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