© 2014 HDR, Inc., all rights reserved.

Mark Forest, P.E., CFM

Practice Leader – Floodplain Management and Modeling

Application of New HEC-RAS 2D

Tool for 1D/2D Modeling of the

Truckee River and Tributaries

June 3, 2015

Key Project Team – HDR Engineering

Mitch Blum, PE, CFM

Keith Weaver, PE

Shalini Kedia, PE

Noel Laughlin, PE

ClientTruckee River Flood Management Authority

Jay Aldean, PE – Executive Director

Acknowledgements

Federal Project - Corps of Engineers

Community Coalition Led Planning Process

Local Sponsors:

o Washoe County

o City of Sparks

o City of Reno

Complex Floodplain Dynamics

Downstream Impacts of Alternatives

o Adjoining County

o Native American Tribal Lands

Project Background

Project

Location

Watershed Area 1720 Sq Mi.

Watershed Originates In Sierra Nevada Range

5 Regulated Reservoirs (water supply with flood control)

Watershed

Flooding History

Winter Events (November to

February)

Warm Rain on Existing Snowpack

“Atmospheric River” Events

Begin cold with snow at upper

elevations

Change to warmer conditions with

rain

Result in melting of the snowpack in

the mid-elevation bands

Produce very high volume events

Impacted by climate change

Hydrologic and Meteorological Conditions that Create Flooding

Constricted Outflow From the Valley

Large Overbank Storage Volumes

Channel Bank Overtopping with Bifurcated Flow

Patterns

Complex Floodplain Dynamics

1940

Mostly Commercial and Industrial

Some Residential

Large Runoff Volume Events

1997 Event Damages; $750 Million

(Well over $1 Billion in Today’s $)

Impacted Development

January 1997

USACE Flood Damage Reduction Project

Original Planning was performed with HEC-

RAS 3.0 to 4.1 as 1D unsteady

Calibrated to 1997 event (120 year event)

Truckee River

Combination of 1D reaches with storage

areas

8 reaches

1100 cross sections

70 storage areas

170 lateral structures

4 Inline structures

38 bridges and culverts

Model Set-Up

Results

200-Year and

500-year events

more complex

Used for

economic

analysis

Complex Flow Dynamics Challenge 1D Models

Converting from a HEC-RAS 1D model to

an HEC-RAS 1D/2D model

HEC-RAS 5.0

Need to correctly analyze 200- and 500-

year events

Need to better analyze impacts of projects

o USACE Flood Damage Reduction Project

o Southeast Connector Roadway

Model Conversion

2‐dimensional Hydrodynamic Flow Routing

Similar to the use of a Storage Area

Linked 1D/2D Capability

Independent 2D Domain for Overbank or Channel

Full Saint Venant or Diffusion Wave Equation

Solution Options

Implicit Finite Volume Solution Algorithm

1D and 2D Coupled Solution Algorithm

Unstructured or Structured Computational

Meshes with Variable Sizes in Domain

Detailed Hydraulic Table Properties for

Computational Cells and Cell Faces

Multi‐Processor Based Solution Algorithm

64 Bit and 32 Bit Computational Engines

What is it RAS 2D?

Source: HEC-RAS 2D User Manual

1D/2D Integrated Modeling Tool

o Public Domain

o No License Fees

o Upwardly Compatible

o Extensively Tested

o Computationally Efficient

o Multiple / Large Terrain Processing

Alternatives Evaluated

o New Code v. Existing Codes

Unique Approach to 2D Solution

FEMA Acceptability

How is it Different?

Most 2D Software Packages

Simplify the Terrain

Sub-Grid Level Detail is Important

Computational Mesh with Sub-Grid Terrain

Data (Terrain Detail is Utilized)

Gridding Process Defines Hydraulic

Property Tables

o Elev-Wetted Perimeter (Face)

o Elev-Area (Face)

o Roughness (Face)

o Elev-Volume (Cell)

Cell Face is a Detailed Cross Section

Able to Capture Complex Hydrodynamics

How is it Different?

Source: HEC

Source: HEC

Sub-Grid Level Detail

Unstructured Mesh

User Defined Cell Faces (Break

lines) to refine mesh

Computationally Efficient

Multi-Processor Solution

64 and 32 Bit Compatible

Validation Process

Advantages

Sub-Grid Level Detail

Unstructured Mesh

User Defined Cell Faces (Break

lines) to refine mesh

Computationally Efficient

Multi-Processor Solution

64 and 32 Bit Compatible

Validation Process

Advantages

Installed over

100 Temporary

Gages to

Capture the

Event

Used as Model

Validation

Mississippi/Ohio River Flooding May 2011 – Forced Levee Breach

Source: HEC

Imperfections in LiDAR Data translate to

model errors

Addition of an improved river terrain data

set

o Used Red and Green Lasers

LiDAR or Topographic Data Quality

RAS Mapper terrain processing

Use of RAS Mapper Interpolation Tool

Terrain Features

Dense development should be analyzed

with buildings correctly represented

Incorporation of Buildings

Example Flow Distribution at Boundary

Build and stabilize 1D reaches

Add and connect lateral structures

Assign boundary conditions

Assign minimal coefficients for

lateral structures for stabilization

Add breaklines for linear features

Re-Stabilize with connected 1D/2D

Calibration Refinements

o Manning’s n value polygons

o Coefficients

o Connections

Steps

Collection of Bathymetric LiDAR and survey

was critical and delayed progress

Waited for breakline tool to facilitate grid

refinements

Bridge and lateral connection refinements

being made

Would like to show you a comparative analysis ….

Dam Breach

Tidal Boundary Condition

Rainfall-Runoff Simulation

Example Simulations

Other enhancements include:

o Simplified physical breach

o Modified breach progression

o Added Xu and Zhang

o Breach parameter calculator

Dam Breach

5000 square kilometers

17 month simulation

Full dynamic solution

Coastal Boundary Condition Example

Does not include

infiltration - Precipitation

entered as “excess”

Code will be used in

HEC-HMS also

Precipitation Input

© 2014 HDR Architecture, Inc., all rights reserved.© 2014 HDR Architecture, Inc., all rights reserved.© 2014 HDR Architecture, Inc., all rights reserved.© 2014 HDR, Inc., all rights reserved.© 2014 HDR, Inc., all rights reserved.© 2014 HDR, Inc., all rights reserved.© 2014 HDR, Inc., all rights reserved.