Post on 01-Aug-2020
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