NGHS Symposium

1. Mohamed Yossef
Good Modelling Practice for Rivers
Practical notes

2. Rivers, difference with respect to Seas
•Planform
•Floodplains
•Structures
•Islands
•Flood wave
11 November 2014

3. Modelling – general overview  good modelling practice
Define modelling strategy
construct the model
construct grid
create topography
hydraulic structures
hydraulic model
define numerical parameters (e.g. Dt)
define physical parameters
roughness
eddy viscosity & closure model
initial condtions
define boundary conditions
define outputs
morphological model
define fixed layers (sediment thickness)
define parameters
sediment size
sediment transport formula
morphological boundaries
morphological acceleration factor
other models (e.g. WAQ)
analysis
absolute
comparative

4. Data for modelling
•Model Construction data  make a model
•e.g. land boundaries, bed levels, etc.
•Boundaries (forcing)  run the model
•Needed to run the model
•e.g. river discharge
•Observations  confidence in the model
•Needed to calibrate and validate
•e.g. water level at stations
4

5. Modelling strategy
•Models to mimic reality  think why!
•Models to answer a question/group of questions
•Make a strategy  make your model fit for purpose
•Define area to model
•Define boundaries
•Define important processes
•Define time-scale
•Define analysis scenarios
•Make your model …
•Attention to grid construction
•Boundary conditions
•Settings
•Analyse and present results
11 November 2014

6. Strategy for the DVR-Model
• Requirements
• Morphological model for the Rhine
• Analyse seasonal and long-term
variations
• Evaluate impact of measures on
reach-scale as well as branch scale
• Calculate dredging volumes
• Model is fast (run 40 years in a week
or so)
• Choices:
• No. of cells in cross-direction
• Choice of boundaries
• Separation between domains
• Physical process
11 November 2014

7. 7
Grid – avoid stair-cases
Avoid stair-cases along alluvial section

8. Grid construction – bifurcation
11 November 2014
Delft3D-FLOW: Structured grid
D-Flow FM: Flexible Mesh
Delft3D 4
Delft3D Flexible Mesh

9. Grid construction – local refinement
11 November 2014

10. Grid construction – floodplains
11 November 2014
Mesh A B C D
Cells in cross-section
~ 13 8 16 32
active points 122,328 22,881 59,331 187,955
A
B
C
D
Delft3D-FLOW: structured
Delft3D-FLOW: structured
D-Flow FM
D-Flow FM

11. a) Delft3D-FLOW: Structured mesh
b) D-Flow FM
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)
Grid construction – optimisation 1
Source: Damir Bekić et al. (Water Resources Department University of Zagreb, Croatia)

12. a) Structured grid (Delft3D-FLOW)  does not follow the river -> requires dense mesh b) Unstructured mesh (D-Flow FM)  Follows the river -> Coarser mesh c) D-Flow FM allows weir schematization  Coarser mesh
Levees
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
Source: Damir Bekić et al. (Water Resources Department University of Zagreb, Croatia)
Grid construction – optimisation 2

13. Attention to bed level interpolation
dense data
sparse cross-sections
dense cross-sections

14. River Flow 2012 Mohamed F.M. Yossef September 7, 2012 14
Boundary condition: quasi-steady discharge for river morphology
0
1000
2000
3000
4000
5000
6000
7000
8000
1993 1994 1995 1996
Time
Q (m3/s)
Q1 Q3
Q2
0
1000
2000
3000
4000
5000
6000
7000
8000
1993 1994 1995 1996
Time
Q (m3/s)
morfac
• Repeat a yearly schematised hydrograph using a sequence of
steady discharges
• Apply a “morphological factor” to speed up morphology (same
morphological changes in shorter flow period): factor 50 to 200

15. River Flow 2012 Mohamed F.M. Yossef September 7, 2012 15
Boundary condition – discharge schematisation
0
2000
4000
6000
8000
10000
12000
14000
16000
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035
Date
Q (m3/s)

16. 11 November 2014
Boundary condition – flood attenuation
Q@x1
Q@x2
t
Q@x1
Q@x2
t
x1 = location x1 upstream
x2 = location x2 downstream
Q
Q
Dynamic simulation QH relation at downstream boundary
Quasi steady Water level time-series at downstream boundary

17. Parameter settings
Choice of sediment transport formula Choice of 2D parameters

18. Analysis of results
2D-behaviour (40-years)
2D-behaviour (detail)
1D-behaviour (40-years)
Dune heights

19. Thank you
Source: Herman Kernekamp (Deltares)

20. Questions
11 November 2014