Download Coastal protection

Coastal protection
GEO3-4306: Coastal Morphodynamics
Egmond and Bergen aan Zee
following the big 1990 storm
Causes of coastal erosion
Episodic:
• Storms / big waves
Persistent:
• Alongshore gradients in sediment
transport
• Sea level rise
• Human interference (e.g., dams in rivers)
• Changes in storm climate
Schematic of dune erosion
Empirical erosion profile
Toward a more process-based approach
The Delft3D/SWAN Part
HRD wind grid
Δt=3hr
1 km
2 km
4 km
Isl
Santa Rosa
and
Schematic of erosion due to sea level rise
Bruun rule: R ≈ S / tan(θ)
• heavily debated
• not generally accepted
Coastal erosion
• Coastal ‘engineering’:
– retain beach material
– functions of the coast
• Human measures interfere with natural
processes
Type of engineering measures
• Hard solutions
– solid, impenetrable barrier between the land
and the sea
– preventing any land / sea interaction
• Soft solutions
– natural material (sand, ….)
– work with nature
Why hard solutions?
•
•
•
•
•
Tradition
Perceived security
High value of the hinterland
Politics (‘visibility’)
….
Seawalls and revetments
Pros:
• Solid physical barrier
• Very visible
Cons:
• Scour
• Downdrift erosion
• Fixes coast (sea level rise)
land
sea
Shore normal structures (groynes)
Shore normal structures (harbour jetty)
Groynes and jetties
Pros:
• Protection of harbours
• Wide beaches on updrift side
Cons:
• Erosion on downdrift side
• Interruption of alongshore drift
• Risk of rip currents
Shore parallel structures (breakwaters)
Breakwaters
Pros:
• Reduced wave activity
• Wide beach in lee side
Cons:
• Erosion elsewhere
• Risk of rip currents
• Vulnerable to damage in storms
• Expensive
Hard solutions
• Primary purpose: protect hinterland from
flooding
• Separates problem (‘erosion’) from cause
(‘alongshore transport’)
• Do not provide sediment to the beach Æ
change location of problem
Soft solutions
• Dune nourishment
– Primary purpose: strengthening of dunes
• Beach nourishment
– Primary purpose: increase beach width
• Shoreface nourishment
– Primary purpose: increase coastal safety
Dutch coastal policy
• “Dynamic preservation”
– Coastal change is allowed except when the
present coastline shifts shoreward of the 1990
coastline
– Combat erosion with soft engineering
solutions (sand nourishments)
– Safety oriented
Coastline changes
240
Cross-shore distance [m]
230
Xtkl(LZK) : 223.64 m tov RSP-lijn
BKL : 198 m tov RSP-lijn
220
210
200
190
180
170
160
1960
1965
1970
1975
1980 1985
Years
1990
1995
2000
2005
any year: MKL (dots); 1980-1990: BCL; 1995-2005: TCL
Shift from beach to shoreface nourishments
Shoreface nourishments
Alongshore (lee) effect
Cross-shore (feeder)effect
Alongshore (lee) effect
• Nourishment acts as a wave filter
• Reduction of wave height and alongshore
current shoreward of nourishment
• Updrift sedimentation
• Downdrift erosion
Cross-shore (feeder) effect
• Nourishment acts as a shoal
• Enhancement of cell circulation patterns
• Enhancement of wave skewness
• Onshore transport over nourishment
• Potentially dangerous end effects (rips)
Shoreface versus beach
• Advantages:
–
–
–
–
cheaper (no pipelines to beach, etc)
more working hours during construction
smaller initial losses
less trouble for tourists
Shoreface versus beach
• Disadvantages:
–
–
–
–
–
effectiveness not well understood
no direct impact on beach width
potentially large up/downdrift effects
risk of rip currents
ecological damage?
Example 1: Egmond case
• Double sandbar system
• Cyclic offshore bar migration (15 years)
Storm waves are
5+ m
Example 1: Egmond case
• Nourishment sand moves shoreward (lee
effect)
• Nourishment stops offshore sandbar
migration
• Little effect on beach volume
• Elsewhere offshore sandbar migration
continues
• Expected lifetime about 6 years
Building with Nature