Rengifo Ortega, Jenny Hagen & Péter Borsányi

Flood hazard mapping

URBAN AREA HYDROLOGY AND FLOOD MODELLING

People need space, so do rivers

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… so do rivers

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http://www.dagbladet.no/2015/09/02/nyheter/innenriks/flom/royken/40924357/

Historical overview

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Flood marks 1675- 1995

1789 largestflood in Norway

1995 Second

largest flood, Glomma

River

1997 National

Flood MappingProgram

2016 Special focus on Urban

Flooding

Since then,

125 rivers mapped on1250 km river length

R&D project on urban floods initiated

NVE’s overall responsibility

Prevention of damages caused by flood (with some exceptions)

Assist local authorities toIdentifyhazard

Analyse and evaluate the risk associated with flood hazard and

Determine appropiate ways to eliminate or control flood hazard

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Mapping Plan

Areas with the largestpotential risk

existing buildings and infrastructure

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Flood mapping methods –steps

Flood estimation

Hydraulicmodelling

1D/2D

GIS Analysis

Flood hazard map

Digital production

and reporting

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Flood estimation

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Hydraulic modelling

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Field data collection• Site visits• Cross sections

Calibration• Watermarks• Boundary conditions

Hydraulic modelling• HEC-RAS (1D and/or 2D)• Mike 11 (1D)• Mike Flood (1D and/or 2D)• etc

Hydraulic simulationsDischarges from flood estimation

Model

Results in

water levels

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0 500 1000 1500 2000 2500350

355

360

365

370

375

380

()

162.7

9

233.1

315 P

326

7.220

7 P4

305.1

19 P

5.5 B

RU

386.4

169 P

7

425.7

35 P

7.5 O

bserv

ert 2

011

445.3

940 P

8

482.5

500 P

9

536.4

312 P

10

581.5

272 P

1160

8.126

563

3.294

1

670.6

488

696.0

299 P

1971

5.841

2 P20

770.6

352 P

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819.1

430 P

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887.8

465 P

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1018

.018 P

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1066

.067 P

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1104

.452 P

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1266

.185 P

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1306

.608 P

3013

32.15

8 P31

1372

.221 P

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1470

.868 P

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1801

.223 P

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1978

.511 P

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2191

.564 P

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2298

.549 P

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2451

.013 P

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2562

.780 P

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2690

.873 P

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2915

.937 P

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Green = Q500Red = Q200Blue = Q20

GIS Analyse 1D

Digital terrain model

Flooded areas

GIS analyse 2D

DTMIrregular mesh

Thiessenpolygons

Flooded Area

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What new in 2D flood analysis?

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Hydrographic LiDAR

Change detection based ondigital elevation model ofdifferences (DoD)

Sediment mapping

Differentiate Manning’scoefficient

Habitat modelling

River bank erosion

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What new in 2D flood analysis?

Knowledge - tools

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Land Use PlanningAvoid development in flood prone areas.

Experiences

• Local involvement is important• Most of the communities are

willing to use the results• Guidance and control from the

government is necessary

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Anne:

Future trends & challenges?

Urban floodingAnne Fleig

HV

Jenny HagenM.Sc. Flood Risk Management, UNESCO IHE, Delft

NVE, Oslo, 01.09.2017

PRE-STUDY FOR “VANN I BY”:SURFACE WATER MODELLING OF THE

AKERSELVA CATCHMENT

Outline

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Introduction

Over 50 % of the world’s population is living in urban areas, Oslo is expected to grow by 20% by 2030 (SSB 2017)Increased focus on remote sensing higher accuracy and better resolutionTechnological development better computing power and more sophysticatedsoftwareChange of focus in hydrological research from rural to urban floods, consideringthe complex man-made water transport systemsCoupled 1D-2D hydraulics used more commonlyHydraulic modelling is used to help understanding the integrated behaviour ofcomplex collection systems, and is used in flood risk analyses

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Objective: Setup a 1D-modell of Akerselva, study velocity and depth at given discharge and couple with 2D model

Study area

• Area 15 km2

• Akerselva 9.78 km long• 150 m drop fron Maridalsvannet to

Hovinbekken• Q min 1.5 m3/s (summer) og 1.0 m3/s

(winter)• 20 waterfalls and 44 bridges• Erosion along shores I prevented by

blocks of rock• Akerselva flooded in November, 2000,

med smaller events in 2006, 2010 and 2014.

