Water Research Laboratory | School of Civil & Environmental Engineering

Laboratory Experiments into the Effect of Reef Width on Extreme Water Levels During Cyclone Events

Kristen D. Splinter, Meixi Huo, and Matt Blacka

Background

• Reef fronted islands are often protected from

ocean swell during modal conditions

• During Tropical Cyclones

» Storm surge

» Large infragravity energy

» Inundation of low lying areas

• Do we understand these processes enough to

design for them in the future?

Wave Flume Physical Model Testing

This project is part of a multi-year research initiative to understand:

• Reef top hydrodynamics

• Revetment design on reefs – Previous talk this session by Matt Blacka

• Overtopping on reefs

Wave Flume Physical Model Testing-

Reef top hydrodynamics

Data collected during the physical model testing included:

• Wave heights in deep water, across the reef rim and throughout the

lagoon;

• Free water surface elevations at several locations throughout the lagoon;

• Video footage of lagoon wave processes such as surf beat and stacking

of bores;

Background – Cook Islands

Cook Islands

Wave Flume Physical Model Profiles

Definitions

Source: Groulay, M. (!996) Wave Setup on Coral Reefs. 2. Set-up on reefs with various profiles. Coastal Engineering. 28, 17-55.

Physical ModelWRL Flume:

1.2 m wide x 44 m long x 1.6 m deep

1:50 scale model

Test Conditions

Reef widths:

75m, 150 m, 300m, 600m

Reef flat: - 0.3 mMSL

Landward edge backed by a 10m tall

smooth revetment (1V:1.5H)

All tests were run for 1000 waves

assuming a JONSWAP spectrum

Table 1: Matrix of Storm Conditions

Modelled in Test Program

Water Level

(m MSL)Hs (m) Tp (s)

0, 0.7 8 11, 13, 15

Wave Transformation - StormsArea where

waves begin to

break

Wave Transformation on Reefs

• Offshore spectra shows groupiness of

waves

• Reef top hydrodynamics

– Incident waves dissipate much of their

energy on the reef edge

– Energy is transferred into low-

frequency components – Surf beat

– Incident band wave bores travel on top

of this surf beat

Reef Top Wave Height

r=𝐻𝑟𝑚𝑠

𝑑𝛾𝑟 = 0.3576 + 0.439𝑒−0.0126𝑥𝑟

VulnerabilityWhat reef widths cause the largest amount of inundation potential during

cyclones and will this change into the future?

Wave Flume Physical Model

Profiles

Mean Wave Setup on Reefs

• Setup increased with reef width up to

300m

– Waves were actively breaking and

dissipating energy on reefs < 300m

• Longer reef width (600m) showed a

decrease in setup at the landward probe

– BEYOND 300m, waves were travelling

as solitary unbroken waves

Results for all 4 reef widths. Most landward probe only.

Example – Infragravity/Surf Beat

Surf Beat on Reefs

• Surf beat defined as the standard deviation of

the low pass filtered (2*Tp) free surface (Seelig,

1983).

• Surf beat increased with

– increasing water level

– with increasing Tp

• Surf beat increased with reef width up to 150 m

for Tp ≧ 13s

• Surf beat decreased with reef width for Tp = 11sResults for all 4 reef widths. Most landward probe only.

𝑋 𝑖 =1

2𝑁𝑃

𝑗=−𝑁𝑃

𝑗=𝑁𝑃−1

𝜂(𝑖, 𝑗)

1% Exceedance of Smoothed Low-

Frequency Water Surface

• 1% exceedance of Surf beat increased

with

– Increasing wave period (Tp)

– decreasing water level

– reef width up to 150 m

• 1% exceedance of Surf beat decreased

– For long (600m reefs)

Results for all 4 reef widths. Most landward probe only.

1% Exceedance of total water level

• Not a term previously looked at by

Seelig (1983) or Gourlay(1994,

1996a, 1996b)

• This term represents the true over-

topping potential

• 80cm increase in TWL for short reef

widths compared to just looking at

surf beat

Results for all 4 reef widths. Most landward probe only.

1% Exceedance of total water level

• 1% exceedance of TWL increased

with

– Increasing wave period (Tp)

– Increasing water level

• 1% exceedance of eta decreased

– With reef width

Results for all 4 reef widths. Most landward probe only.

Key PointsDuring extreme conditions reef/lagoon systems dominated by:

• Wave/Surf Beat and Wave/Wave interaction results in complex and unpredictable

behaviour, altered wave height distributions

• Low Frequency components Increase with Reef width (up to 300)

– Setup increases with increasing reef width up to 300m

– Surf beat is a function of wave period, water level and reef width

– 1% SB -> tended to increase with Tp and reef width (up to 150 or 300m)

• TWL: 1% eta -> tended to increase with Tp and water level, but decrease with reef

width

What does this potentially mean for the future?

• Larger storms (longer period) will likely result in more inundation (high water

levels)

• SLR – the impact of water level is mixed. In some instances it decreases

the impact and in others in increases.

– Higher water levels: increase surfbeat and wave heights on reef

– Lower water levels: increase setup

• Longer reefs are more vulnerable to the low-frequency components (setup,

surfbeat)

• Shorter reefs are more vulnerable to incident waves

Ongoing WorkExpanding flume modelling data set with additional testing that includes:

• Increased range of reef profile characteristics (reef slope and reef rim)

• Increased range of incident wave conditions

• Modelling of specific events/locations with observed extreme events

• Comparison of flume model results with several numerical models

• Revetment design and overtopping

End goal:

• Improvements of empirical equation parameters and design equations for reef

environments

We are looking for interesting data, new ideas and potential collaboration.

Womenincoastal.org

Thank you for your time.