Estimates of Flows to

Meet Salinity Targets

RESOURCE 

EVALUATION 

REPORT

SFNRC Technical Series

2008:2

for Western Biscayne National Park

National Park Service

U.S. Department of the Interior

South Florida Natural Resources Center

Everglades National Park 
Estimates of Flows to meet 

Salinity Targets for 

Western Biscayne National Park 

June, 2008 Department of the Interior 
EXECUTIVE SUMMARY 

This paper uses several methods to estimate the flow volume of freshwater needed to reach 

salinity targets in Biscayne National Park. The salinity targets were developed previously, 

based on a determination of desired ecological conditions in a seagrass-dominated area of 

10,000 acres in the western Bay zone of Biscayne National Park (DOI Discussion Paper April 

2006). The seasonally-based salinity targets are: less than 30 ppt from November through 

March, from 15 to 25 ppt from March through August, and less than 20 ppt from September 

through October. 

Analytical and empirical methods were applied to arrive at estimates of the flows necessary to 

reach these target salinities. It was determined that approximately 960,000 acre-feet/year of 

freshwater flows would be required to meet the salinity targets described above in the 10,000 

acre area of seagrass habitat. In the absence of adequate circulation models to provide greater 

detail, an analysis of seasonal targets was done at a basic level: about 37 K acre-ft per month is 

needed during the dry season, and 149 K acre-ft per month in the wet season. 

Recent time series of flows into Biscayne Bay were analyzed. A time-series comparison of the 

target with the existing flows showed that some transient peak-flow freshwater deliveries met or 

exceeded the targets. It is, however, apparent that the stable estuarine conditions desired in 

Biscayne Bay are not achieved by current freshwater inflows, both because the total volume is 

too little and because the timing and distribution are too unnatural. The restoration of natural 

timing of flows could produce stable estuarine conditions, but without an increase in the volume 

of water available, the salinity targets cannot be achieved throughout the year. Increasing the 

total volume of flows, and in particular providing adequate flows throughout the dry season, 

would provide significant benefits to the ecological system in Biscayne National Park. 

1
INTRODUCTION 

In the discussion paper, entitled “Ecological Targets for Western Biscayne National Park” (April 

2006), the Department of the Interior presented descriptive and quantitative ecological targets in 

the estuarine zone based on biological communities in Biscayne National Park. Two key 

elements of the ecological targets paper are pertinent to the development of hydrologic targets 

for Biscayne National Park. 

Element 1: Target Area 

There are many possibilities for a target area for restoration within Biscayne Bay and Biscayne 

National Park. The Southern Estuaries team from the Comprehensive Everglades Restoration 

Plan’s (CERP) Restoration Coordination and Verification (RECOVER) committee developed 

salinity performance measures for Biscayne Bay (see www.evergladesplan.org) that proposed 

and defined such an area. These performance measures suggested a nearshore target area that 

reached 250m from the coastline during the dry season, and 500m from the shoreline during the 

wet season (Figure 1.) This target area is appropriate for some important ecological functions in 

the estuarine zone. However, the 250/500m nearshore bands do not take into consideration all 

available information about the current and historical geomorphology of the Bay which help 

define the extent of the estuarine zone in Biscayne National Park. 

For the current analysis, maps of substrate type in the Bay were examined, as were maps of the 

current distribution of seagrasses within the Park. Substrate type is a good indicator of the both 

the historical and the future bottom community: areas that are currently hardbottom are more 

likely to have supported hardbottom communities such as soft corals and sponges, whereas areas 

that are covered with soft sediment are an indication of the presence of seagrasses in the past. 

Examination of this information revealed an area of 10,000 acres in the Western Bay which 

shows evidence of the influence of significant freshwater flow, and which has supported 

productive estuarine seagrass communities in the past. Anecdotal and paleoecological evidence 

indicates that this 10,000 Western Bay Zone is probably significantly smaller than the estuarine 

area that was affected by freshwater flows in the historical past (Wingard et al., 2004). However, 

the re-establishment of stable estuarine conditions in the 10,000 acre Western Bay Zone would 

provide significant restoration of the natural values of Biscayne National Park. 

Additional information and maps of the geographic areas of the Bay referenced in this document 

can be found in the April 2006 document. 

Element 2: Desired Ecological Conditions for the Western Bay Zone of Biscayne National 

Park 

The desired condition for the Western Bay Zone of Biscayne National Park is defined as a range 

of salinities that is consistently estuarine for support of a productive, diverse benthic community 

based on seagrass. These environmental conditions also support Federally-listed endangered 

species, such as the American crocodile (Crocodylus acutus) and West Indian manatee 

(Trichechus manatus), and create productive nursery habitat that sustains local and regional (e.g. 

Florida Keys) fishery resources. Species which would be supported under these conditions 

include gamefish such as the spotted seatrout (Cynoscion nebulosus), forage fish such as 

mojarras (Eucinostomus spp.), and mollusks like the Eastern oyster (Crassostrea virginica). The 

2
decline during the last several decades of the abundance of these species, as well as the increased 

presence of marine species such as bonefish (Albula vulpes) and permit (Trachinotus falcatus), in 

the Western Bay Zone is thought to be due to the loss of sufficient extent and stability of 

estuarine conditions. 

A more detailed description of desired ecological conditions can be found in the April 2006 

Discussion paper cited above, or in SFNRC Technical Report 2006 (1). It should be emphasized 

that the desired ecological and salinity conditions described for the western Bay zone of 

Biscayne National Park are not equivalent to pre-drainage conditions. Rather, the pre-drainage 

estuarine area is likely to have extended farther east, where submerged aquatic vegetation and 

soft bottom substrate can still be found as far out as Featherbed Bank (Figure 1). 

The current discussion paper utilizes several methods to estimate the freshwater flows needed to 

achieve the desired salinity targets and produce stable estuarine conditions over the 10,000 acre 

Western Bay Zone of Biscayne National Park. 

3
HYDROLOGIC TARGETS FOR BISCAYNE NATIONAL PARK 

Two pragmatic metrics exist for the physical conditions needed to reach the target ecological 

conditions for Biscayne National Park: 1) measurement of salinities in the estuarine zone and 2) 

quantification of the flows themselves through the coastal structures. 

Though quantification of flows is easily attained, how these flows influence the salinity 

distributions throughout the Western Bay Zone (WBZ) is a complex physical question that 

depends on currents, winds, vertical and horizontal shear, insolation, tidal exchange, and mixing 

rates, among other variables. The coastal freshwater flows to Biscayne Bay are almost entirely 

managed and are a calculated parameter in current water management planning tools. We 

explored the link between these managed freshwater flows and the salinity in the WBZ using a 

variety of estimations. 

