Impact of small bathymetric changes on large-scale hydrodynamics · 2018-10-29 · Impact of small...

Galveston (TX, USA), 10/18/2018

Impact of small bathymetric

changes on large-scale

hydrodynamics Robert Hagen, Janina Freund and Frank Kösters

PECS 2018

19F2004A

www.baw.de | www.mdi-de.baw.de/easygsh/

Modeling area

Imag

ery

rep

rod

uced

fro

m t

he G

EB

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_2014 G

rid

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ers

ion

20150318, w

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Impact of small bathymetric changes on large-scale hydrodynamics| Robert Hagen; Janina Freund; Frank Kösters

10/18/2018

Introduction Results Analysis Summary


Objective

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Reference 2006

Forcing-induced differences Scenario:

Scenario:


Scenario: Bathymetry 2006

Forcing 2010

Scenario: Bathymetry 2010

Forcing 2006

Research with coarse, large-area coastal models implies, that changes in bathymetry result in large scale differences in

tidal dynamics (e.g. Jacob et. al., 2015)

These changes have neither been confirmed with long simulation periods, high-resolution approaches nor have they

excluded meteorological influence. Also, calibrated estuaries and high-resolution wetting and drying have been

neglected

Bathymetry-induced differences


Grid resolution in focus area (German Bight: 30m to 2,000m)

3D approach with focus area

Estuaries included until tidal weir

Increased accuracy with subgrid technology

Goal: 20 year hindcast (1996-2015) available for download

Model Setup


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725 km

450 miles

German

Bight

Model-area:

full domain

Model-area:

Partial focus area

Elbe estuary


The principle behind subgrid models

A cell is divided m times

Subedges receive depths

The cell has a depth distribution

-25

-20

-15

-10

-5

0

5

10

AREA

VOLUME

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Ba

ckg

rou

nd

s b

y B

KG

To

pP

lusO

pe

n w

ww

.bkg

.bu

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; G

rid

© E

asyG

SH

-DB



Advanced wetting and drying with subgrid

Cells can be partially wet

Exact representation of water volume

Better representation of tidal flats

Integration of groins and training walls

These are hydraulically efficient

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Training wall

Dry area

Partially wet

Completely wet

© b

y E

asyG

SH

-DB



Bathymetry changes in the mouth of the Elbe estuary: 2010 - 2006


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deeper shallower



Forcing

Bathymetry

Wate

r le

vel

[m]

Tid

e R

an

ge

[Th

b]

Tide Mean Water [Tmw]

Ebb Flood

- Tide High Water: Highest water level in tide cycle

- Tide Low Water: Lowest water level in tide cycle

- One tide cycle contains 1x Thw and 2x Tnw

- Tide Range: Difference between Thw and Tnw(1,2)

Tide High Water [Thw]

Tide Low Water [Tnw]

Comparing the forcing of 2006 and 2010

Impact of small scale bathymetry changes on large scale hydrodynamics | Robert Hagen; Janina Freund; Frank Kösters

10/18/2018

Time [h]



Influence of nodal tide

Large area variations in deep water result from nodal tide.

Nodal Tide varies around 3cm between 2010 and 2006

Meteorology suggests 2006 to be windier, therefore the tidal range

is larger in 2010 even though astronomy decreases tidal component

amplitudes at the same time.

Nodal tide ( 3cm)

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Increase

Decrease



Differences (Tidal range), scenario - reference

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Δforcing-induced differences Δbathymetry-induced differences


de

cre

ase

inc

rease

Tidal range has

increased from

1 to 8 cm.

Changes affect

large-scale

hydrodynamics.

Tidal range has

increased and

decreased.

Changes affect

local

hydrodynamics.


Differences (Flood current velocity magnitude ), scenario - reference

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Δforcing-induced differences Δbathymetry-induced differences


de

cre

ase

inc

rease

Flood current

velocity is not

affected.

Changes affect

local

hydrodynamics.

Flood velocity

has increased

and decreased.


Correlation between

- bathymetry

- tidal range

Correlating bathymetry differences with hydrodynamics


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hig

her

lo

wer

shallower deeper

(Δvalue | Δdepth)


de

cre

ase

incre

ase




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R = -0.10

de

cre

ase

incre

ase




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de

cre

ase

incre

ase

R = -0.21




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R = -0.29

de

cre

ase

incre

ase


Color-coding differences of bathymetry and hydrodynamics


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de

cre

ase

incre

ase


Changes of tidal range due to deepening of bathymetry in the Elbe estuary


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deep

er

sh

all

ow

er

decrease

increase



Mean tidal range due to shallower bathymetry in the Elbe estuary


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deep

er

sh

all

ow

er

decrease

increase


Mean tide velocity due to deepening of bathymetry in the Elbe estuary


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deep

er

sh

all

ow

er

decrease

increase


Mean tidal velocity due to shallower bathymetry in the Elbe estuary


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deep

er

sh

all

ow

er

decrease

increase


2010 2006

Bathymetry in the mouth of the Elbe estuary: 2006 and 2010

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Background © by BKG TopPlusOpen www.bkg.bund.de




A North-Sea model with high-resolution in shallow water has been built to reproduce hydrodynamics from 1996 to 2015

A well calibrated reference run (2006) has been varied with the forcing (scenario 1) and bathymetry (scenario 2) of 2010

Results:

The forcing produced large-scale differences in tidal dynamics (by meteorology)

Small-scale bathymetry variation produces local changes in hydrodynamics

Direct correlation of local hydrodynamics with bathymetry variation is rather weak

Outlook and future tasks

Extend the temporal space to analyze larger bathymetry differences, because large-scale implications were not found

Summary and Outlook

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1996 2006 2010 2015



References

Web references

EasyGSH-DB: http://mdi-de.baw.de/easygsh/ ; (bathymetry, sediment and hydrodynamic data available for download)

WMS: TopWebPlus (grey); www.bkg.de ; (available for everyone)

WMS: Imagery reproduced from the GEBCO_2014 Grid, version 20150318, www.gebco.net ; (available for everyone)

Other references

JACOB, B.; STANEV, E. V. and ZHANG, Y. J.: Local and remote response of the North Sea dynamics to morphodynamic

changes in the Wadden Sea. In: Ocean Dynamics, Vol. 66, 5, 671-690, doi: 10.1007/s10236-016-0949-8, 2016.

10/18/2018

http://mdi-de.baw.de/easygsh/



http://www.bkg.de/

http://www.gebco.net/

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