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

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

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

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.

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

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

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

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

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

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.

Delft3D Bathymetry & Roughness

Spatial distribution of

roughness field

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

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.

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

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

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

Comparing D-Flow FM: Flow Event 2010

Comparing D-Flow FM: Flow Event 2010

Downstream of GS Ormož

Comparing D-Flow FM: Flow Event 2010

Comparing D-Flow FM: Flow Event 2011

Comparing D-Flow FM: Flow Event 2011

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

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

Flow Event 2011: Iteration on Roughness

Simulating Flood & Breach Event of 2012

Simulating Flood & Breach Event of 2012

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

Simulating Overtopping of Virje Otok-Brezje

Inundation during 2012 flood

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

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.

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

Thank you!!!