STRUCTURES HARD 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
High and dry Drink Mai Tai
Im all wet, Darn neighbor Brett
Modified Shoreline
Original Shoreline Transport direction
PLANFORM EVOLUTION
Groin L
y ( 0, t ) = tan b x
BC; such that waves are normal to shore right at groin. Implies no alongshore transport
4Gt y ( x, t ) = e
where
x2 4 Gt
x x erfc 4Gt
tan b
erfc( z ) = 1 erf ( z )
Next slide for what solutions look like
PLANFORM EVOLUTION
Degrees indicates wave angle
PLANFORM EVOLUTION
For groin length, L, can determine area of planform when bypassing just occurs as
L2 A( t *) = Gt * tan b = 4 tan b
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
0 Q V Q V x x + t dx + x x + t dx = 0 0 G
xG
x
where Q is the sediment transport rate and V is the volume of sediment. TERM1: TERM2:
Q ( xG ) = Q ( x 0 )
xG x0
t dx
x0
V
V Q ( x 0 ) = Q ( xG ) dx t xG
Add terms together and set equal to zero as above
V V Q( xG ) + Q( x0 ) = Q( x0 ) dx + Q( xG ) t dx t x0 xG
xG x0
or
V V dx = dx x t t 0 xG
xG
x0
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
Sediment accumulation
Breakwater
Island acts like a breakwater
Sediment accumulates behind breakwater for two reasons: 3) Diffraction of waves 4) 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
Energy Transfer
No wave
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
Sediment transport
Weak transport
Sediment transport
The waves that drive the current and transport are weaker behind the structure causing sediment to accumulate
Waves traveling towards shore at angle
SEDIMENT ACCUMULATION: DETACHED BREAKWATER
Simple Engineering Formulas X L = Breakwater length X = Distance from shore L
Sediment Accumulation L/X < 1.0 L/X < 1.0 L/X < 0.4 to 0.5 L/X = 0.6 to 0.67 L/X <1.5 SPM(1984) Gourlay (1981) Dally and Pope(1986) Suh and Dalrymple (1987) Ahrens and Cox (1990) Tombolo Formation L/X > 2.0 L/X >1.5 to 2 L/X > 1 SPM(1984) Dally and Pope(1986) Suh and Dalrymple (1987)
PERCHED BEACH
y2 B yo h2 h1 y1
Look at volumes to determine
3 3 5/3 5/3 V = Byo + AN y1 AF y2 5 5