Groundwater modelling Schoonhovenseveer-LAngerak (SLA)

Regional to local modeling for dike stability

1 november 2016

Schoonhovenseveer - Langerak

Riverkilometers 965-983

983

965

Overview area with location of instable dikes

Dike instability Water pressure exceeds weight of dyke body

Problem • Sandy layers form connection between river and dike

• Water board Rivierenland prepares dike stability improvement • Increasing height and width of dikes not always possible

• Other measures needed to lower water pressure

• Effect and efficiency of measures estimated in scenarios

• 3D dynamic groundwater model is needed

• Existing model (MORIA model) not suitable yet

• → Deltares refined the MORIA model

Stability under normal conditions

River

Sand layer

First aquifer

Dike

Instability under extreme conditions

River

Sand layer

First aquifer

Dike

Dike may become instable

Possible measure (1): increase river bed resistance

Sand layer

First aquifer

Dike

Possible measure (2): dam walls

River

Sand layer

First aquifer

Dike

Possible measure (3): horizontal drainage

River

Sand layer

First aquifer

Dike

Groundwatermodelling Waterschap Rivierenland

• Modellering Ondergrond Rivierenland Interactief en Actueel (since 2009)

• iMOD/ modeldatabase

• MORIA consortium: Waterschap Rivierenland, Provincie Gelderland, Vitens

• Several improvements have been made in recent years

MORIA versie 3.2

Used on regional scale level

Average highest groundwaterlevel (m below groundsurface)

Regional Exploration Freshwater Rivierenland (Delta program)

For local applications: further detailing necessary

Sand extraction

Refinement of MORIA for the SLA project

• Holocene from 1 to 7 hydrogeological layers

→ 4 quasi-3D model layers

• From 25x25 to 5x5 meter resolution

• More detail in riverbed elevation

• More detail in other water courses

• Stability measure could be modelled (Grontmij, 2013)

• Groundwater monitoring filters

• Allocated vertically in the new layers

Example cross section

River cross section

GeoTOP and channel belts (sand)

• GeoTOP (TNO, 2012)

• Horizontal extent of hydrogeological units

• Basic parameter values (K-values)

• Channel belts (“zandbanen”) (Cohen et al., 2012)

Lek bottom elevation

Model layers

Hydogeological unit Sand (yellow) or clay/peat (gray)

MORIA model

SLA model

1 Antropogenic AN

1

1

2 Echteld sand/clay 1 EC1

2

3 Nieuwkoop peat NV

4 Echteld sand/clay 2a EC2a

3

5 Echteld sand/clay 2b EC2b

4

6 Basic peat (“basisveen”) BV

7 Wijchen clay WK

8 First aquifer (“WVP1”) WVP

2 5

3

4 6

5

6 7

7

8 8

9 9

Model layers 10 t/m 18

Vertical schematisation: use of iMOD

GEOTOP profiel

SECTION8KR.SPF (o.b.v. geotechnisch lengteprofiel, geïnterpoleerd met iMOD)

GeoTOP

Cross section

iMOD

Vertical schematisation

Insnijding in belangrijke

tussenzandlaag

Old MORIA model New SLA model

Surface water

kolkgaten

Old MORIA model New SLA model

Residuals: d(model, monitoringdata)

• Green: after calibration

• Red: before calibration

• Average: 0.018 meter

Scenario riverbed adjustment (Grontmij)

Effect during MHW (extremely high river level) on head in sandy layer

Scenario horizontal drainage (Grontmij)

Scenario dam walls (Grontmij)

Scenarios (Grontmij, 2013) • River bed adjustment (closing holes) is only effective at one location

• Horizontal drainage has 20-50 cm effect on hydraulic head underneath the dike

• This is potentially a usefull measure

• Dam walls are highly effective

• Effect is strongest at centre of dam wall

Discussion • Model suitable to evaluate scenarios

• Always local research needed before measures are constructed / carried out

• Resistance between river bed bottom and first aquifer is very low:

• Erosion holes occur probably more frequently

• The basic peat layer is discontinuous?

• There still is a gap between monitoring data and model results:

• Still not enough detail?

• Problems in monitoring wells: position, construction, leaks?