Managing the urban impacts on rivers in Mediterranean climates:

Extremes of flood, drought, and sediment loads

Derek B. BoothU C S A N T A B A R B A R A

dbooth@bren.ucsb.edu

P R E S E N T A T I O N O V E R V I E W

1. What constitutes a “Mediterranean-type

stream”?.

2. What watershed changes result from

urbanization?

3. What are the effects of watershed

urbanization on Mediterranean streams?

Issaquah Creek

watershed

2004

0

1

2

3

4

5

6

7

8

9

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

Month

Mo

nth

ly A

ve

rag

e R

ain

fall (

inc

he

s)

Ventura, CA

Issaquah, WA

Climate: Annual totals: Issaquah ~55”/yr

Ventura ~14”/yr

“The maximum intensity of precipitation…at intervals of 10 to 100 years is greater in portions of the San Gabriel and San Bernardino Mountains in southern California than anywhere else in the continental United States.”

Western Regional Climate Center (http://www.wrcc.dri.edu/)

“The maximum intensity of precipitation…at intervals of 10 to 100 years is greater in portions of the San Gabriel and San Bernardino Mountains in southern California than anywhere else in the continental United States.”

Western Regional Climate Center (http://www.wrcc.dri.edu/)

(59 sq. mi)

Issaquah Creek maximum recorded flow

USGS 11113500 SANTA PAULA CREEK NEAR SANTA PAULA, CA

Annual

maximum

discharges

2005

(42 sq. mi)

Recurrence interval, years (annual series)

10-f

old

gre

ate

r

“Extreme” storms are much more extreme.

Regional “growth curves” (ratios of large flood discharges to reference flood discharge):

2 to 4×

20 to >40×

Comparison to Other PNW Studies

0

200

400

600

800

1000

1200

1400

1600

1800

2000

BURP (1

995)

This

Stu

dy

Kin

g Cou

nty

(199

5)

Mad

ej (1

982)

Sla

ymak

er (1

993)

Rei

d (1

981)

Pau

lson

low (1

997)

Pau

lson

hig

h (1

997)

Sed

imen

t P

rod

ucti

on

(to

nn

es k

m-2

yr-1

)

(270-390)

Forested/Logging

Mixed

Land UseUrban

40x

Sediment production from

Pacific Northwest watershed studies

~50t/km2/yr

Sediment loads are much more extreme...

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

18,000,000

1928

1932

1936

1940

1944

1948

1952

1956

1960

1964

1968

1972

1976

1980

1984

1993

1997

2001

2005

Year

An

nu

al

se

dim

en

t y

ield

(to

nn

es

)Total sediment

Coarse (>0.0625 mm) sediment

Calculated sediment load for Sespe Creek at Fillmore

[USGS gage 11113000]

Average yield = 1,109,000 t/yr (1,645 t/km2/yr)

Only 8 events have

exceeded 2x the

“average” sediment load

in this 80-year period.

And, ~50% of

the total load

delivered in

just 3 events.

..and the “extreme” events are very extreme.

TO SUMMARIZE:

1. What constitutes a “Mediterranean-type

stream”?.

• The driver: episodic disturbances (with strongly

episodic inputs of water and sediment) in a semi-arid

climate.

• The responses:

Braided channel form

Rapid channel changes (planform and cross-section)

Large fluxes of water and sediment, prone to dis-equilibrium

P R E S E N T A T I O N O V E R V I E W

1. What constitutes a “Mediterranean-type

stream”?.

2. What watershed changes result from

urbanization?

3. What are the effects of watershed

urbanization on Mediterranean streams?

The “classical model”:

P R E S E N T A T I O N O V E R V I E W

1. What constitutes a “Mediterranean-type

stream”?

2. What watershed changes result from

urbanization?

3. What are the effects of watershed

urbanization on Mediterranean streams?

from Hollis 1975

2 to 3-fold

increases in

Qpeak

The “traditional,” humid-region story:

Typi

cal r

ange

of u

rban

izat

ion

Range of “significant” floods

25-yr Q

From Hawley & Bledsoe, J. Hydrol. 405 (2011) 69–82

>3x incr.

Urbanizing Non-urbanizing

The same story in SoCal streams…?

from Hollis 1975

Actual increaseis more than 3-fold

Much greater hydrologic response to urbanization.

Arroyo Simi

25-yr flood

But…

Peak annual discharges, Los Penasquitos Creek (USGS gage 11023340),

San Diego County.

1965–1980: limited urban development

1981–1997: rapid urbanization

1998–2012: near-buildout (20.1% impervious)

Flood-frequency analysis, Los Penasquitos Creek.

Hydrologic response is not necessarily determined by imperviousness

(particularly for large infrequent events).

Arroyo Simi:

Downstream channel response

The classic single-thread, humid-region

channel evolution model (CEM):

Type/phase 1-3: initial downcutting,

bank failure, and widening

(from Hawley et al. 2012)

Type/phase 4-5: “excess” in-stream sediment,

aggradation, and restabilization

Type/phase 4-5: “excess” in-stream sediment,

aggradation, and restabilization

…but in Mediterranean streams (recall, more water and

less sediment), does this ever actually occur?

~25’

Channel evolution does not necessarily follow the rules...

S U M M A R Y

1. Mediterranean-type streams are distinctive,

particularly in the episodic nature of their

channel-modifying events.

2. Urbanization affects all streams, but humid-

region paradigms may be enticingly

misleading.

3. Recognition of the differences is widespread;

understanding, however, still lags (but is

growing).

With many thanks to colleagues

Pete Downs, Tom Dunne,

Stephanie Moret, Robert Hawley,

Brian Bledsoe, Eric Stein,

Catalina Segura, Liz Gilliam, Glen

Leverich, and Scott Dusterhoff