Thaiënne A.G.P. van Dijk (Deltares & University Twente) Kees van der Tak, Erwin van Iperen (MARIN) 15 February 2012

Morphodynamics of the North Sea bed using echo-sounding time series

- Applied to validating a re-survey policy involving shipping grounding dangers based on AIS data

2

Presentation

•  Introduction •  Sea bed morphology (marine bedforms) •  Echo sounding data

•  Methods and Results: quantification sea bed dynamics •  Vertical dynamics NCS •  Morphodynamics of individual marine bedforms

•  Methods and results: modelling grounding dangers from AIS data (MARIN) •  Regular grounding danger •  Object grounding danger

•  Validation and optimisation of a re-survey policy

•  Conclusions

Introduction

Part 2 Application

Part 1 Morphodyn

3

Rationale

•  Dynamic bedforms •  Local studies of seabed dynamics in

the North Sea (e.g. offshore wind farm sites)

•  Quantification of vertical dynamics NCS wide and detailed analyses of bedforms provide knowledge on the spatial variation of sea bed dynamics

Bathymetry [m -GLLWS]

+ 2.72 m

- 71.07 m

Introduction

Bathymetry NCS (TNO, 2004)

4

Marine bedforms

sand banks

sand waves

Introduction

5

Marine bedforms

(e.g. Ashley, 1990; Knaapen et al., 2001)

Bedform Wavelength

(m) Height (m)

Orientation (degrees to tidal current)

Morphodynamic time scale (order of years)

Offshore sand banks (tidal ridges)

1 000 – 10 000 5 - 50 0 - 30 centuries

Long bed waves 1 000 – 2 000 1 - 10 60 centuries Sand waves 100 – 1 000 1 – 10 90 years-decades megaripples 7 – 40 < 1 90 hours

Introduction

6

Echo sounding data

•  All bathymetric data in NLHO’s Bathymetric Archive System (BAS) •  NLHO & RWS •  SBES & MBES •  Late 1980s – June 2010

•  Non-digital data NLHO (fair sheets)

NLHO BAS

7

Quantification dynamics NCS: methods Part 1:

Morphodyn

•  Echo sounding dataset: Digital Elevation Model (DEM) •  Time series of DEMs: Vertical dynamics NCS

dz/dt (vertical dynamic trend) label (e.g. n, stdv)

•  Vertical dynamics [m/year] based on the analysis of observations of NCS

•  25 x 25 m cells

x

z

y

NLHO survey overview 2002

8

Vertical dynamics NCS Part 1:

Morphodyn

•  First version: survey areas dominant

9

Correction vertical trend

•  removal of differences between surveys

Part 1: Morphodyn

10

Vertical dynamics NCS

Coast is dynamic: •  tidal inlets and

tidal channels •  Estuaries NCS less dynamic

Relatively dynamic on NCS: •  north of Texel •  west of Texel •  southern part with bedforms •  anthropogenic

Part 1: Morphodyn

11

Vertical dynamics NCS (simplified)

Non-dynamic areas NCS: •  > 30 m deep •  north of Wadden islands •  zone offshore North-Holland •  locally offshore South-Holland

and Zealand

Dynamic areas NCS: •  Long bed wave field (n. Texel) •  Sand wave field (w. Texel) •  Southern NCS with bedforms

Part 1: Morphodyn

Vertical dynamics of German Bight

•  Bed elevation range (m)

•  data 1982 - 2004

(Winter, 2011)

Part 1: Morphodyn

13

Morphodynamics individual bedforms Part 1:

Morphodyn

2

1.  Sand waves west of Texel

& ‘TWIN’ area 2.  Long bed waves north of

Texel 3.  Shoreface-connected ridges

north of Ameland

3

1

1

14

Morphodynamics individual bedforms: method

•  Semi-automated method for the analysis of the morphology and morphodynamics of rhythmic bedforms (Van Dijk et al., 2008)

Morphodynamics individual bedforms

1.  Sand waves west of Texel

Part 1: Morphodyn

Sand wave migration rate: •  west of Texel: 19 m/yr •  NCS: 0 – 5 m/yr

L [m] H [m] min av max min av max 100 345 800 0.3 1.4 3.2

1.  Sand waves west of Texel

16

TWIN profile 1 (southwest to northeast)

-36

-34

-32

-30

2500 2700 2900 3100 3300 3500

distance along profile [m]

bed

elev

atio

n [m

-LA

T]

Jan-91Jun-92Apr-94May-95Mar-99Feb-01May-01Mar-03Apr-04Apr-05Oct-06

SW NE

Morphodynamics individual bedforms Part 1:

