2D Modeling of Urban Flood Vulnerable Areas

Transcription

1 2D Modeling of Urban Flood Vulnerable Areas Sameer Dhalla, P.Eng. Dilnesaw Chekol, Ph.D. A.D. Latornell Conservation Symposium November 22, 2013

2 Outline 1. Toronto and Region 2. Evolution of Flood Management 3. Flood Delineation 4. Urban Flood Vulnerable Areas 5. Model Types and model selection 6. 1D Hydraulic modeling 7. 2D hydrodynamic modelling 8. Case Study Spring Creek Sameer Dhalla (Overview) Dilnesaw Chekol (Technical)

3 The TRCA's area of jurisdiction includes: 3,467 sq. km: 2,506 on land and 961 water-based. TRCA Jurisdiction This area is comprised of nine watersheds including: Etobicoke Creek Mimico Creek Humber River Don River Highland Creek Rouge River Petticoat Creek Duffins Creek Carruthers Creek The TRCA s jurisdiction also extends into Lake Ontario to a point defined by the Territorial Divisions Act, R.S.O The population in 2004 within TRCA s jurisdiction is approximately 4,300,000 (37% of Ontario s population).

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5 The Evolution of Floodplain Management Pre-Hazel: Infrastructure Era: Floodplain Protection: Development Near Water and Channelization to Facilitate Development Water Control and Conveyance Prohibition of Development in Floodplains

6 What is a Floodplain? An area, usually low lands adjoining a watercourse, that has been or may have been covered by flood waters. How Do We Delineate Floodplains? All procedures are outlined in the Ministry of Natural Resources Flood Plain Management in Ontario - Technical Guidelines Key Components to Developing Flood Lines: 1.Hydrology Modeling 2.Hydraulic Modeling 3.Flood Plain Mapping 6

7 Hydrology Modeling How much water flows down the river during various storm events 1.TRCA has completed Hydrology Studies for each of the 9 watersheds 2.Hydrologic computer models were developed that include physical parameters such as: Watershed area Land Use / Geology River Length 3. Metrological (stream and rainfall data) is added to the model to determine flow rates 7

8 Hydrology Modeling Results Lower Carruthers, Ajax Lower Don, Toronto Flow Rate (m 3 /s) normal Hurricane Regional flows in most of our watersheds are significantly higher than the 100 year flow resulting in 44 urban areas vulnerable to the Regional Flood. Flood Frequency (years) 8

9 Urban Flood Vulnerable Areas 44 FVA Clusters 6,000 Flood Vulnerable Structures 850 Flood Vulnerable Roads 35,000 People at Risk

10 Markham, Woodbine and Steeles Mississauga, Hwy 427 at Burnhamthorpe GO Train on the Don River Toronto, Black Creek, Jane and Wilson

11 Hydraulic Modeling Calculate water surface elevations to define flood plain extents and velocities. 1. TRCA has completed Hydraulic Studies for each of the 9 watersheds and manages approximately 500 floodplain maps 2. Mostly 1D - Flows are contained within a defined valley and flow in a longitudinal direction 3. In urban situations where flows are not contained and spill (water moves in a longitudinal and lateral direction), 2D modeling is necessary 11

12 Hydraulic Modeling Uses include: 1.Regulation, ensure new development is located outside the Regional Floodplain. 2.Characterise flooding. Flood depths, velocities, potential damage, people effected 3.Flood Warning 4.Flood Remediation 12

13 2D Model Animation Lower Don

14 Model Types and Model Selection Models only approximate complex natural process Models recognize only their own assumptions Model types are classified according to their dimensionality one dimension (1D) (Unidirectional flow: means variables (velocity, depth, etc.) change predominantly in one defined direction, x, along the channel), two dimensions (2D) (compute the horizontal velocity components (Vx and Vy) or, alternatively, velocity vector magnitude and direction throughout the model domain) Model selection need to consider the compatibility of model assumption and the reality that needs to be approximated or represented

15 Model Types and Model Selection (cont d)

16 Model Types and Model Selection (cont d) Model types that TRCA uses HEC-RAS Delft3D Mike Flood (Mike 11 & 21) HEC-RAS MIKE FLOOD MIKE 11 MIKE 21 Delft3D

17 1D Modeling Solves energy. Assumptions, Assumes that flow is parallel to main channels (Unidirectional flow ) Constant Water Surface Elevation on a given cross section: Suitable for: Confined flow and mostly unidirectional No need of detailed velocities With many complex structures

18 1D Modeling (cont d) Advantages, Accurate hydraulic description in rivers with 1D flow; Less computational points relative to 2D model, ie. less computational time; Easy to analyse and extract results Hydraulic structures well represented Disadvantages : Flow paths must be known beforehand; No detailed flow descriptions in floodplains.

