2 november 2016

Modelling of morphodynamic impacts oncoasts during extreme events

Ap van Dongeren

With Kees Nederhoff, Maarten van Ormondt, StephanieSmallegan, Lodewijk de Vet, Huub van Verseveld andRobert McCall

Coastal flood risks• Recent and historic low-frequency, high-impact events

demonstrated coastal risk (Xynthia, 2010; Xaver/St. Nicholasstorm, 2013; St. Agatha, 2014; Sandy, 2012; UK storms 2013/4)

La Faute sur Mer,FR

Chesil Beach,UK

Sandy,NY

1953 Flood,NL, BE, UK

Chesil Beach, UK

Hurricane Matthew Impact

Impact Hurricane Matthew. At Vilano Beach FL, 10/9, video credit Tom KaneSource: https://twitter.com/StuOstro/status/785532989497368576?s=02

Coastal flood risk in Europe and beyond

• Coastal zones will experience increased risk of flooding, erosion anddamage due to the combination of

• Increased hazards due to climate change• Ongoing coastal development• Without adaptation, flood damage on European coasts increase

up to 11 billion Euros per year (IPCC, AR 2014)

• Coastal authorities need to• Assess risk in coastal regions• Develop a Disaster Risk Reduction (DRR) strategy

• Risk assessment needs proper tools ->numerical predictive models• Understand the system• Investigate effect of changes in forcing and/or solutions

Deltares’ open source morphodynamical models

• Delft3D large scalemorphodynamical model• Time scale > days• Driven by stationary wave

conditions

• XBeach local scalemorphodynamical model• Storm-scale• Surfbeat• Non-hydrostatic

Soon: merge of stationary and surfbeat drivers into Delft3D-FM

Representing short and long waves

Governing equations in surfbeat mode

yx wc Ac A c A DAt x y

q

q s¶¶ ¶¶

+ + + = -¶ ¶ ¶ ¶

y rx r rrr w

c Sc S c SS D Dt x y

q

q¶¶ ¶¶

+ + + = - +¶ ¶ ¶ ¶

0sz hu hvt x y

¶ ¶ ¶+ + =

¶ ¶ ¶

2 2

2 2bx x

hFu u u u uu v g

t x y x y h x ht hnr r

æ ö¶ ¶ ¶ ¶ ¶ ¶+ + - + = - - +ç ÷¶ ¶ ¶ ¶ ¶ ¶è ø

2 2

2 2by y

h

Fv v v v vu v gt x y x y h y h

t hn

r ræ ö¶ ¶ ¶ ¶ ¶ ¶

+ + + + = - - +ç ÷¶ ¶ ¶ ¶ ¶ ¶è ø

xyxxx

xy yyy

SSFx y

S SF

x y

¶æ ö¶= - +ç ÷¶ ¶è ø

¶ ¶æ ö= - +ç ÷¶ ¶è ø

Long waves

Short waves

And

Rollers

Wave forcing

Comparing different modes

Type Stationary Surfbeat NonhydrostaticDriver Sea states

(wave heights)Wave heightvariations

-

Hydrodynamics Steady currents Currents andlong waves

Long and shortwaves

Morphodynamics moderate waveconditions

extremeconditions

Bed loadtransport only

Computational time Short Medium LongTime-scale Years to

decadesHours to days Minutes to

hoursExample

Model development philosophy

• Develop new functionality in cooperation with researchers andclients

• Quick time to market: apply models to challenging environments

• Benefits:• Increased confidence in predictive capability.• Strengthening the position of contractors and consultants

• Approach:• Cooperation through research contracts and coops which

exchange knowledge, data and tools.• Dissemination through publications, courses and open-source

software.

How does that work in practice? Some examples

• United States• Santa Rosa, FL• Mississippi Coast• New York/New Jersey Coast

• Netherlands• Ameland

• United Kingdom• Gravel coast

Santa Rosa FL Barrier Island Impact

XBeach modeling ofSanta Rosa Island, Floridadamaged by Hurricane IvanMcCall et al., CE 2010

Challenge: overwash/sheet flow sediment transport

Restoration of Mississippi Barrier Islands afterhurricane impact

Delft3D+SWAN model domains

• What is island’s long term behavior?• What is the effect of hurricanes?• How can we best manage restoration

of the islands?• Do they help protect the main land?• End client: National Parks Service

• Detailed XBeach models of Ship Island, MS.

