Calibration of a 2D hydrodynamic model for flood inundation extent using aerial photographs: A ...

90 Pages·2017·6.17 MB·English

by  Alexandros Chatzakis

TRITA-HYD 2017:03

© Alexandros Chatzakis 2017
Royal Institute of Technology (KTH)
Department of Civil and Architectural Engineering
Division of Hydraulic Engineering
SE-100 44 Stockholm, Sweden
Abstract
Alteration of rainfall patterns is one major impact of climate change. Rainfall events
with big precipitation volumes under short periods of time are predicted to become
even more frequent in higher latitude regions, including Sweden. One characteristic
example of such an intense rainfall occurred between the 5th and 6th of September
2015 in Hallsberg, a city in central Sweden, where approximately 105 mm of rain fell
under 24 hours, causing severe flooding in the city. In order to be able to predict
flood cases like the aforementioned one, hydrodynamic models are employed to
simulate floods and investigate rainfall scenarios so that the competent authorities
can take precaution measures. However, due to lack of calibration data most of flood
models are not validated and are comprised of substantial uncertainty.
This report aims to study the Hallsberg flood event in September 2015 by
calibrating a hydrodynamic model using aerial photographs for the flood inundation
extent. The utilized model is MIKE 21, which is a 2D overland flow model developed
by DHI. Contrary to the common practice in flood studies where inclusion of the
infiltration capacity is implemented with an arbitrary reduction of the rain volume,
the infiltration module of MIKE 21, which is a new development in the model, was
utilized. Apart from the inundation extent, the outputs were also evaluated for the
water depth in two points based on a photograph captured from the streets of the
affected area, the description of the course of events for the timing of flood’s
culmination and the water volume on the pixels that were erroneously simulated as
flooded.
The results presented a high degree of agreement with the observations. The
parameter of surface resistance, expressed as Manning’s “M”, was found to be of
paramount importance with the suitable values for undeveloped areas being below
5. In addition, the culverts’ limited capacity played an important role in the flooding
of the city and hence including them in the simulations is crucial. Finally, utilization
of the infiltration module resulted in a higher accuracy of 8.3% although it can be
considered more of an arbitrary deduction of water as some of the parameters used
in it are not physically well justified.

Keywords
MIKE 21, flood modelling, calibration, infiltration module, inundation, aerial photographs
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Sammanfattning
Ändringar i nederbördsmönster är en tydlig konsekvens av klimatförändringen.
Regnhändelser med stora volymer nederbörd under korta tidsperioder förutses bli
alltmer frekventa i regioner vid högre breddgrader, däribland Sverige. Ett
karaktäristiskt exempel av en sådan händelse skedde mellan den femte och sjätte
september 2015 i Hallsberg. Ca 105 mm regn föll inom loppet av 24 timmar vilket
orsakade stora översvämningar i staden. För att kunna förutse översvämningar så
som den tidigare nämnd och möjliggöra vidtagning av förebyggande åtgärder
används hydrodynamiska modeller för att simulera vattenflöden och undersöka
möjliga scenarion av nederbörd. Emellertid, på grund av avsaknaden av data för
kalibrering av modellerna medför användandet av dem en signifikant osäkerhet.
Syftet med den här rapporten är att undersöka översvämningen i Hallsberg i
september 2015 genom att kalibrera en hydrodynamisk modell med hjälp av
flygbilder för översvämningens utbredning. Den använda modellen, MIKE 21, är en
2D modell över ytavrinningen utvecklad av DHI. Praxis vid studiet av
översvämningar är att inkludera infiltrationsförmåga med ett godtyckligt avdrag av
nederbörden. Här används istället infiltreringsmodulen för MIKE 21, vilket är en ny
del som har utvecklats i modellen. Förutom översvämningens utbredning
utvärderades även resultaten utifrån vattendjupet vid två punkter baseras på ett
fotografi från gatorna i det drabbade området. Utvärdering av resultaten gjordes
också mot tid av översvämnings kulm från beskrivning av händelses förlopp samt
vattenvolym vid pixlarna som felaktigt simulerades som översvämmade.
Resultatet visade på en hög grad av samstämmighet med gjorda observationer.
Parametern ytans råhet, uttryckt som Mannings ”M”, visade sig vara av stor
betydelse med lämpliga värden för underutvecklade områden under 5. Därtill
spelade kulvertarnas begränsade kapacitet en viktig roll vid översvämmandet av
staden. Att inkludera dessa i simuleringarna var därför avgörande. Slutligen,
användandet av infiltreringsmodulen resulterade i en högre noggrannhet av 8.3 %,
även om det kan anses vara ett godtyckligt vattenavdrag då vissa av de använda
parametrarna inte är fysiskt välmotiverade.
Nyckerlord:
MIKE 21, översvämnings modellering, kalibrering, infiltration modul, flygbilder
v

