of a model run and the age
of each particle leaving the mouth of the estuary is recorded. In this
way we may vary factors such as 1) river runoff, 2) wind forcing, 3) tidal amplitude and 4) Long Island Sound salinity structure and measure the corresponding influence on mean residence time of
water within the upper and lower basins of the Housatonic. This
figure schematically shows the distribution of particle clouds in the
the upper and lower basins in the Housatonic.
In this
figure we show the evolution of the salinity field (blue=fresh; red=salty) over a tidal cycle. The red crosses represent particles.
As tidal height rises, water fills the estuary and particles are
advected towards the head. As the tide ebbs, water is flushed
from the estuary. For the case of low runoff, particles are trapped
in the upper basin. For high runoff (spring conditions) particles
are rapidly flushed. We find that for summer runoff conditions,
residence time in portions of the deep, upper estuary hole may
exceed 2 weeks.
Residence time in the upper basin is shown increase non-linearly with decreasing river flow. There is also a dramatic increase in residence time between upper and lower basins. These features are shown in these plots of mean residence time of
particles versus runoff magnitudes.