Salinity 

In several ways, salinity is the best metric to use as a base for the calculation of flows needed to 

produce the target ecological conditions. Evidence of the requirements of a number of species 

presented demonstrates that salinity is a key habitat factor for the bay ecosystem. 

Figure 2 summarizes the optimal salinity ranges for Biscayne National Park ecosystem 

indicators, including primary producers, primary consumers, and predators. While these 

estuarine species can survive at least for short periods in a wide range of salinities, the majority 

of these indicator species prefer salinities between 5 and 20 ppt for growth and reproduction. 

Based on this observation and taking into account that other species (such as seatrout and 

oysters) may require periods of time with slightly higher or lower salinities, we propose the 

following salinity targets to achieve the ecological goals for the WBZ of Biscayne National Park: 

• From November through March (early dry season to late dry season), average daily 

salinities should not exceed 30 ppt. It is particularly critical to measure and track 

salinities during this time period in order to determine the spatial pattern of estuarine and 

marine conditions within the Western Bay Zone. Current salinities in the Park frequently 

exceed 30 ppt in much of the Western Bay Zone during this time period (Biscayne 

National Park, 2006). Re-establishment of salinities under 30 ppt would create 

conditions important to the recovery of important fishery species with life-cycles that 

require estuarine conditions: 

o recreational and commercial fish species that rely on the forage fish for prey, such 

as adult sea trout, as well as snapper and grouper species 

o forage species (mojarras, pinfish), post-larval juvenile shrimp, and oysters, which 

rely on brackish water as a refuge from marine predators. 

• From March through August (late dry season - early wet season), average daily salinities 

should range between 15-25 ppt. This would allow recovery of: 

o key spawning habitat for sea trout, adult habitat for forage species (mojarras, 

silver perch) 

o a healthy, productive, and diverse seagrass community that can be sustained in a 

zone that is subject to freshwater runoff 

o an extensive brackish water refuge from marine predators. Seagrass cover is a 

required feature of nursery habitat for important juvenile fish species. 

4
• From September through October (late wet season), daily salinities should average less 

than 20 ppt. Creation of these conditions would provide: 

o a benefit to juvenile crocodiles that have a stringent physiological requirement 

for low salinity conditions. 

o conditions that promote the recovery of important forage fish species in coastal 

mangroves that do best at oligohaline to mesohaline conditions (such as 

sheepshead minnow, gold-spotted killifish). 

o indirect benefits to all upper trophic level species that consume these forage 

fish in the mangrove zone, including wading birds, mammals, and crocodiles. 

There are considerations other than the average daily salinities which are important ecologically. 

The salinity changes should be gradual and reflect changes in hydropattern that approximate a 

natural system. All vegetation, fish, and invertebrate species benefit from gradual changes in 

salinity that avoid physiological stress. The salinity gradient should extend away from the 

coastline, from lowest salinities nearest the coast to higher salinities towards the sea. 

And perhaps most importantly in an estuary: 

o at no time should daily average salinities exceed 30 ppt. 

This threshold defines estuarine conditions, as compared to marine conditions, and so is a bare 

minimum requirement. Exceeding the threshold of 30 ppt in the Western Bay Zone results in 

environmental conditions that negatively affect all of the Park’s estuarine resources at some 

point in their lifecycle. 

From Salinity Metrics to Estimates of Freshwater Target Flows 

Salinity provides a dynamic link between the biological and physical coastal environments 

because it is an accurate and integral measure of the net results of the total freshwater inputs, 

mixing rate of marine and freshwater flows, wind mixing, net evaporative losses, and amount of 

tidal exchange. The freshwater inflows needed for maintenance of ecologically-required target 

salinities can be calculated in a number of ways. Ideally, a computer-based simulation that 

provides estimates of the spatial and temporal salinity distributions under various conditions 

would be used to arrive at estimates of freshwater flows to meet spatially-dependent salinity 

targets. A verified hydrodynamic model of Biscayne Bay that is forced by observed atmospheric 

and marine inputs and that is coupled with a hydrologic model to provide surface water and 

groundwater inputs would be such a tool. Though tools like this are currently under 

development, at this date an operational tool is not yet available. Therefore, we used several 

alternative approaches, including statistical models, dynamic box models, other modeling 

studies, and static volumetric estimation based on analytic estimates of water budgets and the 

balance of advective/diffusive processes. These different methods provide a range of freshwater 

flow quantities within the WBZ given the salinity targets described above. The limitations and 

advantages of each are discussed and reasonable approximations of freshwater flow quantity are 

provided. 

Spatial Distribution and Timing: Much of the available information on flow is based on current 

canal discharges, which do not mimic natural conditions either in spatial distribution or in 

5
timing. The analyses of required volumes of water included here assume the current distribution 

system, where the freshwater reaching Biscayne Bay is delivered via canals (through structures 

S-22, S-123, S-21, S-21A, S-20G, S-20F, S-20, and S-197). The current distribution system is 

less than optimal for achieving estuarine salinities in the nearshore area of the Bay, because 

much of the freshwater is transported from these point-source discharges via plumes that bypass 

the WBZ and are transported offshore to the marine environment. The optimal distribution 

system to target the WBZ would be that which existed in the pre-drainage past: a large volume of 

surface water elevated inland behind the coastal ridge which induces a large, broad groundwater 

seep into the marsh all along the coastline, with surface waters entering into the bay via many 

dozens of small creeks. This type of distribution system is ideal since it delivers fresh water to 

the WBZ in a highly efficient manner, resulting in a larger impact on nearshore salinities for the 

same volume of water than would be provided by a series of large point-source discharges. 

The desired persistent salinity gradient oriented parallel to the coastline can be most 

economically maintained by the steady flow of waters away from the coast and all along the 

coastline, as would be provided by a coastal freshwater/brackish marsh such as the historic 

coastal wetlands of BISC (this phenomenon is explained more fully in Appendix A). An 

approximately constant freshwater flux is likewise desired at the historic river and creek mouths 

in order to maintain the estuarine salinity targets and avoid ecological damage that is similarly 

caused by cessation of flows or large pulses of freshwaters. Under current conditions, pulsed 

discharges of large volumes of freshwater are typical following large rain events and often result 

in locally low salinities near canal discharge points. The desired spatial and temporal 

distributions apply to all of the target flows derived in this section. 

Flow Volume: 

Until sufficient results are compiled from the desired hydrological models, which are coupled 

to a range of inflow conditions, some alternate performance measures and targets can be 

developed to estimate flow volumes that produce target salinity values. We examine five 

different methods to estimate flow volumes: 1) RECOVER Southern Estuaries sub-team 

performance measures, 2) Advection-Diffusion estimates, 3) Hypersalinity prevention estimate, 

4) TABS-MDW hydrodynamic model estimate, and 5) Volumetric estimate. The RECOVER 

performance measures are currently accepted for use in the design of CERP projects: the 

additional estimates examined here provide information to test the utility of and/or potentially 

modify the RECOVER targets. Estimates 2) and 3) are rough estimates of target flows across 

wet and dry seasons, gleaned from simple calculations of the flows required to maintain a 

persistent salinity gradient parallel to the coastline, and no periods of hypersaline conditions. 

