Scenarios1. Tidal influence

2. Extreme storm surge (wave overtopping, max. limit 200 l/s/m, period 2 h)

Outlook• calibration and validation of 3D model

• transfer and exchange of data with a numerical hydrodynamic sea water model (salt concentration and sea water level) and with a hydrological water balance model

• comparison with transient concentration data acquired from multi-level groundwater probe

• investigation of the influence of groundwater pumping on saltwater intrusion

• investigation of effects of climate-change-induced changes in sea level and salt concentration on saltwater intrusion

Saltwater Intrusion and Storm Surge Processes in Coastal Areas under Climate Change: A Modelling Study in Northern Germany

M. Herold a, J. Yang b, T. Graf b, T. Ptak a a Applied Geology, Geoscience Centre, Georg-August-Universität Göttingen, Goldschmidtstr. 3, 37077 Göttingen, mherold@gwdg.de

b Institute of Fluid Mechanics and Environmental Physics in Civil Engineering , Leibniz Universität Hannover, Appelstr. 9A, 30167 Hannover

Problem description

Numerical developments • code HydroGeoSphere (Therrien et al., 2008): at the time coupling of surface and subsurface processes not possible when simulating density-dependent flow

half-automated three step procedure developed:

1. Maximum water level behind dyke is determined using a density-independent coupled surface-subsurface model, depending on specific wave overtopping

2. Water level and model geometry are used to determine time-dependent position of submerged nodes and according water levels, in which density differences are considered

3. Density-dependent subsurface model uses this information as transient source terms, thus replacing the coupling to the surface model

ResultsTidal influence

Study site

Acknowledgements:The study was supported by the Ministry of Science and Culture of Lower Saxony within the network KLIFF – climate impact and adaptation research in Lower Saxony.

1. Subsurface saltwater intrusion into groundwater under sea level rise

2. Wave overtopping and advective transport from land surface into groundwater under sea level rise

• hydraulic conductivity homogeneous

and heterogeneous (Fig. 4)

• surface flow and variably saturated subsurface flow

• groundwater recharge constant (300 mm year -1)

• saltwater density typical for estuary = 1018 kg m -3

Fig. 5 Tidal range Bremerhaven

Fig. 1 Schematic representation of model scenarios: red arrows = density-dependent transport of saltwater, blue arrows = freshwater, not to scale

Fig. 6 Maximum pond level rise inland of dyke during simulated extreme wave overtopping (infiltration already considered during wave overtopping)

groundwater flow

groundwater recharge

wave overtopping

subsurfacesaltwater intrusion

dyke

1

2

Tidal range (Bremerhaven)

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 5 10 15 20 25 30 35 40

Time [h]

Sea

lev

el [

m]

Ponding Curve

1.04 m

0

0. 5

1

2 3 4 5time [h]

wat

er le

vel [

m]

Flood retreat curve

0

0.5

1

0 5 10 15 20 25 30time [h]

Wat

er le

vel [

m]

Fig. 4 Hydraulic conductivity zonation

Fig. 7 Infiltration from inland pond after end of storm surge into the unsaturated and saturated subsurface

Fig. 8a Mean standard deviation of pressure head

Fig. 8b Mean standard deviation of saltwater concentration and enlarged detail of affected area

• model run 200 days

• mean standard deviation calculated

• pressure head: maximum ± 1.34 m at sea boundary, ± 0.1 m 7 km inland

• confining layers (A and B, Fig. 8a) not influenced significantly

• saltwater concentration: majority of model area not influenced (mean standard deviation below 1 mg l-1, Fig. 8b)

• maximum mean standard deviation close to sea boundary in highly conductive sand layers: up to 707 mg l -1 (Fig. 8b)

• Lower Saxony, Germany (Fig. 2)• cross section perpendicular to North Sea coastline (Fig. 3)• mostly Quaternary sand and gravel, interspersed with clay

lenses• 20 m thick layer of clay (former tidal flat) covers half of

cross section

Fig. 2 Location map Fig. 3 Geological cross section

[m]

Results cont'd Influence of sea level rise

Influence of storm surge

Fig. 10 Distribution of saltwater concentration after 3 days (top) and after 10 years (bottom)

X:Y:Z=1:1:0.04

• 60 cm sea level rise

• 0.5 isoline of relative concentration is moved by up to 160 m inland

• infiltration of saltwater behind dyke into the subsurface (advective transport)

• low concentrations (max. 6 mg/L, Fig. 10 top) and very slow transport through less conductive top layer

• Fig. 10 bottom: plume shrinkage caused by groundwater flushing from inland X:Y:Z=1:1:0,04

concentration [mg l-1]

dep

th [m

]d

epth

[m]

X:Y:Z=1:1:0.04

3D model

2D model

model domain = catchment + estuary

• full coupling of flow and transport between surface and subsurface domain possible with improved HydroGeoSphere

• required node spacing requires very long run times

• only applicable to small models; here: same procedure as in 2D cross section model

• flow model comprises whole model domain, transport only modelled in area close to coastline and river (cf. Fig. 11)

• surface flow and variably saturated flow

• heterogeneous hydraulic conductivity

Fig. 11 3D model area

Fig. 12 Water depth of surface water bodies (coastline civil works and protection measures dominate natural surface flow pattern. If they are not properly implemented in themodel, low-lying areas behind the dyke are being inundated)

water depth [m]

>

<

concentration [mg l-1]

X:Y:Z=1:1:0.04

dep

th [m

]

BA

X:Y:Z=1:1:0.04

dep

th (m

)

pressure head [m]

11 km

Fig. 9 Relative concentration contour lines after sea level rise