Presentation by Roel van der Veen (WSRL) at the iMOD International User Day, during Delft Software Days 2016. Tuesday 1 November 2016, Delft.

1. Groundwater modelling
Schoonhovenseveer-LAngerak (SLA)
Regional to local modeling for dike stability
1 november 2016

2. Schoonhovenseveer - Langerak

3. Riverkilometers 965-983
983
965

4. Overview area with location of instable dikes

5. Dike instability
Water pressure exceeds weight of dyke body

6. 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

7. Stability under normal conditions
River
Sand layer
First aquifer
Dike

8. Instability under extreme conditions
River
Sand layer
First aquifer
Dike
Dike may become instable

9. Possible measure (1): increase river bed resistance
Sand layer
First aquifer
Dike

10. Possible measure (2): dam walls
River
Sand layer
First aquifer
Dike

11. Possible measure (3): horizontal drainage
River
Sand layer
First aquifer
Dike

12. 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

13. Used on regional scale level
Average highest groundwaterlevel (m
below groundsurface)
Regional Exploration Freshwater Rivierenland
(Delta program)

14. For local applications: further detailing necessary
Sand extraction

15. 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

16. Example cross section

17. River cross section

18. 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)

19. Lek bottom elevation

20. 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
46 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

21. Vertical schematisation: use of iMOD
GEOTOP profiel
SECTION8KR.SPF (o.b.v. geotechnisch lengteprofiel, geïnterpoleerd met iMOD)
GeoTOP
Cross section
iMOD

22. Vertical schematisation
Insnijding in belangrijke
tussenzandlaag
Old MORIA model New SLA model

23. Surface water
kolkgaten
Old MORIA model New SLA model

24. Residuals: d(model, monitoringdata)
• Green: after calibration
• Red: before calibration
• Average: 0.018 meter

25. Scenario riverbed adjustment (Grontmij)
Effect during MHW (extremely high river level) on head in sandy layer

26. Scenario horizontal drainage (Grontmij)

27. Scenario dam walls (Grontmij)

28. 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

29. 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?