These two estimates are minimal targets, but they must be achieved first in order to reach other, 

more voluminous, desired restoration target flows. The flows needed to achieve the restoration 

targets are calculated in 4) an analytical estimate, and 5) 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. 

1) RECOVER Target Estimates. There are a number of RECOVER performance measures that 

apply to specific areas of Biscayne Bay for which flow or salinity targets have been developed 

by the Southern Estuaries sub-team, and are currently in use as targets for CERP projects. For the 

purpose of estimating target flows for Central to Southern Biscayne Bay, the current Southern 

Estuaries Salinity Performance Measure (PM) is applicable. This PM specifies a persistent 

salinity gradient parallel to the southern coast of Biscayne Bay at 250 m (dry season) and 500 m 

6
(wet) from shore by meeting oligohaline to mesohaline nearshore targets, and it was estimated by 

Meeder et al. (2002) that about 65 K acre-ft/month in the wet season and 21 K acre-ft/month in 

the dry season (470 K acre-ft/yr) are required to meet these salinity requirements. Alleman 

(2003) arrived at a similar figure of 40 K acre-ft/month in the wet season and 23 K acre-ft/month 

in the dry season (325 K acre-ft/yr) for the RECOVER PM targets from a historical data 

analysis, which lends support to the range of this estimate. In addition, the Southern Estuaries 

Salinity PM stipulates persistent flows of 1.25 K acre-ft/month (15 K acre-ft/yr) out of Snapper 

Creek and into Central Bay to maintain the ecosystem found at the creek mouth. Thus the 

RECOVER total for these target flows for South Bay and nearby waters is 66 K acre-ft/month 

for the wet season and 22 K acre-ft/month in the dry season, for a total volume of 485 K acreft/yr. Note that estimates of flows needed to reach estuarine salinity targets for the 10,000 acre 

WBZ will be much greater because it is 5,800 acres larger than the area used in the RECOVER 

performance measure for southern Biscayne Bay. 

2) Advection Diffusion Estimate. Due to urban coastal development, the only area in which 

CERP projects could restore 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 under the aforementioned optimal distribution system, a 

one-dimensional advection versus diffusion approach would be applicable. As developed in 

Appendix A, a persistent salinity gradient can be maintained by balancing the advection of 

freshwater flows away from the coast with the diffusion of salt from the marine waters offshore 

towards the fresher waters inshore. Given these assumptions, it is found that a sufficient net 

seaward flow to overcome shoreward diffusive effects all along the park shoreline is over 60 K 

acre-ft/month, regardless of season, or 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, 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. In light of this relatively high rate of mixing, to 

maintain just the 250 m/500 m salinity targets they estimated between 60-117 K acre-ft/month ( 

700 – 1,400 K acre-ft/yr) of freshwater needed to be provided along the coastline through the 

marshes between Shoal and Turkey Points. Since the area considered for this exercise was 

confined to the nearshore zone, the estimate for the full 10,000 acres would likely be much 

higher still. 

3) Hypersalinity Prevention Estimate. Another type of rough estimate may be developed by 

considering the volumes required to prevent hypersalinity in the bay. The estimated flow needed 

to avoid reaching the hypersalinity threshold gives a lower bound on the amount of freshwater 

needed to maintain living natural resources characteristic of any current areas of the Bay, and 

provides a context for the estimates of the flows required to reach restoration goals. 

The net water budget is, 

dV/dt = P – E –FWin + GWin – GWout + SWin + SWout

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, 

7
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 mid-bay perpendicular to the long 

axis to the bay. Consequently, South Bay frequently has been frequently been observed to be 

hypersaline in recent years while North Bay has not experienced hypersalinity periods. 

Even with a large annual rainfall, there is a net annual loss of water to evaporation for 

Biscayne Bay. Considering the entire 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 acreft per year over the 141,000 acres, or about 1.25 ft per acre. Though these E and P estimates are 

highly variable and not equally 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 average freshwater 

input of 92 K acre-ft/month (1,100 K acre-ft/yr) from canals would at first glance seem more 

than sufficient to protect against hypersalinity. However, parts of South Bay now routinely 

become hypersaline, which indicates that the 1,100 K acre-feet/yr is not distributed adequately in 

time and space. To compound matters, groundwater levels in the Biscayne Coastal Wetlands are 

maintained at artificially low stages to provide flood protection in the urban area, and are even 

further reduced entering into the dry season to benefit agricultural interests – such practices 

ensure that freshwater flows to the bay via groundwater are minimized. 

For South Bay alone on an annual basis, at least 125 K acre-ft/yr would, therefore, be required 

to offset the evaporative net loss of freshwaters and prevent hypersalinity. Most of this water, at 

a rate of about 16 K acre-ft/month, is required during the dry season when precipitation is scarce. 

During these periods, with no rainfall or canal discharges available, net salinity increases in 

coastal waters have been observed in excess of 0.15 ppt per day. These estimates of freshwater 

flows would prevent hypersaline conditions, but would not reach target restoration salinities. 

4) TABS-MDS 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 TABS-MDS 

(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-salinities 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 

8
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 range of their circa-1900 salinities (Black Point, 5-18ppt; 

Featherbed, 25-35ppt; No Name Bank 18-30ppt). Parrish and Alleman concluded that the total 

(surface and ground) freshwater flow rate under such a ‘natural’ distribution necessary to 

maintain these salinities at these sites in South Bay was approximately 1,500 cfs. This 

instantaneous rate equates to about 91 K acre-ft per month to South Bay, or 1,100 K acre-ft/yr.

5) Volumetric Estimate. These 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 – St)/Sm * 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 RECOVER’s wet/dry seasonally-variable salinity 

targets within Biscayne National Park, with 1600 acres within the 250 m zone at 5ppt/15ppt and 

1600 acres within its 250-to-500 m zone at 10ppt/20ppt, and an average depth of 1.5 ft and 3.0 ft, 

respectively. When applied seasonally in the equation above these figures produce dry season 

estimates of a 16 K acre-ft/month, and a wet season estimate of 25 K acre-ft/month, for a total 

annual target flow of 244 K acre-ft/yr, 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 20 K acre-ft and 33 K acre-ft per 

month for dry and wet seasons, with an annual total of 325 K acre-ft/yr for the limited 3200 acre 

area. 

