principles of fluvial geomorphology relevant to river restoration esm 233: river restoration tom...
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Principles of Fluvial Geomorphology Relevant to
River Restoration
ESM 233: River Restoration
Tom Dunne
Spring 2011
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Aspects of Fluvial Geomorphology Relevant to River Restoration
• Types and functions of river channels in various parts of a drainage basin.
• Scaling and other properties of channel cross sections
• “Stability” versus dynamism• Planform. “Channel patterns”• Channel shifting: processes, rates• Nature of sediment budget• Relations between channel and floodplain
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Upland zone: High sedimentsupply and low storage.
Alluvial transport zone: sediment transport rate ≈ sediment supply rate. Significant transient sediment storage in valley floors and tributary fans. Multi-threaded channels in upper, steeper reaches; single-thread, meandering channels on lower gradients. ‘Free’ alluvial landforms.
Alluvial accumulation zone: sediment transport capacity decreasing downstream; floodplain aggrading.
Outlet: fans; deltas, estuaries.
Length scale Amazon to Atascadero. Depends on plate tectonics,…. Again and always! [ESM 203]
Geomorphic concept 1Channels and floodplains in different parts of a drainage basin function differently
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Geomorphic concept 2: Scaling and other channel properties
• Channel characteristics are responses to outputs from the watershed and to local conditions (incl. land management)
– Change any of these and the channel will change its form and/or behavior
D.R. Montgomery and J.M.Buffington, Channel processes, classification, and response. In:River Ecology and Management (Eds. R.J. Naiman and R.E. Bilby), Springer Verlag, 1998
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Channel cross sections are scaled largely by flow
magnitude supplied by the watershed:
Downstream changes of channel cross-section characteristics with
bankfull discharge, Green R. basin, WY
10.00bQ~bv
40.0bQ~bd
55.0bQ~bw
Bankfull
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Scaling of channel geometry of gravel-bed rivers in Britain
[Richards, Prog. In Phys. Geog., 1987]
w’=wb/D50
d’ =db/D50
v’=vb/√((ρs/ρw)-1)gD50
Q’=w’d’v’
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Elaboration and implications of scaling
• What kinds of flows fill channels?– Medium-sized ‘high flows’ that occur on average once every
year or couple of years – The “bankfull discharge” is typically the “1- to 2-year flood”.
We say that the average recurrence interval of bankfull discharge of most channels is about 1-2 years. We don’t understand it very well, we use the empirical result in deciding how big channels “should” be when they are re-designed.
– There are exceptions, but they are rare in lowland streams in mid latitudes over a wide range of river sizes
– But elsewhere …..
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Concept of “bankfull discharge” is fraught with much confusion as to whether or not the Qb has a fixed
recurrence interval (inverse of frequency)
Lesson: Keep your mind and eyes open! Look around at real rivers in your region, and don’t accept graphs from textbooks
Recurrence interval of bankfull discharge (yr)
1 100 10,000 1,000,000
Drainage area (km2)
20
10
0
Wet mt. forest basins –v.coarse bed material
Mid-latitude lowlandsWet tropical lowlands
Wide, sand-bed rivers in sub-humid regions?2.5
1.5
If I had to estimate ….
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Implications
• If the size of the channel-scaling flows change, expect bankfull width and depth of the channel to change– Change in flows could be due to
• Persistent change in climate/flow regime• Flood control reservoir upstream• Flow diversion
• If riparian vegetation is dense– decreases in flood sizes allow vegetation to invade
and narrow the channel – if flows increase, widening is resisted and channel
likely to become more trough-like
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Channel cross-section change in response to altered discharges
Widening after bank vegetation removal and increased storm runoff from an urbanizing area, Seattle WA
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Geomorphic concept 3
• Channel planforms are also responses to upstream supplies and local conditions: meandering, braiding, straight
Mackinaw R, central Illinois. Nature Conservancy, v 54(2), Summer 2004
Yakima R. WA
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Environmental range of channel patterns (Leopold, Wolman and Miller, 1964, Fluvial Processes in Geomorphology)
Discriminant envelope moves up/down as larger data sets accumulate, but general pattern survives. Braided rivers are generally steeper for a given bankfull discharge
Implication: If a newly designed channel falls above the discriminant line, there is a good chance it will braid
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Qbankfull = 1750 cfs; s = 0.0025
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Geomorphic concept 4• Planforms create patterns of flow, sediment transport,
cross-section characteristics and bed texture associated with the water and sediment having to flow around a bend in the channel.– These perturbations may also be forced on the channel by
large immobile objects, like large woody debris, boulders, bedrock outcrops
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Secondary (cross-channel) flow in meander Bar
Pool
Riffle
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Bar formation and bank undercutting, Popo Agie R., WY
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Sand bars and river bends, W. Venezuela
Pool
Riffle
Bar
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Pattern of cross sections around a meander: asymmetry increases with curvature
riffle
riffle
pool
pool
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Channel morphology and aquatic habitat (Trush, McBain, and Leopold, PNAS, 2000)
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Geomorphic concept 5• River bend geometry is roughly scaled by flow
Shields, F. D. (1996)Hydraulic and hydrologic stability, In: River Channel Restoration, (eds. A. Brookes and F. D. Shields), Wiley, pp 23-74.
