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6.44k
from the western shoreline (encompassing approximately 6400
acres), the green the area from 2km to 5km from shore, and the
blue >5km from shore. The thin lines denote an envelope of +/-1
standard deviation of the residuals from the fitted curve.
18 South Florida Natural Resources Center Technical Series (2006:1)
substantially less salinity reduction effect, so while it takes a
rate of only 300 K acre-ft/yr (415 cfs) to lower mean salinities
by over 20 ppt over 30 days time in the nearshore region, to
reduce them a further 5 ppt appears to take about 700 K acreft/yr (970 cfs) more. This is mostly a reflection of conservation
of volume - the increased volumes of freshwaters displace the
mixed and marine waters to sea as the bay’s volume stays the
same - coupled with the efficiency of tidal exchange and turbulent diffusion.
The South Florida Water Management Model (SFWMM)
driven by a 35-year time series of environmental inputs was
chosen to represent the current flows in the quantification
of beneficial waters. Choosing a model to represent the current flow conditions serves several purposes: to enable the
reconstruction of flows in the past, to permit these flows to
be comparable relative to some common water management
operations, and to isolate local rainfall effects from the impact
of the water management practices that are at the core of the
estimate of beneficial waters. While a number of reasonable
SFWMM model variants (Alt7r, Base2000, etc.) exist that
could be utilized for this study, model run Alt7r5e was chosen.
It is not expected that the use of any of the modern variants
would significantly change the outcome of the analysis.
Estimation of "Beneficial" Flows
protection of fish and wildlife because all flows that exceed the
target, even if they would be beneficial if delivered at another
time, are excluded from the total beneficial quantity (Fig. 9).
Superimposed on the time series flows are the estimated
target flows discussed in the previous section, both for the
preferred 10,000 acre target (red wet/dry seasonal step function) and the RECOVER SE-6 3,200 acre target (yellow wet/
dry seasonal step function) (Figs. 10 and 11). While most of
Time Series of Flow to Biscayne National Park and Target Flow
The estimate of “beneficial” flow volumes is based on the
methods described in draft CERP Guidance Memorandum 4.
Coastal discharges computed from the SFWMM Alt7r model
runs were compared to the 10,000 acre target flow estimates
to arrive at the classification of beneficial flows in accordance with Guidance Memorandum 4. The beneficial water
calculation of the Guidance Memorandum provides a minimum estimate of the water volume currently available for the
exceedence in %
Figure 9. Exceedence curve for flows to southern Biscayne Bay
based on the SFWMM Alt7r monthly time series (1965-2000) of
coastal flows.
Figure 10. The daily time series of flows into southern Biscayne
Bay (measured= green, modeled Alt7r5e = blue) with the 10,000
acre seasonal volumetric target (red) and 3,200 acre target
(yellow).
Monthly Time Series of Flows to Biscayne Bay
mean flow for 2x2 alt7r = 317.8 (301.2 beneficial) Kaf/yr
mean flow for observed = 490.8 (455 4 beneficial) Kaf/yr
mean flow 10k ac target
mean flow 250/500m targ< t =
1066.4 Ka /yr based on Habitat area
244.2 K if/yr based on RËCOVER PMs
Figure 11. The monthly time series of flows into southern Biscayne Bay (measured= green, modeled Alt7r5e = blue) with the
10,000 acre seasonal volumetric target (red) and 3,200 acre target (yellow). The monthly aggregation reduces the impact of
peak daily flows on the beneficial flow estimate and increases
the volume by approximately 10% of the available flows.
Ecological and Hydrologic Targets for Western Biscayne National Park 19
Monthly Reserved Flows to Southern Biscayne Bay based on Alt7r
397.0 out of 415.8 ± 382.5 Kaf/y is beneficial
140----- 1----- 1----- ,----- 1----- 1----- .-
100 -
o ■ 1 ' u « n u *■ i u i u l u u a i i
1965 1970 1975 1980 1985 1990 1995 2000
Figure 12. The monthly time series of beneficial flows to southern Biscayne Bay through S22, G93, S123, S21, S21A, S20F, S20D,
and S197 as estimated from SFWMM Alt7e. Total average annual
flows are 416 K acre-ft/yr using these criteria.
the flow time series is lower than the 10,000 acre target, it is
clear that in the daily time series (Fig. 10) there are many peak
flows which exceed the target. For the 3,200 acre target, only
about 20% of flows would fit the definition of a beneficial
flow. However, for the preferred 10,000 acre target, about
83% of the flows, (390 K acre-ft/yr of the 475 K acre-ft/yr
total of daily observed flows,) classify as beneficial flows. The
monthly aggregation (Fig. 11) reduces the cropping of the
peaks by about 10%, since the pulsed flows following storm
events rarely last more than a few days and do not have a
great impact on the monthly mean. The monthly aggregation
estimates result in 455.4 K acre-ft/yr out of 490.8 K acre-ft/yr
or 92% of the observed flows being classified as beneficial for
the 10,000 acre target. Figure 12 provides the time series of
modeled beneficial flows. The seasonal distribution of these
monthly aggregations demonstrates that the flows in the dry
season (November-May) are almost always entirely classified
as beneficial, and graphically demonstrates the importance of
the timing of these beneficial flows (Fig. 13).
An alternate view of this same time series is made possible
by using the salinity metric, where the observed and modeled
flows have been transferred to salinity space via the flow volume versus salinity anomaly expression developed earlier and
normalized to a 1km strip of coastal waters along the western
shore to enable comparison. Though limited to 35 ppt (there is
no evaporation in this example, hence hypersaline conditions
are not possible), the daily flows result in salinity that is higher
than the mean of the salinity targets about 75% of the time for
the preferred 10,000 acre target (Fig. 14). The most significant
advantage of using salinity as an ultimate flow target indicator
or metric is not only ecologically critical in the estuary, but
Alt7r Monthly Mean Flows to Biscayne Bay, 1965-2000