Alan F. HamletDennis P. Lettenmaier

JISAO Center for Science in the Earth System Climate Impacts Group

and Department of Civil and Environmental EngineeringUniversity of Washington

January, 2004

Effects of Land Cover, Topography, and Climate on Pacific Northwest Flooding and Flood Forecasting

http://www.hydro.washington.edu/Lettenmaier/Presentations/2004/hamlet_coastal_management_jan_2004.ppt

Hydroclimatology of the Pacific Northwest

Annual PNW Precipitation (mm)

Elevation (m)

The Dalles

WinterPrecipitation

SummerPrecipitation

(mm)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

10 11 12 1 2 3 4 5 6 7 8 9

Month

No

rmal

ized

Str

eam

flow

SnowDominated

Transient Snow

Rain Dominated

Hydrologic Characteristics of PNW Rivers

Temperature warms,precipitation unaltered:

•Streamflow timing is altered• Annual volume stays about the same

Precipitation increases,temperature unaltered:

•Streamflow timing stays about the same•Annual volume is altered

Sensitivity of Snowmelt and Transient Riversto Changes in Temperature and Precipitation

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

19

73

19

73

19

73

19

73

19

73

19

73

19

74

19

74

19

74

19

74

19

74

19

74

Water Year

Flo

w (

cfs

)

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

19

73

19

73

19

73

19

73

19

73

19

73

19

74

19

74

19

74

19

74

19

74

19

74

Water Year

Flo

w (

cfs

)

Characteristics of Flooding Events West and East of the Cascades

Coastal and Transient Snow Basins(West of the Cascades)

Flooding frequently occurs in Nov-Dec when intense rain storms with temperatures above freezing are most likely.

In so-called “Rain on Snow” events that produce severe flooding, the presence of snow is actually not the major driver. Instead, intense and sustained precipitation over enlarged basin areas (due to warm temperatures) with fully saturated soils produce the major component of the runoff in the largest events.

In moderate flooding events, snow melt and precipitation tend to be more comparable in their contribution to peak streamflows and antecedent snowpack is more important.

Cold Warm

Effective basin area contributing direct runoff to the river channel system increases in warm winter storm events.

Skagit River Basin

Snow Melt Dominant Basins (East of the Cascades)

Flooding mostly occurs in spring when snow melt peaks.

Severe flooding can result from extraordinarily heavy snowpacks over large spatial areas (e.g. WY 1997), rapid snowmelt due to extremely warm or clear weather, or from a combination of sustained snow melt and heavy precipitation (e.g. the Vanport Flood in 1948).

Moderate snowmelt floods can have much longer duration in comparison with flooding produced by individual rain storms.

Note that huge snowpacks do not necessarily produce severe flooding in spring (e.g. WY 1999).

Effects of Land Cover on Flooding in the Pacific Northwest

Urbanization (increased impervious surfaces and removal of active soil storage during development)

•Altered streamflows:Increased magnitude and “flashiness” of peak flowsMore rapid recession and lower base flows in late summer

•Stream channel erosion and instability

•Capacity problems in storm water drainage systems

•Ecological problems due to erosion, scouring, or increased nutrient and sediment loadings

Source: Booth D.B., 2000, Forest Cover, Impervious-Surface Area, and the Mitigation of Urbanization Impacts in King County, WAhttp://depts.washington.edu/cwws/Research/Reports/forest.pdf

Typical Effects of Urbanization on a Small Watershed Des Moines Creek

Effects of Logging and Road Networks

•Loss of forest canopy increases total snow accumulation

•Increased exposure to wind and solar radiation increases melt rates

•Road building and culverts alter natural drainage networks creating “pipes” to the stream channel which increase peak flows during moderate flooding events

•Loss of vegetation can produce larger sediment loads or trigger debris flows

•Effects of logging and road building are roughly additive.

Source: Storck, P., 2000, Trees, Snow and Flooding: An Investigation of Forest Canopy Effects on Snow Accumulation and Melt at the Plot and Watershed Scales in the Pacific Northwest, Water Resources Series Technical Report No. 161, Dept of CEE, University of Washington

Effects of Forest Canopy on Snow Accumulation

Loss of canopy increases the snow water equivalent and increases the rate of melt.

