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Volumetric Estimate. These rough estimates can be contrasted with a simple volumetric estimate of the freshwater flux
needed to maintain a constant salinity (in the absence of wind
mixing), which could be estimated by:
F = (Area • Depth) • [ Sm~S< I *X
where the product of Area and Depth is the volume of the target location, Sm is the marine salinity, St is the target salinity,
and X is the tidal exchange factor. Geometries and the desired
conditions determine all parameters except for the tidal exchange factor. Though the tidal exchange factor will be variable with space (both on/offshore as a function of distance
from tidal inlets, and along the bay axis due to bathymetric
variations) and even time (spring/neap tides, seasonal sea
level fluctuations), a conservative estimate of 15% daily water
exchange for nearshore conditions may be sufficiently representative of mean conditions in Biscayne National Park. Lee
and Rooth (1976) estimated the residence time in southern
Biscayne Bay during the summer months to be on the order
of a week; if it would take seven days for a parcel of water to
be exchanged, that would mean about 1/7 of the volume there
(15%) is exchanged daily, neglecting mixing efficiency. In reality, the tidal mixing factor will be a function of the distance to
the openings to the ocean, the rate of wind-induced mixing,
and the distance from local embayments and shoals which restrict exchange. In contrast to the weekly residence time scale
in Biscayne National Park, residence times in Northern Biscayne Bay are typically a few days (about 33% exchange daily),
and may be as long as many months in Card and Barnes Sound
at the extreme south end of the bay (<1% of waters exchanged
daily by the tides).
A first volumetric estimate is based on SE-6’s seasonallyvariable salinity targets within Biscayne National Park, with
1600 acres within the 250 m zone and 1600 acres within the
250-to-500 m zone, and an average depth of 1.5 ft and 3.0 ft,
respectively. When applied seasonally in the equation above
these figures produce a total annual target flow of 240 K acreft/yr (330 cfs average), given the daily mixing rate for the area
of 15%. Since the volume estimate is directly proportional to
the mixing rate, it is very sensitive to its value. To demonstrate
the sensitivity of this estimate to the size of the mixing rate; if
the estimate was increased to 20% the resulting flows would
be approximately 325 K acre-ft/yr (450 cfs).
The second volumetric estimate presented here is based
on the 10,000 acres of SAV habitat that are found in the WBZ,
which we believe is a preferable target to the 250 m/500 m
salinity targets since it is representative of the geomorphic underpinnings and the ecological potential of the Bay, not just
the distance from the shoreline. The wet season/dry season
salinity targets of 15 ppt/30 ppt over the 10,000 acres of grass
beds included with the same 15% net tidal exchange can be
added to the simple volumetric estimate to give a net total of
about 1.1 M acre-ft/yr (1500 cfs).
Summary of Freshwater Flow Targets
These rough estimates of target flows have produced a range
of values (Table 2) that encompass either the smaller RECOVER target area or the larger 10,000 acre target. The diffusiveprocess-based estimates span the range from the mid-100's K
acre-ft/yr to several M acre-ft/yr, but are sensitive to the magnitude of the effective diffusivities used. As a lower bound on
the problem, it was shown that approximately 100-200 K acreTable 2. Estimates of the average annual flow volume required to reach the salinity ranges that support the biological targets.
Method of Quantification
Estimated average
annual flow volumes
(Kac-ftyr-1)
Target Area
Estimated flow
volume per unit area
(ac-ft yr-1 / acre)
Notes
RECOVER 325 3200 acres 102
Th is estim ate provides flow s fo r RECO V ER SE-6
250/500m regio n and utilizes th e lim ited salinity
observations availab le in th e W BZ; A lle m a n (2003)
RECOVER 475 3200 acres 148
Th is estim ate provides flow s fo r RECO V ER SE-6
250/500m ta rg e t area; M eeder et al. (2002)
Hypersalinity prevention 125 N A NA
Th is estim ate prevents h ypersalin ity in th e Bay but does
not attem p t to satisfy salin ity targe ts
Advection Diffusion 800 -1400 3200 acres 250-438
Th is estim ate is based on a ra n ge o f d iffu sivities
(A = 1 m2/s - A = 12 m2/s) ap p lied using an advection
dispersion eq u a tio n and ap p lied to th e RECO V ER SE-6
250/500 m ta rg e t area
Hydrodynamic Model 1090 -1 0 ,0 0 0 Acres 109
Uses TA BS-M D S m odel to calculate flow s needed to
achieve ca. 1900 p a leo -sa lin ity targe ts from W in g a rd , et
al. (2004)
Volumetric 1100 10,000 acres 110
Th is estim ate provides flow s fo r th e 10,000 ac SA V zo n e
using an estim ated effective tid al m ixin g o f 15%
16 South Florida Natural Resources Center Technical Series (2006:1)
ft/yr are required just to offset evaporation and avoid hypersaline conditions in the bay, so the actual target flows should be
well in excess of that. The volumetric estimates arrived at an
estimate of 1.1 M acre-ft/yr for the full 10,000 acre target area.
This is consistent with other estimates and is supported by
estimates of the flows in the much smaller 3,200 acre target area (325 K acre-ft - 1.6 Maf per year) required to meet
a similar salinity requirement. The fourth column of Table
2 provides the quantity of water per acre calculated to meet
salinity targets, further demonstrating the consistency of the
estimates. Thus the 1.1 M acre-ft/yr, seasonally varying flow
target represents a reasonable estimate of the required freshwater flows and will be adopted as the standard against which
beneficial waters will be measured for this report. This estimate may be supplanted or refined by subsequent analyses
that more properly take into account the dynamic nature of
the flows within Biscayne Bay.
Estimation of Current Flows
Hydrologic pattern has been altered by regional drainage, canal construction and operation, and urban development, as
well as construction of roads, levees, and other hydrologic
barriers to surface flow. The bay currently receives freshwater
inflow almost entirely as surface water in the form of canal
flows, with only minor overland flow and very little groundwater flow.
Groundwater. When there are no surface flows or rainfall
available, groundwater is the only possible source of freshwaters and is vital to counteract the onset of hypersaline conditions. Although the contribution of groundwater to total flows
may have been quite large during pre-drainage conditions as