8 coastal protection prt · * basic cross-section (rubble mound revetment) * definition structure...

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Hans Hanson, Lund University, Sweden


BEACH EROSION?


2000

Land

Sea

BEACH EROSION! WHY? WHAT TO DO!

2012

2


REACTIONS TO EROSIONNo Action Retreat

Accomodation Stabilization


Key Elements:

• Planning

• Timing

REACTIONS TO EROSION

Main Considerations:

• Economical

• Technical

• Environmental

• Legal

• Institutional

• Social

• Political


ECONOMIC FACTORS (JUSTIFYABLE)

* Initial Cost + Operations & Maintenance

* Economic Sustainability (GNP)

* Financing (IC) & Cost Recovery (O&M)

* Economical Viability (IRR)

TECHNICAL FACTORS (FEASIBILITY)

* Level of Skills

* Availability of People & Equipment

* Availability of Material

* Availability & Accuracy of Data

* State of Transportation Systems

REACTIONS TO EROSION

3


NO ACTION

Isolated or Temporary Events

* GENERAL

Unlimited Development

Unregulated Land Use

* ADVANTAGES

No Direct Costs (protection)

No Side Effects

No Structures

* DISADVANTAGES

Continued Erosion/Flooding

Indirect Costs (property)


RETREAT

Lower Degree of Development

* GENERALGradual Evacuation

Balance Distance & Duration

Change Property Value

* ADVANTAGESNo Side Effects

No Structures

* DISADVANTAGESLoss of Land and Investments

Major Investment

Major Social Implications

Consider Setback Lines!


ACCOMODATION* GENERAL

No Stabilization

Adaptation (e.g., houses on piles)

Development at Own Risk

New Land Use Planning & Implementation

Major Investment

4


STABILIZATION

In Areas with High Degree of Development

* GENERAL

High Cost of Protection

High Economic Benefits

Could Move Problem

Cut Costs with Setback Lines


Hard Structures

Intermediate Structures

Soft ’Structures’


* TYPE OF PROBLEM

Dune Erosion During Storms

Continuous Beach Erosion

* DESIRED RESULT

Erosion Protection

Recreational Beach

* ENVIRONMENT

Degree of Exploitation

DESIGN ASPECTS

5


* CLIMATIC FACTORS

Water Levels Waves

Currents Temperatures

Solar Radiation Ice

* ECONOMIC FACTORS

Capital Cost Maintenance Cost

Indirect Costs/Benefits

* LEGAL AND SOCIAL CONSEQUENCES

* LAND USE, ECOLOGY, AESTHETICS

DESIGN ASPECTS


STRUCTURAL DESIGN ASPECTS

* BASIC CROSS-SECTION (Rubble Mound Revetment)

Geotextile!

Foot!

Crest!


Geotextile!!

Temporarily Excavated

Storm/Winter Profile

Secure the Foot!

6


Geotextile!!

Storm/Winter Profile

Secure the Foot!


* STRUCTURAL PROPERTIES

STRENGTHIndividual FLEXIBILITY

Cooperative

DESIGN ASPECTS


•DESIGN DETAILS

Toe Protection

7


•DESIGN DETAILS

Geotextile/Filter


Flank Erosion


Crest Elevation (Run-up Height)

Pekalongan

8


* MATERIAL vs. ENVIRONMENT

Metal: CorrosionGalvanism

Concrete: Chemical DeteriorationArmor Corrosion

Wood: Rot DamageMarine Organisms

Synthetics: Aging

Sand Blasting

Ice

DESIGN ASPECTS


•MISCELLANEOUS FACTORS

Costs

Availability of Material

Vandalism

Side Effects

Cooperate vs. Individual Approach


DECISION CONSIDERATIONS

* DATA & FACTS

* ONGOING PROCESS

* DESIRED RESULTS

* COSTS

* EFFECT ON CONTINUED EROSION

* ENVIRONMENTAL CONDITIONS

9


PROTECTIVE MEASURES

* SEAWALLS/REVETMENTS/BULKHEADS

* PROTECTIVE DUNES & EARTH DAMS

* DETACHED BREAKWATERS

* GROINS

* PROTECTIVE BEACHES

* ARMORING VEGETATION

* SLOPE ADJUSTMENT


SLOPE PROTECTION (SEAWALLS):

L11-10


Types of ‘SEAWALLS’:

* SEAWALL * REVETMENT

* BULKHEAD

10


SEAWALLS:

* BASIC CROSS-SECTION (Rubble Mound Revetment)

* DEFINITION

Structure placed on land,

parallel to shoreline

to separate land from water.