Sources: Storck, P., 2000, Trees, Snow and Flooding: An Investigation of Forest Canopy Effects on Snow Accumulation and Melt at the Plot and Watershed Scales in the Pacific Northwest, Water Resources Series Technical Report No. 161, Dept of CEE, University of WashingtonBowling, L.C., P. Storck and D.P. Lettenmaier, 2000, Hydrologic effects of logging in Western Washington, United States, Water Resources Research, 36 (11), 3223-3240

Modeling studies (Storck 2000) and comparative analysis of observations in paired catchments (Bowling et al. 2000) show that large scale clearcutting results in increased flood peaks on the order of 10% for small basins in the transient snow zone of the Cascades.

Effects of Harvest Strategies on Magnitude of Flood Peaks

Bowling, L.C. and Lettenmaier, D.P., 1997, Evaluation of the Effects of Forest Roads on Streamflow in Hard and Ware Creeks, Washington, Water Resources Series Technical Report No. 155, Dept of CEE, University of Washington

Bowling and Lettenmaier (1997) estimated that the 10-yr flood peak increased ~10% in two small transient snow basins due to road networks alone.

Roads and logging together were estimated to increase the 10-yr flood peak on the order of 20% in the same two small transient snow basins.

Effects of Roads Networks on Peak Flows

Effects of Climate Variability on Flooding in the Pacific Northwest

A history of the PDOwarm

coolwarm

A history of ENSO

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Pacific Decadal Oscillation El Niño Southern Oscillation

150000

200000

250000

300000

350000

400000

450000

190

0

191

0

192

0

193

0

194

0

195

0

196

0

197

0

198

0

199

0

200

0

Ap

r-S

ept F

low

(cfs

)

Effects of the PDO and ENSO on Columbia River Summer Streamflows

Cool CoolWarm Warm

PDO

150000

200000

250000

300000

350000

400000

450000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Probability of Exceedence

Flo

w (

cfs)

WarmPDO/WarmENSO

WarmPDO/ENSONeut

WarmPDO/CoolENSO

CoolPDO/WarmENSO

CoolPDO/ENSONeut

CoolPDO/CoolENSO

Naturalized Summer Streamflow at The Dalles

Selection Criteria:

•Unregulated Streams

•Daily Flow Records

•Records 57-65 Years Long

Pacific Northwest Streamflow Records Selected for Flood Analysis

Daily Flow Data w/ Threshold (1986-1996)Dungeness R near Sequim, WA

0

1000

2000

3000

4000

5000

6000

10

/1/8

5

3/1

9/8

6

9/4

/86

2/2

1/8

7

8/9

/87

1/2

4/8

8

7/1

1/8

8

12

/27

/88

6/1

4/8

9

11

/30

/89

5/1

8/9

0

11

/3/9

0

4/2

1/9

1

10

/7/9

1

3/2

4/9

2

9/9

/92

2/2

5/9

3

8/1

3/9

3

1/2

9/9

4

7/1

7/9

4

1/2

/95

6/2

0/9

5

12

/6/9

5

5/2

3/9

6

Date

Flo

w (

cfs

)

daily flow

threshold

Daily Flow Data w/ Threshold (Oct.1,1990-Nov.30,1990)Dungeness R near Sequim, WA

0

1000

2000

3000

4000

5000

6000

10

/1/9

0

10

/4/9

0

10

/7/9

0

10

/10

/90

10

/13

/90

10

/16

/90

10

/19

/90

10

/22

/90

10

/25

/90

10

/28

/90

10

/31

/90

11

/3/9

0

11

/6/9

0

11

/9/9

0

11

/12

/90

11

/15

/90

11

/18

/90

11

/21

/90

11

/24

/90

11

/27

/90

11

/30

/90

Date

Flo

w (

cfs

)

daily flow

thresholdData Processing Methods

•Determine mean annual flood for each basin

•Set threshold and reset value

•Determine number of peaks above threshold for each climate category

•Estimate probability of event above threshold for each basin and climate category

