Datasets:
patrickbdevaney
/
mia9

Dataset card Data Studio Files
xet
 Community
1
text
stringlengths
0
6.44k
may be estimated analytically. It is also possible to derive a
more refined, seasonally-varying volumetric estimate driven
by ecosystem requirements that parameterizes the mixing and
flow in the bay in order to arrive at more robust target flows.
Advection Diffusion Estimate. Due to urban coastal development, the only area in which CERP projects could restore
14 South Florida Naturai Resources Center Technical Series (2006:1)
coastal marsh conditions and natural spatial distribution of
flow to the park is from Deering Estate to Mangrove Point. If
water could be distributed all along the 26 km of park coastline at a steady rate, the one-dimensional advection versus diffusion example, developed in Appendix A, which maintains
a persistent salinity gradient, can be applied. A sufficient net
seaward flow to overcome shoreward diffusive effects along
the park shoreline is over 800 K acre-ft/yr.
Other estimates of required volumes to reach target conditions have been developed independently as well. To just meet
the 250 m- and 500 m-from-shoreline salinity requirements
put forth by RECOVER SE-6, another advection versus diffusion estimate was developed by Downer, Klochak and Mullins
(2005), and Nuttle and Downer (personal comm.). They used
long-term averages of modern salinities measured at several
points at different distances from the coast in Biscayne National Park and an assumed logarithmic shape of the seaward
salinity gradient to arrive at an effective diffusivity of 12 m2/s.
To maintain just the 250 m/500 m salinity targets, they estimated between 0.7 - 1 .4 M acre-ft/yr of freshwater needed to
be provided along the coastline through the marshes between
Shoal and Turkey Points. Since the area considered was confined to the near shore zone, the estimate for the full 10,000
acres would likely be much higher.
Hypersalinity Prevention Estimate. Another type of rough
estimate may be developed by considering the volumes
required to prevent hypersalinity in the bay. The net water
budget is,
^ = P - E- FWln+ FWout + GWinGWout+ SWin + SWM
where V is the total volume of the coastal basin, P is precipitation, E is evaporation, FW is fresh surface water, and GW
is the groundwater volume. The net seawater volume, SWin
- SWout, over several tidal periods will be small except when
there are significant freshwater inputs or outputs, since any
excess of freshwater will be moved to sea, and any evaporation-induced deficit of estuarine water within the bay will be
replaced by seawater if no surface or groundwater is available.
A deficit of water induced by any excess of evaporation over
precipitation (P-E < 0) can be replaced by seawater which
will drive the salinities even higher by adding more salt to the
bay, or by freshwater flows which will maintain or lower the
salinity.
The outcome of this dynamic process depends largely
upon the efficiency with which the tides move seawater into
the bay, mix with the bay waters, and export this mixed water
back to sea. Biscayne Bay is a semi-enclosed shallow basin
with an average depth of about 10 ft and an area of 141,000
acres. All exchange with ocean water is limited to certain areas
(Safety Valve, Government Cut, Baker’s Haulover Cut, Norris
Cut, Bear Cut, and the ABC Creeks), with the 9 km opening at
Safety Valve by far the largest source of ocean waters (Wang
et al. 2003). The tidal mixing in Biscayne Bay is generally
efficient, with a tidal prism (inter-tidal volume) of about 250
K acre-ft - this means that, in theory, the entire volume of the
bay could be exchanged with only six tidal cycles (three days).
In practice the less-voluminous North Bay is even more easily
flushed by virtue of the many cuts opened to the Atlantic, while
South Bay is not flushed as easily, with exchange restricted by
the three narrow ABC Creeks to the east and at the northern
end by the shallow Featherbed Banks that stretch into midbay perpendicular to the long axis to the bay. Consequently,
North Bay has not experienced hypersalinity periods (and was
an oligohaline lagoon before the opening of Baker’s Haulover
Cut irreparably changed it in the 1920’s (Harlem 1979)), while
South Bay frequently has been hypersaline in recent years.
Even with a large annual rainfall, there is a net annual loss
of water to evaporation for Biscayne Bay. Considering the bay
as a whole, the estimated mean evaporation rate of 1.66 m/yr
(Royal Palm measurements) contrasts with 1.27 m/yr (Mowry
Canal, chosen for its proximity to the bay) of precipitation, giving a net evaporative loss estimate of about 180 K acre-ft per
year over the 140,000 acres, or about 1.25 ft per acre. Though
these E and P estimates are highly variable and not applicable
to all areas of the bay, it clearly illustrates the importance of
the distribution of flows, and the different exchange rates at
work in Biscayne Bay. With an evaporative loss of only 16% of
the bay’s total volume, a total mean freshwater input of 1.1 M
acre-ft/yr from canals, about 80% of the bay’s total volume,
should protect against hypersalinity.
For South Bay alone on an annual basis, about 125 K acreft/yr would, therefore, be required to offset the loss of freshwaters to evaporation and prevent hypersalinity only. Most of
this water is required during the dry season or droughts when
precipitation is scarce. During these periods, net salinity increases in coastal waters have been observed in excess of 0.15
ppt per day. This estimate of freshwater flows would prevent
hypersaline conditions, but would not approach target restoration salinities. Even more useful may be its demonstration
of the importance of the timing and spatial distribution of this
flow.
Hydrodynamic Model Estimate. The use of a hydrodynamic
model for Biscayne Bay to estimate the necessary freshwater
flows is advantageous since it can incorporate explicitly the
impact of tidal exchange, mixing, bathymetry, and coastal
currents as well as freshwater flows on the nearshore salinities at different points in the Bay. A 3-D version of the TABSMDS (RMA10; see Brown, et al., 2003) hydrodynamic model
for central and southern Biscayne Bay was recently used by
Alleman and Parrish (2005) to calculate the volume of water
necessary to reach the paleo-salinties estimated by Wingard,
et al., (2004) from cores taken at three sites between Shoal
Point and Turkey Point, two of which are within the proposed
10,000 acre target zone. The freshwater input distribution
from the Natural System Model (NSM462) was increased until the modeled freshwater volumes for the years 1965-2000
produced salinities at these sites that were largely within the
Ecological and Hydrologic Targets for Western Blscayne National Park 15
range of their circa-1900 salinities (Black Point, 5-18ppt;
Featherbed, 25-35ppt; No Name Bank 18-30ppt). Parrish and
Alleman found that the total (surface and ground) freshwater flow rate under such a ‘natural’ distribution necessary to
maintain these salinities at these sites was approximately 1,500
cfs, or about 1090 Kaf/yr on average.