Principles of Fluvial Geomorphology Relevant to

River Restoration

ESM 233: River Restoration

Tom Dunne

Spring 2011

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

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

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

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

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’

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 …..

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 ….

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

Channel cross-section change in response to altered discharges

Widening after bank vegetation removal and increased storm runoff from an urbanizing area, Seattle WA

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

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

Qbankfull = 1750 cfs; s = 0.0025

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

Secondary (cross-channel) flow in meander Bar

Pool

Riffle

Bar formation and bank undercutting, Popo Agie R., WY

Sand bars and river bends, W. Venezuela

Pool

Riffle

Bar

Pattern of cross sections around a meander: asymmetry increases with curvature

riffle

riffle

pool

pool

Channel morphology and aquatic habitat (Trush, McBain, and Leopold, PNAS, 2000)

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

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.

Geomorphic concept 6

• River bends force channel migration

• Channel migration builds floodplains and creates their sedimentation patterns, topography, and hydrology

River bend processes(Tuolumne R. Technical Advisory Committee, 2000)

Flat Floodplain (Butzer)

Develop where there is an abundance of bed material accumulating on bars and outer bank material is erodible

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

Channel migration through bank erosion and overbank erosion, Green R., WA

Incipient cutoff

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

Channel shifting creates a diversity of floodplain habitats (aquatic and terrestrial)

Tuolumne R. near junction with San Joaquin R., (1937)

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.

Floodplain habitats(Tuolumne R. Technical Advisory Committee, 2000)

Geomorphic concept 7

• Watershed and riparian forest trees can play a major role in affecting channel geometry and planform

Log jam, Nisqually R., WA

(Collins & Montgomery

Restoration Ecology, 2002)

Log jams in Queets and Nisqually Rivers, Washington

[D. Montgomery, GSA Today, 2004]

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

Stream channel incising after field drainage that concentrated flow (increased Q) and scoured bed material, reducing slope

(perturbation of LHS of Lane expression)

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.

Channel morphology and aquatic habitat (Trush, McBain, and Leopold, PNAS, 2000)

Floodplain habitats(Tuolumne R. Technical Advisory Committee, 2000)

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.