DHI Powerpoint UK

CWRA Conference: 2013 Alberta Floods

2D Hydraulic Modeling for Urban Flood Mapping

Prepared by: Presented by:

Patrick Delaney & Qiao Ying, DHI Eric Fontenot, DHI

Peter Onyshko, Alberta ESRD

Joe Groeneveld, HATCH Background

• 1D hydraulic modeling of Elbow River using HEC-RAS indicated flooding from a 100 year flood event would have multiple flow splits through developed areas of downtown Calgary • Alberta ESRD and the City of Calgary commissioned a 2D hydraulic modeling study in order to gain additional knowledge and confidence about the potential flooding in this area • A team of Hatch and DHI was selected to conduct the study • The proposed modeling approach involved the development of a coupled 1D and 2D hydraulic model using MIKE FLOOD

• Why a 2D model? − In existing HEC-RAS model the overland flow paths (outside of the main river channel) are ‘prescribed’ in both direction and width

• Why a 2D model? − Flood inundation area was based on peak water levels within the model domain and extrapolated to areas that were outside of the model domain

• What is coupled 1D and 2D hydraulic modeling? − Uses a 1D model to simulate flow in the main river channel − Uses a 2D model to simulate flow in the floodplain − Dynamically exchanges flow between the 1D and 2D models based on calculated water levels

Standard 1D approach

Coupled 1D-2D approach

• Benefits of coupled 1D and 2D hydraulic modeling? − Makes use of existing HEC-RAS model and cross-sections − Allows formal representation of bridge structures in 1D model − Solves for 2D flow in developed areas where flow directions are uncertain − Provides a more detailed representation of flooding depths and flow velocities in developed area − Able to explicitly represent the influence of buildings

• Convert existing 1D river model from HEC-RAS to MIKE 11 • Construct the MIKE FLOOD model by coupling the 1D MIKE 11 model to the 2D MIKE 21 model • Calibrate the MIKE FLOOD model • Run design flood events and map results

Conversion of HEC-RAS model

Plan View HEC-RAS Plan View MIKE11

• HEC-RAS Cross-Section MIKE 11 Cross-Section

Construction of MIKE FLOOD Model

• Side channels were removed • Side channel hydraulics described in M21, 2D model representing conveyance outside of main river channel

• Cross-sections were extracted from DEM at ~ 10 m intervals • Cross-sections were trimmed to represent Insert image of MIKE 11 main channel* network with trimmed cross-sections. If possible, include one of the ‘old’ * Except at inflow boundary cross-sections in the image for comparison

• Generate 2D model grid using 2 m grid cell size • Define lateral links from 1D to 2D model along left and right banks of the river • Grid cells belonging to the river channel and building footprints were removed from

Construction of MIKE FLOOD Model

• Boundary Conditions − Upstream – Constant inflow of 300 cms • Peak flow from 2005 flood event − Downstream – Constant water level of 1039 m • Water level at the confluence with Bow River estimated based on HEC-RAS modelling results − Model run for 6 hours to achieve steady flow conditions

MIKE FLOOD Model Calibration

2005 Flood Event • City of Calgary high water marks (HWM) • City of Calgary aerial photo • ESRD high water marks

ID (ESRD) Location HWM WL left bank U/S side of 9th Ave. bridge (Ch. 1075m on 05-ELB-5a StampedeToBowRi 1039.681 05-ELB-3 left bank 35m D/S side of 9th Ave. bridge (Ch. 1125m on 05-ELB-5b StampedeToBowRi 1039.486 right bank 8m D/S of MacLeod Trail (Ch. 971m on 05-ELB-4 05-ELB-4a Between22AvSw) 1045.021 right bank 15m U/S of MacLeod Trail (Ch. 929m on 05-ELB-4b Between22AvSw) 1045.091 05-ELB-2 10m U/S on left bank of 1st St (Pattison) (Ch. 705m on 05-ELB-3a Between22AvSw) 1045.852 12m D/S on left bank of 1st St (Pattison) (Ch. 728m on 05-ELB-3d Between22AvSw) 1045.832 at the end of 1st St near outfall E-25 (Taken in line 05-ELB-2a with BM) 1048.067 © DHI Calibration Data

• City of Calgary HWM − Values are offset by an average of -0.66m from ESRD data − Try to calibrate the trend, but not the values • City of Calgary aerial photo − Photo taken after peak water levels − Not fully representative of maximum flood inundation extents

HWM WL MFLOOD Difference ID (ESRD) Location (m) (m) (m) left bank U/S side of 9th Ave. bridge (Ch. 1075m on 1039.681 1039.68 -0.001 05-ELB-5a StampedeToBowRi left bank 35m D/S side of 9th Ave. bridge (Ch. 1125m on 1039.486 1039.49 0.004 05-ELB-5b StampedeToBowRi right bank 8m D/S of MacLeod Trail (Ch. 971m on 1045.021 1045.017 -0.004 05-ELB-4a Between22AvSw) right bank 15m U/S of MacLeod Trail (Ch. 929m on 1045.091 1045.179 0.088 05-ELB-4b Between22AvSw) 10m U/S on left bank of 1st St (Pattison) (Ch. 705m on 1045.852 1045.982 0.13 05-ELB-3a Between22AvSw) 12m D/S on left bank of 1st St (Pattison) (Ch. 728m on 1045.832 1045.728 -0.104 05-ELB-3d Between22AvSw) at the end of 1st St near outfall E-25 (Taken in line 1048.067 1048.133 0.066 05-ELB-2a with BM)

Comparison of CoC HWM Water Level vs. MIKE Flood Calculatd Water Level

1052

1050

1048

1046

1044

1042

1040 MIKE Flood Cal. Water Level Water Cal. Flood MIKE 1038 1038 1040 1042 1044 1046 1048 1050 1052 CoC HWM Water Level

Modeling the 100 Year Flood Event

Boundary Conditions − Upstream inflow ramped up to 699 cms − Downstream water level set at 1042.25 m • extracted from HEC-RAS model of Bow River

Flood inundation areas

Flood depths

Flow Velocities

Flow Split at 5th Street

Flooding in Mission and Erlton Areas

Detailed Flow Velocity Map

• Integrated model was found to be in good agreement with 1D study • Additional detail of integrated 1D-2D model added understanding to flood behavior and risk • Additional understanding can be used to help make flood protection and mitigation decisions