Geotextile


* FUNCTIONAL DESIGN CONSIDERATIONS

Distance From Shoreline (Landward Limit)

Length & Location Alongshore

Water Levels on Sea- & Landward Sides (Height)

Overall Shape (Use, Run-Up, Reflection, ...)

Soil Stability

Availability of Materials

Economic Limits

Environmental Concerns

Institutional Constraints


* ADVANTAGES

Prevent Dune/Bank Erosion

Prevent Flooding

* DISADVANTAGES

High Initial Costs

Down-Drift Erosion

No Beach-Berm Protection

Decreased Beach Access

Increased Wave Reflection

* CHOICE OF MATERIALS

Rubble Concrete Slabs

Concrete Waste Gabions

Filter Cloth

11


Maybe not so good!Land

Sea

SEAWALL – AN APPROPRIATE ALTERNATIVE?


Seawall Originalshoreline

Incoming waves Reflected waves Erosion Deposition

Impact of Seawall?


RUBBLE - CONCRETE SLABS/WASTE

* GENERAL

Medium - High Energy Coasts

* ADVANTAGES

Economical (local material)

Long Life

High Durability

Low Run-Up

Good Flexibility

* DISADVANTAGESRequires Heavy Equipment

Requires Slope Adjustment (1:2 - 1:3)Aesthetics

12


* GENERAL REQUIREMENTS

Fill Compacted and Free From Rocks

Filter (Cloth and Gravel)

Shape of Rubble/Concrete (L < 3B)

Toe Scour Protection (max Hunbroken)


CONCRETE SLABS

*GENERAL

Medium - Low Energy Coast

* ADVANTAGES

Low Weight

Availability

Aesthetics


* DISADVANTAGES

Require Slope Adjustment

Require Fill

Require Quality Casting

Sensitive to Hydraulic Pressure

Difficult to Repair

High Run-Up

Vulnerable to Ice

* GENERAL

Moderate Waves

Interconnecting

Manpower Placing

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GABION SEAWALLS

* GENERAL

Moderate - Low Energy Coasts

Simple Equipment

Very Flexible

Easily Repairable

Susceptible to Wear

Corrosion Prone

Initial Overfill


COMMON SEAWALL FAILURES

Slope Toe Scour

Deformation

Wash-Out of Fine Sediments

Failure in Over-Steep Slopes

Crest Erosion (Overwash)

Slope Collapse (Soil Instability)

Slope Blow-Out (Hydraulic Pressure)

Material Failure

Vandalism


BULKHEAD – PILING

* GENERAL

Medium - High Energy Coasts

* TYPES

Cantilever

Anchored

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* ADVANTAGES

Durability

Good for Steep Slopes

Low Maintenance

"Discreet"

* DISADVANTAGES

Require Special Equipment & Personell

High Noise Level

High Wave Reflectivity


CHOISE OF MATERIALS

* WOODSoft Soil

Non-Cohesive Back-Fill

Precover Anchor

Minimize Gaps between Sheets

Filter Cloth

Drainage Holes

*STEELHard Soil

Drainage Holes


REVETMENT - SANDBAGS

* GENERAL

Low Energy Coast

* ADVANTAGES

Easy to Install

Easy to Remove

Easy to Repair

* DISADVANTAGES

Short Duration

Easily Punctured

UV-Sensitive

Toe Erosion

15


REVETMENT - CONCRETE ELEMENTS

* GENERAL

Low Energy Coast

Require Compacted Slope

Gives Smooth Slope

Filter Cloth Important

Watch for Toe Erosion

Watch for Flank Erosion

12-09


PROTECTIVE DUNES & EARTH DAMS

* DEFINITION

Natural or Artificial Dunes or Earth Dams,

Maintained at Proper Dimensions

* GENERAL

Medium - Low Energy Coasts

Preferably Vegetated


* ADVANTAGES

Protect Against Flooding

Cost Effective

Stockpile Material for Beach

Aesthetics

* DISADVANTAGES

Vulnerable to Continuous Erosion

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Hybrid – Dike in dune

Noordwijk, NL


FILTER CLOTH STABILIZATION

* GENERAL

Low Cost

Small Waves Only

Aesthetics


SLOPE ADJUSTMENT

* ADVANTAGES

Economical Increased Availability

* DISADVANTAGE

Temporary

* GENERAL

Requires Vegetation Requires Crest Space

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GROINS

•DEFINITION

Narrow Structure of Limited Length, Constructed (Near)Perpendicular to the Beach.