The second volumetric estimate presented here is based on the larger area of 10,000 acres of 

SAV habitat that are found in the WBZ, which we believe is a preferable target to the 250m/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. A similar application of the 

volumetric estimate to the aforementioned wet season/dry season salinity targets of 20 ppt/30 ppt 

(using 20 ppt as the mean of the 15-25 ppt range for the late wet season) over the 10,000 acres of 

grass beds included with the same 15% net tidal exchange provides a dry season estimate of 37 

K acre-ft/month and a wet season estimate of 110 K acre-ft/month. Integrated over a year, the 

9
10,000 acre are therefore requires a net total of about 960 K acre-ft/yr to meet the salinity targets 

outlined previously. 

This second volumetric measurement also provides a means of estimating the amount of 

freshwater flow necessary to just maintain estuarine conditions, <30 ppt, throughout the year. 

Assuming at least an adequate dry season flow volume for 12 months, the volume to prevent 

marine conditions from dominating in the WBZ is estimated to be 440 K acre-ft per year. 

Summary of Freshwater Flow Targets 

These rough estimates of target flows have produced a range of values (Table 1) that encompass 

either the smaller RECOVER target area or the larger 10,000 acre target. The diffusive-processbased estimates span the range from 60 to 120 K acre-ft/month, but are sensitive to the 

magnitude of the effective diffusivities used. As a lower bound on the problem, it was shown that 

approximately 16 K acre-ft/month 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 37 K acre-ft/month in the dry season and 110 K acre-ft/month 

in the wet season 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 (22 K acre-ft / 

66 K acre-ft per month in the dry/wet season) required to meet a similar salinity requirement. 

The dry season monthly estimate of 37 K acre-ft/month also represents the flow required to 

simply maintain estuarine conditions. The fourth column of Table 1 provides the annual quantity 

of water per acre calculated to meet salinity targets, further demonstrating the consistency of the 

estimates. Thus the 37 K acre-ft / 110 K acre-ft per month flow targets represent a reasonable 

estimate of the required dry/wet season freshwater flows to meet ecological targets in the 10,000 

acres area and will be adopted as the standard estimate, at least until such time that subsequent 

analyses are available that more properly take into account the dynamic nature of the flows 

within Biscayne Bay. 

10
Estimates 

Average 

annual flow 

columes (K 

ac-ft per 

year) 

Target 

Area 

(acres) 

Estimated 

flow 

volume per 

unit area 

(ac-ft per 

acre of 

habitat) 

Notes 

RECOVER 325 3200 102 The estimate provides flows for 

RECOVER 250/500m region and utilizes 

the limitd salinity observations 

available in the WBZ; Alleman (2003) 

RECOVER 475 3200 148 Provides flows for 250/500m targets 

area; Meeder et al. (2002) 

Hypersalinity 

prevention 

125 NA NA Prevents hypersalinity in the Bay but 

does no attempt to satisfy salinity 

targets 

AdvectionDiffusion 800-1,400 3200 250-438 Based on a range of diffusivities (A=1 

m2/s to A=12 m2/s) applied using an 

advection-dispersion relation and 

applied to the RECOVER 250/500m 

target area 

Hydrodynamic 

Model 

1090 ~10,000 109 Uses TABS-MDS model to calculate 

flows need to achieve ca. 1900 paleosalinity targes from Wingard et al. 

(2004); Alleman (2005) 

Volumetric 960 10,000 96 Provides flows for 10,000 ac WBZ 

using an effective tidal mixing of 15% 

Table 1. Estimates of the average annual flow volumes required to enter Biscayne Bay 

between the S-22 and S-197 structures in order to reach the salinity ranges that support the 

biological targets. 

Estimation of Current Flows 

The hydrologic pattern in Biscayne National Park 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 predrainage conditions as anecdotal evidence suggests (Kohout and Kolipinski 1967), studies show 

that the modern fresh groundwater inputs into Biscayne Bay are very small (<10% of the surface 

flows; Langevin 2001). In addition, the saltwater intrusion line in south Florida has been stable 

or has encroached further inland over the past two decades (Sonenshein 1995) despite efforts to 

protect the water supply from saltwater intrusion, and hypersaline conditions are commonplace 

during droughts. Both of these observations support the understanding that groundwater flow to 

11
Biscayne Bay is limited under current conditions. Because of the relatively small contribution 

groundwater makes to the total water budget and the limited availability of observed data, 

groundwater flows were not accounted for in this analysis. However, because of the importance 

of groundwater flow during the dry season and in drought conditions, these flows could be 

included in the estimates of mean annual water volume if a reliable means for quantifying the 

groundwater flows to the bay existed. Work underway to estimate groundwater flows may 

provide additional information for estimating comprehensive flow volumes in future analyses. 

Surface Water. Canal flow estimates are derived from the head and tail water elevations across 

the coastal flow control structures maintained by the SFWMD and are stored in its DBHYDRO 

database. The observed flow data from the coastal control structures S197, S20, S20F, S20G, 

S21, S21A, S123, S22, S25B, G93, S26, S27, S28, S29, and S29Z for the time period 1985-2005 

were examined. On average, 1,210 K acre-ft/yr (accurate to about +/- 5%, (Alleman, pers 

comm.)) of total surface freshwater flows enter any part of Biscayne Bay. For just the waters 

entering the boundaries of Biscayne National Park (direct flows through S20F, S20G, S21A, 

S21, and S123 at the northern coastal boundary are included, as are indirect flows from S22 

Central Bay, S20 into Card Sound, and S197 into Barnes Sound all of which eventually pass 

through park waters), the average freshwater flux is much less, about 534 K acre-ft/yr or 44% of 

the total. These flows either directly or indirectly into Biscayne National Park in South Bay will 

be the focus of this discussion. 

Figures 3 and 4 show the volume of flow contributed by each of the structures relative to each 

other. Of the annual average of 534 K acre-ft of canal flows that are discharged to southern 

Biscayne Bay from 1985-2005, 138 K acre-ft (26% of all annual flows) entered directly into 

Biscayne National Park through C-103 (S-20F), 113 K acre-ft (21%) through C-1 (S-21), 73 K 

acre-ft (14%) through C-102 (S-21A), and a minor amount through Military Canal. In addition, 

there were indirect flows to the park waters through C-100 (S-123) (46 K acre-ft, or 9%), and C2 (S-22) (100 K acre-ft, 19%). Additional freshwater eventually enters the park through its 

southern boundary at the entrance to Card Sound. The freshwater in Card Sound and Barnes 

Sound section comes primarily from discharges from the Sea Dade Canal (S-20, 18 K acre-ft/yr, 

3%) and the C-111 Canal (S-197, 28 K acre-ft/yr, 5%) into Manatee Bay in western Barnes 

Sound, with some additional unquantified contributions from overland runoff from extensive 

freshwater and coastal wetlands contiguous with the mainland shoreline of these two basins. 

