URBAN AREA HYDROLOGY AND FLOOD MODELLING

Transcription

1 URBAN AREA HYDROLOGY AND FLOOD MODELLING Flood hazard mapping Rengifo Ortega, Jenny Hagen & Péter Borsányi

2 People need space, so do rivers 2

3 so do rivers 3

4 Flood marks Historical overview 1789 largest flood in Norway 1995 Second largest flood, Glomma River 1997 National Flood Mapping Program 2016 Special focus on Urban Flooding Since then, 125 rivers mapped on 1250 km river length R&D project on urban floods initiated 4

5 NVE s overall responsibility Prevention of damages caused by flood (with some exceptions) Assist local authorities toidentify hazard Analyse and evaluate the risk associated with flood hazard and Determine appropiate ways to eliminate or control flood hazard 5

6 Mapping Plan Areas with the largest potential risk existing buildings and infrastructure 6

7 Flood mapping methods steps Flood estimation Hydraulic modelling 1D/2D GIS Analysis Flood hazard map Digital production and reporting 7

8 Flood estimation 8

9 Hydraulic modelling 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 9

10 Hydraulic simulations Discharges from flood estimation Green = Q500 Red = Q200 Blue = Q20 Model ( ) Results in water levels P P P5.5 BRU P P7.5 Observert P P P P P P P P P P P P P P P P P P P P P P P P P41 10

11 GIS Analyse 1D GIS analyse 2D Flooded areas Digital terrain model Irregular mesh DTM Thiessen polygons

12 Flooded Area 12

13 Hydrographic LiDAR What new in 2D flood analysis? Change detection based on digital elevation model of differences (DoD) Sediment mapping Differentiate coefficient Manning s Habitat modelling River bank erosion 13

14 What new in 2D flood analysis? 14

15 Knowledge - tools Land Use Planning Avoid development in flood prone areas. 15

16 Experiences Local involvement is important Most of the communities are willing to use the results Guidance and control from the government is necessary

17 Anne: Urban flooding Anne Fleig HV Future trends & challenges? 17

18 PRE-STUDY FOR VANN I BY : SURFACE WATER MODELLING OF THE AKERSELVA CATCHMENT NVE, Oslo, Jenny Hagen M.Sc. Flood Risk Management, UNESCO IHE, Delft

19 Outline 19

20 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 resolution Technological development better computing power and more sophysticated software Change of focus in hydrological research from rural to urban floods, considering the complex man-made water transport systems Coupled 1D-2D hydraulics used more commonly Hydraulic modelling is used to help understanding the integrated behaviour of complex collection systems, and is used in flood risk analyses Objective: Setup a 1D-modell of Akerselva, study velocity and depth at given discharge and couple with 2D model 20

21 Study area Area 15 km 2 Akerselva 9.78 km long 150 m drop fron Maridalsvannet to Hovinbekken Q min 1.5 m 3 /s (summer) og 1.0 m 3 /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 Flood zone map not developed (only1000-year flood for a dambreak study) 21

22 Methods: Overview 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 roughness Limited 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 22

23 Methods: Main steps 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 23

24 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 with discharge) Structures (bridges, culverts, weirs excluded 2D cell size max 5 m 24

25 Methods: Boundary conditions 1D-model: Based on flood event in August, 2010 Upstream: Discharge from gauging station Maridalsvannet Downstream: normal depth or water level at outlet to sea ( 2D-model: Precipitation: Synthetic 24h rainfall series generated from max observed precipitation in Oslo in August

26 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 26

27 Results: 1D-model 182 georeferenced XS interpolated with m spacing XS locations preparing for including structures later Elevations adjusted to match DEM Time Step calculated from Courant-Friedrichs-Lewy stability criterion, setting Courantnummer=1 27

28 Results: Depth and velocity Wl and V at Grandalen Used for coupling 1D and 2D model domains 28

29 Results: Coupling to 2D-grid 2D-model: Composed of 5 sub-grid linked with virtual lateral structures HEC-RAS cell size: 5m x 5m (recommended minimum for urban flow simulations) DEM: 0.5m x 0.5m 29

30 Results: Simulations 1D simulation time min 1D-2D simulation time: min 30

31 Results: Flow depth and velocity 31

32 Limitations and uncertainty Lack of data and assumptions River banks move with discharge Friction varies along and across Stormwater collecting network neglected Uncertainties er mainly realted to: Model structure Parameters (Manning s n) Quality of data used for boundaries DEM includes building at terrain Rough resolution (5m): loosing details 32

33 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 greatly improve the system by representing an important feature of urban floods 33

34 Recommended literature Chen, A., Djordjevic, S., Leandro, J., Evans, B. and Savic, D Simulation of the building blockage effect in urban flood modelling. 11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, Chow, V.T Open Channel Hydraulics. New York, NY, USA: McGraw-Hill Book Company, Inc. Coon, W.F 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), Benchmarking 2D hydraulic models for urban flood simulations. Proceedings of the Institution of Civil Engineers: Water Management, Vol 161 (1), DOI: Mark, O., Weesakul, S., Apirumanekul, C., Aroonet, S.B. and Djordjevic, S Potetial and lmitations of 1D modelling of urban flooding. Journal of Hydrology, Vol 299 (3-4), DOI: Saltveit, S.J. and Braband, Å 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 Flood inundation modelling: A review of methods, recent advances and uncertainty analysis. Environmental Modelling and Softwar,e Vol 90 (2017), DOI: Flood maps in PDF format WebGIS 34

35 Questions? 35