Coastal Processes &
Structures
OCE 582 November 19 & 24 2003
Coastal Sediment
Properties
Important for:
dredging
environmental questions
beach fills
scour protection
sediment transport predictions
Dredging
A hydraulic dredge entrains sediment from the bottom and pumps it
through a pipe
Entrainment and the pumping controlled by the properties of the
sediment
Sediments are classified for entrainment as:
fluid: very soft muds (easily entrained)
loose: loose silt and sand (easily entrained)
firm: dense sand and stiff clay (needs a cutterhead dredge to
loosen)
hard: rock or corals (unsuitable for dredging)
Dredging
Sediment can also be classified as cohesive, noncohesive, or mitigated
Cohesive sediments get transported through the pipe as lumps and nodules
Noncohesive sediments disperse as a slurry, which is more easily pumped
through the pipe
Mitigated sediments consist mainly of noncohesive sediments with a small
amount of clay, which increases the transport efficiency of the pipe
Dredging “Rules”
1. The diameter of the pump limit the size of the material which can be
pumped
2. Oversize material is prevented from entering the pipeline of a suction
dredge by
grid placed across the draghead
cutterhead
3. Degree of cohesiveness allows the sediment to stand in near-vertical
banks while being dredge increasing efficiency
4. Hydraulic dredges may not be efficient - mechanical dredges may be
required:
grab bucket operated from a derrick for softer sediments
dipper dredge (power shovel on a barge) for harder sediments
Environmental
Environmental problems are associated with the handling and deposition
of sediment
Most frequently associated with dredging operations
BUT, concerns occur anytime sediment is introduced into the marine
environment
Burial of bottom-dwelling organisms most problematic
Other issues: blockage of light to all organisms and the toxicity
of the sediments
Environmental
Environmental problems are associated with the handling and deposition
of sediment
Most frequently associated with dredging operations
BUT, concerns occur anytime sediment is introduced into the marine
environment
Burial of bottom-dwelling organisms most problematic
Other issues: blockage of light to all organisms and the toxicity
of the sediments
Environmental
Sediment property most critical is: grain size
Turbidity in the water column depends on fall velocity
Fall velocity is a function of grain size
What grain sizes are most problematic? why?
Environmental: What
do you do?
Know the local, state, and Fed regulations
Conduct an early site investigation including sediment sampling
Assess the sediment for size, composition and toxicity
Beaches
Beach fills have 2 functions: 1) upland protection and 2) increase
recreational space (temporary)
Sand size or greater is ESSENTIAL
Sand size is required for recreational purposes
Beaches
Source for beach fills is called “borrow” material
Try to match the grain size distributions of native and borrow
materials; if not possible - skew to a coarser borrow
Can select a borrow with small amount of fines because they tend to be
lost in the dredging and placement process
Borrow > 2 mm is not suitable for recreational
Color and odor must also be considered in selection of borrow
Scour Protection
Scour: localized removal of bed material below its natural elevation or
bathymetry
Occurs: marine structures (jetties, seawalls, bridge pilings, seafloor
installations)
Prevention: revetment aka scour blanket
Revetment: e.g., broken rock (rip rap) that provides a layer resistant to
scour
Revetment properties: size distribution, density, porosity, permeability
Scour Protection
Density: must be heavy enough to resist motion under design currents
What density do we mean here?
Porosity and layer thickness must be able to dissipate fluid energy
Permeability must be adequate to relieve pore pressure
Scour Protection
Riprap ideally: dense, durable blocky
Bad riprap: coral, limestones, shales
Sediment Transport
physics of sediment transport poorly known
functions relate fluid properties (shear stress) to sediment properties
(shear resistance)
shear resistance is approximated by; grain size, grain density, angle of
repose
empirical methods generally provide better information:
tracers (trace and/or heavy minerals)
optical sensors
Coastal Survey
Estimate of Velocity
Shore Protection
Issues
storm damage reduction
coastal erosion
ecosystem restoration
Storm Damage
Flooding: probably causes more damage than any
other mechanism; caused by storm surge - more of
a problem in the NE US
hurricanes
nor’ easters
Wave damage: damage is a nonlinear function of
wave height - more of a problem on the west coast
US
Storm Damage Mitigation
After a devastating flood in 1953, the Dutch began the Delta Project: barriers (dams)
across the estuarine openings to the North Sea. It is one of the largest coastal
engineering projects ever completed.
Coastal Erosion
Storms create short-term erosional events
Natural recovery after the storm and seasonal fluctuations may not be in balance
Shore protection projects moderate the long-term average erosion rate of shoreline change
from natural or manmade causes
Reduced erosion means a wider sediment buffer zone between the land and
the sea
Erosion mitigation = damage reduction from flooding and wave
attack
Natural shorelines remain stable and mitigate upland damage
Control of coastal flooding and erosion is a myth
We cannot control nature
Mitigation (reduction in levels of flooding and erosion) means storm damage reduction
Therefore risk-based design is essential
Natural Beaches
Minimum Beach Width
Naturally open and pocket beaches have a minimum stable
beach width
Ymin is defined as the horizontal distance between the mean
highwater (mhw) shoreline and the landward boundary
Minimum Beach Width
Erosion Mitigation Structures
Infrequent storms will reach the foredune, cliffs, structure or vegetation line.
Beach stabilization structures are designed to provide the minimum, dry beach width
for shore protection under extreme conditions
3 common beach erosion mitigation structures: headland breakwaters, nearshore
breakwaters, and a groin field
Each has a Ymin required for design located in the gap area with greatest wave
energy
EST method determines the probability distribution of minimum dry beach widths
including the minimum for normal storm conditions
Functional designs depend on two factors: minimum dry beach width (or volume) and the
natural, sediment transport processes at the site design
Natural Beaches
Empirical Design
Parameters
For Chesapeake Bay Yg/Lg = 1.7
it takes several years to reach the predicted embayment shoreline
shape
Hardaway, Thomas, & Li (1991) present minimum design parameters for
medium wave energy shorelines
Offshore Breakwaters
Breakwater
Breakwater
Y = Distance of breakwater from nourished shoreline
Ls = Length of breakwater structure
Lg = Gap distance between adjacent breakwater segments
ds = Depth (average) at breakwater structure below mean water level
When the breakwater is long and/or located close to shore, conditions
favor tombolo formation
Ls/Y > 1-2 for tombolo formation
Dally and Pope (1986) recommend:
Ls/Y is 1.5 to 2 for a single breakwater
Ls/Y = 1.5 for a segmented breakwater
Breakwater
short breakwaters at greater distance from shore favor salient formation
for salient formation: Ls/Y is 0.5 to 0.67
tombolos do not form: Ls/Y = 0.12
permeable structure systems (partly submerged, large gaps) also allow
sufficient wave energy to minimize the chance for tombolo formation
Ahrens and Cox (1990) defined a beach response index, Is = exp (1.72 - 0.41
Ls/Y):
Permanent tombolo formation, Is=1
Periodic tombolos, Is=2
Well-developed salients, Is=3
Subdued salients, Is=4
No sinuosity, Is=5
Empirical Results
Coastal Structures
Seawalls & Dikes
Bulkheads
Revetments
Coastal Structures
Seawall
New Tech. Structures
Pre-cast, concrete units
Geotextile filled bags
Beach drains
Talks
Sampling Disturbance
Mallik Gas Hydrates
Storegga
Ekofisk Pore Pressure
Ekofisk Subsidence
Hurricanes
Intro to Tsunamis
PNG Tsunamogenic Landslides
Puerto Rican Trench Tsunamis