Because no other significant or quantifiable source of surface or groundwater exists, these 

coastal structure flows into southern Biscayne Bay are considered in this analysis to be the only 

freshwater inflows along the coast. 

The temporal variability of these flows and how they relate to the flow targets outlined above 

is of the utmost importance for the discussion of ecosystem restoration goals. The time series 

(1985-2005) of the South Bay flows (from S-22 in the north to S-197 in the south) and targets is 

shown in Figure 5. The average monthly flows from these input sources are depicted in yellow in 

Figure 6, as are the target flows (red) of 37 K acre-ft / 110 K acre-ft for the dry/wet season in the 

10,000 acre WBZ region. The flows necessary to maintain estuarine conditions are shown as a 

dashed line. The 1st quartile (lowest 25%) of monthly flows, representing typical dry conditions 

during the 20 year time period, is depicted in green. Figure 7 shows the monthly deficit (target 

minus actual) of flows to Biscayne National Park in blue, with the dry conditions’ deficit 

depicted in green. Though the wet season flow deficit is larger, when the same relationship is 

shown in Figure 8 and expressed as a percentage of the total mean monthly flow available to 

South Bay (blue), it is seen that the relative magnitude of the deficit increases throughout the dry 

season, peaking in April at over 250%, and is proportionally higher than the wet season deficit. 

During dry periods (green) these trends remain consistent. During a mean year, the fresh water 

12
deficit is a total of 20 K acre-ft (average of 5 K acre-ft/month) during the early dry season and 

485 K acre-ft (60 K acre-ft/month) during the late dry and early wet season. An inspection of the 

time series and the targets reveals that during the 20-year time period, monthly wet season flows 

met or exceeded the target less than 10% of the time; meeting late dry season targets was even 

more infrequent. Paradoxically, early dry season statistics come closer to the targets due to 

seasonal water management practices that unnaturally reduce groundwater stages in southern 

Miami-Dade by inducing large outflows to Biscayne Bay during November and December (the 

southern “agricultural drawdown”; Kearns et al., 2008). 

Southern Biscayne Bay therefore is thus currently in a state of almost constant water deficit. 

Ongoing deleterious effects on the estuarine organisms within the western reaches of the Bay are 

to be expected, since the estuarine ecological functions in the Bay are inhibited both by the 

shortfall in freshwater volumes as well as the unnatural timing of those limited flows that are 

available. Though it appears that an adequate volume of fresh water is currently available to the 

bay on an annual basis to at least maintain the bare minimum estuarine conditions, the timing of 

this flow is inadequate to do so. 

Salinity. The salinities present in Biscayne Bay are directly dependent upon these freshwater 

fluxes. Under the current water management scheme, large plumes of relatively freshwaters 

(<25ppt) extend away from the canal mouths towards the bay axis during periods of high rainfall. 

These fresher waters are then mixed into the other bay waters and are subject to partial exchange 

with marine waters (35 ppt) through tidal processes. The result in a typical year is an average bay 

salinity less than marine (<35ppt) during the wet season and approaching or exceeding marine 

during the dry season, though during years with less-than-average canal run discharges it is 

common to observe hypersaline (>37ppt) conditions through large portions of southern Biscayne 

Bay, including the western shoreline. 

Time series of salinity data have been collected by Miami-Dade Department of Environmental 

Resources Management (DERM), Florida International University (FIU), and NPS at scattered 

points at different intervals within Biscayne Bay for more than 10 years. The salinity at a given 

station is largely a function of the efficiency of tidal exchange at that location (usually related to 

the distance from the ocean with its typical salinity of 35-37 ppt), the freshwater surface flow to 

the bay (mostly local but some remote influences dependent on location), the time history of 

evaporation and precipitation in the bay, the volume of intra-bay transports, and any wind events 

within the past few weeks that greatly influence mixing rates and on/offshore transports. These 

individual time series offer little help in assessing the synoptic distribution of spatial gradients 

within the bay, and very few are in the WBZ that is the region of greatest interest for salinity 

targets due to their ecological importance there. Taken as a whole, however, these salinity data 

can help elucidate the net result of all the influences on salinities in the bay. 

If these observed data are integrated over 30 days, and grouped by their general location 

within the bay and their distance from the coastline (approximating the effect of both distance 

from the freshwater flows and the ocean influences), some interesting general trends emerge 

when correlated against the integrated observed flows from the coastal structures (Figure 9). 

Nearshore (<2 km from shore, but more than 0.7 km from any canal mouth to avoid aliasing 

from any freshwater plume emanating from it) there is a dramatic decrease in the monthly 

salinity with increasing flow. However, with increasing flows there is a proportionally 

decreasing influence on the salinity, with a fairly well-defined 1/xn

 shape but with a significant 

random error about the mean. Beyond a flow rate of about 25-35 K acre-ft/month there is 

substantially less salinity reduction effect, so while it takes a flow rate of 25 K acre-ft/month to 

lower mean salinities by greater than 20 ppt over 30 days time in the very nearshore region, to 

reduce them a further 5 ppt appears to take about 60 K acre-ft/month more. This is consistent 

13
with the increased volumes required to meet wet season salinity targets, and 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. 

An important conclusion drawn from these results relative to the WBZ is that it would be 

expected that the northeastern corner of the WBZ would be most difficult to affect with 

additional flow volumes. Since this area is the farthest from the shoreline as well as from any 

existing source of fresh water output, this area would be an ideal location for monitoring efforts 

for future restoration programs that seek to redistribute large volumes of fresh water. 

DISCUSSION 

Prior to the significant changes in the freshwater flow patterns in south Florida caused by the 

creation of a water control system in the early 20th century, Biscayne Bay was a true estuarine 

system. Large amounts of freshwater in the form of both surface and groundwater were present 

throughout most of the year and supported a wide range of flora and fauna. When these 

freshwater sources were diminished and their distribution altered by water management 

practices, the vegetation in the bay, as well as the juveniles of many fish and invertebrate 

species, were adversely affected and the ecosystem in the bay changed drastically. The 

ecosystem that exists today in Biscayne Bay is largely marine in nature, as the volume, timing, 

and distribution of freshwater flows are insufficient to maintain an estuarine environment over 

ecologically-significant temporal and spatial scales. In keeping both with the Everglades 

restoration efforts and the NPS mandate to preserve unimpaired the nation’s natural resources 

within the parks, this document provides targets for desired salinity conditions in Biscayne 

National Park in terms of salinity, and provides a range of estimates for the restoration target 

flows required to reach the desired salinity conditions that are necessary for the ecological targets 

within the park. 

