Enhancing coastal wetland resilience to SLR: just add water?


By U. S. Fish and Wildlife Service - Northeast Region via Wikimedia Commons

In Brief

Salt water encroachment and sea level rise are making Everglades restoration increasingly difficult. Geological data indicate that for more than three millennia southeast Florida hosted extensive freshwater wetland habitat extending to within 50 m of the Biscayne Bay shoreline; however, the current area is much smaller. Over the last 70 + years, the region has been subject to salt water and mangrove encroachment triggered by acceleration in the rate of sea level rise.  A 1994 study to determine if increasing freshwater delivery could reverse or minimize salt water encroachment concluded water levels in the delivery canals were too low to mitigate salt water encroachment. Subsequently, the U.S. Army Corps of Engineers and the South Florida Water Management District initiated measures to increase canal head and placed breaches in a coastal levee to increase water delivery.  Results of an ongoing study of molluscan assemblages, conducted in the coastal wetlands of Biscayne Bay, indicate that even though freshwater delivery was increased, salt water encroachment was not stopped or reversed but actually increased.  Under the current rate of sea level rise and concomitant with salt water encroachment, our data indicate the open waters of Biscayne Bay will migrate inland and over existing coastal marsh habitat, ultimately extending to the L31E coastal levee within 50 to 100 yr. Increasing freshwater delivery may slow this trend, but is not a viable, long-term solution to salt water encroachment triggered by rising sea level. To limit engineering challenges and construction costs, strategic withdrawal should be immediately implemented as a viable solution to enhance Everglades wetland resilience, even under conditions of accelerating sea level rise that will ultimately result in its conversation to open water near the end of this century.


Key Concepts

  • Enhanced freshwater delivery for restoration of Everglades coastal wetlands is necessary to offset the effects of sea level rise and salt water encroachment

  • If enhanced freshwater delivery is accepted as the most practical solution, a balance must be achieved in which the volume of freshwater necessary to restore coastal wetland habitat does not exceed elevation thresholds that trigger urban flooding

The Problem

Freshwater management activities designed to mitigate salt water encroachment (SWE) for freshwater vegetation in Biscayne Bay coastal wetlands needs to be done at a level that could flood urban communities on the edge of these wetlands. This paper discusses the results of changing water delivery on vegetation and makes suggestions regarding viable solutions to future management challenges.

Decreasing water delivery. The coastal wetlands along the western shoreline of Biscayne Bay are located at the southern boundary of the Greater Everglades Ecosystem watershed (Figure 1) and have been affected by anthropogenic changes in water storage and flow patterns associated with an aggressive drainage program that began in the late 19th century1. By the early 1970s, these alterations had resulted in a 70 % reduction of historic water delivery to Miami-Dade County2.

 

Solecki & Walker, 2001
Fig. 1. Water management in the Greater Everglades Ecosystem.

 

Consumptive uses of water have since increased with accelerating population growth and the water quality delivered to Biscayne Bay has declined with increased urbanization and agriculture3. Prior to 1900, Biscayne Bay coastal wetlands received Everglades water by way of sheet flow through 13 transverse glades that cut across the Atlantic Coastal Ridge and by ground water discharge through the Ridge’s highly porous Miami Limestone. By the 1960s, the transverse glades had been drained by canal construction, causing Everglades water to be delivered to Biscayne Bay as point sources, thereby bypassing the coastal wetlands (Figure 2a). Reclaimed land in the low transverse glades was first utilized for agriculture but has since been converted to canal-side homes. Established residential use of the natural flow-ways is a limiting factor in future water management changes. Construction of the L31E storm levee and borrow canal that parallels the southern Biscayne Bay coastline is also a significant impediment to freshwater flow in to the southeast Everglades. Built in the 1960s, this water control feature effectively blocked sheet flow of Everglades freshwater to the coastal wetlands of Biscayne Bay (Figure 2b).

