lecture 17-10-24 bank erosion dean
DESCRIPTION
river bank erosion mitigation techniques.usually applied for high velocity stream riversTRANSCRIPT
Review – Adapted from Brierley and Fryers. An approach to River Characteriza<on • Valley Confinement
• River morphology • Planform (number of channels, sinuosity, stability)
• Floodplain characteris<cs • Channel Size
• Channel Morphology • Bank Morphology • Bed Morphology
• Bars • Bedforms
• Ripples • Dunes
• Func<on of local hydraulics – depth, shear stress (velocity), sediment supply.
Bank erosion processes • Existence of cohesion because silt/clay dominate upper
part of many banks – Creates Stability • Stability controlled by strength of basal materials • Gravity contributes to stability and instability • Vegeta<on contributes to bank strength
– Smith (1976) • Silty banks with no veg [erosion rate = 265 kg/hr] [lateral erosion rate = 162 cm/hr] • Silty banks with 17% root reinforcement and 5 cm of root reinforcement [erosion rate =
0.01 kg/hr] [lateral erosion rate = 0.018 cm/hr]
• Weakening mechanisms • Prewe[ng • Desicca<on • Freeze-‐thaw
• Processes – Hydraulic ac<on
• Fluvial entrainment • Undercu[ng
– Mass failure • Slab failures • Rota<onal failures
Smith (1976), Effect of vegeta7on on lateral migra7on of anastomosed channels of a glacier meltwater river. GSA Bulle7n 87: 857-‐860. Thorne (1982), Processes and mechanisms of river bank erosion, in Gravel-‐bed Rivers, Hey et al. eds. Wiley, 227-‐271.
Bank morphology
• Bank morphology records the balance of erosional and deposi<onal processes associated with different transport, alignment, and flow energy at different discharges
• Bank angle primarily is determined by the type of the bank material – Cohesive material forms steeper banks
Measure Stability of Bank Using Factor of Safety:
Factor of Safety: Resis<ng Forces/Driving Forces Fs> 1 STABLE
Fs< 1 UNSTABLE Resis7ng Forces: cohesion and fric7on f (µw (pore- water pressure), Φ‘ (effective angle of internal friction), c
(cohesion of soil), σ (normal stress)) Driving Forces: f (bank height, slope, weight of bank material (soil + water), surcharge)
Review of impacts in Simon and Collison 2002
Shear Strength of the Soil (sediment)
fn (cohesion, bank height, slope angle, water content)
τf = c’ + (σ-µw)tanφ’
shear strength
effec<ve cohesion normal
stress
effec<ve angle of internal fric<on
pore-‐water pressure
Mohr-‐Coulomb Equa<on for the shear strength of soils
Review of impacts in Simon and Collison 2002
Cohesion Increased (or Decreased) directly with: • Clays • Roots • Cemen<ng of minerals Other factors that add (or reduce) cohesion: • Water Content as it relates to
• pore pressure (matric suc<on) • fric<on angle
hip://www.cals.ncsu.edu/course/zo419/lectaids.html
Vegeta<ve Effects on Bank Stability
Hydrologic Impacts Stabilizing (reduce pore water pressures)
Intercep<on and ET Destabilizing (increase pore water pressures)
Concentrate flow in certain areas – i.e. stem flow
Mechanical Impacts Stabilizing
Root reinforcement-‐ added cohesion Increase normal stress
Destabilizing Surcharge – weight of the trees on the bank.
Review of impacts in Simon and Collison 2002
(Remember:(ψ = µa - µw))
Simon and Collison 2002; Pollen and Simon 2005
Characterize added cohesion from roots and their distribu<on in the
bank
Cr – soil cohesion
ε = k(τe-‐τc)
Use a jet-‐test in the field – Test the erodibility of sediment
Greg Hanson
erosion rate (m/s) erodibility
coefficient (m3/N-‐s)
effec<ve and cri<cal shear stress (Pa)
(Hanson 1990; Constan<ne et al 2010)
M = E * ub’
Suggests that E is directly related to k Constan<ne et al., 2010
Bank Erodibility and Migra<on Rates Empirically derive meander migra<on rate – many meander evolu<on models employ a coefficient of bank erosion E. Typically determined through planform changes – thus, unclear physical meaning.
M = migra<on rate E = coefficient of bank erosion – typically determined by historic planform changes Ub
’ = difference between depth-‐averaged near-‐bank velocity and the cross-‐sec<onally average velocity.
ε = k(τe-‐τc)
erosion rate (m/s) erodibility
coefficient (m3/N-‐s)
effec<ve and cri<cal shear stress (Pa)
M = E * ub’
Sacramento River Test show: • Vegeta<on plays a minor role • Bank material proper<es dominate • Limit to migra<on is the erosion of the unconsolidated basal layer
BUT, this rela@onship may vary with: • The size of the river • Roo<ng depth and bank height • Timeframe over which these variables are measured
Photo: M.B. Singer
Root Zone
Bank Toe
Coarse-‐grained bar deposits – and basal sands (bedload)
Fine-‐grained overbank deposits
Bank angle
Bank morphology generally dictated by: 1) Sediment mixture (homogenous, cohesion) 2) Vegeta<on 3) Mass movement mechanisms
What can we infer from bank morphology?