The spatial focus of the discussion of ecologic targets includes the Western Bay Zone (WBZ) 

of Biscayne National Park – the 10,000 acre area along the western shoreline which contains the 

portion of the ecosystem that most benefits from freshwater flows. The shallow waters of the 

WBZ contain thousands of acres of seagrasses as well as a fringing mangrove forest. The desired 

condition, or overarching goal, for the western zone of Biscayne National Park is the existence of 

stable estuarine conditions that persist through the dry season, to be achieved through more 

natural timing and distribution of freshwater flows. These stable estuarine conditions support a 

productive, diverse benthic community based on seagrass. These conditions will also support 

endangered species and sustain productive nursery habitat for local and regional fishery 

resources. 

The appropriate restoration area to consider was discussed in this document. The existing 

RECOVER wet season performance measures for Southern Biscayne Bay focus on a narrow 

(500 m) strip of coastline that encompasses 3200 acres of park waters. The more-inclusive 

approach used here is to focus on existing geomorphological information to define an area of soft 

bottom suitable for seagrasses: this approach seeks to extend the area already identified by 

RECOVER to the wider WBZ. This target habitat in the WBZ includes roughly 10,000 acres of 

park area. This larger region was chosen as the target area for stable estuarine conditions because 

it is based on bay geomorphology, a factor that is fundamental to bay ecology. 

The ecological targets for the WBZ were based upon an approach that includes the benthic 

community, endangered species, and important fishery resources in the western bay. Because 

seagrass is important nursery and growth habitat for indicator species, a fundamental resource 

14
management and restoration goal is to maximize coverage by SAV beds at sustainable levels. 

Under appropriate salinity and water quality conditions, it is expected that this area will support 

excellent SAV growth where sediment and water depth are appropriate for such growth. One 

explicit restoration target is an increase in the vitality and diversity of the WBZ seagrass 

community, with wigeon grass as the dominant SAV species at the mangrove edge within the 

nearshore ecotone and shoal grass becoming co-dominant with turtle grass through much of the 

rest of the WBZ. Another explicit target is the restoration of the community of seagrassassociated fauna that have been largely extirpated from South Bay, and the enhancement of 

habitat for others, such as crocodiles and pink shrimp that will likewise benefit substantially 

from the target salinity conditions. 

These ecological targets require freshwater flows that produce mesohaline conditions 

throughout most of the year at the bottom of the bay, with salinities ranging from 5 to 20 ppt 

over the soft bottom areas of the WBZ that have the substrate necessary to sustain SAV. In 

particular, in order to preserve the estuarine character of the WBZ, the measured salinity should 

not exceed 30 ppt anywhere in the zone. The ecological and salinity targets that link mesohaline 

conditions and associated seagrass and faunal communities for this area are not currently being 

met because current freshwater deliveries are insufficient in terms of quantity, timing, and 

distribution. 

Simple volumetric estimates of the restoration target flows to reach these salinity goals in the 

10,000 acres of the tidally-driven system result in monthly flows of 37 K acre-ft/month in the dry 

season and 110 K acre-ft/month in the wet season. This results in a target annual flow of 960 K 

acre-ft/yr. Other types of flow target estimates – diffusive, empirical, semi-empirical – discussed 

in this document fall close to this range as well. In the absence of more complete hydrological 

modeling results which could reduce the range of estimates, the volumetric estimate will suffice 

as a flow target for comparison against the existing flows. Future work should focus on 

hydrological modeling results that will not only help refine the volumetric estimates, but also 

provide information concerning the expected spatial and temporal distribution of the freshwater 

flows, including work to improve the distribution, timing and quantity of flow through the 

coastal wetland and mangrove shoreline areas of the Park. 

The existing flows analyzed here are comprised of the managed water flows through the 

control structures at the end of the canals that empty directly in or adjacent to the WBZ. 

Groundwater flows were omitted from hydrologic analysis in this assessment because the built 

system has vastly reduced them and the likelihood of generalized groundwater increases to 

Biscayne is very small. Groundwater flows could potentially be beneficial in the dry season; 

however, for the last several decades early dry season groundwater flows have been actively 

eliminated from the study area by water management operations. 

A comparison of the canal discharges from S-22 south to S-197 indicated that the waters 

reaching Biscayne National Park are well below the volumes determined by the salinity 

requirements for ecological targets. The mean deficit of fresh water flows to meet those 

restoration salinity targets is 5 K acre-ft/month (20 K acre-ft total) during the early dry season 

and 60 K acre-ft/month (485 K acre-ft total) during the late dry and early wet seasons. The 

percentage of the deficit as a function of the mean monthly volume of water available to BISC 

rises throughout the dry season and peaks in April at over 250%. During dry conditions (when 

canal discharges are within the lowest 25% of flows) these deficits are exacerbated, with the 

April deficit exceeding 350%. The frequency with which the flow targets have been met over the 

period of record is extremely low, less than 10% of the time. 

The historical record of salinity in Biscayne National Park indicates that the current timing 

and distribution of canal discharge waters is largely ineffective at maintaining estuarine 

conditions or even preventing hypersalinity during the dry season. Volumetric estimates of the 

15
required flow to maintain minimal estuarine conditions of <30ppt are 440 K acre-ft per year, 

which is currently available on an annual basis from the water management system but has such 

an unnatural timing and distribution that these flows fall far short of maintaining the estuary. 

Without the pre-drainage groundwater flows and historic creeks that used to provide waters to 

South Bay during the dry season, there is not enough flow to South Bay to prevent evaporationdriven hypersalinity. The situation is even more pronounced in Barnes and Card Sounds, located 

immediately to the south of Biscayne National Park. With tidal inflows restricted to those 

spilling from South Bay over the shallow Cutter Bank at the mouth of Card Sound, characteristic 

long residence times (months), and with few freshwater surface inputs (C-111), Barnes and Card 

Sounds quickly become hypersaline during the dry season and periods of mild drought. 

This paper has discussed the ecological targets for Biscayne National Park and provided 

annual estimates of freshwater flows needed to reach them. A gross estimate of how the annual 

flow is distributed between the wet and dry seasons was also provided. As restoration projects 

develop to provide additional flows to Biscayne National Park, further analysis will be needed to 

develop metrics for the seasonal and interannual variability associated with hydrologic 

restoration targets for the park, as well as to address spatial variability within Biscayne Bay. 

These ecological and hydrologic targets are critical for evaluation of potential benefits of 

restoration projects for Biscayne National Park and to assess progress toward ecosystem 

restoration. 

16
Key Technical Conclusions and Management Implications for Biscayne National Park 

1. To promote restoration of estuarine habitats (seagrasses) and estuarine species, salinities 

in the Western Bay Zone should range between 15-25 ppt from March through August 

(late dry season-early wet season), and should be consistently under 20 ppt during the end 

of the wet season (September-October). This report uses a variety of estimates to 

conclude that, given the current drainage canal-based distribution system along the coast, 

the volumes of water required to reach these targets are approximately 37 K acre-ft per 

month from December through April, and 110 K acre-ft per month from May through 

November, for a total annual volume of 960 K acre-ft. 