 

a: South Florida Water Management District. b: google Earth Base Map
Figure 2. a. Major water control canals that deliver freshwater into the Biscayne Bay watershed as point sources. These canals were all cut through the transverse glades; topographic depressions in the Atlantic Costal Ridge that historically served as natural outlets for Everglades water to drain into Biscayne Bay. b. Location of sediment cores recovered along Mowry Canal and Turkey Point transects.

 

Sea level rise. To complicate the effects of upland drainage on Everglades coastal wetlands, sea level rise (SLR) began to accelerate near 1900 from a long term (~3 kyr). average rate of < 1 mm yr-1 during the Late Holocene, to a regional rate of 9 mm yr-1 during the last decade (Figure 3). This dramatic increase in the rate of SLR is associated with what is now termed the Anthropocene Marine Transgression4,5 and has produced a transgressive stratigraphic sequence along the western coastline of Biscayne Bay. The sequence reflects the effects of SWE, which caused replacement of freshwater marshes with salt tolerant mangroves6 and ultimately open water.
 

Authors
Figure 3. Sea level record for the Holocene. The left curve represents sea level rise during the Holocene Epoch until 190022. The sea level record since 1900 is shown in the right curve. Note that the two curves are at different scales. At the onset of the Anthropocene Marine Transgression (~1990), the rate of SLR accelerated to 2 mm yr-1 (right panel)25, to 2.6 – 2.9 mm yr-1 by 200026 and to 9 mm yr-1 by 200917. Predictions of future rates of SLR vary d27, e28 and f29.

 

Time series analyses of aerial photography indicate that as recently as 1938, freshwater wetlands were located within 50 m of the southern Biscayne Bay shoreline6. The photo-interpretation is supported by sediment core profiles in which marls containing freshwater molluscan assemblages underlie recently deposited mangrove-rich sediments containing marine mollusks. In a 1996 study, Meeder and collaborators5 examined five basins in the Southeast Saline Everglades (SESE) using stratigraphic methods and radiometric dating to distinguish between the effects of SLR and the historical decrease in freshwater delivery on the rate and magnitude of SWE. Four out of the five basins displayed recent increases in the rate of SWE, with the distance of encroachment most closely related to the elevation of high tide. The contact between fresh and marine-influenced sediment along the shore-normal profile was determined to have migrated landward at rates up to 71 m yr-1. In the fifth basin, Taylor Slough, a major drainage pathway between the Everglades and Florida Bay, scant evidence of SWE was observed. At the time of the study, freshwater delivery to this basin, though reduced from historical volumes, was sufficient to maintain freshwater sawgrass communities that generated peat fast enough to sustain a marsh surface elevation above high tide.

Restoration activities. By the 1940-50s, evidence of SWE in Dade County was well documented as agriculture and water fields along the coasts were impacted7. In response, water control structures were established at the mouths of major canals diverting freshwater to Biscayne Bay and briefly mitigating SWE. More recently however, increased urban consumptive use and salt water intrusion into the groundwater forced movement of County water fields further west.

Of course, this is not a complete history of Biscayne Bay restoration activities and does not address problems such as solid waste disposal into Biscayne Bay prior to construction of the offshore outfall8, nor many other issues that did not directly affect the western shoreline of south Biscayne Bay. An important event was the formalization of the Greater Everglades Restoration Plan and its legal authorization95. In all cases though, the elevation of spring tide appeared to be the principle factor controlling the distance of SWE5. With both SWE and the inland expansion of mangroves continuing apace, more landward coastal environments, including the coastal White Zone, either migrated landward or were overtaken by the retreating mangrove front11,12.

