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Predicting long waves and their coastal

impacts

Keynote lecture IAHR 2015

Prof. Dano Roelvink

UNESCO-IHE Institute for Water Education (0.8)

Deltares (0.2)

Delft University of Technology (0.0)

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Contents

• Background on infragravity waves

• Dune erosion and overwashing

• 2D hurricane impact studies

• Modeling: surfbeat vs wave-resolving mode

• Runup on beaches

• Coral reef environments

• Tidal inlets

• Harbour oscillations

• Global IG wave prediction

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Principle sketch -

physics

20 s < T < 200 s

Ew(T)

u(T)

bound wave

motions dominated by

long waves not short

waves

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Delft3D Surfbeat

• 2D wave energy balance computed using mean wave direction

computed by SWAN

• Reniers, A.J.H.M., Roelvink J.A. and Thornton, E.B. (2004).

Morphodynamic modelling of an embayed beach under

wave group forcing. Journal of Geophysical Research, 109,

C01030.

• Reniers, A.J.H.M., MacMahan, J.H., Thornton, E.B. and

Stanton, T.P. (2006). Modelling infragravity motion on a

ripchannel beach. Coastal Engineering, 53, 209-222.

• Application restricted to almost perpendicular wave incidence

• Rather complex code in periphery of big software system

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XBeach: motivation

• 2004 Hurricane season hit Florida coast 4 times

• Congress awarded multi-million project MORPHOS3D to develop new

physics-based model system to assess hurricane impacts

• Scope: wind-surge-waves-nearshore processes-impacts

• Play ‘what-if?’ games around Corps of Engineers projects

Figure 1 Pre- and post hurricane Ivan, Perdido Key, Florida (source: USGS)

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XBeach - approach

• open source code available for free on internet (xbeach.org)

• easy to use

• Short-wave averaged but long-wave resolving modeling of

waves, flow and morphology change in time-domain

• Swash and overwash motions

• Dune erosion, overwashing, breaching and full inundation

• Domain from outside surfzone to backbarrier

• Driven by boundary conditions from surge and spectral wave

models

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Combined short/long wave

propagation and decay

Principle

test:

overwashing

by LF waves

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• Describes Xbeach model and a number of validation studies

• Makes an effort to quantify skill, find single set of ‘factory

settings’

• Shows importance of long waves, avalanching

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Detailed beach process model

plus avalanching

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Example: LIP11D Delta Flume tests

Short

wave

height

Long

wave

height

Short

wave

orbital

velocity

Long

wave

orbital

velocity

Wave

setup

Beach

profile

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Profile development

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Erosion volume and dune retreat

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18th month meeting – 23-27th Nov. 2009, Montpellier

MICORE.EU Field measurement sites in

Europe

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Xbeach

Surfbeat mode

• Short wave averaged

• Wave action balance plus

NSWE

• Incident band swash

neglected or parameterized

• Skewness and asymmetry

parameterized

• Dano Roelvink et al. (2009). Modelling

storm impacts on beaches, dunes and

barrier islands. Coastal Eng, Vol 56,

Issues 11-12, Nov 2009, Pages 1133-1152.

Nonhydrostatic mode

• Short wave resolving

• NSWE plus nonhydrostatic

pressure correction

• Incident band swash

resolved

• Skewness and asymmetry

resolved

• Built in by Pieter Smit (TUD)

• One layer version of SWASH

• Marcel Zijlema et al. (2011). SWASH: An

operational public domain code for

simulating wave fields and rapidly varied

flows in coastal waters. Coastal Eng, Vol.

58, Issue 10, Oct 2011, Pages 992-1012.

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Overwash case: Santa Rosa Island

Hurricane Ivan: 16 September 2004

Start of hurricane

Hurricane landfall

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Pre- and post-storm Santa Rosa Island

Overwash case: Santa Rosa Island

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Overwash case: Santa Rosa Island Robert McCall, Dave Thompson

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Similar erosion and

deposition patterns:

• erosion of dune face

• deposition on island

Overwash case: Santa Rosa Island

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Fire Island Lodewijk de Vet

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Camp Osborne Kees Nederhoff

MODELING THE EFFECTS OF

HARD STRUCTURES ON DUNE

EROSION AND OVERWASH

a case study of the impact of Hurricane

Sandy on the New Jersey coast C.M. Nederhoff1,2, Q.J. Lodder2,3, M. Boers1, J.P.

den Bieman1, J.K. Miller4

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Long Island Huub van Verseveld

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2D XBeach (multi-dir) tends to

underestimate IG runup

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Single_dir vs multi_dir

Spatial plot of the room-mean-square short wave height for a multi_dir and single_dir simulation.

