US Army Corps of Engineers

BUILDING STRONG®

Managing Skagit River Basin Dams for

Flood Risk Management

Ken Brettmann, PE

Senior Water Manager, Western

Washington

Army Corps of Engineers, Seattle District

16 February 2016

BUILDING STRONG®

Outline

Seattle District Corps of Engineers Operation of

Skagit Basin Dams

Overview of Flood Risk Management

Hydrologic Uncertainty - Examples

Future Concerns/Challenges

BUILDING STRONG®

Seattle District Corps Operating Projects

LAKE

WASHINGTON

SHIP CANAL

MUD MOUNTAIN DAM

CHIEF JOSEPH DAM

LIBBY DAM

ALBENI FALLS DAMHOWARD HANSON

DAM

PUGET SOUND

RECOVERY

BUILDING STRONG®

Skagit River Basin Map

BUILDING STRONG®

flood storagetotal capacity

Skagit Basin Flood Storage

120 kaf 74 kaf

Ross Upper Baker

194,000 acre-feet of flood

storage space reserved each

flood season

BUILDING STRONG®

What Causes Flooding in the

Skagit and Other Western

Washington Basins?

The Good News – We know what causes

flooding: Atmospheric Rivers

The Bad News – Hydrologic uncertainty is

often significant (the details matter!)

BUILDING STRONG®

Atmospheric Rivers

Rivers in the Sky

Narrow, long water vapor plumes originating in the

tropics

BUILDING STRONG®

Significant

flooding above

this line

The biggest peaks are

in the fall and winter

Seasonality of Skagit River Annual Peaks (1958-2012)

BUILDING STRONG®

Flood Risk Management

Goal - prevent loss of life/minimize property damage

Operate reservoirs to avoid filling prematurely during a flood event, recover filled space as soon as possible to prepare for the next flood event

Operations consider forecasts, observations, operational constraints, and uncertainty

BUILDING STRONG®

Flood

space

BUILDING STRONG®

Flood stage

Downstream levee

capacity

BUILDING STRONG®

Hydrologic Uncertainty

Impacts current operations – managing

floods today

Impacts the future – managing future

events

►Inter-annual variability

►Climate change

BUILDING STRONG®

Minor flooding – Mt.

Vernon

Significant

flooding – Mt.

Vernon

BUILDING STRONG®

Minor flooding –

Mt. Vernon

Significant

flooding – Mt.

Vernon

Downstream peak is higher

BUILDING STRONG®

BUILDING STRONG®

Downstream

peak is lower

BUILDING STRONG®

Forecast inflow

Observed inflow

Forecast gets higher as event starts

BUILDING STRONG®

Forecast inflow

Observed inflow

Forecast gets higher as event

starts

BUILDING STRONG®

Forecast flows

Observed flow

Mt. Vernon channel

capacity

BUILDING STRONG®

Observed Cumulative Precipitation –

2006 FloodRainfall

0

2

4

6

8

10

12

14

16

18

11/2/2006

0:00

11/3/2006

0:00

11/4/2006

0:00

11/5/2006

0:00

11/6/2006

0:00

11/7/2006

0:00

11/8/2006

0:00

11/9/2006

0:00Time (hr)

Prec

ip (in

ches

)

SKYW

M ARW

DARW

Central

Cascades: heavy

rain intensifies

North Cascades:

rain tapers off

11/2 11/911/3 11/4 11/5 11/6 11/7 11/8

18”

0”

6”

12”

Atmospheric river shifts slightly south

Skagit avoids catastrophic

flood

Flood of record: Skykomish

and Carbon Rivers

12” in 1 day!

BUILDING STRONG®

Skykomish RiverNovember 2006 – record 129,000cfs

Courtesy of Professor Paddle Website

BUILDING STRONG®

Future Concerns

Change in magnitude and/or frequency of

fall/winter floods

Flood season shift

►conflict between flood space and reservoir

refill

BUILDING STRONG®

Different peaks here?

Different peaks here

during refill season?

How will the future be different?

BUILDING STRONG®

Questions?