HARD STRUCTURES

• Designed with erosion mitigation in mind

• Typically not coincident with placement of sand, but can be

• Can often have adverse effects on shoreline of planform

GROINS

• Vertical barrier extending from dunes (typically) offshore

• Meant to trap alongshore drifting sediment

• Impounds updrift thus it causes erosion downdrift

• To minimize downdrift erosion, can place a groin field rather than single one

• Less interest lately as they lead to the screw your neighbor problem

SCREW YOUR NEIGHBOR PHENOMENA

Original Shoreline

Modified Shoreline

High and dryDrink Mai Tai

Transport direction

I’m all wet, Darn neighbor Brett

PLANFORM EVOLUTION

Groin

L

bx

ty tan,0

BC; such that waves are normal to shore right at groin. Implies no alongshore transport

bGt

x

Gt

xerfcxe

Gttxy

tan

4

4),( 4

2

Next slide for what solutions look like

where )(1)( zerfzerfc

PLANFORM EVOLUTION

Degrees indicates wave angle

PLANFORM EVOLUTION

For groin length, L, can determine area of planform when bypassing just occurs as

b

b

LGttA

tan4

tan**2

Colorscale is the log10 of the area

PROOF THAT UPDRIFT ACCRETION = DOWNDRIFT EROSION

Suppose a groin exists at a location xG along a straight beach. Apply the sand conservation

equation from –x0 to x0, where the distance x0 from the groin is outside the region of the groins

influence. The total integral is broken into two parts as

00

0

dxt

V

x

Qdx

t

V

x

Qx

x

x

x G

G

where Q is the sediment transport rate and V is the volume of sediment.

TERM1: dxt

VxQxQ

Gx

x

G

0

)()( 0

TERM2: dxt

VxQxQ

x

x

G

G

0

)()( 0

Add terms together and set equal to zero as above

dxt

VxQdx

t

VxQxQxQ

x

x

G

x

x

G

G

G

0

0

)()()()( 00

or

dxt

Vdx

t

Vx

x

x

x G

G

0

0

since the sediment transport rate at x0 and –x0 are equivalent because they are

outside the region of influence of the groin. Thus, the volume deposited updrift is equivalent to the volume deposited downdrift.

SEDIMENT ACCUMULATION: DETACHED BREAKWATER

Breakwater

Sediment accumulation

Island acts like a breakwater

Sediment accumulates behind breakwater for two reasons:

1) Diffraction of waves2) Sheltering of sediment carrying currents and waves (main reason)

SEDIMENT ACCUMULATION: DETACHED BREAKWATER

Tombolo

If sediment accumulates until it reaches breakwater, a feature known as a tombolo is formed. This is usually not desirable.

Tombolo

SEDIMENT ACCUMULATION: DETACHED BREAKWATER

Diffraction: The transfer of energy along a wave crest.

Wave

No wave

Energy Transfer

Causes waves to have a round shape from edge of structure

SEDIMENT ACCUMULATION: DETACHED BREAKWATER

Sheltering: The blocking of wave energy that drives along shore currents

Waves traveling towards shore at angle

Sediment transportSediment transport Weaktransport

The waves that drive the current and transport are weaker behind the structure causing sediment to accumulate

SEDIMENT ACCUMULATION: DETACHED BREAKWATER

Simple Engineering Formulas

L

XL = Breakwater lengthX = Distance from shore

Sediment Accumulation Tombolo Formation

L/X < 1.0 SPM(1984) L/X > 2.0 SPM(1984)

L/X < 1.0 Gourlay (1981) L/X >1.5 to 2 Dally and Pope(1986)

L/X < 0.4 to 0.5 Dally and Pope(1986)

L/X > 1 Suh and Dalrymple (1987)

L/X = 0.6 to 0.67 Suh and Dalrymple (1987)

L/X <1.5 Ahrens and Cox (1990)

PERCHED BEACH

B

h1h2

Δyo

y1y2

Look at volumes to determine

3/52

3/51 5

3

5

3yAyAyBV FNo