1. Pilot Case Study Using D-Flow Flexible Mesh
Hydraulic Study of River Drava (Croatia)
Sanjay Giri , PhD
Department of River Dynamics and
Morphology
Deltares, The Netherlands
Delft Software Days
19 June 2014
Damir Bekić, PhD, Dipl.Ing.
Igor Kerin, mag.ing.aedif.
Ana Mioč, mag.ing.aedif.
Water Resources Department
University of Zagreb, Croatia

2. Method: Finite differences
Time step: Same Dt
Computational speed
Numerical Method: Finite volume
Time step: Same Dt
Expected Computational speed
Delft 3D
D-FM
Lower flexibility
-> Denser mesh -> (higher number of calculation
points)
-> Lower time step Dt
-> Computationally less efficient
Higher flexibility
-> Coarse mesh -> (less number of calculation
points)
-> Higher time step Dt
-> Computationally more efficient
Model
SAME MESH (NO OF CALCULATION POINTS)
for both models
DIFFERENT MESH (NO OF CALCULATION POINTS)
Lower number of calculation points with satisfactory
results
Mesh and model capabilities and flexibility
Goal: by comparing D3D and D-FM models answer two main
questions.
Questions for River Drava study:
1. What are the alterations in model results?
2. Which model is computationally efficient?
Overview

3. Outline
1. Introduction
a. Drava pilot case area
b. Flood event 5th November 2012
c. HPP Formin outlet canal levee breaches
d. Original Delft3D model
2. Delft3D and D-FM meshes, geometry and breach schematization
3. Results and comparison
a. Flow event 2011 (calibration of original D3D)
b. Flow event 2010 (verification of original D3D)
c. Flood event 2012
d. Simulating 2012 Levee breach and overtopping in D-FM
4. Conclusions

4. Study Area
Study area is located on the Drava river in SE part of Europe at the border between
Slovenia and Croatia, and between the cities Maribor and Varaždin.

5. Flood Event of November 2012
 The River Drava section in Austria, Slovenia and Croatia (Qavg.HPP-
Varaždin=330 m3/s)
 5/11/2012: the highest flood in the last 60 yrs (Qmax.HPP-Varaždin= 3311 m3/s):
• unexpected transformation of the flood wave in the downstream area
• in 1966: Q=2843 m3/s, in 1998: Q=2221 m3/s
• large flooding in Slovenia and Croatia, levee breach and overtopping
• large amount of damage in Slovenia and Croatia

6. HPP
Qmax
[m3/s]
Time at peak
DQ
[m3/s]
Dt
[hrs]
Labot 2592 5/11/2012 15:00
Vuhred 2945 5/11/2012 16:00 +353 1
Ožbalt 3040 5/11/2012 17:00 +95 1
Zlatoličje 3170 5/11/2012 18:00 +130 1
Formin 2840 5/11/2012 23:00 -330 5
Varaždin 3311 6/11/2012 08:00 +471 9
Čakovec 2085 6/11/2012 21:00 -1226 13
Dubrava 1930 6/11/2012 23:00 -155 2
• Sudden flow increase on HPPs
• Propagation speed 12.5 km/hour
(45 m/s)
• Peak flow on HPP Varaždin was
471 m3/s higher than the peak
flow on HPP Formin.
Flood Event of November 2012

7. Flow hydrographs in 2010, 2011, 2012
Qin = QHPP Formin + QDravinja + QPesnica
Qout = QHPP Varaždin
200-years return period.
Comparing to the theoretical
flood waves
Flood Event of November 2012

8. During the 5/Nov/2012 flood wave two levee breaches of the HPP Formin outlet
canal and overtopping and breach of levee Virje Otok-Brezje occurred.
The HPP Varaždin peak flow was 471 m3/s higher than the peak on the HPP
Formin.
Peak flows of Dravinja 109 m3/s and Pesnica 85 m3/s + overtopping of the levee
Virje Otok-Brezje→insufficient for the flow increase of 471 m3/s on the HPP
Varaždin
Flood Event of November 2012