Other factors that play a major role in bank morphology: § Coarsening vs Fining Upward Profiles § Locally sourced resistant/forcing
elements § Recent history of erosional deposi<onal
processes
However, generally bank morphologies are simple. Many have bank-‐aiached bars that create a low-‐sloped, stepped morphology.
Modeling Bank Stability/Failure • Takes slope stability approach • Best for steep banks • Model accounts for bank material, bank geometry, added cohesion from roots, and
groundwater • Also incorporates fluvial erosion
Developed by Andrew Simon and others at the USDS-‐ARS
hip://www.ars.usda.gov/research/docs.htm?docid=5044
Rela%ve Effects of Mechanical and Hydrologic Impacts Vegeta%on
Simon and Collison 2002
Mechanical effects – root tensile strength, root distribu<on, root diameter/area Hydrologic effects – stem flow (water infiltra<on around trunk), canopy intercep<on, pore-‐water pressure (suc<on).
q Increased Fs due to tree cover (winter 2000)
q Decrease in Fs with large rainstorm
q Fs begins to rise again due to higher matric suc7ons (greatest under trees due to transpira7on)
q Soil moisture deficit protected bank through January un7l next storm.
q Early season winter rain resulted in failure because there was no intercep7on from the canopy.
Role of Invasive Vegeta@on In Channel Narrowing
• Rapid invasion of Tamarisk and Russian Olive
• Incised channel with sandy banks (no cohesion)
• NPS interested in vegeta<on removal and channel recovery
(Pollen-‐Bankhead et al 2009 – Canyon de Chelly)
shape aiributes
• Symmetrical – May be erosional channel or cross-‐over between bends
• Asymmetrical – Typical of one side of channel erosional and one side is deposi<onal
• Irregular • Compound
Channel Cross-‐sec<on Form • Width (B) • Mean depth (h) • Cross-‐sec<on area (A) • Weied perimeter (P) • Hydraulic radius (R) • Maximum depth (hmax) • Bed width (Bbed)
• Width/depth (B/h) • hmax / hmean
• asymmetry indices – A* = (Ar -‐ Al)/A
• Ar –area to right of center • Al – area to lev of center
– A2 = 2x (hmax -‐ h) / A • X – hor dist from center to max depth
From Knighton
Channel size • Channels with steep slopes and channels transpor<ng large
volumes of coarse bedload with braided channels are typically wide and shallow
• Channels, especially sand channels, with flashy discharge are typically wide
• Channels with dense riparian vegeta<on are narrower and deeper than with sparse vegeta<on
• Regime theory and hydraulic geometry -‐-‐ be aware of regional se[ng of the data and condi<on of the channels that were measured
• Is it possible to predict channel geometry??? – i.e. channel width??? Given certain hydrologic parameters and drainage basin aiributes.
Hydraulic geometry rela<ons: at-‐a-‐sta<on
The mathema<cal form of these rela<ons is:
B = aQb
h = cQf
U = kQm
Note: Q = B h U = ackQ(b+f+m)
This plot is an at-‐a-‐sta<on hydraulic geometry plot, because it depicts changes that occur at one place on the channel
The hydraulic geometry of streams: �
n B = aQb �n b = 0.26 �n b = 0.5 �
n h = cQf �n f = 0.40 �n f = 0.4 �
n U = kQm �
n m = 0.34 �n m = 0.1 �
at-a-station downstream �
… or use regional relations between drainage basin area and width�
(Leopold, 1994)�
We do not yet have a good physically-based model that predicts channel width. The downstream hydraulic geometry is a good predictor of channel width, But there is a significant degree of scatter, and natural variability�n B = aQ0.5 �
Given a certain hydrology, and certain drainage basin aiributes, how else can we approach the problem of predic<ng channel geometry?
Methods to es<mate channel-‐forming discharge: 1. Bankfull discharge – a field-‐based channel a[ribute
Original observa<ons: heavily grazed meadow, single-‐thread meandering stream, rapid migra<on rate, maintenance of channel conveyance over a decade of channel movement
Big Creek, UT: meandering form, very slow migra<on rate
Florence Creek, MT: coarse-‐bedded, straight channel, very slow migra<on rate
Original field iden<fica<on criteria (top of the flat-‐lying alluvial surface), now expanded to include non-‐geomorphic aiributes (i.e., lower eleva<on of perennial vegeta<on, stain lines)
Methods to es<mate channel-‐forming discharge: 1. Bankfull discharge – a field-‐based channel a[ribute
“Although several criteria have been iden7fied to assist in field iden7fica7on of bank-‐full stage … considerable experience is required to apply these in prac7ce, especially on rivers that have in the past undergone aggrada7on or degrada7on.”