2. To maintain minimal estuarine conditions, the fresh water reaching the southern Bay 

must have sufficient volume, adequate timing, and effective distribution to maintain 

salinities of less than 30 ppt (daily average) all year round in the Western Bay Zone. 

Salinities in the Western Bay Zone currently surpass this threshold, and cause a loss of 

estuarine ecological function. This loss of estuarine function may be reversed given 

adequate changes in fresh water deliveries to the Bay. 

3. Analyses of existing flows indicate that essentially all the water currently reaching 

Biscayne National Park via the current distribution system is needed by the ecosystem to 

reach desired restoration conditions, including a healthy benthic community, endangered 

wildlife (American Crocodile) and important fishery resources in the Western Bay Zone. 

4. Modifications to the distribution system that will produce a steady flow of waters away 

from the coast all along the shoreline, are needed to most efficiently create estuarine 

conditions with a given volume of water. These modifications will also serve to avoid the 

ecological damage that is caused by rapid cessation of flows due to management practice 

or large point-source pulses of freshwaters following storm events. 

17
FIGURES 

Figure 1. Biscayne National Park, showing the Western Bay Zone that was described based on the 

current and potential distribution of submerged aquatic vegetation on the bay bottom. 

18
Figure 2. Optimal salinity ranges (units in ppt) for Biscayne National Park ecosystem indicators. 

Figure 3. Average distribution of total annual canal flow (1,210 K ac-ft) to all of Biscayne Bay by 

SFWMD structure for 1985-2005. The highlighted portions represent those structures which discharge a 

total of 534 K ac-ft into Southern Biscyane Bay. 

19
20

Figure 4. Location and the annual average (percent of total) of canal discharges to southern 

Biscayne Bay. 
Figure 5: The monthly flows to Biscayne National Park from 1985-2004. The blue are 

observed flows in K acre-ft/month, while the red are the flows required to meet salinity and 

ecological targets. The time series shows that target flows are met only 8% of the time in the 

wet season, and 4% of the time in the dry season. The green line represents a minimum flow 

that would be required to just barely maintain estuarine conditions throughout the year. 

21
 Figure 6: The average monthly flows into Biscayne National Park waters (yellow), the 

target flows required to meet ecosystem goals (red), the average monthly flows during 

dry periods (green; for the lower quartile of flows). 

22
Figure 7. The average monthly flow deficit (target minus actual) for Biscayne 

National Park is depicted in blue; the deficit for the driest 25% of the record is 

shown in green. 

23
Figure 8. The average monthly flow deficit (target minus actual) for Biscayne 

National Park expressed as a percentage of the average flows available for each 

month for all conditions (blue) and the driest 25% of the period of record (green). 

24
Figure 9. Observed south Biscayne Bay salinity data integrated over 30 days and grouped 

according to their distance from the coastline vs. flow rates expressed as K acre-ft/yr. the 

thick red curve denotes the area away from canal mouths but within 2 km from the western 

shoreline (encompassing approximately 6400 acres), the green line denotes the area from 2 

km to 5 km from shore, and the blue line denotes >5km from shore. The thin lines denote 

an envelope of +/-1 standard deviation of the residuals from the fitted curve. 

25
LITERATURE CITED 

Alleman, R. 2003. Historic salinity contours. PowerPoint presentation to Regional Evaluation 

Team Southern Estuarine Team on May 21 2003. South Florida Water Management District, 

West Palm Beach, Florida. 

Alleman, R. and D. M. Parrish. 2005. Hindcasting salinity in Biscayne Bay. Pp. 201-202. In: 

Proceedings of the Florida Bay and Adjacent Marine Systems Science Conference. University 

of Florida, IFAS. December 11-14, 2005. Duck Key, FL. 

Biscayne National Park. 2006. Salinity sampling in Biscayne Bay. Annual Report. A report to 

the U.S. Army Corps of Engineers for the Monitoring and Assessment Plan of the 

Comprehensive Everglades Restoration Plan for RECOVER Assessment Team, Southeast 

Estuary Subteam. Homestead, FL. 

Brown, G.L., R. McAdory, G.H. Nail, M. S. Sarruff, C. Berger, and M.A. Granat, 2003. 

Development of Two-Dimensional Numerical Model of Hydrodynamics and Salinity for 

Biscayne Bay, Florida. US Army Corps of Engineers Coastal and Hydraulic Laboratory 

Report ERDC/CHL TR-3-10. 173 pp. 

Downer, C., J. Klochak, and T. Mullins. 2005. Biscayne Bay Coastal Wetlands Alternative P 

Specifications. Prepared for Comprehensive Everglades Restoration Program, Biscayne Bay 

Coastal Wetlands Project Delivery team. 

Department of the Interior 2006. Ecological Performance Measures for Western Biscayne 

National Park. Discussion paper prepared by the office of the Director of Everglades 

Restoration Initiatives, Office of the Secretary, Florida International University, Miami FL. 

Kearns, E.J., A. Renshaw, and S. Bellmund, 2008. Environmental Impacts of the Annual 

Agricultural Drawdown in Southern Miami Dade County. Joint Assembly of the American 

Geophysical Union, Ft. Lauderdale, FL. 

Kohout, F. A. and Kolipinski, M. C. 1967, Biological zonation related to groundwater discharge 

along the shore of Biscayne Bay, Miami, Florida; In: G. H. Lauff (Ed.). Estuaries: American 

Association for the Advancement of Science, Publication no. 83: 488-499. 

Langevin, C. D. 2001. Simulation of Ground-Water Discharge to Biscayne Bay, Southeastern 

Florida. U.S.G.S. Water-Resources Report 00-425, 127pp. 

Lee, T. and C. G. H. Rooth. 1976. Circulation and Exchange Processes in Southeast Florida’s 

Coastal Lagoons. Biscayne Bay Symposium, University of Miami Sea Grant Report No. 5. 

Rosenstiel School of Marine and Atmospheric Science, Miami, FL. 

Meeder, J. F., P. W. Harlem, and A. Renshaw. 2002. Restoration of the Black Creek Coastal 

Wetlands and adjacent nearshore estuarine zone of Biscayne Bay. Report to the South Florida 

Water Management District, West Palm Beach, Florida. Southeast Environmental Research 

Program, Florida International University. 

SFNRC Technical Report 2006 (1). Ecological and Hydrologic Targets for Western Biscayne 

National Park. South Florida Natural Resources Center, Everglades National Park, 

Homestead, FL. SFNRC Technical Series 2006:1: 25 pp. 

