2 nd annual dredged material management summit minimizing sedimentation in harbors and marinas 6...

2nd Annual Dredged Material Management Summit

Minimizing Sedimentation In Harbors and Marinas

6 October, 2005

Minimization Of

Sedimentation

In Harbors And Marinas


Eric Smith and John Headland, Moffatt & Nichol



PRESENTATION TOPICS

• Why do We Dredge?

• Why Does Sedimentation Happen?

• How Can We Minimize Sedimentation?

• Examples

• Conclusions



• To Deepen Waters For Vessel Access (New Work Dredging

• To Maintain Depths Subject To Sedimentation (Maintenance Dredging)

• Dredged Material Can Be Managed By Managing Sedimentation

• Reducing Sedimentation Keeps Sediment In The Natural System

WHY DO WE DREDGE?



• Sediment Movement and Tidal/Wave Conditions Are Balanced in Most Natural Systems

• Many Natural Systems Are Equilibrated And Free of Sedimentation or Scour

• Sedimentation Occurs When:

There Is Sediment (Mud, Silt or Sand) In The Water Column

Flow Velocities Are Not Strong Enough To Keep Sediment Moving (Usually a result of deepening for navigation/harbor development)

WHY DOES SEDIMENTATION HAPPEN?



• Build Marine Facilities In Naturally Deep Water (i.e. Don’t Deepen)

• Otherwise, Simply:

Keep Sediment Moving (KSM) Through The Harbor

Keep Sediment Out (KSO) of the Harbor

• Both Strategies Involve Controlling Flow Conditions

HOW CAN WE MINIMIZE SEDIMENTATION?



FLOW-THROUGH HARBORS/MARINAS

Berths Parallel To Flow

Newark Bay, NJ Wilmington, DE



SEDIMENTATION IN DEEPENED CHANNELS

Qs in = uin cin hin Qs out = uout cout hout

hin

hout

Because Qs out < Qs in

Sedimentation

Qs in, uin

Sediment Transport In Sediment Transport Out

Qs out, uout

Dredged AreaUn-Dredged Area



Sedimentation = Sediment In – Sediment Out

ΔQs = Qs in – Qs out

ΔQs = cin Uin din – cout Uout dout

Where: ΔQs = Sedimentation Rate, c = sediment concentration

• No Sedimentation When U,d are constant

• Sedimentation When Uout < Uin (i.e., dout > din)

SIMPLIFIED PHYSICS OF SEDIMENTATION FOR FLOW THRU HARBORS & CHANNELS



Strategy is to:

• Place harbor & channels within naturally deep water

• Increase velocity in deepened area by engineering works

Flow Training Structures That Maintain Velocities

By flow augmentation with jets or injection dredging

KSM IN FLOW-THRU HARBORS & CHANNELS



KSM- FLOW TRAINING STRUCTURES MAINTAIN VELOCITY IN A DEEPENED AREA

Yangtze River, China Shooters Island, NY



KSM- FLOW TRAINING STRUCTURES

Delaware Bay Mississippi River



KSM- FLOW AUGMENTATION WITH SCOUR JETS



KSM- FLOW AUGMENTATION WITH PROPELLOR JETS

Propeller Jet System, Scour Systems



• Systems Have Been Installed:

King’s Bay, GA

Savannah, NC

Linden, NJ

Gray’s Harbor

• Each Installation Has Eliminated Dredging

• Best-Suited For Berthing Areas

• System Keeps Sediment In The Natural System

KSM- PROPELLOR/SCOUR JETS



HARBOR/MARINA BASINS

Port Elizabeth Channel, NJ Manten, Holland

Quiescent Basins



SIMPLIFIED PHYSICS OF SEDIMENTATION ININ BASIN HARBORS



ΔQs = c p x Qe

Where:

c = sediment concentration outside of harbor

p = basin trapping efficiency

Qe = exchange rate through entrance of harbor

• Best Way To Reduce Sedimentation is To Reduce Qe

SIMPLIFIED PHYSICS OF SEDIMENTATION ININ BASIN HARBORS



FLOW PHENOMENA IN HARBOR BASINSQe Depends on 3 Mechanisms:

Horizontal Eddy Exchange (Can Be Reduced) Tidal Filling (Cannot Be Changed) Density Currents (Can Be Reduced)



HORIZONTAL EDDY EXCHANGE- HARBOR BASINS



SALT/FRESH WATER DENSITY CURRENTS- HARBOR BASINS

Saltier Water

Fresher Water

Harbor BasinChannel

Fresher Water Saltier Water

Harbor BasinChannel

Incoming Tide:Brings Saltier/HeavierWater To Basin

Outgoing Tide:Brings Fresher/LighterWater To Basin

Even A Small Change In Salinity (i.e., 1-2 ppt) Can ExchangeA Tremendous Volume Of Water, More Than Tide Or EddyExchange!!



