The University of the West Indies Organization of

American States

PROFESSIONAL DEVELOPMENT PROGRAMME:

COASTAL INFRASTRUCTURE DESIGN, CONSTRUCTION AND MAINTENANCE

A COURSE IN

COASTAL ZONE/ISLAND SYSTEMS MANAGEMENT

CHAPTER 3

COASTAL PROCESSES 1

By PATRICK HOLMES, PhD Professor, Department of Civil and Environmental Engineering

Imperial College, London

Organized by Department of Civil Engineering, The University of the West Indies, in conjunction with Old Dominion University, Norfolk, VA, USA and Coastal Engineering Research Centre, US Army, Corps of Engineers, Vicksburg, MS , USA.

Antigua, West Indies, June 18-22, 2001

THE PURPOSE OF

COASTAL ENGINEERING

RETURN ON CAPITAL INVESTMENTBENEFITS:

Minimised Risk of Coastal Flooding - reduced costs and disruption to services in the future.

Improved Environment, Preserve Beaches - visual, amenity, recreational…...

COSTS:

Construction costs & disruption during construction.

Costs linked to avoidance of risks. (e.g., higher defences, higher costs, including visual/access impacts, but lower risks)

ORIGINS OF COASTAL PROBLEMS

1. Define the Problem.• This needs to be based on sufficiently reliable INFORMATION and requires a DATA BASE. For example, is the beach eroding or is it just changing in shape seasonally and appears to have eroded after stormier conditions? Beaches change from “winter” to “summer” profiles quite regularly.

• Many coastal problems result from incorrect previous engineering“solutions”. The sea is powerful and “cheap” solutions rarely work well.

• If the supply of sand to a beach is cut off or reduced it will erode because beaches are always dynamic. It is a case of the balance between supply to versus loss from a given area.

• A DATA BASE need not be extensive but in some cases information is essential and there is a COST involved.

• A photographic record of a coastal area - photographs taken at the same locations at the same times of the year - is cheap and very helpful. More extensive data bases need surveys and other measurements, with increasing costs.

STEPS TOWARDS A SOLUTION

2. A FEASIBILITY STUDY.

• The size of the study is linked to the size of the problem.

• Defines/sketches the options for a solution in sufficient

detail to give an order of magnitude of the potential costs.

• Indicates the availability of expertise and materials needed.

• Provides a basis for discussion with and decisions of the

client.

MARINE “FORCES”

TIDES:

• Regular and predictable because they are generated by the attractions of the Moon and the Sun acting on the oceans.

[High tide to Low tide ≈ Low tide to High tide, typically 6 hours and 26 minutes, but modified by the local land masses and coastal shape.]

WAVES:

• Generated by winds blowing over the ocean surface. Therefore they are not regular - they are RANDOM. This leads to the need to design for EXTREME EVENTS.

STORM SURGES:

• Increases in Mean Sea Level also generated by the wind, therefore also RANDOM.

WATER LEVELS AND MOTIONS

Mean Sea Level - Increasing due to Global Warming (+0.5m by 2050?)

Tides - Moon and Sun, Regular and Predictable.

“Spring Tides” “Neap Tides” - due to the changing influence of the moon and the sun every two weeks higher tides, Springs, and alternate two weeks lower tides, Neaps.

MEAN SEA LEVELHIGHEST ASTRONOMICAL TIDE

LOWEST ASTRONOMICAL TIDE

HIGHEST ASTRONOMICAL TIDE

H.A.T.

• Easily found by examining one year’s predicted tidal heights for the site and selecting the highest predicted level. This will be accurate to within a few millimeters.

• It may be necessary to measure tide levels at a site to relate them to predicted tidal levels and times at the nearest port for which predictions are available.

• It would also be useful to note L.A.T. - Lowest Astronomical Tide - indicates the width of a beach at low tide etc.

• Levels MUST be related to the land-based vertical datum use for the design.

LAND-BASED FACTORS

WHAT LAND-BASED FACTORS

CONTROL THE DESIGN?

COASTAL ACTIVITIES

Agriculture Fisheries Forestry

Commerce Transport Tourism

Infrastructure Environment

Special Sites Sand/coral Mining

Waste-water Disposal

Quantify Scale and Economic Importance

WINDS AND WAVES

WINDS - variable in speed and direction, seasonal.

IMPORTANCE:

Wind Loading.

Wind-induced “SET-UP” of the sea surface.

Wave Generation.

EXTREMES - as design criteria

“Return Period” - usually 50 years for design of coastal structures, the “50 year Return Period”

WIND INDUCED STORM SURGE

WIND

SHEAR FORCE ON THE WATER SURFACE

“STORM SURGE”

Set up is related to the SQUARE of the wind speed - more extreme winds create a much larger set up the FIFTY YEAR RETURN PERIOD.

WATER SURFACE SLOPES UPWARDS IN RESPONSE

EXTREME EVENTS

0 2.0 4.0 6.0 8.0

Wave Height (m)

100

10

1

0

Ret

urn

Perio

d (Y

ears

)

Design Return Period 50 years.

