US Army Corps of Engineers

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Workshop on Probabilistic Flood 

Hazard Assessment (PFHA)

Panel 8: Combined Events Flooding

Christopher Dunn, P.E., D.WRE, Director

Hydrologic Engineering Center, CEIWR-HEC

Institute for Water Resources

31 January 2013

www.hec.usace.army.mil

Assessing Levee System Performance 

Using Existing & Future Risk Analysis Tools
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Combined Events Flooding

Panel 8 focuses on identifying and evaluating 

combined event scenarios within a risk 

informed framework. Combined events can 

include flooding caused by seismically induced 

dam or levee failure; flooding caused by 

combinations of snowmelt, rainfall, and ice; 

flooding caused by combinations of coastal and 

riverine events; basin or system wide 

performance and impacts; human and 

organizational factors; and many other 

scenarios.
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Need for System Approaches with

Risk Analysis

■ ER 1105-2-100, Planning Guidance Notebook, 22 April 2000, requires 

systems approaches, "The planning process shall address the Nation’s 

water resources needs in a systems context..."

■ ER 1105-2-101, Risk Analysis for Flood Damage Reduction Studies, 3 

January 2006, requires risk analysis for all flood damage reduction studies, 

 "All flood damage reduction studies will adopt risk analysis..."

■ EC 1110-2-6067, USACE Process for the National Flood Insurance 

Program (NFIP) Levee System Evaluation, 31 August 2010, defines the 

objective of a system evaluation: "verify that the levee system performs as 

an integrated set of features and components functioning individually and 

collectively to provide reasonable assurance..." 

■ USACE Actions for Change, Themes 1 - 4, required comprehensive 

systems approaches that include integrated sustainable solutions and 

decisions are risk-informed. 
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■ Design Standard Paradigm. (People tend to be risk adverse.)

■ It can’t be done. (i.e. Lack of understanding by the practitioners.)

■ What is the value added? (How do we make decisions differently?)

■ It costs too much. 

■ How do we communicate to the Stakeholders?

■ How do we communicate to the Decision Makers?

■ Even for the well informed, terminology/practice continues to change.

• Risk Based

• Risk Analysis

• Risk and Uncertainty

• Risk Management

• Risk Assessment

• Risk Informed

• Probabilistic Risk Assessment

• Probabilistic Flood Risk Assessment

Risk Analysis Hang-ups
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Demonstration Case Study 

Using HEC-FDA

■ Process for Conducting Risk Impact Analysis for 

Proposed Modifications to the Sacramento River 

Flood Control Project Levees

■ 408 Policy Guidance

● Full range of loading 

conditions

● Impact on system 

performance

● Must include a risk 

analysis
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SRFCP System

■ 1,300 miles of 

levees

■ Protects 800,000 

Acres

■ Significant 

Upstream Storage 

Reservoirs

■ Project Report No. 

71 - documents the 

entire process
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Uncertainties

■ Hydrologic: Inflow hydrographs at hand-off 

locations, reservoir operations. Uncertainty 

based on equivalent period of record.

■ Hydraulic: Topographic data, roughness 

coefficients, weir coefficients, breach 

characteristics, downstream boundary, discharge 

at index locations.

■ Operational: Levee system performance (based 

on levee failure criteria), flood fighting activities 

(not considered).
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HEC-FDA – Does not address as an 

interrelated system

Little Creek

Jones Creek

Big 

River

Damage 

Reach 1

Damage 

Reach 2

Damage 

Reach 4

Damage 

Reach 3
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Watershed Analysis Tool (HEC-WAT)

An overarching interface that allows the PDT to perform water resources 

studies in a comprehensive, systems based approach by building, 

editing and running models commonly applied by multi-disciplinary 

teams and save and display data and results in a coordinated fashion.
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HEC-WAT Framework

HEC-WAT

Simulation

With Default

Program

Order

HEC-ResSim 

Plug-In

HEC-RAS 

Plug-In

HEC-HMS HEC-ResSim HEC-RAS HEC-FIA

HEC-HMS 

Plug-In

HEC-FIA 

Plug-In

Model Results (simulation.dss)

Basic HEC-WAT

■ The initial set of models and tools to 

be used during the analytical process 

in HEC-WAT were:

●Hydrology (HEC-HMS)

●Reservoir Operations (HEC-ResSim)

●Hydraulics (HEC-RAS)

●Consequences (HEC-FIA)

■ Data is shared through 

a common DSS file.

■ Version 1.0 provides 

for an event or P-O-R 

analysis.

Hydrology

Hydraulics

Reservoir Operations

Consequences
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FRA Compute Option

■ CEIWR-HEC began researching and creating a tool within the 

WAT that would perform risk management with a life-cycle 

approach (Flood Risk Analysis (FRA) compute option).

■ Provides a systems and life-cycle approach to plan formulation 

for assessing risks and uncertainties in simple systems as well 

as complex, interdependent systems.

■ Provides an effective tool for risk communication.

■ FRA will apply the Monte Carlo simulation & allow for a lifecycle type computation of consequences (economic and lossof-life) and associated performance indices.