Sonenshein, R. S. 1995. Delineation of Saltwater Intrusion in the Biscayne Aquifer, Eastern 

Dade County, Florida. USGS Water-Resources Investigations Report, 96-4285. 

26
Wang, J.D., E. Daddio, M.D. Horwitz, 1978. Canal Discharges into South Biscayne Bay. Report 

to the Department of Environmental Resources Management, Metropolitan Dade County. 57 

pp. 

Wang, J. D., J. Luo, and J. S. Ault. 2003. Flows, salinity, and some implications for larval 

transport in south Biscayne Bay, Florida, Bulletin of Marine Science 72: 311-329. 

Wingard, G. L., T. M. Cronin, C. W. Holmes, D. A. Willard, G. Dwyer, S. E. Ishman, W. Orem, 

C.P. Williams, J. Albietz, C. E. Bernhardt, C. A. Budet, B. Landacre, T. Lerch, M. Marot, and 

R. E. Ortiz. 2004. Ecosystem history of southern and central Biscayne Bay: Summary report 

on sediment core analyses – Year Two. Open File Report 2004-1312. U.S. Geological Survey. 

27
APPENDIX A: ADVECTION VERSUS DIFFUSION 

A one-dimensional flow of water and salt in the x direction can be expressed in a steady-state, 

vertically-mixed form as: 

D(US)/dz = d/dx (A dS/dx) 

where S is the salt content, U is the horizontal velocity, and A is the horizontal turbulent 

diffusion coefficient. If one assumes that U is independent of the distance x from the coast 

(which is a very reasonable assumption for a flow distributed all along a coastline, and an 

unreasonable assumption for a point source flow), and that A is likewise independent of x (a 

poor but pragmatic choice) then: 

U dS/dx = A d2S/dx2

Given the assumptions, the analytical solution is exponential. The importance of this solution is 

that, in the absence of other transient forcing, a steady flow offshore gives a persistent 

exponential gradient located near the coast. As the speed of the flow increases, this gradient will 

move farther offshore and will become sharper (larger magnitude). As the mixing becomes more 

intense or efficient (i.e., the magnitude of A increases) the gradient will move closer to shore and 

the gradient’s magnitude will decrease. The ratio of A/U is the length, or e-folding scale, and, as 

such, is a good estimate of the width of the offshore gradient region. While the velocity U along 

a coastline can be determined by metering out a known volume of water at a known rate along a 

length of shoreline, the horizontal turbulent diffusion coefficient A is not as simple and is often 

several orders of magnitude greater than equivalent molecular diffusivities. It is a function of the 

flow and resulting friction in the area and, as such, will be dependent on the tides, winds, and 

topography, and can vary by several orders of magnitude. 

The advection dispersion estimate provided on page 21 is derived from a horizontal diffusivity 

of A = 1 m2/s and steady offshore velocity U= 0.001 m/s for 26 km of coastline with an average 

depth of 1 m. The value of the diffusivity A has been shown by Wang et al. (1978) to vary from 

0.5 m2/s to 5 m2/s along the western shoreline, producing a theoretical range of net offshore 

velocities from 0.0005 m/s to 0.005 m/s. These velocities translate to freshwater fluxes of 400 K 

acre-ft/yr to 4 Maf/yr, respectively – a considerable span of values. However, the diffusivity is 

highly variable with time and space, including dependencies on wind speed, current speed, water 

depth, and the distance to the shoreline. Since the shallow areas adjacent to the coastline are not 

subject to the largest tidal velocities and wind/wave effects, they will likely have effective 

diffusivities on the lower end of the range in all but the most extreme (storm) events. The 800 K 

acre-ft/yr target flow estimate was arrived at by a conservative evaluation of these factors and 

assuming an average diffusivity in the Western Bay Zone of 1 m2

/s. 

28
 United States Department of the Interior 

 National Park Service 

 Biscayne National Park Everglades National Park 

 9700 SW 328 Street 40001 State Road 9336 

 Homestead, FL 33033 Homestead, FL 33034

In reply refer to: L54 

July 2, 2008 

Ms. Carol Ann Wehle 

Executive Director 

South Florida Water Management District 

3301 Gun Club Road 

West Palm Beach, FL 33406 

Dear Ms. Wehle: 

The National Park Service received a letter from your agency in April of 2005, requesting technical 

information relevant to the establishment of reservations for Biscayne Bay. In response to this letter 

and to more recent ongoing conversations with your agency, the National Park Service has developed 

the attached document, entitled “Estimates of Flows to meet Salinity Targets for Western Biscayne 

National Park”. The present document represents technical work that follows on a previous 

document, titled “Ecological Targets for Western Biscayne National Park”, transmitted to your agency 

on June 16, 2006. We hope that these technical analyses will be helpful as your agency addresses 

projects that affect Biscayne National Park resources, including water management operations, Florida 

State water law processes such as reservations and Minimum Flows and Levels, and the design of the 

Biscayne Bay Coastal Wetlands project. 

The technical analysis in the attached document supports the National Park Service’s broad 

responsibility for the preservation of our nation’s natural and cultural resources. In the context of the 

ecosystem restoration efforts in South Florida, this translates into the responsibility for determining the 

ecological and underlying physical conditions that represent the restored natural resources of the South 

Florida Parks. 

The attached document represents a joint effort by Biscayne National Park resource management staff 

and staff at the South Florida Natural Resources Center at Everglades National Park. During the early 

evolution of this document, valuable comments and input were received from staff at the U.S. Fish and 

Wildlife Service Office of Ecological Services in Vero Beach. 
We anticipate that continuing collaboration with your staff will be beneficial in developing further 

metrics associated with the hydrologic restoration targets for Biscayne National Park. We are looking 

forward to continued cooperation in the establishment of restoration targets for our South Florida 

National Parks, and to working with your agency to provide the needed water for restoration of these 

nationally important natural areas. 

Sincerely, 

__________________________ ________________________ 

Mark Lewis, Superintendent Dan B. Kimball, Superintendent 

Biscayne National Park Everglades National Park 

Cc: Kameran Onley, U.S. Department of the Interior 

Michael Collins, SFWMD Governing Board Member 

Chip Merriam, SFWMD Deputy Executive Director, Water Resources Management 

John Mulliken, SFWMD Director, Water Supply 

Cecelia Weaver, SFWMD, Director of the Florida Keys Service Center 

Beth Carlson Lewis, SFWMD Office of Counsel 

Joan Lawrence, U.S. Department of the Interior 

Terrence “Rock” Salt, U.S. Department of the Interior 

Robert Johnson, U.S. National Park Service 

Paul Souza, U.S. Fish and Wildlife Service 

Pamela Repp, U.S. Fish and Wildlife Service