Evaluation of the South Florida Water Management District’s Plan to Increase Freshwater Flows to Florida Bay

Evaluation of the South Florida Water Management District’s Plan to Increase Freshwater Flows to Florida Bay

Categories: Special Report: Florida Bay

By Dr. Rajendra Paudel, Hydrologist, The Everglades Foundation and

Dr. Stephen Davis, Wetland Ecologist, The Everglades Foundation

 

Why does Florida Bay need more fresh water?

Florida Bay is the ultimate recipient of freshwater flow from the Everglades, which was historically fed by rainfall and spillover from Lake Okeechobee. After the construction of the Central and South Florida Project, instead of freshwater flowing south from the Lake into the Everglades, most Lake water considered “excess” is dumped to the Caloosahatchee River (to the west) and St. Lucie River (to the east) where it is damaging the ecology and economy of the communities surrounding these estuaries.  Because lake water has been diverted and the remnant Everglades dammed, not enough water reaches the Everglades.  As a result, Florida Bay is starved for freshwater needed to maintain a healthy salinity balance for seagrass and the numerous species of fish, shellfish, birds, marine mammals, and sea turtles that depend on this critical habitat.  Today, the fate of Florida Bay is entirely dependent on local rainfall and therefore very susceptible to droughts.

During the summer of 2015, a drought in South Florida led to several months with no freshwater flow to Florida Bay through Shark River Slough and Taylor Slough of Everglades National Park. This produced high salt (“hyper-salinity”) conditions in the upper central region of Florida Bay where these two drainage basins converge and triggered the beginning of a massive seagrass die-off that continued to expand through the first half of 2016 (see Figure 1).

What solutions does the South Florida Water Management District propose to increase the flow of freshwater into Florida Bay?

In response to public outcry over the seagrass die-off and recognizing the fact that more freshwater flow into the bay is needed to resolve the problem, the South Florida Water Management District (SFWMD) recently presented a plan “that will become a major part of saving the bay[1].”  The proposal has some complex elements, but the principal features are:

  1. Completion of planned and under-construction components of the South Dade Project and the Modified Water Deliveries Project;
  2. Lowering of canal stages near Everglades National Park;
  3. Adding new pumps in the Frog Pond and use them to pump water directly from canals into Everglades National Park;
  4. Modifying infrastructure along the boundary of Everglades National Park to allow introduction of water from canals directly into the Park’s marshes.

The plan was announced in July 2016 by the SFWMD, though the development of the main elements came out of the South Dade Investigations[2], which focused primarily on improving flood protection in the L-31N basin along the eastern boundary of Everglades National Park.   The July 2016 proposal was put forth as a means of doubling flow into the headwaters of Taylor Slough, which is one of the important contributors but not the sole contributor of freshwater inflow to Florida Bay.  The claim is that increasing flow to Taylor Slough will increase flows to Florida Bay, thereby promoting recovery of seagrass beds badly damaged in 2015[3].

The SFWMD provided Everglades Foundation with the Regional Simulation Model (RSM) inputs of their plan, thus allowing us to run the model and conduct an independent analysis of the results. We considered all details of the plan to understand where the additional water is coming from, where the water is going, when the water is delivered, and how much of that water is making it into Florida Bay.  This provides a technical basis for the conclusions.

How much flow increased and where?

The modeled results of the SFWMD plan indicate that flows near Taylor Slough Bridge, just downstream of where the plan pumps water into the marshes, nearly doubles, with an average increase of more than 20,000 acre-ft per year (or 6.5 billion gallons; see Figure 2)

We analyzed the water budget, which accounts for all of the flows of water that cross a defined boundary. That way, internal flows like recirculation of water at the S-332 structures are fully accounted for.  Roughly speaking, on average, about 17,000 acre-ft per year comes from intercepting the seepage leaving from Everglades National Park to the developed areas, and about 4,000 acre-ft comes from increasing the seepage out of Everglades National Park; that is, about 80% of the water in this plan is coming from seepage that is moving eastward that has already left Everglades National Park.  About 20% of the water is from increasing the seepage out of Everglades National Park.  SFWMD’s plan would decrease the drainage of water from the canals to the South Dade Agricultural Area with increased pumping at the S-332s (Figure 3).  However, lowering canal stages in L-31N also extracts water out of Everglades National Park above the headwaters of Taylor Slough.

One contention posited at public meetings by the SFWMD is that the increased flows in Taylor Slough come from diverting damaging flows at S-197, the southernmost structure in C-111 that releases water into Biscayne Bay. Structure flows indicate that S-197 discharges decrease by about 4,000 acre-ft per year on average in their plan, far less than the quantities of water that are pumped into Taylor Slough.  Therefore, the source of the additional flow in Taylor Slough is likely decreased beneficial seepage towards Biscayne Bay, not a decrease in harmful canal discharges to Barnes Sound.