9. Levee breach and overtopping
UPSTREAM LEVEE BREACH (L1)
• 1.3 km downstream from the HPP Formin
powerhouse, 150 m wide
• left bank erosion - 50 m wide and 8 m tall
• 12 m of sediment deposits in the canal
DOWNSTREAM LEVEE
BREACH (L2)
• 6.3 km downstream from the HPP
Formin powerhouse, 200 m wide
• 300,000 m3 of sand deposits in the
outlet canal
• flows to the outlet canal was evident
even after the flood wave passage

10. Original Delft3D model
Model domain includes:
• River Drava old channel;
• the HPP Formin outlet canal;
• Dravinja and Pesnica streams.
Upstream boundary and discharge insertion location:
• Powerhouse of the HPP Formin (discharge boundary);
• Dam at HPP Formin (discharge insertion) ;
• Dravinja (discharge insertion);
• Pesnica (discharge insertion).
Downstream boundary:
• water level at HPP Varaždin dam.

11. Delft3D Bathymetry & Roughness
Spatial distribution of
roughness field

12. Control cross sections
• 41 on the River Drava old channel
• 17 on the HPP Formin outlet canal
Model control point g.s. Ormož :
• Control Points (CP)
• Recorded water levels on g.s. Ormož
(hourly data).
• Peak discharge at HPP Varaždin.
• Start time of overtopping of the Virje
Otok-Brezje levee (L3).
Delft3D model calibration and validation
Simulation scenrios
• SIMULATION A: with breaches of the
outlet canal included (L1+L2)
• SIMULATION B: no breaches of the
outlet canal

13. Delft3D model calibration and validation
Model calibration
results for 6/2011
Model verification
results for 9/2010
Water level at Ormož
• For the simulation A (solid line) the
difference between computed and
measured WL is within 10 cm.
• The simulation B result (dashed line) shows
that the water level rising at the Ormož is
significantly slower, and the peak water
level 60 cm lower than the recorded level.

14. a) Structured mesh (Delft3D, D-FM1)
b) Unstructured mesh (D-FM2)
Number of net nodes: 331082
Number of net links: 660523
Maximum orthogonality:0.35 (poor)
General smoothness: 1 (good)
Maximum local smoothness:8 (poor)
Number of net nodes: 80860 (4.1 times less)
Number of net links: 177659 (3.7 times less)
Maximum orthohonality:0.014 (good)
General smoothness:1 (good)
Maximum local smoothness:10 (poor)
D-FM Mesh Generation

15. a) Structured grid (D3D) -> does not follow the river -> requires dense mesh
b) Unstructured mesh (D-FM) -> Follows the river -> Coarser mesh
c) D-FM allows weir schematization with TC -> Coarser mesh
Levees
• Thin Dams/Weirs
(with TC)
• Pump
In (a) and (b) levees and groynes are modelled within bathymetry, while in (c) mesh is more
coarse as there is no need for longitudinal elements to be covered in bathymetry
D-FM Schematization

16. 1. Convert existing Delft3D into Delft Flexible Mesh model
2. Compare Delft3D and D-FM without changing mesh in D-Flow FM for three events:
a. Flood event in 2010 used for verification of Delft 3D model
b. Flood event in 2011 used for calibration of Delft 3D model
c. Flood event on 2012
3. Set-up of a new Delft3D Flexible Mesh grid and model using the new graphical user
interface (RGFGRID + Delta Shell) and apply the advantages of the new software
(Flexible mesh, thin dikes and weirs with time control)
4. Compare results (output results and computation time) of all existing models with
measured data (discharges and water levels) for three recorded hydrological
events:
a. Flood event in 2010 used for verification of Delft 3D model
b. Flood event in 2011 used for calibration of Delft 3D model
c. Flood event on 2012 (additionally replicate breach using weirs with TC)
5. Use same computer system for all
simulations:
Methodology

17. Comparing D-Flow FM: Flow Event 2010

18. Comparing D-Flow FM: Flow Event 2010
Downstream of GS Ormož

19. Comparing D-Flow FM: Flow Event 2010

20. Comparing D-Flow FM: Flow Event 2011

21. Comparing D-Flow FM: Flow Event 2011

22. 1. More detailed roughness distribution of floodplains could partially improve rising
stage of hydrograph, but the main problem with 1h lag time and peak discharge
could not be resolved with roughness re-distribution in the original model.
2. Roughness distribution in D-FM is much easier as one can define roughness
distribution in Delta Shell environment.
3. This is done by drawing roughness polygons (gradient and contours) over defined
mesh and different layers such as open street maps, Bing maps, or any other geo-
referenced background layer.
Flow Event 2011: Roughness Variability

