Shoreline Change in Response to Sea Level Rise on the Florida Southwest Coast

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1 Shoreline Change in Response to Sea Level Rise on the Florida Southwest Coast Jim Houston 1 6 Southwest Counties Shoreline Change Absalonsen and Dean (2010) 1 Director Emeritus, Army Engineer Research and Development Center, Corps of Engineers

2 Shoreline Change Data Shoreline change data from Absalonsen and Dean (2010) Shoreline width measurements since ~ 1870 with alongshore spacing of only ~ 300 m Cross Section of a Beach Profile Monument MHWL Shoreline Width Two Periods Early Total ~ 1870 to ~1970 ~ 1870 to ~ 2010 (prior to most beach (includes beach nourishment) nourishment)

3 Early Period, ~ 1870 to ~ 1970 Recession largely explained by effects of inlets (passes) Average gain ± 11.3 meters

4 Manatee County Shoreline Change Advancement Absalonsen and Dean (2010) Recession

5 Lee County Shoreline Change Absalonsen and Dean (2010) Advancement Recession Passes

6 Total Period, ~ 1870 to ~ 2010 Average gain ± 13.8 meters

7 East Coast, ~ 1850 to ~ 2010 Houston and Dean (2014) Average gain ± 5.9 meters

8 Advance Advance Inlet cut, 1892 North jetty, Inlet induced Absalonsen and Dean (2010)

9 Explain Shore Change by Sand Budget Longshore Transport Gains and Losses Inlet Shoal Growth Ft Pierce Inlet Longshore Transport Beach Nourishment Sea Level Rise

10 Factors Affecting Shoreline Change 1. Sea level rise 2. Beach nourishment - NOAA tide gauges - Florida Department of - Church and White (2009) Environmental Protection - Bruun rule (2008) - Coastal Planning and Engineering (2013) 3. Inlet Shoal Growth 4. Longshore Transport - Hine et al (1986) - Taylor Engineering (2002) - Davis and Gibeaut (1990) Determined contributions that these factors made to shoreline change

11 Shoreline Change Southwest Coast Early Period ~ 1870 to ~ 1970 Confidence Interval (Standard Deviation) No overlap

12 Shoreline Change Southwest Coast Total Period ~ 1870 to ~ 2010 Confidence Interval (Standard Deviation) No overlap

13 Shoreline Change Southwest Coast Total Period ~ 1870 to ~ % Confidence Interval No overlap

14 Shoreline Change By County Sum of factors (sea level rise, beach nourishment, inlets, longshore transport) What should have happened Figure 1 Figure 2 Average recession of about 30 meters ~1870 to ~ 2010 Average advance of about 36 meters What actually happened ~1870 to ~ 2010 Figure 2 minus Figure 1 is a measure of the sand required to produce what was actually measured

15 (Figure 2- Figure 1) Converted to m 3 /m-yr ~ 1870 to ~ 2010 Every county has gained sand not explained by the four factors

16 Must be a Sand Source Beach quality sand is not presently flowing down rivers to the southwest coast Not from bay interiors inlets almost universally produce shoreline recession, not advance Dean (1987) hypothesized two possible sources of sand 1. Carbonate sand 2. Onshore sand transport production due to wave asymmetry Dean (1987)

17 On East Coast Houston and Dean (2014) found opposite trends % Carbonate Sand Shoreline Change Opposite trends Source must be silica sand Shoreline Change Early Period before beach nourishment

18 Southwest Coast - % of Carbonate Sand Uncorrelated with Sand Gain % carbonate sand Sand gain that would account for measured shoreline change Uncorrelated (0.09 coefficient of determination) The source is silica sand, not carbonate sand Silica sand

19 Onshore Sand Transport Equilibrium profile theory says profiles rise with rising sea level Bruun rule Shore must recede to provide sand to raise the profile (assumes constant sand size) Dean equilibrium Wave asymmetry causes a landward stress and sand-size variation results in onshore transport, raising the profile and advancing the shoreline (assumes sand available) Bruun (1962) Dean (1987)

20 Barrier Island Formation and Onshore Davis (1994) said, most (barrier islands), appear to have formed as the result of some type of landward transport Transport Pizzuto (1986) concluded from Delaware Bay measurements that, results contradict the notion implied by the Bruun Rule that sediment is carried offshore as sea level rises. Rather, sandy barrier sediments may be supplied from offshore

21 Dean Equilibrium Concept Measurements since 1896 on the Dutch coast show onshore transport and shoreline advance in agreement with the Dean equilibrium concept the Bruun effect is generally of minor importance Sea level rise rate has to triple to be comparable to onshore transport (Stive et al 1990) Central Dutch coast Sea level rise Closure depth Stive et al 1990 Similar results are obtained for the Florida east coast with Dean equilibrium being consistent with measured shoreline change in 10 counties and the Bruun rule in 2 (Houston and Dean, 2014)

22 Bruun Rule Versus Dean Equilibrium on Southwest Coast Assume Bruun rule No overlap of confidence intervals No overlap Southwest Coast Assume Dean equilibrium Overlap of confidence intervals Overlap Southwest Coast

23 Bruun Rule Versus Dean Equilibrium For 6 Southwest Florida Counties Line of Perfect Agreement With Measured Data ~ 1870 to ~ 2010

24 Bruun Rule Versus Dean Equilibrium for 6 Southwest Florida Counties Confidence Intervals ~ 1870 to ~ 2010

25 Offshore Sand Source. Period Volume of Sand Gain (m 3 /m-yr) Early Period 2.7 ± 0.8 Total Period 2.8 ± 0.9 USGS (1999) Measured sand volume from closure depth to a distance about 2 km offshore Average annual sand volume gain that would explain measured shoreline change Transect Volume (m 3 /m) Caladesi-Clearwater 2600 Sand Key 1400 Indian Rocks Beach 2300 Treasure Island 3200 Long Key Anna Maria Key 3200 Longboat Key 2400 Siesta Key 2600 Casey Key 0

26 Summary Bruun rule is inconsistent with average shoreline advance in southwest Florida over the past ~ 140 years Advance is consistent with the Dean equilibrium concept The Dutch central and most of the Florida east coast shorelines also advanced in agreement with Dean equilibrium Dutch coast Florida east coast

27 Conclusions Shorelines can either retreat (Bruun rule) or advance (Dean equilibrium) in response to sea level rise Traditional View Dutch central and much of Florida east and southeast coast for ~ 140 yrs Average shoreline advance or retreat depends on offshore sand supply and the local sea level rise rate ( improved inlets almost universally cause recession)

28 Response of the Florida and Dutch coasts to sea level rise opens up a fruitful area of research The Future Relationship between shoreline change, sand supply, and rate of relative sea level rise? Notional Concept Courtesy Bob Dean Courtesy Bob Dean Equation that Unifies Bruun Rule and Dean Equilibrium Working to determine whether this equation can explain past shoreline change and estimate future shoreline change for projected rates of sea level rise

29 The End Thank You Jacksonville Beach