How much water gets to Florida Bay?

The proposed plan increases the pumping at S-200 and S-199; however, a substantial fraction of water returns back towards the C-111 canal (13,000 acre-ft between S-177 and S-18C, and 12,000 acre-ft between S-18C and S-197 canal sections). If one looks at the flows approaching Florida Bay (the total of transects T23B and T23C in Figure 3), flows increase from 238,000 acre-ft per year to 256,000 acre-ft per year, about an 8% increase in total flows. These new operations will send approximately 2,000 acre-ft less water to Florida Bay across transect T23C in the Panhandle region. Overall, there is a shift of about 6,000 acre-ft of water from the Panhandle region and S-197 to the Taylor Slough (T23B) transect. In the SFWMD proposal, 42% of the flow is in the western section, while the base is about 37%. While the net increase is very modest, the distribution of flow is shifted westward, which is a definite benefit.

In summary, SFWMD’s proposal increases a net annual average flow by about 18,000 acre-ft (an 8% increase) of water across Taylor Slough and Eastern Panhandle (see Figure 3 for T23B and T23C transects). However, it doesn’t increase net flows into Shark River Slough which are essential to freshening the western margin of Florida Bay in dry years.

Does Florida Bay improve during droughts?

As for the SFWMD contention that this will improve seagrass habitat, one important consideration is when does the water get to Florida Bay? Specifically, the question is does the freshwater come under dry conditions, or on top of already wetter conditions?  Figure 4 is a flow duration curve of the total flows across Transects 23B and C.  The interpretation of these results is that nearly all of the increase in flows come during wetter conditions.  That is, flows will be higher in typical or above-average wet seasons and above-average dry seasons, but there is little change during below-average wet seasons and typical or below-average dry seasons.

Moreover, if one looks at a drought situation, this conclusion is confirmed. While no two droughts are exactly alike, the 1989 dry season is an important comparison to 2015, as the 1989 drought contributed to a seagrass die-off in Florida Bay.  In Figure 5, we see that the changes in flows are extremely small. Therefore, there is no basis to conclude that this plan will improve drought conditions, like the 2015 drought.

In summary, while the plan has some modest flow benefits during wet conditions, it will not likely change flows during dry years. That’s because the source of the water is, ultimately, the marshes of Everglades National Park.  During droughts, those marshes are dry and do not supply additional water.  The plan does not create “new water” by carrying water from a wet period to a dry period.  Rather, the plan redistributes the water during wet periods, the only time that water is available in the presently managed state of the Everglades.

What about water quality?

Since the L-31N/C-111 canal stages are lowered in the proposed plan, it will alter the exchange of flows between canal and the agricultural fields and therefore the phosphorus loadings. There is a proposed connection of the S-200 high-head canal to L-31W to push water towards the headwaters of Taylor Slough which could ultimately change the phosphorus loading rates into Taylor Slough. Figure 6 shows that flows from the agricultural areas to the canals increase at low flow rates, but decrease at high flow rates, though generally, the changes represent small quantities of water.  To determine the water quality impacts, we would need further information about the water quality characteristics as a function of flow.  The SFWMD has contended that there is no water quality problem; we do not have sufficient information to make a determination.

A second water quality issue is related to direct surface water discharges from canals into marshes along the L-31W canal. The plan as proposed has point discharges, and these point flows will result in localized disruptions to flora and fauna, as they are entirely inconsistent with natural Everglades flow patterns.   Therefore, the plan does contain water quality issues that need to be addressed.

Does the proposed plan restore Florida Bay?

In our opinion, no. The freshwater needs of Florida Bay are much greater than what is made possible through these proposed actions.  Although SFWMD’s plan may produce a modest increase in additional water to the bay, some of this benefit comes from a re-distribution of water that is already in the Everglades. Much of the “new” water seems to originate from less water reaching the South Dade agricultural fields and flowing toward Biscayne Bay.  Further, by focusing solely on Taylor Slough, this effort neglects the significance of Shark River Slough in benefitting Florida Bay.  There are several published studies[4] that have demonstrated the significance of flows from Shark River Slough in freshening western Florida Bay both historically, at present, and in a restored Everglades. The SFWMD and U.S. Army Corps of Engineers recognized these conclusions in their 2002 Florida Bay and Florida Keys Feasibility Study[5], stating that “[Florida] Bay salinities and nutrient loadings are impacted by the quantity and quality of coastal transport and the distribution of flows from Shark River Slough, Taylor Slough and lower C-111”.

How can we deliver more freshwater to Florida Bay?

