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

1. Wave Transformation, Water Levels
and Coastal Flooding
Ap van Dongeren
Deltares, Delft, Netherlands

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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 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. 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. 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