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Coastal Processes 1 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 INVESTMENT BENEFITS: 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. HIGHEST ASTRONOMICAL TIDE MEAN SEA LEVEL 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 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. EXTREME EVENTS Similar Extrapolation for Extreme Winds. 100 Design Return Period 50 years. 10 50 year Design 1 Wave, H = 6.2m Return Period (Years) Period Return One year’s data 0 0 2.0 4.0 6.0 8.0 Wave Height (m) 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. 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: BREAKING H1 H2 H2 > H1 REFRACTION: SPREADING FOCUSSING HH HB HH > HB SHORELINE 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 SURF ZONE - SWASH ZONE OVERTOPPING BROKEN WAVES FLOODING ! BACKWASH SEDIMENT TRANSPORT ON COASTS UNI-DIRECTIONAL FLOW: FLOW SUSPENDED LOAD BED LOAD WAVE-INDUCED TRANSPORT: q WAVES OFF qOUT SEDIMENT qIN qSHORE ALONGSHORE TRANSPORT ∆ q = qIN + qOUT + qSHORE + qOFF EFFECTS OF COASTAL STRUCTURES A POCKET BEACH: q = 0 q = 0 STABLE BEACH BARRIERS: (OFTEN MAN-MADE!) SUPPLY BLOCKED BY HARBOUR EROSION ACCRETION 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 REDUCED REFLECTION AND OVERTOPPING ENERGY ABSORBTION POTENTIAL EROSION STABLE FOUNDATIONS WAVE WALL - REDUCED STABLE CREST OVERTOPPING ROCK ARMOUR SLOPE FILTER LAYER TOE STABLILITY TOE STABILITY ARMOUR: NATURAL ROCK (BLENDS), MAN-MADE UNITS (ARTIFICIAL) 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. 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 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.
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