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
b
Applied Geology, Geoscience Centre, Georg-August-Universität Göttingen, Goldschmidtstr. 3, 37077 Göttingen, mherold@gwdg.de
Institute of Fluid Mechanics and Environmental Physics in Civil Engineering , Leibniz Universität Hannover, Appelstr. 9A, 30167 Hannover
Problem description
Scenarios
Results cont'd
1. Subsurface saltwater intrusion into groundwater under sea level rise
1. Tidal influence
Influence of sea level rise
Tidal range (Bremerhaven)
2
2. Wave overtopping and advective transport from land surface into groundwater under sea
level rise
Sea level [m]
groundwater
recharge
wave overtopping
• 60 cm sea level rise
1.5
1
0.5
0
-0.5
0
5
10
15
20
25
30
35
40
X:Y:Z=1:1:0.04
• 0.5 isoline of relative concentration
is moved by up to 160 m inland
-1
-1.5
dyke
-2
Fig. 9 Relative concentration contour lines after sea level rise
Time [h]
concentration [mg l-1]
Fig. 5 Tidal range Bremerhaven
2
Influence of storm surge
• infiltration of saltwater behind dyke into
the subsurface (advective transport)
Flood retreat curve
groundwater flow
1
1.04 m
0.5
• low concentrations (max. 6 mg/L, Fig. 10
top) and very slow transport through less
conductive top layer
0.5
0
Study site
•
•
•
•
2
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
3
time [h]
4
5
Fig. 6 Maximum pond level rise inland of dyke during
simulated extreme wave overtopping (infiltration
already considered during wave overtopping)
0
0
5
10
15
time [h]
20
25
30
• Fig. 10 bottom: plume shrinkage
caused by groundwater flushing
from inland
3D model
• full coupling of flow and transport between surface and
subsurface domain possible with improved HydroGeoSphere
model domain = catchment + estuary
• code HydroGeoSphere (Therrien et al., 2008): at the time coupling of surface and subsurface
processes not possible when simulating density-dependent flow
X:Y:Z=1:1:0.04
X:Y:Z=1:1:0,04
Fig. 10 Distribution of saltwater concentration after 3 days (top) and
after 10 years (bottom)
Fig. 7 Infiltration from inland pond after end of storm
surge into the unsaturated and saturated subsurface
Numerical developments
depth [m]
Fig. 1 Schematic representation of model scenarios: red arrows = density-dependent transport of saltwater, blue arrows =
freshwater, not to scale
1
Water level [m]
Ponding Curve
water level [m]
1
subsurface
saltwater intrusion
depth [m]
2. Extreme storm surge (wave overtopping, max. limit 200 l/s/m, period 2 h)
2D model
half-automated three step procedure developed:
• 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)
1. Maximum water level behind dyke is determined using a density-independent coupled
surface-subsurface model, depending on specific wave overtopping
• surface flow and variably saturated flow
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
• heterogeneous hydraulic conductivity
3. Density-dependent subsurface model uses this information as transient source terms, thus
replacing the coupling to the surface model
water depth
[m]
Fig. 2 Location map
>
Fig. 3 Geological cross section
[m]
• hydraulic conductivity homogeneous
and heterogeneous (Fig. 4)
Results
Tidal influence
Fig. 11 3D model area
A
<
B
depth (m)
• surface flow and variably saturated subsurface flow
• groundwater recharge constant (300 mm year -1)
• saltwater density typical for estuary = 1018 kg m -3
pressure head [m]
• model run 200 days
X:Y:Z=1:1:0.04
• mean standard deviation calculated
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 the
model, low-lying areas behind the dyke are being inundated)
11 km
Fig. 4 Hydraulic conductivity zonation
• pressure head: maximum ± 1.34 m at sea
boundary, ± 0.1 m 7 km inland
Fig. 8a Mean standard deviation of pressure head
concentration
[mg l-1]
• saltwater concentration: majority of model
area not influenced (mean standard
deviation below 1 mg l-1, Fig. 8b)
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.
X:Y:Z=1:1:0.04
depth [m]
• confining layers (A and B, Fig. 8a) not
influenced significantly
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
• maximum mean standard deviation
close to sea boundary in highly conductive
sand layers: up to 707 mg l -1 (Fig. 8b)
• investigation of the influence of groundwater pumping on saltwater intrusion
Fig. 8b Mean standard deviation of saltwater concentration
and enlarged detail of affected area
• investigation of effects of climate-change-induced changes in sea level and salt concentration
on saltwater intrusion