beach fill and soft engineering structures · 2010. 11. 11. · beach fill only coastal engineering...
TRANSCRIPT
Chapter 11
Beach Fill and Soft Engineering Structures
Solutions to Coastal Erosion
Soft Structures
Hard Structures
Retreat
No such thing as low cost coastal protection
Beach Nourishment
Beach Filling
Beach Nourishment
Sand placed on beach to replace sand that has eroded from a beach
Buys time
Cures symptom
Increases storm protection
Adds recreational value
Beach Fill
Only coastal engineering remedial measure that adds sand to the system. Can be use alone or with structures.
Sources Submarine, subaerial
PlacementHydraulic or mechanical placement over the profile, on the dry beach. Split-hull dredge.
CompatibilitySize and material
Loss of Beach Nourishment
Three mechanisms for sand loss:Profile equilibration: out-of-equilibriumSpreading out lossesBackground erosion
Fill Material
Bodge, JWPCOE, 2006
Overfill Factor
Native sand is in some equilibrium with the waveclimate
Overfill FactorFill sediment size distribution will approach that of the native sediment. Finer sand/coaser sandlost--Krumbein & James (1965)
Beach Fill Compatibility
mean grain size
std dev
Sand finer than is lost.
Dean’s Method: Less conservative; smaller overfill factors.
Mean of fill to be the same as borrow
Same mean size for both
Compatibility of Fill Material
Problem: Sediment range is small. Well-sorted sediments a problem.
Given: mean diameter of native material, and mean diameter and std. deviation of fill material,find K
Overfill Factor on Single Curve
Bodge, JWPCOE, 2006
3rd Way: Equilibrium Profile CompatibilityUse equilibrium beach profileto determine the fill volumesrequired
Coarse fill, intersecting profile
Fine Fill, non-intersecting profile
Finer Fill, submerged profile
Calculate Fill Volumes: Non-Intersecting Profile
Substituting for
Effect of Decreasing AF
Same volume for all profiles
Increasing the Sand Volume: AF=0.1, AN=0.2 m1/3
Fill Volumes for h*/B=2
Fill Volume for h*/B = 4
Additional Dry Beach Width
Beach Fill Planform Response
Effect of Fill Length
An Approximate Retention Rate (Dette)
Solve for k
Dette et al. (1994)
Multiple NourishmentsUsing Pelnard-Considere solution
Note that the length of time between renourishments grows.
Multiple Nourishments
Erosional Hot Spots
Causes: Irregular refraction, offshore borrow pits, grain size variation, variation in fill rates, headland effects, coastal structures
Sea wall at Honolulu
Fill in Front of Seawall
Consider AN = AF
Now that threshold is reached, then normal beach fill calculations holdfor additional dry beach width.
Star denotes virtual origin of native profile
Perched Beach
What is the volume of sand needed?
Calculate volume using equilibrium profiles
Problems: Sand lost offshore; scour at wall,drowning hazard at offshore end, end losses
Concept: Anchoroffshore end of profile
Fill Volume
V is a function of h_1, A_N, A_F, B
Perched Beach vs Natural Beach
Sorenson and Beil (1988)
Lower fig: perched beach, offshore losses, landward scour,
Tybee Island Fill Project
Beach fill after one yearfollows offshore contours, sorefraction due to the offshorebathymetry is important
Grand Isle, Louisiana
Effects of two offshore borrow pits
Drag Scraper
Nearshore excavation of sand
Borrow pit left behindHow does it fill?
Offshore Berms
Purpose: Provide offshore feeder to beach Reduce wave height
If berm offshore of depth of closure, inactive
Example: Silver Strand, CA
113,000 m3, h= 5-8 m, mound height = 2m
Dean Number ~ 4-5, onshore motionaccording to Wright & Short beach stage
Offshore Berm
No motion in four years. Stabilized coast. Otay (1994) Dean No. = 2
Beach Drains
www.shoregro.com
Why they should work:
Decrease backwash
Increase fall velocity
Reduce outflow from beach
Filters sand from surf zone
Turner & Leatherman (1997) review: Not much proof of effectiveness
Reduction of water table
Plating-out Effect
Example: Cs=0.05;
Problems
Pumping rates are highStorm damage to equipment
Pump-free system: Davis et al., ICCE (1992)Drains installed in beachAppears there is water table reductionNo morphological change
Vegetation as Shore Protection
Mangrove shorelines
Submerged Grasses (Real or Artificial)
ReefsCoralWorm
Kayangel, Palau