Post on 21-Mar-2017
transcript
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
27
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
28
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
29
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)
30
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!