The Comprehensive Everglades Restoration Plan (CERP) contains provisions affecting water sources and volumes, and therefore quality and quantity of water delivered to Biscayne Bay9. Of the 68 projects listed in CERP, the following have had the most impact on Biscayne Bay: Biscayne Bay Coastal Wetlands Project, the C-111 Spreader Project, the South Dade Waste Water Reuse Project, the L31N Seepage Management Project and the Lake Belt Storage. The objective of the Biscayne Bay Coastal Wetland Project is to restore the historic water supply patterns through the coastal wetlands and into southern Biscayne Bay. The Project includes three major restoration objectives: (1) restore historic coastal wetland hydrology, (2) re-establish a near shore estuarine zone and (3) reduce nutrient loading to the Bay.

Implementation of the Biscayne Bay Coastal Wetland Project has included the installation of many more culverts through the L31E levee and more recently the storage of additional water along the coast in order to decrease SWE. Increased water storage was also achieved by pumping from the major Mowry Canal and resulted in increasing canal stage by at least 30 cm. Again, this increase in water delivery into the Biscayne Bay was designed to restore the historic coastal wetland hydraulic regime. This act of adaptive management, as well as future actions based upon monitoring data, are key to the success of restoration. However, increased coastal storage and delivery is ultimately limited by water availability and urbanization.

An ongoing investigation has documented that neither the delivery of freshwater to Turkey Point or the increased water delivery at the Mowry Canal site since 1996 has had any effect of SWE13,14. At the Turkey Point site, SWE has continued to migrate inland an additional 875 m between 1996 and 2016 (43.9 m yr-1), again documenting the ineffectiveness of water delivery further south in a much broader wetland. SWE at the Mowry Canal site resulted in the development of a marine-influenced mangrove peat surface layer at the L31E that is five times as thick as was identified in 1996. This indicates increased water delivery has not decreased salinity. These findings bring to question the validity of the concept that increasing freshwater delivery will decrease SWE.

Future changes in water management are expected to reduce Everglades water delivery to Biscayne Bay from present sources, replacing it with reclaimed wastewater. In addition, plans call for the ‘rehydration’ of coastal marshes by using canal water presently being delivered directly into Biscayne Bay by diversion into small tidal creeks in the wetlands15. This activity is presently functional only along the Biscayne Bay reach, which includes the Mowry Canal site.

 
Will “Just add water” work in the future

The Central Everglades Planning Project (CEPP)16 expedites storage, treatment, and conveyance of “new” water from Lake Okeechobee to the southern Everglades. CEPP is now the central focus of the Everglades restoration effort. The State is implementing a few of the less costly CEPP components, such as increasing the capacity of key water control structures and facilitating more water delivery through the Northeast Shark River Slough and into Everglades National Park. The focus of the CEPP on the interior Everglades suggests there will be less water available in the future for Biscayne Bay coastal restoration. Water will come from diversion of major canals presently discharging as point sources into Biscayne Bay and reclaimed waters15. These activities require major construction, including pump installation and construction of treatment wetlands to increase water quality prior to delivery to coastal wetlands. At present, only a minor volume of canal water diversion is occurring and although some treatment wetlands have been constructed, there is little or no use of treated waste water. These two management tools are likely to take decades to complete because of project prioritization and funding constraints.

Future adaptive management of Biscayne Bay coastal wetlands is totally dependent upon the availability of freshwater and the rate of sea level rise (SLR), presently 9 mm yr-1 (ref. 17). There’s nothing that can be done to alter the rate of SLR in an effort to facilitate the restoration of Biscayne Bay, thus restoration must adapt to changes in sea level. At some future time, freshwater delivery currently directed to the Biscayne Bay’s coastal wetlands will be delivered directly into the landward expanding, open-water estuarine zone of an evolving Biscayne Bay. This will result in a decline in existing ecosystem services as the open water estuarine habitat of Biscayne Bay expands into the SESE. The cumulative effect of these changes is unknown but will likely be substantial.