(Biedenharn et al. 2008)
Harrelson et al. 1994. Stream channel reference sites: an illustrated guide to field technique. General Report No. RM-‐245, U. S. Forest Service.
DATE 1920 1930 1940 1950 1960 1970 1980 1990 2000
0.2
0.3
0.4
0.5
140.0
160.0
MEAN CHANNEL WIDTH, IN METERS
SECONDARY CHANNEL AREA, IN SQ. METERS
CHANNEL WIDTH CHANGE OVER TIME
1920 1930 1940 1950 1960 1970 1980 1990 2000 98 102 106 110 114 PRE-‐DAM POST-‐DAM
BANKFULL CHANNEL WIDTH, IN METERS
Allred and Schmidt, 1999 – Green River near Green River, UT.
Methods to es<mate channel-‐forming discharge: 2. Specified recurrence of peak -‐-‐ objec7vely defined
from hydrologic data
1 10 100
Return Period of Bankfull Flow (years)
Wolman and Leopold, 1957 WY, MT, MD, NC, SC, CT
Kilpatrick & Barnes, 1964 AL, GA, NC, SC
Leopold, Wolman, Miller 1964 IN, NB, MS, MD
Williams, 1978 CO, UT, NM, OR
ALL n = 107
2 @ 200 yrs -->
75% of obs within box 1.06 5
19% of obs: 1.36<RI<2.2 yr
“because of the uncertain7es …, it is recommended that discharges … [of 1-‐3 yr recurrence] be compared to the bank-‐full stage in the field to verify that they do have morphological significance.”
(Biedenharn et al. 2008)
Soar. 2000. Channel restora7on design for meandering rivers. Ph. D. thesis, University of Nohngham, UK.
“assuming a priori that Qri is related to either Qbf or Qef should be avoided in channel design” (Shields et al., 2008) Many studies show that the 1-‐3 yr recurrence
has liPle to do with the bankfull discharge!!! Bad approxima@on!!!
Methods to es<mate channel-‐forming discharge: 3. Effec7ve discharge – a calculated value based on
sediment transport data
But many studies have shown that effec<ve discharge is not equivalent to bank-‐full discharge and that the effec<ve discharge may not always be a direct surrogate for the channel-‐forming flow
(Andrews, 1980)
(Wolman and Miller, 1960)
(Baker, 1977)
Effec<ve discharge “is the best basis for channel restora<on design” (Shields et al., 2008)
The channel-‐forming discharge
Long-‐term average channel form depends on the <me-‐averaged magnitude of erosion and deposi<on (recovery) processes. Rivers where recovery processes are faster typically are adjusted to more common floods. Channels with less riparian vegeta<on and with highly variable, ephemeral flow are more likely to have disequilibrium morphologies.
(Poff et al., 1997)
Wolman and Gerson, 1978)
Method for determining effec<ve discharge (method in montecarlo.xls)
1A) Obtain discharge data for at least 10-‐15 year period. well established that mean daily discharge masks role of short-‐dura<on
events. Thus, hourly or 15-‐min data should be used for small streams if data exist. USGS Instantaneous Data Archive hip://ida.water.usgs.gov/ida/index.cfm 2) Develop sediment ra<ng curve, for transport of those sediment sizes that form the channel boundary
bed material only or include sizes that comprise the banks and natural levees? 3) Determine the transport by each discharge event 4) Establish discharge bins and sum total transport for each bin 5) Determine sensi<vity of Qef to bin size 6) Iden<fy modal value
Accoun<ng for Bank Stability/Erosion Improves Width Predic<ons
Eaton and Millar 2004
“unconstrained” “constrained”
Problems associated with effec<ve discharge calcula<ons (Shields et al., 2008)
1. Computed values are sensi<ve to the number of increments used to build the discharge histogram
2. Effec<ve discharge is just one flow that over-‐simplifies the actual flow regime and its history
3. Limited applicability to unstable channels and those where a catastrophic event has occurred during the period of record. Flow frequency and sediment-‐transport rela<ons may have changed as system adjusts to this event.
In an unstable channel that adjusts its form to a changing hydrologic and sediment supply regime, Qbf does not equal Qef. “Therefore, the expression “bank-‐full discharge” should never be used to refer to Qri or Qef.
References
B-‐F: 93-‐108
B: 162-‐180
Knighton, 165-‐187
L: 126-‐182
Mueller, E. R., and Pitlick, J. (2005), Morphologically based model of bed load transport capacity in a headwater stream. Journal of Geophysical Research, 110, F02016, doi:10.1029/2003JF000117.
Parker, G., et al. (2003), Effect of floodwater extrac<on on mountain stream morphology. Journal of Hydraulic Engineering 129:885-‐895.
Pitlick, J., and Cress, R. (2002), Downstream changes in the channel geometry of a large gravel bed river. Water Resources Research 38(10), 1216, doi:10.1029/2001WR000898.