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 dry Drink Mai Tai Transport direction I’m all wet, Darn neighbor Brett

PLANFORM EVOLUTION Groin L BC; such that waves are normal to shore right at groin. Implies no alongshore transport Next slide for what solutions look like where

PLANFORM EVOLUTION Degrees indicates wave angle

PLANFORM EVOLUTION For groin length, L, can determine area of planform when bypassing just occurs as Colorscale is the log 10 of the area

PROOF THAT UPDRIFT ACCRETION = DOWNDRIFT EROSION Suppose a groin exists at a location x G along a straight beach. Apply the sand conservation equation from –x 0 to x 0, where the distance x0 from the groin is outside the region of the groins influence. The total integral is broken into two parts as where Q is the sediment transport rate and V is the volume of sediment. TERM1: TERM2: Add terms together and set equal to zero as above or since the sediment transport rate at x 0 and –x 0 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 waves 2)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 transport Weak transport 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 X L = Breakwater length X = Distance from shore Sediment AccumulationTombolo Formation L/X < 1.0SPM(1984)L/X > 2.0SPM(1984) L/X < 1.0Gourlay (1981)L/X >1.5 to 2Dally and Pope(1986) L/X < 0.4 to 0.5Dally and Pope(1986) L/X > 1Suh and Dalrymple (1987) L/X = 0.6 to 0.67Suh and Dalrymple (1987) L/X <1.5Ahrens and Cox (1990)

PERCHED BEACH B h1 h2 Δyo y1 y2 Look at volumes to determine