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The University of the West Indies Organization of American States

PROFESSIONAL DEVELOPMENT PROGRAMME: COASTAL INFRASTRUCTURE DESIGN, AND MAINTENANCE

A COURSE IN COASTAL DEFENSE SYSTEMS I

CHAPTER 3

INTRODUCTION TO COASTAL ENGINEERING

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

Organized by Department of , 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. St. Lucia, West Indies, July 18-21, 2001

1

THE PURPOSE OF COASTAL ENGINEERING

RETURN ON CAPITAL INVESTMENT BENEFITS: Minimised Risk of Coastal Flooding - reduced costs and disruption to services in the future. Improved Environment, Preserve - 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)

P.Holmes, Imperial College, London 2 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 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 is powerful and “cheap” solutions rarely work well. • If the supply of 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.

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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.

P.Holmes, Imperial College, London 4 MARINE “FORCES”

TIDES: • Regular and predictable because they are generated by the attractions of the Moon and the Sun acting on the . [High 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 surface. Therefore they are not regular - they are RANDOM. This leads to the need to design for EXTREME EVENTS. STORM SURGES: • Increases in Mean also generated by the wind, therefore also RANDOM. P.Holmes, Imperial College, London 5

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.

HIGHEST ASTRONOMICAL TIDE MEAN SEA LEVEL LOWEST ASTRONOMICAL TIDE

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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 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 MUSTbe related to the land-based vertical datum use for the design.

P.Holmes, Imperial College, London 7

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

P.Holmes, Imperial College, London 8

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”

P.Holmes, Imperial College, London GLOBAL WARMING - HIGHER WINDS?

IF WIND SPEED IN EXTREME STORMS INCREASE, WAVE HEIGHTS, WAVE ENERGY ETC. INCREASE NON-LINEARLY

WIND SPEED U + 5% +10% 1.0 1.05 1.10 1.0 1.103 1.21 (U2) WAVE ENERGY 1.0 1.22 1.46 (U4) (H2) ROCK ARMOUR 1.0 1.34 1.77 (U6) (H3)

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WIND INDUCED

WIND

SHEAR FORCE ON THE WATER SURFACE

WATER SURFACE SLOPES UPWARDS IN RESPONSE

“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.

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EXTREME EVENTS

Similar Extrapolation for Extreme Winds.

100 Design Return Period 50 years.

10

50 year Design 1 Wave, H = 6.2m

Return Period Period (Years) Return One year’s data

0 0 2.0 4.0 6.0 8.0 Wave Height (m)

P.Holmes, Imperial College, London 11 WAVES

UNIFORM WAVES - SAME HEIGHT AND SAME LENGTH

Speed (m/s) Height (m)

Length (m)

Sea Bed

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.

P.Holmes, Imperial College, London 12

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)

P.Holmes, Imperial College, London 13

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]

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WAVES IN SHALLOW WATER

CHANGE: HEIGHT, DIRECTION AND (EVENTUALLY) BREAK SHOALING: BREAKING H1 H2

H2 > H1

REFRACTION:

SPREADING FOCUSSING

HH

HB HH > HB SHORELINE P.Holmes, Imperial College, London 15

WAVES : AND BEACH

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

Hb

db ≈ Hb db SURF AND : MAXIMUM RUN-UP

SURF ZONE - SWASH ZONE OVERTOPPING BROKEN WAVES FLOODING !

BACKWASH

P.Holmes, Imperial College, London 16 ON

UNI-DIRECTIONAL FLOW:

FLOW SUSPENDED LOAD BED LOAD

WAVE-INDUCED TRANSPORT:

q WAVES OFF

qOUT

NT DIME SE qIN

qSHORE ALONGSHORE TRANSPORT ∆ q = qIN + qOUT + qSHORE + qOFF

P.Holmes, Imperial College, London 17 EFFECTS OF COASTAL STRUCTURES

A : q = 0 q = 0

STABLE BEACH

BARRIERS: (OFTEN MAN-MADE!)

SUPPLY BLOCKED BY HARBOUR

EROSION ACCRETION

NET DIRECTION OF SEDIMENT MOTION P.Holmes, Imperial College, London 18

BEACH CONTROL STRUCTURES

GROYNES:

BY-PASSING - BY DESIGN

INCREASING COMPLEXITY - HIGHER EFFICIENCY ?

DETACHED BREAKWATERS:

TOMBOLA

P.Holmes, Imperial College, London 19 PROTECTION - SEA WALLS,

WAVE REFLECTION OVERTOPPING REDUCED REFLECTION AND OVERTOPPING

ENERGY ABSORBTION

POTENTIAL STABLE FOUNDATIONS

STABLE CREST WAVE WALL - REDUCED OVERTOPPING ROCK ARMOUR SLOPE

FILTER LAYER

TOE STABLILITY TOE STABILITY

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

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BEACH NOURISHMENT

VOLUME TO BE ADDED PER UNIT LENGTH OF BEACH Y x h = Ax2/3

NEW PROFILE h

ORIGINAL PROFILE

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. P.Holmes, Imperial College, London 21

BREAKWATERS BEACH, BERTH AND MOORING PROTECTION

ARMOUR LAYER W OVERTOPPING UNDERLAYER W/2 TO W/20

FILTER LAYER CREST

TOE STABILITY CORE

PREVENT LOSS OF FINES FROM THE CORE

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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: HIGHER STORMS

P.Holmes, Imperial College, London Xtra

What can go here to improve amenity Power boat/jet-ski ramp value? Car parking Coffee shop/bar Sailing dinghy/wind-surfer ramp Scuba/diving centre

Segregate activities - safety

P.Holmes, Imperial College, London DATA COLLECTION “KIT” LEVEL 1.

Level, Staff and Tape. Digital Camera Hand-held GPS Digital Compass Yacht-type Echo Sounder (12v supply) Sample Bags

Reference Grid (x,y) Survey Land Levels (z) Bathymetric Survey (-z) referenced to MSL Datum Photographic Record (minimum 3 monthly and after any storm) Sample Beach and Nearshore Material

P.Holmes, Imperial College, London