1 Introduction to Natural Channel geomorphic processes ... · Natural Channel Processes 1...
Transcript of 1 Introduction to Natural Channel geomorphic processes ... · Natural Channel Processes 1...
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NaturalChannel
Processes
1 Introduction to geomorphic processes
in natural stream channels of Southern California
Stream Naturalization Workshop
May 29, 2008
Tom Moody, P.E.
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How do we get from this…
…to this?
THE QUESTION…
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HISTORIC CONDITIONSMaps, photos, anecdotal evidence, some field data; Useful for trends and causes
May not be applicable in today’s climate, hydrology, development
Ballona Creek, 1927
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ANALYTICAL APPROACHESQuantifying/modeling
fluvial processes
V
= 1.486/n
R2/3 S1/2
V
= (8g/ƒ)1/2 (R S )1/2
ρ1 /γ
+ x1 V12/2g
+ z1 = ρ1 /γ
+ x2 V2
2/2g + z2 + hL
Sf
= S0
–
∂y/ ∂x –
V/g ∂V/ ∂x –
1/g ∂V/ ∂t
Q = VA R = A/WP
Q = CLHe1.5
V1
= (2g ( Z + Ha
+ y1
))1/2
y1
/y2
= ½
(1+8F12)1/2
-1)
τ = ωRS
The value of computer models is insight, not numbers.
Useful assessments….but where do we begin?
HEC-RAS
HEC-6
HEC-FFA
SAM
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5A VIEW OF THE RIVER
An empirical approach:
Natural stream channels are created and maintained by the water and sediment of their watersheds.
Stream Functions:
• Convey flood flows• Transport sediment• Dissipate energy
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6 THE NATURE OF RIVERS
Not this!Channels are constantly adjusting to changes in sediment and water inputs.
This!
FORM FUNCTION
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Stream Corridor Restoration: Principles, Processes, and Practices, 1998
Stream channel form depends on:
Watershed size Landscape (valley, slope) Geology Climate
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CHANNEL DIMENSIONTerrace is
an abandoned floodplain
Sevier River, UT
Channelcarries moderate, frequent flows.
Floodplain is a level area near a river channel, constructed by the river in the present climate and overflowed during moderate flow events.
Abandoned floodplainor terrace Floodplain
Channel
Hillslope
VALLEY CROSS-SECTION
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ALLUVIAL FEATURES
Low flow channel transports coarser sediments
Floodplain/terraces spread flood flows, dissipate energy
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STEEP VALLEYSSteep stream slopes (>4%)V-shaped channels
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MODERATELY STEEP VALLEYSSteep stream slope (2- 4%)U-shaped channel
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12 BROAD, LOW-GRADIENT VALLEYS
Gentle stream slopesBroad overbank zones
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SEDIMENT SUPPLY
Wide, shallow channels: greater sediment supply, weaker banks.
Narrow, deep channels: Lower sediment supply, stronger banks
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Channel meander dissipates energy.
CHANNEL PATTERN
Meander is inversely proportional to slope.
Alaska
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Steep channels;little meander.
MEANDER & SLOPE
Low gradient channels;More meander.
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MEANDERING IS COMMON
TO ALL STREAM CHANNELS
STRAIGHT CHANNELS WORKTO CREATE MEANDER
CHANNEL MEANDER
Walla Walla River
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SECONDARY CELLS
These currents are responsible for scour and deposition along the channel.
(Leopold, Wolman, & Miller, 1964)
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18ALLUVIAL FEATURES
Point bars form on inside of meandersRiffles located between meanders
Channels with less meander;
Less well-developed point bars.
Animas River, CO
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Channel profile is related to channel meander pattern.
CHANNEL PROFILE
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Riffles, runs, & drops dissipate energy;
provide natural grade control
Runs and pools dissipate energy through turbulence
Wet Beaver Creek
North Fork Virgin River
CHANNEL BEDFORMS
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Step-pools in steep channels.Slope > 4 %
BEDFORMS & SLOPE
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22Continuous
riffles in moderately
steep channels.Slope 2 - 4 %
Bear Wallow Creek
Riffle-pools in low-gradient meandering
streams.Slope < 2 %
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In many regions, channel beds are dominated by sand.