Morphodyn

1.  Sand waves south-west on the NCS (‘TWIN’ area) •  time series of 11

datasets

Sand wave migration rate: •  ranges between -4.5 and

+0.064 m/yr •  Average 1.5 m/yr to SW Sand wave growth: •  <0.2 m/yr

L [m] H [m] min av max min av max 140 270 380 2.5 4.8 7.3

Morphodynamics individual bedforms

2.  Long bed waves north of Texel & Vlieland: •  migration 12.4 m/yr to NE (ranges from 10.5 to 18.4 m/yr)

-30

-28

-26

-24

0 3000 6000 9000 12000

distance along profile (m)

bed

elev

atio

n (m

-LAT

)199020032009

SW NE

12

34 5

Part 1: Morphodyn

L [m] H [m]

min av max min av max 744 1125 1409 2.7 3.4 4.3

Morphodynamics individual bedforms

3.  Shoreface-connected ridges north of Ameland and Schier: •  migration ~0 m/yr •  growth ~ 0 m/yr

-5

0

5

0 5 10 15 20 distance along profile (km)

seab

ed e

leva

tion

arou

nd z

ero

(m)

1997,1998 2006

SSW NNE

Part 1: Morphodyn

L [m] H [m]

min av max min av max 4084 4614 5154 2.9 4.3 5.5

19

Application to validating survey policies

NLHO’s re-survey policy for the NCS, 2007

Rationale:

•  NLHO: hydrographic measurements for reliable nautical mapping

•  Accuracy requirements defined by the International Hydrographic Organisation (IHO)

•  No guidelines for the monitoring frequency (re-surveying policy)

•  Increasing efficiency without diminishing safety

Part 2: Application

gcap

tain

.com

20

Modelling grounding dangers from AIS data

•  (predicted) water depth

•  estimate the probability of unknown objects at the seabed

•  Distribution unknown objects at the sea bed

0.00.10.20.30.40.50.60.70.80.91.0

0 5 10 15 20

prob

ababilit

y

height  of  obstruction

all

0.00.10.20.30.40.50.60.70.80.91.0

0 5 10 15 20

prob

abability

height  of  obstruction

all

new

•  Known objects at the sea bed

↓

•  Known new objects at the sea bed

Two types of danger of running aground: •  limited water depth (regular grounding) •  unknown objects at the seabed (object grounding)

Modelling grounding dangers from AIS data

•  If the actual margin > desired margin → no grounding danger (= 0) •  If the actual margin < 0 → ship certainly grounds •  If the actual margin < desired margin and > 0 → probability of grounding

Critical situation Non-critical situation

•  Calculate the danger of running aground using safety margins

AIS-database

•  Automatic Identification System •  observation every two minutes

•  Traffic is allocated to cells

(1 x 1 km) •  Per cel: number of ships with

draught in classes of 1 m •  Danger when depth is smaller

than the critical depth

AIS-data of 1 week in July 2009

Part 2: Application

23

Regular grounding danger

Fig 8-1 MARIN-rapport 23407.620/4r

MARIN’s regular grounding danger on NCS for all ships per km2 per year

•  approach channels to harbours •  shallow shipping lane •  small areas

•  4 spots DWRE •  north of Terschelling

Part 2: Application

•  all ships year-1 km-2 with respect to average water depth (actual margin ma < 0)

•  ma = WD(t) – draught •  Critical water depth =

draught + UKC + 2 [m]

§ 262,800 obs/year § 10000, i.e. 1 in 26.28 observations is a ship with a

negative margin § 1 obs ~ 1 ship

Simplifications: •  Tides were neglected •  Waterdepth in 1 x 1 km

cells (min-mean-max)

24

Object grounding danger

Fig 8-6 MARIN-rapport 23407.620/4r

MARIN’s object grounding danger on NCS for all ships per km2 per year

•  traffic lanes •  anchorage areas

Part 2: Application

Assumption: •  Known objects are not a danger,

since these are marked.