Challenge: wave climate schematization -> selection of hurricanes with impact

New York/New Jersey Coast: hurricane Sandy Impact

• Wilderness Breach, LI: breachdevelopment• Ph.D. Maarten van Ormondt• M.Sc. Lodewijk de Vet

• Rockaway Impact• M.Sc Huub van Verseveld

• Mantoloking, NJ: effect of seawalland breaching• Ph.D. Stephanie Smallegan• M.Sc. Kees Nederhoff

Hurricane Sandy: Wilderness Breach opening

Hurricane Sandy: Wilderness Breach opening

• Xbeach model of Fire Island, NY.• Predicts occurrence of breach.• Lodewijk de Vet (Coastal Seds ‘15)

Challenges:• Effect of land roughness on

breach formation

Wilderness Breach – Fire Island (NY)Wilderness Breach formed during Hurricane Sandy• 2 year morphodynamic model (Nov 2012 – Oct

2014)• Delft3D (calm conditions) and Xbeach (storms)• Storms are responsible for breach growth AND

partial closure due to spit formation

Challenges:• Tidal gradients• IG wave-induced swash

transports control breachgrowth and net import ofsediment to flood deltas

Sandy: the effect of buildings on coastal erosion

• What is the effect of buildings hard structures on erosion?• Relevant for Dutch coastal towns• Case study at Camp Osborne, Normandy Beach, NJ.

• Erosion effects increaseddue to presence ofbuildings

• Back-wash erosion of waterspilling from Bay to Ocean.

Challenges:• Sediment transport around and

over unerodible surfaces

Nederhoff et al. Coastal Seds 15

Sandy: the effect of buildings on coastal erosion

Sandy: the effect of a sea wall

• Ph.D. Stephanie Smallegan: effect of seawall and backflow from bay

Smallegan et al. CE 2015

Seawall:limited damage

No seawall:Inundation and backflow

Impact Modelling of Hurricane Sandy on the Rockaways, NY

Configuration Area of Application

Building ground elevation

• Can we relate observed damages to modelled impacts ofinundation, flow velocity and erosion using a BayesianBelief Network?

Using numerical model to feed Bayesian Network

Challenges: damage and physical data

Meeting, Venue, Date

Answering questions using model date

Poelhekke et al.,Coastal Engineering2016

Hydro-meteo forcing onshore impact site characteristics

Constrain forcing to a particular storm Consider one particular location

And watch theimpacts change

Ameland (Netherlands): small dune row

Problem location: what happens when first dune row breaches?

Coastal erosion

Good reproduction of local dune erosion due toSt. Nicolas storm 5 December 2013

Annual measurements:LiDAR + JARKUS

Model simulation:XBeach for 3 days

Challenges:First time application nearlarge ebb tidal delta

Normative storm (1/2000) will result in breaching

Dune row will breach

But hinterland beyondthe main sea defensewill not flood

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Gravel beaches, PhD Robert McCall

• Gravel beaches occur in many high-latitude areas around theworld

• Considered sustainable forms of coastal defence due to ability toabsorb large amounts of wave energy

• Challenge: Little knowledge ofprocesses occurring on gravelbeaches, particularly during storms

Porlock Bay

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Model development Xbeach-G

Storm processes on gravel beaches differ to those on sandy beaches:

Sand Gravel

Waves Dissipative surf zone,infragravity dominance

Reflective, incidentband dominance

Groundwater Low infiltration rates,limited importance onswash volumes

High infiltration rates,leading to large swashasymmetry

Sediment transport Transport takes place insurf zone and swashzone. Suspendedtransport dominant.

Transport dominated bybed load transport inthe swash zone

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XBeach-G: hydrodynamics

• Depth-averaged hydro-morphodynamic model for gravel coasts

• Phase-resolving wave model, using non-hydrostatic pressurecorrection term in NLSWE (similar to 1-layer SWASH model)• First developed for XBeach by Pieter Smit, TUD

• Groundwater model based on Darcy-Forchheimer-type equations,and non-hydrostatic groundwater pressure gradients

• Validated with Deltaflumeexperiments (not shown today)

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XBeach-G

• Phase-resolving wave model (1-layer SWASH model) + groundwatermodel

• Gravel sediment transport computed with bed load equation, including:o Effect of infiltration/exfiltrationo Pressure gradient / turbulence effect in swash to better describe

uprush / downwash transport balance

• Bed level change computedwith:o Sediment transport

gradientso Slumping over angle of

repose

• One parameterset for 10 stormevents

• Brier Skill Scorevaries from 0.38 –0.99

Model validation: morphodynamics

2 november

Measured pre-stormMeasured post-stormModelled post-storm

Berm building Beach erosion

Model validation: morphodynamics

2 november

Crest build-up Crest lowering Barrier rollover

Challenges ahead: adding processes

• Gravel coasts ->• dynamics of mixed sediments

• Vegetated coast ->• Behavior in extreme conditions

• Structures ->• erosion around and over hard elements• model building collapse directly?

• Coral reefs ->• Sediment transport of carbonate/biogenic sands

• Wind ->• Aeolian sediment transport models

Challenges ahead: a wider look but keep it manageable

• Focus on impacts instead of hazards.• Incorporation of morphodynamic effects (pathways)

in coastal impact analysis• Computational efficiency• Storage and presentation of results

• Always bigger and better and longer• Morphodynamics 1D-> 2DH -> 3D• Nesting of different process-based models

• Validation – good pre- and post field data needed!