Designed to Trap Longshore Drift for:

- Building a Protective Beach

- Retarding Erosion of Existing Beach or

- Preventing Beach Materialto Reach Down-Drift Location


New winter shoreline

Ideal Effect of Groins


* FUNCTIONAL GROIN DESIGN CONSIDERATIONS (1/2)

Longitudinal Section(height, length, permeability)

Beach Alignment

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* FUNCTIONAL GROIN DESIGN CONSIDERATIONS (2/2)

Groin Spacing & Tapering

General Rule: sn = 2 - 3 ln


* ADVANTAGES

Build Out the Beach, Aesthetics

* DISADVANTAGES

Down-Drift Erosion

Require Sand Transport

Do Not Protect Dunes

Do Not Prevent Flooding

Function Best with Coarse Sand

Sensitive to Ice


GROINS?

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+ -

?

GROINS


?

+

GROINS + SAND


* COMMON DESIGN FAILURES

No Dominating Wave Direction

Too Long (Down-Drift Erosion)

Too Close (Force Out Transport)

Too Far Apart (No Interaction, Rip Currents)

Too Impermeable (Inefficient)

Not Anchored Far Enough Landward (Flank Erosion)

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DETACHED BREAKWATERS

•DEFINITION

A Structure Designed to Provide

Protection to an Area or Shoreline

Located Inside of the Structure


* ADVANTAGES

Trap Sand Efficiently

No Structures on Beach

Prevent Offshore Erosion

Limit Wave Energy

* DISADVANTAGES


Down-Drift Erosion

Requires Substantial Sand Transport

Aesthetics(?)


DETACHEDBREAKWATERS?

21


DETACHEDBREAKWATERS?

+- +-



Breakwater Length vs. Distance Offshore

Structure Height (Overtopping)

Breakwater Permeability

Single vs. Segmented Breakwaters

Distance Offshore vs. Breaker Zone Width

Fillet Volumes

Availability of Materials

Economic Limits




* GENERAL

Require Sand Transport

Limited Dune Protection


Function Best with Coarse Sand

* CONSTRUCTION TYPES

Rubble

Concrete Blocks

Concrete Caissons

GabionsSand Bags

Combinations

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Conclusion – Hard measures do not SOLVE erosion problems, they MOVE them!

+?

+

-

-

Zero sum game?

-

+

-Erosion moves down-drift

Cliff/Bluff erosion stops!Beach disappears?

Beach erosion stops!Beach disappears?

Problem moves down-drift!


Construction,

Beach fills reinforcement Vegetated & vegetation earth dams of dunes

Plan Profile

Stabilizing – Soft Protection

13-09


PROTECTIVE BEACHES

* DEFINITION

A Natural or Artificial Beach, Maintained at Proper Dimensions

* ADVANTAGES

Good Protection Against Moderate Waves

No Structures

Moderately Low Initial Costs

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UTFYLLNAD

-+

SAND- BEACH FILL (SAND)


Nourishment Strategies

Berm Nourishment

Beach NourishmentProfile Nourishment


Foto: RIKZ

Foto: RIKZ

Foto: RIKZ

Nourishment Strategies

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BUT......

..won’t the fill disappear again?

No! but...


HOW MUCH SAND IS LEFT?

Eroded volume

Eroded volume

Condition 2015without fill

Condition 2025without fill

Condition 2015with fill

Condition 2025with fill


BUT......

..don’t you have to do it again?

No! but... Yes! but...

..won’t the fill disappear again?

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BUT......

.... worthwhile to repaint?

.... worthwhile to re-pave the streets?.... worthwhile to clear the snow?

..don’t you have to do it again?


WHICH ALTERNATIVES SHOULD WE COMPARE?

Today’s condition......?

?

?

2015

2015


..... or perhaps in 10 years?

!

2025

2025

!

WHICH ALTERNATIVES SHOULD WE COMPARE?

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ALTERNATIVE STRATEGIES

Do you want a rocky beach?