METHODS TO REDUCE FLOW VOLUME EXCHANGE- IN HARBOR BASINS

• Narrow Entrance

• Use Only 1 Entrance

• Change Entrance Orientation

• Use Structures to Reduce fh

Pile Groin

Current Deflecting Wall (CDW)

Modified CDW For Density Currents



CLOSE SECOND ENTRANCE PORT OF BREMEN, GERMANY

SECOND ENTRANCE



IN-SITU TREATMENT- PORT OF EMDEN, GERMANY

SEDIMENTATION REDUCTION 67%



GROIN PILE FIELD- REDUCES fh By 30%

CAN INTERFERE WITH NAVIGATION!



CURRENT DEFLECTING WALL- REDUCES fh By 30%

Kohlfleet Harbor BasinHamburg, Germany



CURRENT DEFLECTING WALL- KOHLFLEET BASIN



CURRENT DEFLECTING WALL- PRINCIPLESWithout CDW

With CDWCDW



CURRENT DEFLECTING WALL- FLOW PATTERNS



KOHLFLEET BASIN CDW REDUCES fh By ~35%



CURRENT DEFLECTING WALL FOR DENSITY CURRENTS



CURRENT DEFLECTING WALL FOR DENSITY CURRENTS

• Creates A Vortex Over The Width of The Basin Entrance

• CDW Decreases Water Exchange In Lower Layers By 40% During Flood Tide

• CDW & Sill Reduce Same to 70%

• Directs More Flow Into The Basin In Top Layers

• Not Yet Tested in Field Conditions



CURRENT DEFLECTING WALL FOR DENSITY CURRENTS- PHYSICAL MODEL TESTS

TIME (Sec)

Dye

Mas

s In

Bas

in (

g)

Without CDW

With CDW & Sill



• Mobilize Sediments Near the Bottom Under Low Pressure

• Used Extensively In Europe To Move Sediments From Quiescent Areas To Areas of High Velocities

• Sediment Is Lifted Only A Few Feet Off Bottom Creating Little Turbidity

• Can Be Used To Move Sediments Up to 1000m

• 2 M m3 Are Managed This Way in Port of Hamburg

KSM- WATER INJECTION DREDGING



KSM- FLOW AUGMENTATION WITH WATER INJECTION DREDGING (WID)

European Water Injection Dredger



KSM- FLOW AUGMENTATION WITH WATER INJECTION DREDGING (WID)

BREMERHAVEN, GERMANY

FAST-MOVING RIVER

QUIESCENT BASIN



• Historical Sedimentation 350,000 m3 per year

• WID Used For 11 Years

• WID Reduced Sedimentation Rate To Zero

• Eliminated Need For Upland Placement

• Keeps Sediment In The System

KSM- WID, PORT OF BREMERHAVEN, GERMANY




FAST-MOVING RIVER

QUIESCENT BASIN



In-Situ Treatment, PORT OF EMDEN, GERMANY

• Harbor Area Fronting Lock Subject To 2.5 cubic meters Per Year (deposited upland and/or offshore)

• The Deposit Starts As Fluid Mud & Eventually Consolidates Into Mud

• IST Consists of Removing Sediment From the Bottom into a Dredge Hopper, Exposing it to Oxygen (Oxygenating it), and Then Depositing it Back on the Bottom

• Sediment Is Not Removed Or Suspended Above Bottom

• Ships Sail Right Thru The Fluid Mud!

• IST, Performed Every 3 Months, Eliminated the Need For Dredging



IST, PORT OF EMDEN, GERMANY

• Suspend Sediment: While Alive, Organisms Act as “Parachutes” to Keep Sediment Suspended

• Fluid Mud: Organisms Form a Slime That Binds Particles To Form A Dense, Stable Suspension

• Mud: Results From Consolidation In Absence of In-Situ Treatment, Organisms Die Owing To Lack Of Oxygen

• IST Oxygenates Organisms In Order To Maintain Fluid Mud In Suspension



FLUID MUD THICKNESS- PORT OF EMDEN, GERMANY

3.3 m thick



FLUID MUD THICKNESS PORT OF EMDEN, GERMANY

210 kHz

15 kHz

210 kHz

15 kHz-10

-5

-5

3.3 m Thick 3.5 m Thick

April July

FLUID MUD IS IN EQUILIBRIUM WITH RIVER/SEDIMENT CONDITIONSTHICKNESS REMAINS CONSTANT OVER TIME!




DEPTHS FLUID MUD THICKNESS



IN-SITU TREATMENT (IST)- PORT OF EMDEN, GERMANY



CONCLUSIONS

• Dredged Material Can Be Managed Thru Sedimentation Management

• There Are A Number of Strategies For Reducing Sedimentation Including New Developments Proven in Europe

• These Strategies Keep Sediment In The System

• These Systems Can Be Very Cost-Effective and Are Equally Applicable To Large and Small Harbors

2 nd annual dredged material management summit minimizing sedimentation in harbors and marinas 6...

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