Transient Snow Basins

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

EwPw EnPw EcPw EwPc EnPc EcPc

Climate Category

Pro

ba

bili

ty o

f F

loo

d E

ve

nt

Ab

ov

e

Th

res

ho

ld

Snow-Dominant Basins

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

EwPw EnPw EcPw EwPc EnPc EcPc

Climate Category

Pro

ba

bili

ty o

f F

loo

d E

ve

nt

Ab

ov

e

Th

res

ho

ld

Effects of Climate Change on the Pacific Northwest

Precipitation Fraction, 2020s

0.5

0.75

1

1.25

1.5

1.75

J F M A M J J A S O N D

Frac

tion

hadCM2

hadCM3

PCM3

ECHAM4

mean

Delta T, 2020s

-1

0

1

2

3

4

5

J F M A M J J A S O N D

De

gre

es

C

hadCM2

hadCM3

PCM3

ECHAM4

mean

Delta T, 2040s

-1

0

1

2

3

4

5

J F M A M J J A S O N D

De

gre

es

C

hadCM2

hadCM3

PCM3

ECHAM4

mean

Precipitation Fraction, 2040s

0.5

0.75

1

1.25

1.5

1.75

J F M A M J J A S O N D

Fra

ctio

n

hadCM2

hadCM3

PCM3

ECHAM4

mean

Four Delta Method Climate Change Scenarios for the PNW

~ + 1.7 C ~ + 2.5 C

Somewhat wetter winters and perhaps somewhat dryer summers

ColSimReservoir

Model

VICHydrology Model

Changes in Mean Temperature and

Precipitation or Bias Corrected Output

from GCMs

Current Climate 2020s 2040s

Snow Water Equivalent (mm)

VIC Simulations of April 1 Average Snow Water Equivalentfor Composite Scenarios (average of four GCM scenarios)

The main impact: less snow

Regulated Flow

Historic Naturalized Flow

Estimated Range of Naturalized FlowWith 2040’s Warming

Naturalized Flow for Historic and Global Warming ScenariosCompared to Effects of Regulation at 1990 Level Development

0

1000

2000

3000

4000

5000

6000

7000

8000

900010

/1

10/2

9

11/2

6

12/2

4

1/21

2/18

3/18

4/15

5/13

6/10 7/8

8/5

9/2

Date

Infl

ow

(ac

re-f

t) Simulated 20thCentury Climate

2020s ClimateChange Scenario

2040s ClimateChange Scenario

Effects to the Cedar River (Seattle Water Supply)for “Middle-of-the-Road” Scenarios

Observed Climate Change:

Trends in Temperature, Precipitation, Snowpack, and Streamflow

Area-weighted Regional Avg=1.5 F/century

Annual Precipitation TrendsFrom HCN stations

Relative Trends in April 1 Snow Water Equivalent 1916-1997

Relative Trend %/yrRelative Trend %/yr

Ele

vatio

n (m

)

Trends in Annual Streamflow at The Dalles from 1858-1998 are strongly downward.

0

50000

100000

150000

200000

250000

300000

350000

1858

1868

1878

1888

1898

1908

1918

1928

1938

1948

1958

1968

1978

1988

1998

An

nu

al M

ean

Flo

w (

cfs)

Annual

5 yr mean

10 yr mean

Linear (Annual)

Some Conclusions Regarding Planning, Project Design

Specifications, and Flood Forecasting

“Past Performance is not a Good Measure of Future Performance.”

Estimates of flood probability distributions and design specifications (e.g. the “100 year” or “1% likelihood” flood) are a complex function of land surface characteristics, interannual and decadal scale climate variability, long-term climate variations (such as global warming), and water management policies, all of which are non-stationary in time.

For convenience, estimates of flood design specifications have traditionally been based on fixed periods of the historic record.

In the case of expensive or long-lived structures or for planning processes that should be robust to climate variability and climate change, the use of the historic record for flood estimation is problematic both because of relatively small sample size and changing conditions over time.

Note that in the case non-stationary conditions over time, longer streamflow records do not necessarily improve estimates of flood frequencies.

Problems with Forecasting Applications Based on Statistical Relationships

Many operational streamflow forecasting applications are currently based on statistical relationships between weather or climate forecasts, snowpack measurements, and streamflow.

When land cover of the basin or climate conditions change, the skill of these forecasts can be impaired. Such problems cannot be resolved in the short term because there is no training data available for the altered conditions.

These problems have serious implications both for short term flood forecasting applications and forecasts used for water management at seasonal time scales.

Current or Projected Land Surface

Conditions

Current or Projected Meteorological Data

Hydrologic Model

Updated or ProjectedStreamflowTime Series

Design CriteriaForecasts

Planning Scenarios

Use of dynamic models can improve estimates of hydrologic design specifications and short-term and seasonal streamflow forecasts.