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 Placement Hydraulic or mechanical placement over the profile, on the dry beach. Split-hull dredge. Compatibility Size and material
Loss of Beach Nourishment Three mechanisms for sand loss: Profile equilibration: out-of-equilibrium Spreading out losses Background erosion
Fill Material Bodge, JWPCOE, 2006
Overfill Factor Native sand is in some equilibrium with the wave climate Overfill Factor Fill sediment size distribution will approach that of the native sediment. Finer sand/coaser sand lost--krumbein & James (1965)
Beach Fill Compatibility Dean s Method: Less conservative; smaller overfill factors. mean grain size std dev Sand finer than is lost. Mean of fill to be the same as borrow Same mean size for both
Compatibility of Fill Material Given: mean diameter of native material, and mean diameter and std. deviation of fill material, find K Problem: Sediment range is small. Well-sorted sediments a problem.
Overfill Factor on Single Curve Bodge, JWPCOE, 2006
3 rd Way: Equilibrium Profile Compatibility Use equilibrium beach profile to determine the fill volumes required 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 Nourishments Using 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 Star denotes virtual origin of native profile Consider AN = AF Now that threshold is reached, then normal beach fill calculations hold for additional dry beach width.
Perched Beach Concept: Anchor offshore end of profile 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
V is a function of h_1, A_N, A_F, B Fill Volume
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 year follows offshore contours, so refraction due to the offshore bathymetry is important
Grand Isle, Louisiana Effects of two offshore borrow pits
Drag Scraper Nearshore excavation of sand Borrow pit left behind How 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 m 3, h= 5-8 m, mound height = 2m Dean Number ~ 4-5, onshore motion according to Wright & Short beach stage
Offshore Berm No motion in four years. Stabilized coast. Otay (1994) Dean No. = 2
Beach Drains Why they should work: Decrease backwash Increase fall velocity Reduce outflow from beach www.shoregro.com Turner & Leatherman (1997) review: Not much proof of effectiveness Filters sand from surf zone Reduction of water table
Example: Cs=0.05; Plating-out Effect
Problems Pumping rates are high Storm damage to equipment Pump-free system: Davis et al., ICCE (1992) Drains installed in beach Appears there is water table reduction No morphological change
Vegetation as Shore Protection Mangrove shorelines Submerged Grasses (Real or Artificial) Reefs Coral Worm Kayangel, Palau