Stream response to hydromodification in a sandy watershed

Stream response to hydromodification in a sandy watershed

Stream response to
hydromodification in a sandy
watershed, Gobernadora Canyon,
Orange County, California
Scott Brown
Geologist/Geomorphologist
and
Barry Hecht, CEG
Principal Hydrologist and Engineering
Geologist
Outline
„ What is ‘hydromodification’?
„ Intro to study site
„ Peak flow
comparisons
„ Examples of channel
response
What is Hydromodification?
„
Increases in peak flow and/or runoff volume
caused by a hydrologic disturbance within the
watershed
„
„
Urbanization creates:
„
„
„
„
Often associated with urbanization
Decreased infiltration
More efficient channel integration
Increases in sediment transport
What are the ‘important flows’?
„
Moderate, high-frequency flows
(Leopold, 1968)
(Hecht and Woyshner, 1984)
(Leopold, 1968)
San Juan Creek Watershed
Gobernadora
Canyon
Watershed Area =
10.5 sq. mi.
Source: Orange County Watershed
and Coastal Resources Division
Soil-geomorphic terrains in the Gobernadora and larger San Juan
watersheds, southern Orange County, California
Watershed Characteristics
„
Gobernadora Canyon
Sandy canyon floor composed
primarily of alluvium
„ Very little gravel or cobbles
„
„
Urbanized in mid- to upper
reaches
Built in several stages
beginning in the mid-1980s
„ Detention to control ~25year flood events
„
GERA
~15%
Urbanized
~45%
Urbanized
1994
2006
Lack of channel integration under
pre-urban conditions
Peak rainfall intensities
4.5
4
WY 2003
WY 2004
3.5
WY 2005
5-year recurrence
Rainfall
(inch
PeakPeak
Rainfall
(inches)
3
2.5
2
1.5
1
0.5
0
1-Hour
2-Hour
6-Hour
Rainfall
(duration)(hours)
Rainfall
duration
*5-year recurrence based on Orange County Hydrology Manual
12-Hour
24-Hour
Peak flows: Urbanized vs. non-urbanized
Chiquita
(non-urbanized)
Gobernadora
(urbanized)
Peak flows are higher in
urbanized watershed
900
Chiquita Creek u/s of first Chiquita Cyn road crossing
Chiquita at Narrows
800
square
PeakPeak
flowflow
(cfs (cfs
per per
square
mile)
'Area 12' tributary
700
Gobernadora Creek @ Lower Gobernadora Gage
Gobernadora Creek above Sulphur Canyon
600
Wagon Wheel @ Riley Park
500
400
300
200
100
0
2/23/1998
2/21/2000
3/15/2003
Order of magnitude difference
Example
Site #1:
Channel
avulsion
8-1-1999
Channel
avulsion
1/1/2002
Channel
avulsion
1/1/2005
Channel response
March, 2003
Example
Site #2:
Channel
incision
Channel response
March, 2003
Channel response
March, 2004
Channel response
March, 2005
Conclusions
„
Hydromodification response can be especially rapid
and prominent in sandy watersheds
„
Engineering geologists should anticipate the potential
for channel response, and recognize that sandy
watersheds will likely require additional stormwater
controls
„
Structural controls are of only limited value in this
setting due to the highly erodible nature of the
alluvium and extent of the channel response. Flow
volume and peak control is the only long-term
solution in this setting.
Questions?
Thanks to:
Rancho Mission Viejo