■ Incorporate new computational methodologies.
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No

HEC-WAT Framework with FRA Compute Option & Cost Analysis

Hydrologic

Sampler

Plug-In

Fragility

Sampler

Plug-In

HEC-RAS

Plug-In

Spreading

Plug-In

HEC-FIA

Plug-In

Cost

Plug-In

Hydrologic

Sampler

Hydrograph Sets Model Results (simulation.dss) Damage

Sets

Peak

Discharge

Inflow

Hydrograph

Breach

Hydrographs

Depths, 

Velocities per 

Grid

Expected

Net Benefit,

CDF

Net

Benefit

Sampler

Benefits

Model

Convergence

(e.g., EAD)

Damages

HEC-ResSim

Plug-In

Reservoir 

Modeling 

Sequence

Fragility

Curve

Sampler

HEC-RAS

Model

Spreading

Model

HEC-FIA

Model

Depths, 

Velocities per 

Grid

Inflow

Hydrograph

Water Surface 

Elevations, Flow

CSRA

Yes

Inflow

Hydrograph

Unsteady

Steady

includes Forecast 

Sampling Option
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FRA Monte Carlo Sampling Sequence

■For each project alternative, a single instance of the 

project life cycle (e.g., fifty years) is simulated by sampling 

annual maximum flood events for the duration of the life 

cycle.

■Sample System-Wide Fragility Functions

■Sample Historic Pool of events with associated 

Hydrograph Set

■ Route Hydrograph Set

● Consequence Area (CA) system Failures are based on hydraulics and 

fragility curves 

● Hydrographs will get adjusted as Dictated by Spills/Failures based on 

hydraulic model

● Determine Flow and Stage at all Consequence Areas
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FRA Monte Carlo Sampling Sequence

(Continued)

■Two-dimensional spreading can be performed in areas 

where needed 

■ Compute Damage/Loss-of-Life for all Consequence 

Areas

■ Repeat
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FRA Sampling Sequence

Computing EAD by Event Sampling

Simple

Monte Carlo

Simulation

Random choice of 

probability U[0,1] 

to "generate" event 

Method 1

Reservoir Analysis

Channel Hydraulics

Levee Behavior

After all realizations are computed 

you now have EAD:

∑=

=

N

i

Damage i

N

EAD

1

( ) 1

Spreading Model (2D?)

Inundation Mapping

Structure Inventory

Damage to Structures

stage

damage Damage(i)

Hydrograph sets

One Realization

Exceedance Probability

Peak Discharge (cfs)
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Hydrologic Sampling - Method 2

● pull out an event, 

use all its 

hydrographs, put it 

back...SHAKE

● pull out an event, 

use all its 

hydrographs, put it 

back...SHAKE

● pull out an event, 

use all its 

hydrographs, put it 

7 back...SHAKE

5

2

6

9

4

1 8

3

10

11

14

13

12

16

15

19

18

17

20

21

200-year

100-year

500-year

22

keep 

events 

whole
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20-years of 50-year life-cycle

after drawing 50 random U[0,1] values

0

100000

200000

300000

400000

500000

600000

200-year 

event
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20-years of 50-year life-cycle

after drawing 50 random U[0,1] values

0

100000

200000

300000

400000

500000

600000

200-year 

event
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FRA Load 

Distribution

Yakima River

Kennewick

Burbank

Pasco

Richland

West

Pasco

Ice Harbor

Priest Rapids

Qi

Qo

Qb

Li

Oct Dec Feb Apr Jun Aug Oct

1947 1948

10

15

20

25

30

35

40

45

50

Plan: BaseCondit:Event1948:RAS-Reach1 River: Columbia Reach: RM 143-Bonn RS: 145.86

Time

Stage (ft)

Legend

Stage
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Inflow Spreading 

& Consequences

Yakima River

Kennewick

Burbank

Pasco

Richland

West

Pasco

Ice Harbor

Priest Rapids

Li

Cn

Cn+1

Oct Dec Feb Apr Jun Aug Oct

1947 1948

10

15

20

25

30

35

40

45

50

Plan: BaseCondit:Event1948:RAS-Reach1 River: Columbia Reach: RM 143-Bonn RS: 145.86

Time

Stage (ft)

Legend

Stage
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Consequence Analysis 

Inundation Mapping on Structure Inventory
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Risk Communication

■ Economic and Environmental Performance

■ Annual Exceedance Probability

■ Conditional NonExceedance Probability

■ Long-Term Exceedance 

Probability

■ Risk Maps

■ Loss-of-life
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Challenges

■ Life Cycle Modeling needs to include rehabilitation, repair 

and flood recovery

■ Consequence evaluation – economic, social, 

environmental, and Loss-of-Life

■ Uncertainty analysis – trade offs between detailed 

modeling and important sources of uncertainty

■ Risk Communication – trade off analysis will likely 

encourage stakeholder support

■ How to reduce computational burden

■ How to model multiple failures

■ How to maintain technologies
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HEC-WAT for the Columbia River Watershed

■ 258,000 sq. miles

■ 2 countries

■ 7 states

■ 1,214 miles

■ 125 tributaries

■ Approximately 

176,000 structures

■ 51 projects

■ 100 fragility curve 

locations

■ 43 consequence areas

■ 128 levee systems; 

449 miles

24
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Conclusion

■ Currently, USACE can conduct risk assessments in a 

systems context with HEC-FDA.

■ HEC-WAT/FRA will be a tool that performs these 

calculations.

■ It will include systems approaches, event sampling, 

alternative analyses, structural and non-structural 

analyses, costs, loss-of-life, agricultural damage 

analyses. 

■ Could be used nationwide for levee evaluations, 

levee assessments, and planning and design 

studies.
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US Army Corps of Engineers

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QUESTIONS?

www.hec.usace.army.mil