Is ‘just adding additional water’ a viable solution to restore coastal wetlands in the Everglades? The two previous attempts, as documented herein, would suggest no, given neither have functioned as designed. Yet, enhanced freshwater delivery must be considered a viable wetland restoration strategy. It must generate a freshwater wedge of sufficient scale to prevent or even reverse SWE without creating adverse flooding. For example, we completed six coastal wetland elevation surveys in the southeastern Everglades that revealed the presence of topographic basins ~ 20 cm below local grade level10,18,19. If not regulated with great accuracy, it is very likely that additional water delivery will result in permanent flooding of these basins. Therefore, pilot projects testing the effects of increased water delivery should be completed prior to widespread implementation. If water availability becomes an issue, the restoration of coastal marshes could be limited to areas identified as most likely to conserve delivered water based upon analyses of salinity during drought conditions20.

An effective wetland restoration solution requires a management strategy that addresses the following limitations:

(1) Delivery canal stage must be elevated to maintain positive head in respect to SLR, requiring more water for storage, but elevated canal stages must also be constrained to prevent urban flooding. The elevation of the freshwater head has the potential to flood human infrastructure upstream of water control structures. Low-lying areas in south Florida already experience urban flooding after heavy rains and/or high tides because there is no place for the water to go, nor can it enter the already saturated bedrock.

(2) Urbanization has already penetrated into the wetlands of the coastal zone along Biscayne Bay and the transverse Glades.

(3) As sea level rises, tidal exchange will become more efficient, requiring more freshwater to maintain a wedge that can effectively control SWE.

(4) Even if canal water can be contained to prevent urban flooding, SLR will cause ground water to rise, as it has in the earlier Holocene in response to SLR21,22.

(5) Once the elevation of sea level is greater than the elevation of the fringing mangroves and thus causes permanent inundation, considerably more water delivery will be required to maintain a freshwater wedge because more efficient tidal exchange will drive more saline water into the hinterland. At present, there is a ~ 5 mm annual deficit between the rate of sea level rise and the rate of fringing mangrove sediment accretion. Thus, the permanent inundation of fringing mangrove is inevitable.

 
The Solution

All things considered, strategic withdrawal of water control structures to facilitate landward migration of wetlands must be viewed as the most viable solution to mitigating the effects of SLR and SWE. In the context of this investigation, strategic withdrawal refers to an informed, science-based plan that allows at-risk critical coastal assets (i.e., fringing mangrove forests) to migrate landward in response to sea level rise. By contrast, an alternate strategy of protect and defend would call for elevating freshwater delivery in an attempt to maintain or sustain the current landscape configuration. In the study area, strategic withdrawal is preferred for two reasons: (1) the loss of the SESE within the next 50 to 100 yr is inevitable no matter what engineering water delivery solution is implemented, and (2) ecosystem function will be prolonged by allowing unencumbered landward migration of coastal communities. As a first step towards preservation, a water control structure could be constructed within the transverse glades and between segments of the Atlantic Coastal Ridge to impede SWE into the Everglades. The existing Atlantic Coastal Ridge would then act as a natural barrier, albeit leaky, to delay the influx of salt water.

Even though fighting SLR via increasing freshwater delivery and strategic withdrawal may not save the Everglades in the long term, we feel that it is imperative to maintain its functions as long as possible. This approach will expand the duration of ecosystem services at a reduced cost given engineering and construction expenses would not be reactive and immediate (i.e., post-disaster event recovery), but spread over decades. These results make it clear that hard decisions are going to be forced upon resource managers within the next few years that will determine the fate of the Everglades. The longer we wait to initiate new restoration activities the less likely they will work as sea level is changing so rapidly that a delay of 20 yr may mean a total redesigning of the delivery projects planned, designed, but not yet constructed.

 
Acknowledgements

We would like to thank the SFWMD for funding the 1996 C-111 study (C-4244), 2016 C111 Restudy (P15AC01625), L31E Pilot Project (C12409), the Historic Creek Study (SFWMD 11679), and the Black Point Rehydration project (SFWMD x204114) which all contributed to this study. This is publication number 874 of the Southeast Environmental Research Center (JFM) and 14 of the Sea Level Solutions Center (RWP), Institute of Water and Environment at Florida International University.

 
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