50 year Design

Wave, H = 6.2mOne year’s data

Similar Extrapolation for Extreme Winds.

WAVES

UNIFORM WAVES - SAME HEIGHT AND SAME LENGTH

Length (m)

Height (m)

Speed (m/s)

Wave Period = Time between successive waves (seconds)

Typically a 10 second wave will travel at 15 m/s. in deep water.

Its length in deep water will be 156m.

Sea Bed

WAVES

RANDOM WAVES different heights and lengths.

Average Wave Height; Significant Wave Height [HS] (m) etc…..

Highest Wave Height; (!) Zero Crossing Wave Period [TZ] (seconds)

WAVES FOR DESIGN

Record waves for Three hours and calculate Hs and Tz for each record

Eight records per day, 2920 records per year. (with luck!)

STATISTICAL analysis to predict

EXTREME WAVE CONDITIONS.

WAVE DIRECTION - a critical parameter for coastal engineering

Can be measured but often has to be derived from wind data.

MINIMUM OF ONE YEAR’S DATA REQUIRED

[TAOS Wave Predictions for the Caribbean]

WAVES IN SHALLOW WATER

CHANGE: HEIGHT, DIRECTION AND (EVENTUALLY) BREAK

SHOALING:H1 H2

H2 > H1

BREAKING

REFRACTION:

SHORELINE

FOCUSSINGSPREADING

HH

HB HH > HB

WAVES : SURF ZONE AND BEACH

BREAKING:WHEN THE WATER DEPTH EQUALS THE WAVE HEIGHT (APPROX.)

Hb

db

Hb≈ db

SURF AND SWASH:MAXIMUM RUN-UP

BACKWASH

OVERTOPPING

FLOODING !

SURF ZONE -

BROKEN WAVESSWASH ZONE

SEDIMENT TRANSPORT ON COASTS

UNI-DIRECTIONAL FLOW:

FLOW

BED LOAD

SUSPENDED LOAD

WAVE-INDUCED TRANSPORT:

qIN

qOUT

qSHORE

qOFF

∆q = qIN + qOUT + qSHORE + qOFFALONGSHORE TRANSPORT

WAVES

SEDIMENT

EFFECTS OF COASTAL STRUCTURES

A POCKET BEACH: q = 0 q = 0

STABLE BEACH

BARRIERS: (OFTEN MAN-MADE!)

ACCRETIONEROSION

SUPPLY BLOCKED BY HARBOUR

NET DIRECTION OF SEDIMENT MOTION

BEACH CONTROL STRUCTURES

GROYNES:

BY-PASSING - BY DESIGN

INCREASING COMPLEXITY - HIGHER EFFICIENCY ?

DETACHED BREAKWATERS:

TOMBOLA

COAST PROTECTION - SEA WALLS, REVETMENTS

WAVE REFLECTION OVERTOPPING

POTENTIAL EROSION

REDUCED REFLECTION AND OVERTOPPING

STABLE FOUNDATIONS

WAVE WALL - REDUCED OVERTOPPING

TOE STABLILITY

ROCK ARMOUR SLOPE

TOE STABILITY

STABLE CREST

FILTER LAYER

ARMOUR: NATURAL ROCK (BLENDS), MAN-MADE UNITS (ARTIFICIAL)

ENERGY ABSORBTION

BEACH NOURISHMENT

Y

ORIGINAL PROFILE

NEW PROFILE

VOLUME TO BE ADDED PER UNIT LENGTH OF BEACH

CONCEPT OF EQUILIBRIUM PROFILE FOR A GIVEN SAND DIAMETER

1. IF IMPORTED SAND DIAMETER = NATIVE SAND DIAMETER THE PROFILES WILL BE IDENTICAL.

2. IF IMPORTED SAND DIAMETER > NATIVE SAND DIAMETER THE NEW PROFILE WILL BE STEEPER, THE BEACH WILL BE MORE STABLE AND A LOWER VOLUME OF SAND WILL BE NEEDED.

[NOTE: TOO STEEP - DANGEROUS FOR BATHING!]

3. COUPLE WITH MEASURES TO CONTROL SAND LOSS ALONGSHORE.

x

h

h = Ax2/3

BREAKWATERS

BEACH, BERTH AND MOORING PROTECTION

CORE

ARMOUR LAYER

UNDERLAYER

FILTER LAYER

OVERTOPPING

CREST

TOE STABILITY

W

W/2 TO W/20

PREVENT LOSS OF FINES FROM THE CORE

COAST PROTECTION AS AN OPPORTUNITY

1. PRIMARY PURPOSE: PROTECT AGAINST

OVERTOPPING/FLOODING

2. FACTORS: VISUAL INTRUSION

ADDED AMENITY - SPORTS

LEISURE/SOCIAL

ENVIRONMENTAL IMPACT

3. MANAGEMENT: MONITOR

4. DESIGN FOR ENHANCEMENT: SEA LEVEL RISE

HIGHER STORMS