Use rock!

Do you want a sandy beach?

Use sand!


Planned retreat Hard protection

POSSIBLE TO CHANGE YOUR MIND?

Soft protection

Abandon Adapt

NO! NO!

NO! Maybe

YES!


* GENERAL

Fine Fill Material: Increased Erosion

Coarse Fill Material: Decreased Erosion

Initial Overfill

Increased Down-Drift Transport

May Focus Wave Energy

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Longshore Transport Characteristics

Deficiency of Material Supply

Characteristics of Native Beach Sand

Evaluation & Selection of Borrow Material

Beach-Berm Elevation & Width

Wave-Adjusted Foreshore Slopes

Beach Fill Transitions

Economic Limits




GRAIN-SIZE DETERMINES PROFILE SHAPE

Fill coarser

Fill same

Fill finer

Fill much finer


PERCHED BEACH

* DEFINITION

An Artificial Beach, Maintained by an

Underwater Rock Sill

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*ADVANTAGES

Builds Out the Beach

Limited Losses of Sand

Less Maintenance than Regular Beach Fill

* DISADVANTAGES

No Flooding Protection

Only Protection Against Mild Waves


VEGETATION

* FUNCTION

Stabilization of Natural or Artificial Dunes and Dams

* ADVANTAGES

Resistive Against Short Storms

Self-Repairing After Minor Failures

Inexpensive (~ 1/10)

Aesthetics

Good Durability

* DISADVANTAGES

Sensitive to Continuous Wave Action

Requires Substantial Maintenance

Initial Protection => Limited Access


* GENERAL

Slope Adjustment (min 1:3)

Right Specie, Distance, Place, & Time

Own Nursery

* ENVIRONMENT

Repeated Accumulation/Erosion

Flooding/Drought

Salt/Fresh Water

Sand Blasting

Limited Nutrition

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ADMINISTRATIVE MEASURES

Hans Hanson, Lund University, Sweden1) Indirectly2) If Significant Transport or Pre-Filled3) Unless Pre-Filled

CHARACTERISTICS OF DIFFERENT PROTECTIVE MEASURES

Seawalls

Protective Dunes & Earth Dams

Groins Detached Breakwaters

Protec-tiveBeach

Vegetation

Perched Beach

Build Out Beach

No Yes1) Yes2) Yes2) Yes No Yes

Protect Dunes/Backland

Yes Yes Yes1) Yes Yes1) Yes Yes1)

Prevent Flooding

Yes Yes No No No No No

Decrease Wave Height

No No Yes1) Yes Yes1) No Yes1)

Increase Wave Reflection

Yes Yes No No NoNo

No

Down-Drift Erosion

Yes No Yes3) Yes3) No No No

Initial Cost High Moderate High -Moderate

High Moderate -Low

Low Moderate

Maintenance

Low Moderate Moderate -Low

Moderate -Low

High Moderate Moderate

Aesthetics No Yes Can Be Can Be Yes Yes Yes

Limited Access

Yes No No No No Can Be No

Manual Labor

Can Be Can Be No No Can Be Yes No


AVAILABLE MEASURES AGAINST COASTAL EROSION.Measure Seawalls Groins Detached

BreakwatersBeach Nourish-ment

Vegetation

Functioning Separates land from water

Decreases the amount of sand leaving the coastal reach. Builds out a beach on up-drift side.

Attenuate the wave energy on adjacent beach. Builds out a beach behind structure.

Replaces lost beach material.

Decreases the susceptibility for erosion. Mends itself after moderate damages.

Prerequisites Significant amount of longshore transport. Dominating transport direction.

Significant amount of longshore transport.

Availability of suitable borrow material.

Favorable conditions for plants.

Limitations Does not create dry beach area. May cause deepening of offshore bottom. May cause downdrift erosion.

Does not directly protect eroding hinterland. May cause downdrift erosion.

Does not directly protect eroding hinterland. May cause downdrift erosion.

Does not decrease erosion rate. Does not directly protect eroding hinterland.

Only slows down erosion.

Economic Considerations

High initial costs.Moderately high maintenance costs.

High initial costs. Moderately high maintenance costs.

Very high initial costs. High maintenance costs.

Low initial costs. High maintenance costs.

Low initial costs. Low maintenance costs.

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8 coastal protection prt · * basic cross-section (rubble mound revetment) * definition structure...

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