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Shoreline Change in Response to Sea
Level Rise on the Florida Southwest Coast
6 Southwest Counties Shoreline Change
1 Director Emeritus, Army Engineer Research and Development Center, Corps of Engineers
Jim Houston1
Absalonsen and
Dean (2010)
Shoreline Change Data
• Shoreline change data from
Absalonsen and Dean (2010)
• Shoreline width
measurements since ~ 1870
with alongshore spacing of
only ~ 300 m
Two Periods
Early Total
~ 1870 to ~1970 ~ 1870 to ~ 2010
(prior to most beach (includes beach nourishment)
nourishment)
Monument
Shoreline Width
MHWL
Cross Section of a Beach Profile
Early Period, ~ 1870 to ~ 1970
Recession largely
explained by effects of
inlets (passes)
Average gain + 5.1 ± 11.3 meters
Manatee County Shoreline Change
Absalonsen and
Dean (2010)
Advancement
Recession
Lee County Shoreline Change
Absalonsen and
Dean (2010)
Advancement
Recession
Passes
Total Period, ~ 1870 to ~ 2010
Average gain + 36.3 ± 13.8 meters
East Coast, ~ 1850 to ~ 2010
Average gain + 47.8 ± 5.9 meters
Houston and Dean (2014)
Absalonsen and
Dean (2010)
Inlet cut, 1892
North jetty, 1926-1929
Inlet induced
Advance
Advance
Longshore
Transport
Longshore
Transport Sea Level
Rise
Inlet Shoal Growth
Beach
Nourishment
Explain Shore Change by Sand Budget
Gains and Losses
Ft Pierce Inlet
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
Shoreline Change Southwest Coast
Early Period ~ 1870 to ~ 1970
No overlap
Confidence Interval
(Standard Deviation)
Shoreline Change Southwest Coast
Total Period ~ 1870 to ~ 2010
No overlap Confidence Interval
(Standard Deviation)
Shoreline Change Southwest Coast
Total Period ~ 1870 to ~ 2010
No overlap 99%
Confidence
Interval
Shoreline Change By County
What actually happened
What should have
happened
• Figure 2 minus Figure 1 is a measure of the sand required
to produce what was actually measured
Average recession of
about 30 meters
~1870 to ~ 2010
Figure 1
Average advance of
about 36 meters
~1870 to ~ 2010
Figure 2
• Sum of factors (sea level
rise, beach nourishment,
inlets, longshore transport)
(Figure 2- Figure 1) Converted to m3/m-yr
~ 1870 to ~ 2010
Every county has gained sand not explained by the four
factors
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)
On East Coast Houston and Dean (2014)
found opposite trends
Opposite trends
- Source must be silica sand
Early Period
– before beach
nourishment
Shoreline Change
% Carbonate
Sand
Shoreline Change
Southwest Coast - % of Carbonate Sand
Uncorrelated with Sand Gain
Sand gain that
would account for
measured
shoreline change
• Uncorrelated (0.09 coefficient
of determination)
• The source is silica sand, not
carbonate sand
% carbonate sand
Silica sand
Onshore Sand Transport
• “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)
Dean (1987)
Bruun (1962)
Equilibrium profile theory says profiles rise with rising sea
level
Barrier Island Formation and Onshore
Transport • Davis (1994) said,
“ … most (barrier islands),
appear to have formed as the
result of some type of
landward 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 …”
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)
• 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)
Stive et al 1990
“Closure
depth”
Central Dutch coast
Sea level rise
Bruun Rule Versus Dean Equilibrium on
Southwest Coast
• Assume Bruun rule
• No overlap of confidence
intervals
• Assume Dean equilibrium
• Overlap of confidence
intervals
No overlap
Overlap
1870-2010
1870-2010
Southwest Coast
Southwest Coast
Bruun Rule Versus Dean Equilibrium
For 6 Southwest Florida Counties
Line of Perfect
Agreement With
Measured Data
~ 1870 to ~ 2010
Bruun Rule Versus Dean Equilibrium for
6 Southwest Florida Counties
Confidence
Intervals
~ 1870 to ~ 2010
Transect
Volume
(m3/m)
Caladesi-Clearwater 2600
Sand Key 1400
Indian Rocks Beach 2300
Treasure Island –
Long Key
3200
Anna Maria Key 3200
Longboat Key 2400
Siesta Key 2600
Casey Key 0
Offshore Sand Source.
• Measured sand volume from
closure depth to a distance about
2 km offshore
Average annual sand volume
gain that would explain
measured shoreline change
USGS (1999)
Period Volume of Sand Gain
(m3/m-yr)
Early Period 2.7 ± 0.8
Total Period 2.8 ± 0.9
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
Florida east coast
Dutch coast
Conclusions
• Shorelines can either retreat (Bruun rule) or advance (Dean
equilibrium) in response to sea level rise
• Average shoreline advance or retreat depends on offshore
sand supply and the local sea level rise rate
(“improved inlets” almost universally cause recession)
2050 2030
2030 2050
Traditional View
Dutch central and
much of Florida east and
southeast coast
for ~ 140 yrs
The Future • Response of the Florida and
Dutch coasts to sea level rise
opens up a fruitful area of
research
• Relationship between
shoreline change, sand supply,
and rate of relative sea level
rise?
Notional Concept
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
Courtesy
Bob Dean
Courtesy Bob Dean
The End Thank You
Jacksonville
Beach