• Flood zone map not developed (only1000-year flood for a dambreakstudy)

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Methods: Overview

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Special in urban areas Higher resolution and accuracy necessary

Challenges:

Limited or no data on collection systems

Limited or no data with cross sections

No data on roughnessLimited data on flow structures

(bridges, culverts, weirs, etc)

HiResTerrain Model• Land Use• Surface

roughness• (From Lidar)

Hydraulic Model• Bathymetry• Boundaries• (From Shallow

water or Saint-Venant eq)

Flow depth, velocity and time

Methods: Main steps

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• Define number of cross sections (considering limitations)

• Using ArcGIS (HEC-GeoRAS extension) for digitizing main channel, banks, cross sections from ortofoto and maps

• Manually edit cross sections in HEC-RAS to match longitudinal profile from terrain model

• Run and stabilize1D model for main river• Couple 2D flow areas to 1D main river• Publish results in ArcGIS (generate maps)• Summarize limitations, uncertainties, etc

Methods: Assumptions and model setup

Flow is 1D in main channel

Cross sections are simplified to trapezoids, about 1m depth

Buildings in main channel are square shapes, vertical walls

River banks are fixed from ortophoto and maps GeoCache-kart (despite of varying withdischarge)

Structures (bridges, culverts, weirs excluded

2D cell size max 5 m

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Methods: Boundary conditions

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Based on flood event in August, 20101D-model:

Upstream: Discharge from gauging station Maridalsvannet

Downstream: normal depth or water level at outlet to sea (www.sehavnivå.no)

2D-model:

Precipitation: Synthetic 24h rainfall series generated from max observed precipitation in Oslo in August 2010

Methods: Model validation

• only limited possibilities for validation

• Satellite images to validate flooded extent (typically obscured by clouds)

• Comparing results with alternative models:

• Existing: Multiple Flow Algorithm from GIS analyses • To be developed: Machine learning algorithms using social video sites (Vimeo, Youtube, Facebook stream) as source or smartphone apps recording flow extent or similar

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Results: 1D-model

182 georeferenced XS interpolated with 3.5 – 10 m spacing

XS locations preparing for including structures later

Elevations adjusted to match DEM

Time Step calculated from Courant-Friedrichs-Lewy stability criterion, setting Courant-nummer=1

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Results: Depth and velocity

Wl and V at Grandalen

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Used for coupling 1D and 2D modeldomains

Results: Coupling to 2D-grid

2D-model:

Composed of 5 sub-grid linked with virtual lateral structures

HEC-RAS cell size: 5m x 5m (recommendedminimum for urban flowsimulations)

DEM: 0.5m x 0.5m

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Results: Simulations

1D simulation time

15-18 min

1D-2D simulation time:

23-25 min

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Results: Flow depth and velocity

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Limitations and uncertainty

Lack of data and assumptions

River banks move withdischarge

Friction varies along and across

Stormwater collecting networkneglected

DEM includes building at terrain

Rough resolution (5m): loosing details

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Uncertainties er mainly realted to:Model structure

Parameters (Manning’s n)

Quality of data used for boundaries

Conclusions

1D model for Akerselva is developed, tested with different boundary conditions and θ-factor

2D model with 5m gridsize couplled via lateral structure covering the first upstram 75m river below Maridalsvatnet

Existing XS will have to be replaced by new ones when available

Manning’s n will have to vary along the reach reflecting varying friction and energy losses

Structures (bridges culverts, etc) will have to be built in the model when available

Transfer capacities and connections of the stormwater transport system will greatlyimprove the system by representing an important feature of urban floods

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Recommended literatureChen, A., Djordjevic, S., Leandro, J., Evans, B. and Savic, D. 2008. Simulation of the building blockage effect in urban flood modelling. 11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008.Chow, V.T. 1959. Open Channel Hydraulics. New York, NY, USA: McGraw-Hill Book Company, Inc. Coon, W.F. 1998. Estimation of Roughness Coefficients for Natural Stream Channels with Vegetated Banks. Denver, CO, USA: U.S. Geological Survey (USGS). Hunter, N.M., Bates, P.D., Neelz, S. et. al (9 more authors), 2008. Benchmarking 2D hydraulic models for urban flood simulations. Proceedings of the Institution of Civil Engineers: Water Management, Vol 161 (1), 13-30.DOI: http://dx.doi.org/10.1680/wama.2008.161.1.13Mark, O., Weesakul, S., Apirumanekul, C., Aroonet, S.B. and Djordjevic, S. 2004. Potetial and lmitations of 1D modelling of urban flooding. Journal of Hydrology, Vol 299 (3-4), 284-299.DOI: https://doi.org/10.1016/j.jhydrol.2004.08.014Saltveit, S.J. and Braband, Å. 2016. Konsekvenser av vannføringsendringer og lave vannføringer på biologiske forhold I Akerselva. Oslo, Norway: Naturhistorisk Museum, Universitet I Oslo, Rapport nr. 55.Teng , J., Jakema, A. J., Vaze, J., Croke, B.F.W., Dutta, D. and Kim, S. 2017. Flood inundation modelling: A review of methods, recent advances and uncertainty analysis. Environmental Modelling and Softwar,e Vol 90 (2017), 201-201.DOI: https://doi.org/10.1016/j.envsoft.2017.01.006Flood maps in PDF format http://www.nve.no/flomsonekart

WebGIS http://atlas.nve.no/ge/Viewer.aspx?Site=NVEAtlas

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Questions?