Preface
This report constitutes my Master of Science thesis which marks the completion of
my education in the Master’s program Environmental Engineering and Sustainable
Infrastructure at the Royal Institute of Technology (KTH) in Stockholm, Sweden.
The thesis was conducted as a collaboration between Sweco Environment AB and
the Division of Hydraulic Engineering in KTH during the period of March to October
2015.
I would like to thank all the people involved in a greater or lesser extent in the
realization of this project. A big thank you is directed to Sweco Environment AB and
Fredrik Ohls for showing trust in me and giving me the opportunity to collaborate
with them, which made the process of conducting my thesis instructive in multiple
levels. Special thanks go to my supervisor in Sweco, Xavier Mir Rigau, for all his time,
support and valuable inputs. I would also like to thank the whole group of VA-
utredningar that made me feel welcomed.
From KTH I would like to thank my supervisor Joakim Riml for having a
harmonious collaboration, for his availability and for his valuable inputs in the
analysis of the results. I would also like to thank Anders Wörman for being my
examiner and his help in formulating the topic of the thesis.
Finally, since this thesis signifies the completion of my master’s studies, I
would like to express my gratitude to KTH and, more specifically, to the faculty and
personnel of EESI program for offering me the opportunity to develop myself by
making sure to provide top quality education.

Stockholm, December 2016

Alexandros Chatzakis

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Table of contents
Abstract ...................................................................................................................... iii
Sammanfattning .......................................................................................................... v
Preface ................................................................................................................... ….vii
1 Introduction…………………………….………………………………………………..1
1.1 Types of floods ................................................................................................ 1
1.2 Flood management practices ........................................................................ 3
1.2.1 Strategies for modelling floods ............................................................. 4
1.2.2 Different approaches for flood modeling ..............................................5
1.2.3 Calibration data for flood models………………………………………………….6
1.3 Aims and objectives ....................................................................................... 7
2 Materials And Methods………………………………………………………………9
2.1 Materials........................................................................................................ 9
2.2 MIKE 21 ........................................................................................................ 9
2.2.1 MIKE 21 Flow Model ........................................................................... 10
2.2.2 Resistance ........................................................................................... 12
2.2.2.1 Resistance in MIKE 21 ................................................................ 16
2.2.3 Flood and Dry ..................................................................................... 17
2.2.4 Infiltration ........................................................................................... 17
2.2.4.1 Infiltration in MIKE 21 ............................................................... 19
2.3 Case study .................................................................................................... 21
2.3.1 Description of the Study area .............................................................. 21
2.3.2 Hydrological conditions and previous floods .................................... 25
2.3.3 The flood event in September 2015 .................................................... 27
2.4 Acquisition and preprocessing of the data ................................................. 29
2.4.1 Bathymetry ......................................................................................... 29
2.4.1.1 Buildings ..................................................................................... 30
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2.4.1.2 Culverts and bridges ................................................................. 30
2.4.1.3 Closing the boundaries .............................................................. 31
2.4.2 Precipitation ....................................................................................... 32
2.4.3 Surface resistance .............................................................................. 34
2.4.4 Infiltration .......................................................................................... 35
2.5 Model setup ................................................................................................. 37
2.6 Calibration .................................................................................................. 38
2.6.1 Surface resistance ................................................................................ 41
2.6.2 Infiltration parameters ...................................................................... 43
2.6.3 Initial Water surface .......................................................................... 46
2.6.4 Culverts ...............................................................................................47
2.6.5 Sink ..................................................................................................... 48
2.7 Performance comparison and uncertainty analysis ................................... 49
3 Results……………………………………………………………………………..……52
4 Discussion……………………………………………………………………………..60
4.1 Results ......................................................................................................... 60
4.2 Uncertainties and limitations ..................................................................... 62
4.2 Investigation of uncertainty in model parameters .................................... 64
4.3 Calibration and Performance Index ........................................................... 65
5 Conclusions………………………………………………………………………..…..67
References ................................................................................................................. 68

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