Presented By:John Covey, PE, CFMAmec Foster Wheeler

Project Partners:Oklahoma City (Client)

Meshek & Associates (PM)Utley & Associates

Lemke SurveyingE7 - ASFPM - 2016

When 2D Meets Urban Flood Risk Assessments

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Project Description

Summary & Objectives

Project Methodology

SWMM Methods Overview

Hydrology

Hydraulics 1D & 2D

Project Data Collection

Project Parameter Details

Results

Flood Risk

Capital Improvements

Lessons Learned

Questions / Discussion

Project Description Summary

To analyze approx. 3.4 sq miles of downtown Oklahoma City.

Objectives

Identify the System

Simulate Urban System Response

Identify & Map Urban Flood Risk Areas

Develop Alternatives & Solutions

Project Description Define the Problem

Past Urban Street Flooding

Challenging Terrain Interconnected sub-

surface & 2D surface

Usability into the future Promote continued

resiliency and planning.

Holistic watershed solution

Project Methods Model Options

PCSWMM 1D/2D

EPA-SWMM

XPSWMM 1D/2D

Bentley SewerGEMS

Innovyze Infoworks ICM

Etc…

Project Methods SWMM Methods Overview

Hydrology - Runoff Block Methods

Infiltration - SCS Curve Number, Green Ampt, etc

Standard Design (SCS Type II Rainfall), User defined, etc

Project Methods SWMM Methods Overview

Hydraulics – 1D Full Saint Venant’s Equation

Project Methods Hydraulics – 1D St. Venant Equation Assumptions

Flow is one-dimensional

Hydrostatic pressure prevails and vertical accelerations are negligible

Streamline curvature is small.

Bottom slope of the channel is small.

Manning’s equation is used to describe resistance effects

The fluid is incompressible

Project Methods Hydraulics – 2D in

PCSWMM

Hexagon or Rectangular Cells

Node / Link System –SWMM5 Engine Analysis

Cells connected via conduits

Varied wetted perimeter

“n” Values

Blocked Obstructions

Project Methods Data Collection

1-foot Lidar

3-inch Aerial Photography

Building footprints

Field Investigation / Measurements Geometry

Connectivity

Quality

Project Methods - Aerial

Project Methods - Topo

Project Methods - Topo

Project Methods – 3D

Project Methods - Rainfall

Project Methods – Historical Rainfall

Model Specifics The Model

>800 Subbasins

Avg. 2.5 acres

>3500 Manholes & Inlets

>3800 1D Stormwater Conduits (88 miles)

2 Historical Events

Precipitation Depth Calibrated NEXRAD

Model Specifics 2D area limited to

Area of Interest and true 2-D flow. Street conveyance comprised of 1D linear flow left as 1D.

Model Specifics Focused 2D Area

1.2 sq miles

>140,000 2D Cells

10ft – 50ft

Varying “n” Values

Blocked Obstructions

Inlet Capacity controls flow into 1D conduits

Results - Depth

Results Model Runtime – 24

hours (24 hr simulation period)

422 Structures impacted by 100yr Floodplain

224 Structures impacted by 10yr Floodplain

Results Capacity

Approx. 30-40% of the system has less than 5yr capacity.

Approx. 60-70% of the system has less than 10yr capacity.

Flood Risk Flood Duration

Duration of facility/structure exposure to flood waters increases potential damage and potential for flood-proofing failures.

Flood proofing response time.

Depth / Water Surface Elevation

Significant factor in determination of damage and risk.

Velocity

As velocity and flood depths increase so does the pressure and force on structures and flood protection.

Products of Depth & Velocity

Intensity (ie Depth * Velocity)

Force (ie Depth * Velocity2)

Water Quality

Flood waters are typically “dirty” with chemicals, waste water, pesticides, oils, debris.

It is easy with 2D.

Risk – Depth & Velocity Damage/Risk Assessment & Requirements

FEMA Damage & BCA

NRCS

APWA

Human Instability & Flood Protection Studies

Risk – Water Surface

Risk – Depth Grid

Risk - Velocity

Risk – Velocity Grid

Risk – Depth * Velocity

Risk – Depth * Velocity2

Capital Improvements Alternatives

Model runtime to significant for proper alternative development.

Developed a 1D surface / subsurface model using 2D model results

1D model runtime reduced to 45 min

4 primary problem areas were focused on.

Focus areas result of significant capacity issues and recent flooding from several events.

Capital Improvements Problem Areas

Capital ImprovementsProblem Area 1

Capital ImprovementsProblem Area 2

Capital ImprovementsProblem Area 3

Capital ImprovementsProblem Area 4

Capital Improvements 2 to 3 alternatives were developed for each problem

area.

Alternatives included increasing pipe size, adding upstream storage, and added inlet capacity

Alternatives resulted in up to 20% removal from 100yr floodplain

Alternative results in up to 30% removal from 10yr floodplain

FEMA Benefit-Cost Analysis was performed

Capital Improvements

Example Comparison of Existing 100yr and Alternative 100yr Floodplains

Lessons Learned 2D model provided effective way to identify flood risk.

2D model very sensitive to inlet capacity location and connectivity.

Alternative development not reasonable using only 2D model given magnitude and runtime.

Given project schedule demands it required the development and use of an adequate 1D model.

Our imagination – the only limitation on how we communicate flood risk.