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HARD STRUCTURES

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

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HARD STRUCTURES

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  1. 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

  2. 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

  3. SCREW YOUR NEIGHBOR PHENOMENA I’m all wet, Darn neighbor Brett High and dry Drink Mai Tai Modified Shoreline Original Shoreline Transport direction

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

  5. PLANFORM EVOLUTION Degrees indicates wave angle

  6. PLANFORM EVOLUTION For groin length, L, can determine area of planform when bypassing just occurs as Colorscale is the log10 of the area

  7. 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 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 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.

  8. SEDIMENT ACCUMULATION: DETACHED BREAKWATER Sediment accumulation Breakwater Island acts like a breakwater • Sediment accumulates behind breakwater for two reasons: • Diffraction of waves • Sheltering of sediment carrying currents and waves (main reason)

  9. 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

  10. SEDIMENT ACCUMULATION: DETACHED BREAKWATER : The transfer of energy along a wave crest. Diffraction Wave Energy Transfer No wave Causes waves to have a round shape from edge of structure

  11. SEDIMENT ACCUMULATION: DETACHED BREAKWATER : The blocking of wave energy that drives along shore currents Sheltering Weak transport Sediment 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

  12. SEDIMENT ACCUMULATION: DETACHED BREAKWATER Simple Engineering Formulas X L = Breakwater length X = Distance from shore L

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

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