beach fill and soft engineering structures · 2010. 11. 11. · beach fill only coastal engineering...

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