Water Research Laboratory School of Civil Environmental Engineering

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 Model WRL Flume: 1. 2 m wide x 44 m long x 1. 6 m deep 1: 50 scale model

Test Conditions Reef widths: 75 m, 150 m, 300 m, 600 m Reef flat: - 0. 3 m. MSL Landward edge backed by a 10 m tall smooth revetment (1 V: 1. 5 H) 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) 0, 0. 7 Hs (m) Tp (s) 8 11, 13, 15

Wave Transformation - Storms Area 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 lowfrequency components – Surf beat – Incident band wave bores travel on top of this surf beat

Reef Top Wave Height

Vulnerability What 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 300 m – Waves were actively breaking and dissipating energy on reefs < 300 m • Longer reef width (600 m) showed a decrease in setup at the landward probe – BEYOND 300 m, 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 ≧ 13 s • Surf beat decreased with reef width for Tp = 11 s Results for all 4 reef widths. Most landward probe only.

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 (600 m 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, 1996 a, 1996 b) • This term represents the true overtopping potential • 80 cm 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 Points During 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 300 m – 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 300 m) • 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 Work Expanding 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.

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Thank you for your time.