Assistant Professor, MSU -Biology and U of M Bioproducts ... · Channel evolution in Driftlessarea...
Transcript of Assistant Professor, MSU -Biology and U of M Bioproducts ... · Channel evolution in Driftlessarea...
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Christian Christian LenhartLenhart, Ph.D., , Ph.D., Assistant Professor, MSU Assistant Professor, MSU -- BiologyBiologyand Uand U of M of M BioproductsBioproducts & & BiosystemsBiosystems EngineeringEngineering
Stream ClassificationStream Classification
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Why classify streams?
� To eliminate variability by stratification (grouping)
� To understand fluvial processes� To facilitate communication amongst
managers� To develop appropriate restoration and
management actions
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Channels behave differently
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Form vs. function
� Darwin’s finches
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Form/function link in streams
� Stream dimensions are adjusted to flow and sediment load (Lane’s equation)
� Sediment transport capacity strongly linked to hydraulic radius
τ = γ * S * R � Shear force = specific weight of
water* slope*hydraulic radius (units of lbs/ft2 or kg/m2)
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E and G channel types – high efficiency;
low width: depth ratio
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Pipe – optimal efficiency
Wide shallow channels have low sediment transport efficiency
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Classification systems
� Schumm (later modified by Simon)� Montgomery and Buffington: geology of
mountain streams� Rosgen – empirically derived classess
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Simon and SchummChannel Evolution Model
� Most southern Minnesota streams are in stages 3-5, especially 4 and 5
� Widening more dominant in main channels
� Slight incision (1-2 ft) can be important
� Small streams - III
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Montgomery and Buffington
� Alternative to Rosgensystem for mountain west streams
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Sediment supply/transport balance
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Rosgen classification scheme
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Classification key for natural rivers
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Entrenchment ratio
� Incision of a stream within its valley� Flood-prone width/bankfull width� Critical to sediment transport processes as
well as bank erosion and water Range : Very entrenched <1.4 - 2.2 (not)Low number = more entrenched� Types A, G, F are entrenched� Types E, C not entrenched (much)
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Width-Depth ratio
� Key to sediment transport processes� Wide streams are less efficient at carrying
sed load; reduced depth decreases shear force – this is why ditches aggrade
� >40 W:D ratio is a braided stream (Type D), cannot carry sediment load
� Narrowest W:D ratio is Type A, E, G (<12), most efficient at transporting sediment
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Sinuosity
� Range From 1.0 to > 1.5 (about 2)� In Midwest, E Type most sinuous� Often first to be altered by changed hydrology
( can change from E to C Type)� Widespread channelization (ag and roads)� Loss of sinuosity increases slope, sediment
transport efficiency; ↓ habitat variability
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Bed materials
� Modifier of Rosgen Types1. Bedrock – uncommon in midwest, does occur2. Boulder – uncommon in Minnesota (except North
Shore of Lake Superior)3. Cobble - Uncommon (North Shore and lower
portions of rivers dropping into MN River valley)4. Gravel – common in undegraded and higher
gradient Midwest streams5. Sand – many Midwest streams6. Silt clay – many Midwest streams
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Type B – high gradient, cobble bed
(Rosgen photo)
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Type C – typical Midwestern stream; not entrenched;
� Slightly entrenched
Ratio > 2.2
Elm Creek near Trimont
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Type E – headwaters stream (E5-6)
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Type G – gully (Rosgen photo)
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Type D – braided channel- Platte River, NE
� Platte River
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Initiation of multiple channels
Elm Creek, MN
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Type F4 (Rosgen photo)
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What are Ditches ?
Often C or E in
MN – Stage II in Siomon CEM
Lily Creek by Fairmont
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Rosgen G6 – Gullies Simon CEM: Stage IIIDominant process: downcutting
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Channel Evolution
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Channel adjustment in Minnesota Elm Creek example
� Widening in lower reaches� Entrenched headwaters� Ditches – aggrading, still entrenched
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“C creep” – channel widening moving upstream
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159131721252933
1854
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E5 C5
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Channel
Widening: Simon CEM – IVRosgen E to C (or F)
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Lower Elm Creek adjustment
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Channel evolution in Driftless area
� Pre-Euro settlement, channels less entrenched
� Farming hillslopes eroded sediment; deposit in valley (3-8 feet at Rush Creek site)
� Channel then downcut over time, and evolved to a stable E type entrenched within a larger valley
� 2007 Flood blew out E stream, widening 2 x
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Rush Creek example
� Trout stream, type E, narrow (50 ft), deep
� 2007 flood widened to 100 feet (now type C , F
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Linkage between stream form and function
� Simon CEM� Stages III-V all have high sediment load� Stage III has highest bedload� Stage IV-V high suspended load� Stage I and VI are stable
Has been tested in Minnesota
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CEM stage and sediment load in MRB
� 58% of channels unstable in Western Corn Belt Ecoregion: Stages III, IV and V (Simon et al. 2008)
� Stage III-V channels have average sediment yield of 243 Tons/y/km2
� Stage I and VI (stable types) had 20.3 tons/y/km2 on average
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Ravines
� Abundant along Minnesota, Blue Earth and bigger rivers
� Often C type channels with G at headcuts and below
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Rosgen link between stream type and fluvial processes
� Types A-B, high transport capacity, little aggradation possible
� Type D – often at alluvial fans at base of steep slopes, streams have high bedload, unable to transport it all
� Type G – downcutting, unstable stream type
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Rosgen form-function
� Type E – narrow width-depth ratio (<12) increases sediment transport efficiency; slight entrenchment (<1.4) allowing frequent floodplain overflow
� Type C – slightly entrenched, higher width –depth ratio (12-40)
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Entrenchment
� Critical to turbidity issue� Reduced floodplain deposition of sediment� Increases sediment transport efficiency
� More precise measure: Bank height to bankfull height (used in BEHI) range 1 – 3 or 4
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Incision disconnects floodplain
Elm Creek near Huntley
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Historic channel
Active channel
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Issues with classification
� Static in time� Doesn’t quantify processes (hydrology,
sediment transport)� How do you know where stream has been
and where is it going?� What are “natural” background rates of
channel erosion
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Identifying bankfull elevation
� Difficult without experience; often higher than first thought
� Need to look at multiple point bars� Look for flat recent floodplain deposits � Verify with regional curve and/or stream gage
data
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Quantifying processes
� Measuring change over time� Monitoring (bank pins, resurvey)� Aerial photo analysis (long-term rates)
� Modeling:� Bank erosion – BSTEM (USDA - Simon et al.)BEHI, BANCS (Rosgen’s WARSSS)� Sediment transport – bed load, suspended load
� RiverMorph (Rosgen - like)� CONCEPTS (USDA – Simon et al.)
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Channel Management issues in MN
� In Minnesota River Basin – Channel widening is main issue ( “C creep” and/or conversion to F)
� Loss of sinuosity has occurred under the radar� In Ravines/gullies- downcutting (bed erosion) is
dominant process� In ditches; overwidening is issue – aggradation
occurs; counteract via 2-stage ditch (an E type within ditch walls)
� Loss of headwaters streams
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Watershed-scale management strategies for stream erosion
� Most techniques are labor intensive & expensive (vanes, rip rap, bioengineering)
� Are there less expensive ways� Ag practices using perennial plants in riparian
zone� Diversion of stream from major bluffs� Watershed management for hydrology
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KeyPerennial crops
New cattle fencing
Oxbow reconnection
Cattle crossing
Cross vane
Willow buffer
Existing fence
Demonstration site design: Phase I