Bed material is determined by watershed geology.
Chinle Wash. AZ
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RIPARIAN VEGETATION
Riparian plant zones relate to hydrologic processes.
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PHYSICAL & BIOLOGICAL COMPONENTS
• Dissipates energy during high flows.
• Enhance water quality • Provides habitat for fish and wildlife.
• Strengthens stream banks to reduce erosion.
• Aesthetics/recreation
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GEOMORPHIC APPROACH
A rigorous empirical process for identifying and
quantifying channel patterns to assess and predict present and future form and function.
Systematic research began with USGS scientists in 1950s
How can we quantify stream processes?
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BANKFULL STAGE
• Common reference point• Geomorphic floodplain deposits• Divides flow from sediment functions• Moderate, frequent flood events (1-2 yr RI)
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GEOMORPHIC PATTERNSMeander length ~ 10.9 channel widths
(Leopold, 1994)
Meander length ~ 4.7 Mean radius of curvature
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GEOMORPHIC PATTERNS
Source: Leopold, et al, 1964; Emmett, 1975; Jackson, 1994
REGIONAL BANKFULL RELATIONSHIP
1
10
100
1,000
10,000
0 1 10 100 1,000 10,000
Watershed Area (mi2)
San Francisco RegionUpper Green RiverSalmon RiverColoradoZuni MtsEastern U.S.
Common reference point allows:• Quantify channel dimensions• Compare stream channels• Identify regional patterns (250+ sites in Arizona, Utah, New Mexico)
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Watershed Area vs Bankfull XS-Area
1
10
100
1,000
0 1 10 100 1,000
Watershed Area (mi2)
Coastal Outliers
Arizona Regional Curve
New Mexico RegionalCurveLos Angeles RegionalCurve
SO CALIFORNIA STREAMS
R2 = 0.89, n = 21
Are they unique? No!
Stream forms are similar to othersData creates useful geomorphic patternsAdditional information needed.
Courtesy: Santa Monica Bay Restoration Commission
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So California Sites
Address development impacts:•Hydrology•Channel form•Sediment supplies
Additional data needed:
Broader geographic distribution
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How do we fix this…
…or this?
THE QUESTION…again
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EFFECTIVE APPROACHES
2. Regional geomorphic patterns suggests initial channel/floodplain dimension, pattern, & profile
1. Historic photos, maps, anecdotes suggest changes and trends;
3. Analytical modeling tests hypotheses and tests alternatives
Las Virgenes Creek, Courtesy Questa Engineering
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WHERE TO START?Regional geomorphic patterns can estimate:• Size and shape of channel• Width and height of floodplains• Meander pattern• Type and spacing of bedforms
So we don’t end up with this!
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35DAYLIGHTING STREAMS
Gently meandering low-gradient streams with broad overbanks and robust riparian vegetation
Centinella Creek
Lafayette Park
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36 BALLONA CREEKProvide sufficient width for flood protection;
Vegetation for habitat, aesthetics, water quality, energy dissipation
Graphic courtesy J. Hall, SMBRC
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PRINCIPLES
URBAN STREAM RENEWAL1. Project should maintain existing benefits;2. Project should provide additional
“restoration” benefits;3. Natural stream dynamics should be
incorporated into structural constraints;4. Project should provide successful
example5. Project should improve science of urban
stream restoration.Floods will still occur!
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PRINCIPLES
LIVING WITH A RIVER
Areas along a river can be shared
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39 NATURAL CHANNEL PROCESSES
Stream channels are created and maintained by watershed processes.
Stream functions: –convey flood flows, –transport sediment, –dissipate energy
Alluvial forms adjust to changing watershed conditions
Riparian vegetation is essential to stream function
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QUESTIONS?
Tom Moody, P.E.Natural Channel Design, Inc.
Flagstaff, Arizona928-774-2336 [email protected]
There’s always hope!