Following a similar Florida Bay seagrass die-off in 1987 and a series of bay-wide algae blooms that persisted into the mid-1990s, the Comprehensive Everglades Restoration Plan (CERP) was authorized in 2000 to restore the flow of freshwater in South Florida. CERP represents the Master Plan for re-building lost storage capacity into the remaining Everglades ecosystem so that harmful discharges to the Caloosahatchee and St. Lucie estuaries can be reduced while simultaneously sending that freshwater south to meet the needs of the Everglades and Florida Bay. Implementation of CERP will greatly increase the flow of freshwater into Shark River Slough and Taylor Slough/Panhandle region, which are both essential in delivering inflows to Florida Bay (see Figure 1).

Several restoration projects are planned or awaiting implementation to increase flows into Florida Bay. Operating the C-111 Spreader Canal Western Project (C-111SC), a CERP project that has been constructed but still awaiting implementation, will “improve the quantity, timing and distribution of water delivered to Central Florida Bay via Taylor Slough”[6].  Fully implemented, the C-111SC will increase total flow volumes by 52% during average year across transects those were slightly north of the T23B and C transects[7]. Although it is not easy to make direct comparison between the models used for C-111SC and this plan, it highlights the benefits of the C-111SC in delivering more water to Florida Bay. The Central Everglades Plan, which is pending congressional authorization, will deliver an annual average of 210,000 acre-ft of new water south from Lake Okeechobee.  Another major CERP project to restore the flow of freshwater to Florida Bay is the EAA Reservoir Project, deemed a high priority when the CERP plan was completed and will also dramatically increase the flow of “new” freshwater from Lake Okeechobee to the south—benefitting the Everglades and salinity conditions across Florida Bay.

These solutions for restoring Florida Bay as well as other near-term operational strategies should have been investigated and prioritized based on cost benefits through an open process involving all stakeholders. It is quite possible that other more efficient and beneficial operational strategies could have been developed for Florida Bay while providing a consistent level of flood protection for the South Dade Agricultural Area. In sum, SFWMD’s proposal is not a stand-alone restoration plan, and to make a meaningful difference in the state of the bay, much more water is needed—especially during dry years.

Figure 1: Everglades National Park map, highlighting major structures, canals, Shark River Slough, Taylor Slough, and approximate area of seagrass die-off in 2015. Shark River Slough represents the largest volume of freshwater flow through the park.

 

Figure 2: Model simulated average annual overland flows through structures and across transects for current conditions (left) and proposed plan (right) for a period from 1965 to 2005. Reference: July 7, 2016, SFWMD presentation, “Modeling Florida Bay Options.”

 

Figure 3: Map showing key structures, canals, and the area of Taylor Slough affected by SFWMD’s proposed fixes. The bar charts represent net annual average flows (1000 acre-ft) across transects including flows through S-331+S-357 and S-197 structure.

 

Figure 4: Flow duration curves for daily flows across T23B + T23C transects in the Current condition (blue) and the SFWMD’s Proposed plan (red).

 

Figure 5: Changes in average annual flows (1000 acre-ft) under the proposed plan from a dry season of a dry year (Nov. 1, 1989 to May 31, 1990).

 

Figure 6: Flow duration curves for daily flows from agricultural areas to L-31N/C-111 canal in the Current condition (blue) and the SFWMD’s Proposed plan (red). Note that the flows represent only positive flows towards canal across transect shown in Figure 3.

 

[1] SFWMD July 14, 2016 “Moving Water Into Florida Bay” Press Release

[2]http://my.sfwmd.gov/portal/page/portal/xweb%20about%20us/miami%20dade%20service%20center#s_dade_investigation  Accessed October 4, 2016.

[3] www.sfwmd.gov/floridabay  Accessed October 4, 2016.  “This is an immediate first step to help reduce salinity levels in the bay and promote the recovery of seagrasses killed during a severe drought in 2015, providing critical relief now while larger Everglades restoration projects are built and completed.”

[4] T.J. Smith et al. 1989. Bulletin of Marine Science. Vol. 44, pp 274-282; J.D. Wang 1998. Estuarine Coastal and Shelf Science. Vol. 46, pp 901-915; J.N. Boyer et al. 1999. Estuaries. Vol. 22, pp 417-430; F.E. Marshall et al. 2009. Estuaries and Coasts. Vol. 32, pp 37-53.

[5] South Florida Water Management District and U.S. Army Corps of Engineers. 2002. Florida Bay & Florida Keys Feasibility Study. 69 pages.

[6]Page 19 of the U.S. Fish & Wildlife Service Coordination Act Report, Annex A of the CEPP Project Implementation Report.

[7] Table 14 of Volume 2 –Annexes A-B, Central and South Florida Project CERP C-111 Spreader Canal Western Project, Final Integrated PIR and EIS.