Management and Assessment of Dredged Material Disposal Near a South Florida Sensitive Coral Environment Christopher J. McArthur, P.E. Member ASCE COPRI

Management and Assessment ofDredged Material Disposal Near a South Florida Sensitive Coral Environment

Christopher J. McArthur, P.E.Member ASCE COPRI

U.S. Environmental Protection Agency Region 4 – Atlanta, GA

Jack StamatesNOAA Atlantic Oceanographic and Meteorological Laboratory – Miami, FL

Dredging 2012 Conference: Dredging in the 21st CenturySan Diego, CA October 2012


History• In the early 1990’s EPA was

requested to designate 3 ODMDS in Southeast Florida – Support maintenance activities of

existing channels– Support deepening/widening of

Port of Miami• Concern for potential impact to

South Florida’s Coral Reefs– Corals are highly sensitive to its

surrounding environment.– It is part of the only living coral

barrier reef in the continental United States and is the third largest coral barrier reef system in the world.

– Southeast Florida counties derive $6 billion annually from reef-oriented tourism and fisheries.

Palm BeachODMDS

Port EvergladesODMDS

Miami ODMDS

Source: Southeast Florida Coral Reef Initiative



Unknowns• Direction of transport of disposal plumes and frequency of

reef directed currents– Located on the continental slope at the edge of the Florida

Current.– Florida Current spin-off eddies were believe to occur as frequently

as twice per week.• Concentration of disposal plumes at reefs

– Reefs are located within 2.5 km of the sites.– Models predicted maximum concentration at reefs in the range of

6 to 10 mg/l.• Acceptable levels of exposure (concentration, duration,

frequency) to suspended sediments due to dredged material disposal.



Interim Protective Measures (Environmental Windows)

• Established by interagency group: EPA, USACE, NOAA, Florida, Port of Miami

• Restrict disposal during onshore current events to minimize disposal plume transport towards the reefs.

• Requires real-time monitoring of currents at the disposal site and application of current velocity criteria.

• Dredging contractor must be ready to cease operations with little advance notice and unknown duration.

• Utilized for the 1995-2005 Deepening Project




Real-time current measurement system



Long-term Approach

1. Establish baseline conditions at reefs to establish guidelines for acceptable suspended sediment concentrations.

2. Track and measure suspended sediment disposal plumes to determine suspended sediment concentration in plumes.

3. Monitor corals for sub-lethal impacts during disposal project.

4. Determine the frequency of plumes reaching the reef.



1. Exposure Guidelines• Intensity

– High suspended solids concentrations place stress on corals– Elevations in SSC due to disposal plumes should not exceed the highest

concentrations that naturally occur

• Duration– Elevated SSC may cause an adverse impact if the corals are exposed to

elevated concentrations for a sufficient amount of time– Elevations in SSC due to disposal plumes should not exceed the length of

the naturally occurring events of equal magnitude

• Frequency– Corals can tolerate occasional heavy sediment concentrations if given

sufficient opportunity to recover– Elevations in SSC due to disposal plumes should not occur more frequently

than the natural occurrence of similar events (magnitude & duration)



ADCP

Current Flow

PAROBS

OBS

PAR

Wind Sensor

Range Marker

PAR

OBS

S4

Site 1

Site 2

Reef

Reef

Measurement of ambient suspended sediment concentrations and physical processes

Acoustic sensors

Optical sensors

Wave &



Winter SSC Frequency Distribution

Suspended Solids Concentration (mg/l)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

Fre

qu

en

cy o

f Occ

ure

nce

(%

)

0

2

4

6

8

10

12

14

16

14 mg/l (99th percentile)

8 mg/l (95th percentile)



Duration above Threshold Concentration (minutes)

0 60 120

180

240

300

360

420

480

540

600

660

720

780

840

900

960

1020

1080

1140

1200

Allo

wab

le O

ccur

ence

s pe

r W

eek

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

SummerFallWinter

Duration and Frequency Guidelines



2. Plume Measurements

• Utilized calibrated optical turbidity probes suspended 5 and 10 meters below a tracking drogue.

• Measured currents and backscatter acoustically from the surface to track the plume.



