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