Simplifications: •  speed = 14 knots (deep ships) thus

overestimation in anchorage areas, but here also higher probability for objects

•  for mean water depth •  Number of ships year-1 km-2

for which margin is exceeded based on probability unknown objects

25

Validation of a re-survey policy

GIS-overlay method: •  re-survey policy •  combined grounding dangers for

ships for water depth and unknown objects

•  morphodynamics (dz/dt) as predicted water depths 2011, 2015, 2020

Part 2: Application

26

Combined grounding dangers

very low

medium

0

high

low

CAT 1 – at least every 2 years

CAT 2 – 4 years CAT 3 – 6 years CAT 4 – 10 years CAT 5 – 15 years

Part 2: Application

27

Validation of categories Category 1

0102030405060

0 1 10 11 20 21 22 23 30 31 32 33

combined grounding danger

occu

rren

ce o

f tot

al

(%)

•  A different distribution

of classes will lead to different occurrences of dangers

CAT 1 – at least every 2 years

CAT 2 – 4 years CAT 3 – 6 years CAT 4 – 10 years CAT 5 – 15 years

very low

medium

0

high

low

Part 2: Application

Category 5

0204060

80100

0 1 10 11 20 21 22 23 30 31 32 33

combined grounding danger

occu

rrenc

e of

tota

l (%

)

28

Refinement of a re-survey policy Part 2:

Application

Steps towards a new plan 1.  Category division based on danger maps 2.  Assignment of re-survey frequencies to

categories based on predictions

Step 2 --> predictions

•  New plan alike existing policy in both division and distribution of categories

•  Some sites show higher dangers (DWRE)

•  Also areas where the re-survey frequency may be lowered

Step 1

29

Predicted water depths

•  Difference maps

•  Linear extrapolation dynamic trend

Part 2: Application

30

Predicted grounding dangers

•  Object grounding 2011 •  Regular grounding 2011

•  2015 •  2020

Part 2: Application

31

Development of grounding danger

•  Differences in combined dangers in the future, based on predicted water depths

2010 to 2015 2010 to 2020 2010 to 2011

Part 2: Application

32

Conclusions

•  Quantified vertical dynamics of the NCS is the first overview study of the NCS •  highly dynamic coastal zone (> 0.3 m/yr) and less dynamic offshore •  dynamic offshore: marine bedforms (0 to 0.3 m/yr) •  non-dynamic: > 30 m water depth, north of Wadden islands, small areas (< 0.02 m/yr)

•  Migration rates of individual marine bedforms: •  sand waves 0 – 5 m/yr NCS; 19 m/yr west of Texel •  long bed waves (1 location) 12.4 m/yr •  shoreface-connected ridges north of Wadden islands are nearly stable

•  Morphodynamics can be used in the application of validating and optimising re-survey policies,

•  variation of the occurrences of high and low dangers within one category pleads for rearrangement of the categories and re-survey frequencies

•  assign re-survey frequencies by predicting the development of dangers in the future.

•  Regular grounding danger largest in approach channels and some local areas (e.g. Deep Water Route East due to sand banks)

•  Object grounding danger largest in traffic lanes and in anchorage areas

33

Acknowledgements

Netherlands Hydrographic Office, Royal Netherlands Navy •  Provided all echo sounding data •  Financed the project

Maritime Simulation Centre Netherlands, Maritime Research Institute Netherlands •  modelled grounding dangers

Rijkswaterstaat •  Provided echo sounding data

•  Provided AIS data to MARIN Netherlands Coastguard

RWS-project 2011

Research project on the sea bed dynamics: •  Quantitative morphodynamics of approach channels and

the river Waal •  Factors controlling the spatial and temporal morphodynamic

variation •  Advice on re-survey plan

--- Thank you for your attention ----

Morphodynamic trend

•  Elevation rate of 1 m/yr •  Goodness of fit = 1

(a) node (25,0)

y = xR2 = 1

0

1

2

3

4

5

0 2 4 6 8 10

time (years)

z (m

)

(b) node (0,0)

y = 0.9214x - 0.6571R2 = 0.8021

0

1

2

3

4

5

0 2 4 6 8 10

(c) node (0,0) for different dt

y = 0.2724x + 0.5286R2 = 0.4908

0

1

2

3

4

5

0 2 4 6 8 10

36

Typen data

MBES

530100 530150 5302005743000

5743050

5743100

595250 595255 5952605805890

5805895

5805900

100 x 100 m 10 x 10 m

SBES

3 redenen van onzekerheid bij vergelijk: •  Breedte SBES straal: eerste reflectie (ondiepste punt binnen

straal) •  Getijdereductie •  Ondiepste punt geselecteerd op minuurtbladen

•  Digitale data uit BAS: reductie tot 1 observatie per 3 x 5 m

•  Gedigitaliseerde minuutbladen lagere resolutie

In addition

Vertical dynamics NCS

1.  Tidal inlet between Vlieland and Terschelling

Part 1: Morphodyn

39

Objective 1: tijdseries

•  Stijgende trend in verticale dynamiek

•  Periode van opneming geen groot effect op nauwkeurigheid

•  Meeste tijdseries korter:

In addition

40

Overzichten bij analyses morfodynamiek (Obj.3)

•  Goodness of fit, R2 •  Aantal surveys in tijdserie

In addition