Time from Disposal (hours:minutes)

00:10 00:20 00:30 00:40 00:50 01:00

Susp

end

ed

Se

dim

en

t C

on

centr

atio

n (

mg/l)

0

20

40

60

80

100

120

Turb

idity

(N

TU

)

0

5

10

15

20

25

30

35

Depth = 5 metersDepth = 10 meters

• Plume concentrations appear to be highly variable from load to load.• Surface plume concentrations appeared to reach background within 30

minutes.

Plume Concentrations



3. Monitor Corals for Sublethal Effects• Objective: Determine if

dredged material disposal is inducing a stress response in hermatypic corals on nearby coral reefs.

• Utilizes Coral Molecular Bioassay developed by Georgia Institute of Technology.– Montastrea faveolata

• Monitoring multiple stations (5 colonies at each) and a reference.



Sublethal Effects• Significant gene expression events occurred

in June and October of 2005. – The June event preceded the start of

dredging – The October event occurred just following

passage of a couple of named storms, including Rita, a major turbidity event.

– June and October (2005) had the highest recorded precipitation during the period

• Salinities < 30 ppt are potentially lethal to coral

• Other than at those times, gene expression, though variable, was unremarkable.

• Conclusion: If dredged material disposal is affecting these study sites, its signal is too small to be seen above the “normal” background level of stress these corals are experiencing.

Reference: Sara Edge. 2007 “Using Microarrays to Quantify Stress Responses in Natural Populations of Coral,” Dissertation: Georgia Institute of Technology.

Sample Processing. A) Extract total RNA and label. B) Hybridize to microarray. C) Compare gene expression patterns from different sites.

AB C1 C2

Colonies were photographed and marked for monitoring.



0

45

90

135

180

225

270

315

0% 5% 10% 15% 20% 25%

<=50

>50 - 100

>100 - 125

>125 - 150

>150 - 200

>200

M agnitude (cm /s)

Duration of Event (hours)

0 2 4 6 8 10 12 14 16 18 20 22

Fre

qu

en

cy o

f E

ve

nt

(%)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

Cu

mu

lative

Fre

qu

en

cy o

f E

ve

nt

(%)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

FrequencyCumulative Frequency

4. Determine Frequency of Plumes Reaching the Reef

• The longest onshore current event lasted 23 hours.• The probability of an onshore current event of any duration immediately

following a disposal event is less than 2 percent (123 events). • The probability of onshore currents lasting for 5 hours or more

immediately following a disposal event is less than 0.5 percent (19 events).

Currents are dominated by the Florida Current with strong north northeasterly flows



Potential for Plume Transport to Nearshore Reefs

• Streakline Analysis– One dimensional and assumes

spatially homogeneous currents.– Less than 1% probability plume will

reach reef within 5 hours.– Less than 3.5% probability plume will

reach reef within 10 hours.• Plume Visitation Frequency

– 2 dimensional and spatially varying currents.

– Contours represent the frequency that a disposal plume will visit the 0.04 km2 grid cell during the five hour travel time.



Study Conclusions• Guideline Development and Plume Measurements

• Background suspended sediment concentrations are highly seasonal.

• Disposal plume concentrations are low and infrequent in comparison to those caused by meteorological events.

• Sublethal effects are small in comparison to background levels of stress corals are experiencing.

• Shoreward directed currents are infrequent and the likelihood of plumes reaching the reefs is very small.



Results• In 2008 the Environmental

Windows in place since 1995 were lifted.

• Miami Harbor Phase III Deepening can move forward without the need for the costly real-time monitoring system and associated risks of project suspension.



Port of Miami

Dredging and Port Construction Around Coral ReefsPIANC Report No 108 (2010)

• Facilitate interaction between the project proponent, regulators and stakeholders

• Environmental Impact Statement• Disposal Site Management and Monitoring Plans

• Technical mitigation measures• Environmental windows

• Baseline surveys• Background turbidity levels and/or light levels• Shoreline and physical processes (currents, waves)

• Predictive numerical modeling• Model validation and calibration

• Receptor tolerance levels• Based on site specific data supplemented by relevant literature values• Should take into account both the magnitude and duration of the loading

• Habitat monitoring during the dredging project



Thank You

[email protected]://www.epa.gov/region4/water/oceans/



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Management and Assessment of Dredged Material Disposal Near a South Florida Sensitive Coral Environment Christopher J. McArthur, P.E. Member ASCE COPRI