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Wave Transformation, Water Levels and Coastal Flooding UFORIC


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Wave Transformation, Water Levels and Coastal Flooding - UFORIC Understanding Flooding on Reef-lined Island Coasts Workshop

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Wave Transformation, Water Levels and Coastal Flooding UFORIC

  1. 1. Wave Transformation, Water Levels and Coastal Flooding Ap van Dongeren Deltares, Delft, Netherlands
  2. 2. 23 februari 2018 The stage Incident wave groups Outgoing free long wave OFLW Run up Overwash Flooding Set up, S-S, IG, VLF wavesWave breaking Reef flat Island Reef slope Reef edge Shoreline
  3. 3. Reef slope • Wave shoaling follows mild-slope theory (Lowe et al., 2005a; Monismith et al., 2015) even though • waves are steep and asymmetrical • bottom slope is steep and complex • Improvements can be made using higher-order theories but probably minor contribution • Research need is wave dissipation due to high friction • Effect of offshore wave directionality and spreading • Data: bathy & rugosity hard to obtain in situ, remote sensing an option 23 februari 2018 N
  4. 4. Reef edge – wave breaking Field observations • Violent dissipation of S-S (0.04-0.2 Hz) energy in plunging breakers • Transformation to IG (0.005-0.04 Hz) and VLF (0.001-0.005 Hz) bands. • On rough complex reefs dissipation by bottom friction > wave breaking (Rogers, Monismith). Consequences for setup Model results: • Surprisingly good results for bulk Hm0 with simple models using a constant γ • γ ranges 0.6-1.1 and trends positively with beach slope but is a free parameter • Details of wave breaking not captured by simple models: • Consequences for wave-induced forcing • Detailed measurements in lab (Buckley et al., 2014) • Need for detailed measurements in the field 23 februari 2018
  5. 5. Reef Edge - IG wave generation • S-S energy transformation to IG wave band • IG waves generated by breakpoint mechanism • Numerical experiment on contribution of offshore bound wave and surfzone (breakpoint) forced waves • Surfzone generation only: no difference in H_IG on reef • Offshore generation only: 70% reduction of H_IG 23 februari 2018 Full Surfzone generated Offshore bound wave crossshorecrossshorecrossshore time
  6. 6. Reef edge – IG energy balance • Largely balance between flux, radiation stress input and bottom friction dissipation • large ε = IG wave bore dissipation? 23 februari 2018 Do IG waves become so large as to dissipate and break on the reef edge? Energybalanceterms
  7. 7. Reef flat – water levels • Tides • Astronomic tides are well understood. Unknowns: effect of (macro) tidal variation on waves and bottom friction transformation in case of macro-tides. • Surge • Driven by inverse barometric effect. Only important in case of “direct hit”, wind-driven set up smaller due to lack of continental shelf. • Wave-induced setup • Proportional to breaking wave height • Inversely proportional to water depth (Becker et al 2013) -> smaller depth -> larger rad stress term -> larger setup • Models underestimate setup levels due to incomplete forcing • Resonance • See next slides 23 februari 2018 Setup almost linearly dependent on wave height and inversely proportional to tide level
  8. 8. Narrow reef flats (<400m) – S-S and IG waves • S-S wave heights display tidal control • High tide: shoreline wave climate dominated by S-S waves • IG waves not tidally controlled and driven by offshore forcing • Low tide: runup elevation determined by set-up. 23 februari 2018 Tide level (m) HS-S HIG η Beet ham, JGR 2015
  9. 9. Wide reef flats (> 400 m) – S-S and IG waves • IG waves dominate spectrum on inner reef flat • IG dissipation due to bottom friction, tidal control • S-S wave dissipation by breaking and bottom friction 23 februari 2018 Offshoretoonshore • Fairly well predicted • Friction is important • Need characterization of coral species in terms of roughness coefficients • Or direct simulation of two-layer flow.
  10. 10. IG wave response on narrow and wide reefs • Analytical model extending breakpoint forcing to IG band predicts shoreline IG wave height (Becker et al 2015) • Response controlled by 23 februari 2018 Smoother/narrower/deeper Rougher/wider/shallower Hf is reef- face wave height
  11. 11. Reef flat - friction • Friction factors much larger than on sandy beaches (by factor 100) • Fw ~ 0.2 (Lowe) – 2! (Monismith) • Fc ~ 0.02 – 0.1, so order smaller. • Fc decreases with increasing depth (Pomeroy et al 2012) • Fc decreases with decreasing frequency (Lowe et al, 2005a) • Fc and fw expected to decrease with declining reef health. • How to translate props of coral reefs species to spatially varying friction fields? • More physical: account for porosity of reefs (in-canopy model)? 23 februari 2018
  12. 12. In canopy model validation on Buckley Labdata (rough elements) • Small modification to S-S wave height transformation • Much better transformation of IG relative to • Reference (smooth bed) • Calibrated (rough bed, cf tuninng) • Overprediction of setup. 23 februari 2018 𝐻𝑟𝑚𝑠 = 0.06