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Praia de Faro (Portugal) runup comparison Roelvink, Vousdoukas

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Surf beat modelling – Delft3D vs XBeach

• Delft3D • Action balance solved

over mean direction from

SWAN

• Several validation

studies for long waves in

ports

• Implicit scheme

• Boundary problems with

oblique waves

• Not parallellized

• Complex code

• Problems with swash

• Xbeach • Action balance solved

using directional

spectrum

OR

using mean direction

from stationary solver

(new)

• Explicit scheme

• Good lateral boundaries

• Good parallel scaling

• Straightforward code

• Very robust in swash

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Modelling long waves in ports

Delft3D-Surfbeat or XBeach?

• Wim van der Molen, this symposium

• excellent results with Delft3D-Surfbeat

• quotes faster runtime due to implicit scheme

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Cascais – IG waves in port Xavier Bertin, Univ. of La Rochelle

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Tidal inlet- Lagoa de Albufeira Xavier Bertin, Maitane Olabarrieta

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Observed water levels, wave heights and

spectra

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Xbeach validation of IG waves

PT1 PT3 PT2

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IG wave modelling at Ningaloo Ap van Dongeren, Ryan Lowe, Andrew Pomeroy

Wave transformation (breaking)

and wave-induced motions across

the reef destroy and pick-up reef

material, drive material and

nutrient transport, as well as

transport and dispersal of larval

fish and other organisms.

Time-varying hydrodynamics

across reefs are still little studied

Recent field studies show

dominance of infragravity wave

motions

Trang et al. , 2010, AGU

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Instrument array and wave data Ryan Lowe, UWA

swell energy decays rapidly, IG motions pick up

and dominate spectrum

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Model (red)-data (black) comparison

(swell)

rapid swell wave

height decay

from C1 to C3

swell variance on

reef well

predicted.

very high bed

friction

dominates

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Model (red)-data (black) comparison (IG)

IG wave height

response well

predicted across

the reef.

IG wave heights

are larger than

swell wave

heights.

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Extreme sea levels at Pacific islands

Commonly cited cause:

tropical cyclones =

storm surge (+ tides)

Cyclone Mena - Rarotonga

Hydrodynamic modelling of wind- and pressure driven

storm surge from “synthetic” cyclones (Fiji)

1 in 100 year storm tide

McInnes, at al. (in review)

Global and Planetary

Change

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Hernani Xbeach model, super typhoon Haiyan Maarten van Ormondt (Deltares)

• XBeach

• Non-hydrostatic

mode

• Peak of the storm

• Offshore water

level: 0.7m, ~0.25m

surge

• Offshore Hs 15 m

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Xbeach Hernani (profile)

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augustus

2015

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Hernani water levels

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augustus

2015

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Effect of infragravity waves at Hernani

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augustus

2015

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XBeach formulations in Delft3D-FM Johan Reyns (UNESCO-IHE)

• Large, complex areas such as Funafuti: unstructured grid only

option

• Too large area to use wave-resolving approach

• Implemented Xbeach wave driver using transport solver within

Delft3D-FM

• Ideal for atolls with lagoon, hit by waves from all directions

• Work funded by SPC, Fiji

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Funafuti

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Example from Kiribati

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Example from Kiribati

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Swell event hitting Côte d’Ivoire, aug 2011 Toualy et al, AJMS 2015

Wave period and

direction

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Prediction of global ocean IG waves Arshad Rawat, Fabrice Ardhuin

• IG waves generated in surfzone

and radiated out treated as

source term in WW3

• Based on empirical relation with

incoming wave conditions along

all shorelines

• Locally validated with data and

2D XBeach simulations

• Explains source of free long

waves at IG frequencies

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Conclusions

• Infragravity waves are a dominant mechanism in many

situations:

• swash zone under swell or storm conditions

• dune erosion and overwashing

• rip currents

• harbour basins

• small tidal inlets

• coral reef systems

• They can be predicted by wave-resolving models but much

more efficiently using a surfbeat approach

• Their effect on total water level during storms or swell events

must be accounted for in early warning systems