Leopold, Wolman and Miller, 1964, Fluvial Processes in Geomorphology
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Examples of riffle spacing formulas
14050
210
29050
290613
.p
.
.r
.
r Ds
Dw.L
Lr = riffle spacing (m)
w = width (m)
Dr50 Dp50 = median particle size on riffle and pool (mm)
w.Lr
364w on steeper gradients
8-9w on gentler gradients
Hey, R.D. and C. R. Thorne (1986) Stable channels with mobile gravel beds, J. Hydraulic Engineering, Am. Soc. Civil Engr, 112, 8, 671-689.
Roy, A G. and A. D. Abrahams (1980) Discussion of rhythmic spacing and origin of pools and riffles, Geol. Soc. Amer. Bull., 91, 248-250.
Higginson, N. N. J. and H. T. Johnston (1989) Riffle-pool formations in northern Ireland rivers. In: Proc. Internat. Conf. On Channel Flow and Catchment Runoff, 638-647.
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Geomorphic concept 6
• River bends force channel migration
• Channel migration builds floodplains and creates their sedimentation patterns, topography, and hydrology
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River bend processes(Tuolumne R. Technical Advisory Committee, 2000)
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Flat Floodplain (Butzer)
Develop where there is an abundance of bed material accumulating on bars and outer bank material is erodible
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Channels with erodible outer banks and large bed-material load build point bars and shift laterally, leaving
evidence of their shifting rate.
Little vertical accretion of fine-grained suspended load
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Channel migration through bank erosion and overbank erosion, Green R., WA
Incipient cutoff
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Diverse off-channel water bodies created by channel migration, leaving lakes and sloughs with differing depths and degrees of connection to channels, and therefore inundation regimes and
biogeochemistry.. Mamore R. Oxbow, Bolivia
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Channel shifting creates a diversity of floodplain habitats (aquatic and terrestrial)
Tuolumne R. near junction with San Joaquin R., (1937)
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Convex Floodplain (Butzer)
Develop where deposition of bed material as bars is slow, cohesive outer banks erode slowly but fine-grained load (silt-clay) is deposited rapidly over bank.
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Floodplain habitats(Tuolumne R. Technical Advisory Committee, 2000)
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Geomorphic concept 7
• Watershed and riparian forest trees can play a major role in affecting channel geometry and planform
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Log jam, Nisqually R., WA
(Collins & Montgomery
Restoration Ecology, 2002)
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Log jams in Queets and Nisqually Rivers, Washington
[D. Montgomery, GSA Today, 2004]
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Geomorphic concept 8:Lane’s channel equilibrium concept
Q * s Qs * D50
Q = bankfull discharge s = channel slopeQs = bed-material load D50 = median bed particle size
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Stream channel incising after field drainage that concentrated flow (increased Q) and scoured bed material, reducing slope
(perturbation of LHS of Lane expression)
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Geomorphic concept 9[Common] Attributes of an alluvial river
(Trush, McBain & Leopold, Proc. Natl. Acad. Sci., 2000)
Spatially complex channel morphology.
Streamflows and water quality are predictably variable.
Channel-bed surface is frequently mobilized.
Alternate bars scoured deeply in 3- to 5-yr floods.
Balanced reach-scale budgets of fine and coarse sediment.
Episodic channel migration or avulsion.
Floodplain inundated and receiving fine sediment ~ once/yr.
Infrequent channel re-setting floods.
Self-sustaining, diverse riparian plant communities.
Naturally fluctuating floodplain groundwater.
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Channel morphology and aquatic habitat (Trush, McBain, and Leopold, PNAS, 2000)
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Floodplain habitats(Tuolumne R. Technical Advisory Committee, 2000)
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Readings for Week 3
• Trush, W. J., S. M. McBain, and L. B. Leopold, Attributes of an alluvial river and their relation to water policy and management, Proc. National Academy of Sciences, 97 (22), 11858-11863, 2000.
• Kondolf, G. M. 2006. River restoration and meanders. Ecology and Society 11(2): 42. [online] URL: http://www.ecologyandsociety.org/vol11/iss2/art42/
• Beechie, T. J. et al. Process-based Principles for Restoring River Ecosystems, BioScience, March 2010 / Vol. 60 No. 3, 209-222.