23. 1. Bathymetry has been obtained from surveyed cross sections. Area between two
cross sections was interpolated!
2. Upstream part of model is located in Republic of Slovenia. Geometry data around
and upstream of GS Borl needed to be transformed in Gauss-Kruger MGI 1901 /
Balkans zone 5 coordinate system so that it could match downstream geometry
data.
Flow Event 2011: Bathymetry

24. Flow Event 2011: Iteration on Roughness

25. Simulating Flood & Breach Event of 2012

26. Simulating Flood & Breach Event of 2012

27. Simulating Flood & Breach Event of 2012
1. Different times of levee breach (L1) and (L2)
2. Replicate floodplain erosion by changing the bathymetry (see figures below)
3. Adding additional elements (weirs) for roads and simulate breach of road at (L1)
4. Testing additional flow (assumption that HPP Powerhouse was not closed)
Additional test simulations

28. Simulating Overtopping of Virje Otok-Brezje

29. Inundation during 2012 flood

30. 2010 2011 2012_A
2012
_B
_dikebreach
Paramete
r
D3D D-FM1 D-FM2 D3D D-FM1 D-FM2 D3D D-FM1 D-FM2 D3D_B
D-
FM2_B
dike br
Method FD FV FV FD FV FV FD FV FV FD FV FV
Roughne
ss
Same distribution Same distribution Same distribution Same distribution
Simulatio
n time
(hours)
96 96 50 50
Courant - 0.7 0.7 - 0.7 0.7 - 0.7 0.7 - 0.7 0.7
Time step 7.5 s
Restricted by
Courant
7.5 s
Restricted by
Courant
7.5 s
Restricted by
Courant
7.5 s
Restricted by
Courant
No of
nodes
437,162
341M
1282N
331082
(actual)
431,082
(maximu
m)
80,860
437,162
341M
1282N
331082
(actual)
431,082
(maximu
m)
80,860
437,162
341M
1282N
331082
(actual)
431,082
(maximu
m)
80,860
437,162
341M
1282N
331082
(actual)
431,082
(maximu
m)
84,300
CPU time
(hours)
8.510 31.416 4.683 7.828 21.600 2.950 9.240 36.850 11.816 10.020 3.983 4.100
D-FM /
D3D
1.00 3.69 0.55 1.00 2.76 0.38 1.00 3.99 1.28 1.00 0.40 0.44
comment
s:
3.7x
slower
1.8x
faster
2.7x
slower
2.6x
faster
4.0x
slower
1.3x
slower
2.5x
faster
2.3x
faster
Computational Efficiency

31. Conclusions
1. At GS Ormož
• Rising stage of hydrograph shows good match with the observations
• Peak water levels and discharges are closer to the observation.
2. Time of levee breach replicated satisfactorily (L1) (L2) (L3)
3. Time of Virje Otok – Brezje (L3) overtopping has been replicated very well.
4. Comparison of 2012 flood with breach model reveals reasonable match at Virje
Otok Brezje longitudinal profile.
5. The peak discharge could be reached only if (a) we assume that the HPP
Powerhouse was fully opened with discharge over 500 m3/s (520 m3/s is the
maximum value); (b) we assume that during the 2012 flood, significant volume
of water accumulated (clogged) on left floodplain upstream of the breach (L1).
This accumulation was the source of additional flow, which rapidly entered into
the HPP canal (causing large floodplain erosion and dike breach). This appears
to have led to an unexpected rise of the peak in the canal and downstream of
the main river channel.

32. Drava pilot study showed that:
1. Grid flexibility in D-FM allows significant reduction of grid numbers, particularly in
case the model domain is complex like in current study.
2. Schematization of weirs with time control option allows to simulate levee breach
more realistically.
3. Computational time seems to be higher due to local fine grid and high velocities (as
the time step is computed in D-FM based on CFL condition).
4. D-FM is a powerful and user friendly (simple to model) software. New Delta Shell
GUI simplified and improved the model set-up (boundary conditions, roughness,
bathymetry, etc.)
5. Post processing tool could be improved to allow more output options.
Conclusions

33. Thank you!!!