  13. 13. Underestimation of set up • Reasonable agreement on set up for Demirbilek case (mild foreshore slope) after tuning • Underestimation of set up on steeper foreshore slopes (Buckley case) relative to theoretical momentum balance linear theory • Need roller model to transfer momentum upward and shoreward. • Roller models are not commonly included in phase resolving models 23 februari 2018
  14. 14. Reef flat – VLF motions (Gawehn et al, 2016) Resonant waves at high water level and low peak frequencies Standing waves at intermediate to high water levels Progressive-growing at intermediate water levels Progressive-dissipative at low water levels. 23 februari 2018 Progressive-Dissipative Progressive-Growing StandingResonant 3.5% 31% 28.5% 37% Gawehn et al. JGR 2016 DEPTH Frequency
  15. 15. Reef flat - currents • Wave breaking generates a radiation stress gradient which is balanced in part by a pressure gradient • In the case of a low bottom frictional resistance, a cross-shore flow can be generated (from advective terms) • Friction is thus the controlling factor 23 februari 2018 • Outstanding questions: • Some obs on fringing reefs, few on atolls • What effect does 2D current field have on wave attack on beaches?
  16. 16. Island - runup • Wave run-up composed of • Mean component (surge, tide, wave setup) • Oscillatory component (IG and S-S) • Contributions vary according to forcing and reef params • Wave runup measurements on reefs are scarce • (Becker, Beetham, Cheriton), flotsam lines (Shimozono) • Wave runup under predicted by models • Wave overtopping • Not systematically measured 23 februari 2018 Runup larger for narrower reefs and steeper beach slopes
  17. 17. Runup contribution per frequency band 21 Runup trends reflect field and laboratory observations (a) (b) (c) (d) (e) (f) (g) Pearson et al JGR 2017 IG dominant on narrow reefs Consistent with Shimozono, not with Beetham?
  18. 18. Shimozono vs. Beetham 23 februari 2018 IG dominant in absolute sense. S-S relatively more important with decreasing width Narrow reef: S-S larger in absolute sense for higher water levels.
  19. 19. Runup characteristics as f(width, beach slope) 23 februari 2018 Narrow reef: IG and S-S with reef flat and beach resonances, large number of runup events Wide reef & steep beach: n-l steepening, IG bores, fewer runups but large damages Wide reef & mild beach: breaking dissipation, fewer and more gradual runups, less damages Shimozono et al, JGR 2015
  20. 20. Runup and overtopping trends per coastal orientation 23 februari 2018 Large change for previously sheltered reef Decrease in overtopping: due to topography? Shope et al., 2016
  21. 21. Island/coast - flooding • Flooding on Roi Namur due to • Two wave events with large wave heights and large wave periods • Spring high tides Causing • Skewed IG waves and • energetic VLF wave heights. • But also one anomalous wave event • Moderate offshore wave height but spring tide • Wave event seen propagating over the reef flat • Phasing of wave components important? 23 februari 2018 Cheriton et al, 2016
  22. 22. Application: Ebeye, RMI 23 februari 2018 Giardino and Nederhoff, World Bank report • On small islands 2D shock- capturing models suffice • Missing/under- researched processes: • Groundwater infiltration • Lagoon flooding • Larger coasts: • Reduced physics flood solvers
  23. 23. 23 februari 2018 Roi-Namur: waves matter Time (years) -> RCP8.5+iceRCP8.5RCP4.5 NO WAVES, JUST SLR Wave-induced flooding will cause significant impacts decades before static SLR+tide will. Storlazzi et al. 2018 submitted WITH WAVES
  24. 24. Round up: knowledge needs • Reef edge • Energy balance, IG “bore dissipation” term • Better description of forcing: roller model • Relative contributions of dissipation by bottom friction and breaking • (consequences for setup and circulation) • Reef flat • Macro-tidal effect on waves and circulation • Characterization of coral species in roughness field or replacement by porosity. • 2D circulation and effect on wave propagation • Trigger parameters for resonance (predictability) 23 februari 2018 • Island coasts • Runup dynamics (S-S/IG/VLF, relative magnitude & phasing, directions and spreading) • Morphodynamic coupled response due to change in forcing • SLR, wave angle, dir. spreading -> erosion and overtopping • Underprediction of setup and wave runup. • Flooding on lagoon side -> triggers. • Interaction ground water/surface water • Reduced complexity flood solvers
  25. 25. Round up: data gaps -> needs • Bathymetry • Shoaling zone through remote sensing • Porosity of reef structures • Topography • Elevation of islands and coasts through remote sensing, LIDAR, Laser • Hope for better SRTM? • Wave breaking • Field observations of breaking process. How?? • Circulation • 2D current fields on alongshore varying bathymetries • Runup and overwash • Observations (long term and high resolution): sensor strings/drones/video? • Flooding • Time variation of flood extents, depths • Ground water/aquifer interactions. 23 februari 2018