19 2D Modeling Solves mass and momentum Assumptions, 2D models make no implicit assumptions about flow direction or magnitude, discharge divisions in splitting channels and the discharge given inflow and outflow elevations can be calculated directly. Suitable for: Flow paths are not well defined or difficulty of visualizing the flow patterns Complex channel-floodplain interaction Threaded rivers and poorly confined flow. Flood Hazard-When detailed velocity and depth patterns are important Complicated nature of overflow along streets and between development Flow attenuation and floodplain storage are significant

20 2D Modeling (cont d) Advantages, Realistic computation of velocities in any direction and determining watershed sheet flow patterns, flow depth -hazard to people Accounts for lateral variation in water surface elevations Better schematization of distributed flow in threaded rivers or unconfined flows Relatively easy to schematise model (i.e can be quick to set up) Beneficial to model impacts of obstructive fill Disadvantages: Costly in computational time; Requires fine grid in rivers/channels in order define conveyance accurately 2D model results are limited by the accuracy of input data Resolution effects may be a problem

21 1D/2D Coupled Modelling Techniques Coupling helps to take advantage of the benefits from both 1D and 2D; Source:DHI Depiction of a general 1D model of the river channel coupled with a 2D model of the floodplain

22 Data Requirements of 2D Modeling Data requirements of flood inundation models can be categorized in to the following four sections. 1.Topographic data of the channel and floodplain to act as model bathymetry, 2.Time series of bulk flow rates and stage data to provide model input and output boundary conditions, 3.Roughness coefficients for channel and floodplain, which may be spatially distributed, and 4.Observed data for model calibration, and validation

23 Where to put resources? Importance of various aspects related to accuracy of 2D Modeling result Source:RMA2 Manual, 2006

24 Representation of Topography Considerable loss of info. Source: Haile & Rientjes, (2005) What is the appropriate DEM resolution to represent the topography of the study area? Low resolution lead to loss of information High resolution results in excessive computational time Lumping Heterogeneity will be ignored Features like dyke and other flow obstacles will be leveled Local storage areas will diminish Actual flow pattern will be poorly represented.

25 Representation of Topography (cont d) Same Upstream discharge hydrograph Downstream boundary condition Roughness Difference DEM resolution Source: Haile & Rientjes, (2005) Results Wider flood extent with lower resolution DEM.

26 Case Study: Spring Creek Part of Etobicoke Creek Highly urbanized Area Flows not contained within valley 2-dimensional MIKE 21 model developed 2D models better represent overland flow and spilling than 1D models like HEC- RAS 1D/2D Coupling

27 2D Model set-up in MIKE 21 Model Domain-study area Channel and overland flow sections Roughness and features like roads, buildings, river banks and dykes Topographic Data DEM, LiDAR, and HEC-RAS cross section data Hydrological Input Define Inflow location in the study area-upstream flow boundaries Point Sources Boundary exit location stage-discharge relation. Simulation Time step

28 Model domain Initial Improved Model domain includes area between north of Queen Street and Highway 407

29 Model Mesh Triangular Quadrilateral Interpolation of a Bathymetry to the Mesh

30 Topographic Data Sources Overland flow sections Channel flow sections Urban areas: flow dynamics and flood propagation will be greatly affected by features like roads, buildings, river banks and dykes as such they must be accounted for in the model set-up.

31 Roughness Roughness values were defined for the MIKE21 grid based on TRCA s standard manning coefficient Flood obstructive features such buildings were delineated and coded in the model domain

32 3 upstream boundary conditions Hydrographs 1 downstream boundary condition stage discharge curve 7 Point sources Hydrographs Land Boundary Vertical wall Boundary Conditions

33 Simulation of Flood Propagation

34 Maximum Flow Depth

35 Maximum Flow Velocity

36 Comparison of 1D and 2D Outputs

37 Conclusion One-dimensional models provide accurate representation of confined flows. Extent of spill areas can be properly determined using the twodimensional models. Appropriate representation of the topography within the 2D model domain is paramount for the accuracy of 2D model results.

38 Thank you for your time. Contact Information: Sameer Dhalla, P.Eng. Dilnesaw Chekol, PhD.

39 Numerical Modelling Numerical methods will be implemented on a discretized representation of space called either a mesh or grid. Computational efficiency of a numerical model is directly related to the number of equations that need to be solved and therefore to the resolution of the grid. Structured grid Unstructured grid