college of earth, ocean, and atmospheric sciences ......fluvial geomorphology and river engineering:...

vasting debris

regardless of

he impacts on

earch. Equallys of channels

rts to enhancento considera-

bris contribu-wasting as a

woody debris

ironmental Ethicsgun. Unpublished

metric analysis of. Band 46, 67-77., L., 1982. Con-basins. Sediment

,sins. U.S. Forest

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xtland, OR , dated

elected landslides

'gon. Unpublishedgon State Univer-

GIDMDRPHDlDGY

ELSEVIER Geomorphology 31 (1999) 229-245

Fluvial geomorphology and river engineering: future rolesutilizing a fluvial hydro systems framework

David 1. Gilvear *Department of Environmental Science, University of Stirling, Scotland, FK94LA, UK

Received I May 1997; received in revised form 30 June 1997; accepted 15 July 1997

Abstract

River engineering is coming under increasing public scrutiny given failures to prevent flood hazards and economic andenvironmental concerns. This paper reviews the contribution that fluvial geomorphology can make in the future to river

engineering. In paricular , it highlights the need for fluvial geomorphology to be an integral par in engineering projects, that

is, to be integral to the planning, implementation, and post-project appraisal stages of engineering projects. It should beproactive rather than reactive. Areas in which geomorphologists wil increasingly be able to complement engineers in rivermanagement include risk and environmental impact assessment, floodplain planning, river audits, determnation of instreamflow needs , river restoration, and design of ecologically acceptable channels and structures. There are four key contributionsthat fluvial geomorphology can make to the engineering profession with regard to river and floodplain management:

1. to promote recognition of lateral, vertical , and downstream connectivity in the fluvial system and the inter-relationshipsbetween river planform, profile, and cross-section;

2. to stress the importance of understanding fluvial history and chronology over a range of time scales , and recognizing the

significance of both palaeo and active landforms and deposits as indicators of levels of landscape stabilty;3. to highlight the sensitivity of geomorphic systems to environmental disturbances and change, especially when close to

geomorphic thesholds, ard the dynamcs of the natural systems; and4. to demonstrate the importance of landforms and processes in controllng and defining fluvial biotopes and to thus

promote ecologically acceptable engineering.

Challenges facing fluvial geomorphology include: gaining full acceptance by the engineering profession; widespreadutilization of new technologies including GPS, GIS, image analysis of satellite and airborne remote sensing datacomputer-based hydraulic modeling and geophysical technques; dovetailing engineering approaches to the study of riverchannels which emphasize reach-scale flow resistance, shear stresses , and material strength with catchment scale geomorphicapproaches, empirical predictions, bed and bank processes, landform evolution, and magnitude-frequency concepts;

* Fax: +44- 1786-467843.E-mail address: djglCistir.ac.uk(DJ. Gilvear).

0169-555X/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved.PIT: SOI69- 555X(99)00086-

230 D.J Gilvear 1 Geomorphology 31 (1999) 229-245

producing accepted river channel typologies; fundamental research aimed at producing more reliable deterministic equationsfor prediction of bed and bank stability and bedload transport; and collaboration with aquatic biologists to determine the role

and importance of geomorphologically and hydraulically defined habitats. (Q 1999 Elsevier Science B.V. All rights reserved

Keywords: fluvial; geomorphology; river engineering; risk assessment; environmental impact; river restoration

1. Introduction

A few decades ago, the relationship between flu-vial geomorphology and river engineering was un-clear. Engineering involved the use of straight trape-zoidal channels , impoundments , embankents, and arange of training Strctures to control rivers and theirflow. Little consideration was given to downstreamenvironmental impacts , and when engineering struc-tures failed , it was normally explained by 'designflood exceedance rather than that the dynamics ofthe geomorphic system had not been taken intoaccount. At the same time, geomorphology was gen-erally concerned with landscape evolution overtimescales that seemed inappropriate to the realm ofthe engineer, and fluvial geomorphology was in itsinfancy. Over the last few decades, however, thedisciplines have been on converging paths. Indeedby 1988 a book had been published entitled FluvialProcesses In River Engineering (Chang, 1988). Amore recent ilustration of convergence is that fluvialgeomorphologists have recently published a guide-book for the US Ary Corps of Engineers (Thomeet aI. , 1997). Perhaps the most important develop-ment over the last decade has been geomorpholo-gists ' move from undertakng relevant or applicableresearch (Gregory, 1985; Hooke, 1988) to studies inwhich outcomes are put into practice (e. , Brookes1992 , 1995). An indication that geomorphology cancontribute to engineering has been its success inassessing the feasibility of using engineering totame ' the mighty Brahmaputra in Bangladesh (e.

Thome et aI. , 1993).The change in the relationship between fluvial

geomorphology and engineering has resulted in parfrom a trend toward process studies , increased pro-fessionalism among geomorphologists , greater quan-tification, adoption of common methodologies andtools (i. , computer-based hydraulic modeling, re-mote sensing, GIS, GPS, etc.) (Cornelius et aI.1994), and the requirements of geomorphologists toseek funding for their studies from research councils.

In addition, the recent interest in geomorphologystems from the desire to minimize flood damage

, therequirement to reduce environmental degradation asa result of river engineering schemes (Hey,

1996), amove toward restoring ' sterile' channelized riverchannel reaches to ecologically valuable and aesthet-ically pleasing watercourses (e. , Larson , 1996), andconcern with regard to the response of river channelsto climate change scenarios (Gilvear and Black1999).

A number of areas in which fluvial geomorphol-ogy is directly relevant to river engineering andmanagement are shown in Table 1. More generally,application of a geomorphological approach involv-ing the following principles would be beneficial toriver engineering.

Principle 1. The river channel functions as a three-dimensional form with longitudinal

, transverse, andvertical dimensions involving changes in morphol-ogy and fluxes of water and sediment.

Principle 2. The river system functions in response

to water and sediment inputs from the upstreamcatchment.

Principle 3. The size , shape, and planform of a rivernormally varies through time, but the dynamics ofnatural channel adjustment varies between and alongrivers.

Principle 4. The geomorphic stability of a riversystem can be upset by such activities as rivertraining, removing riparian vegetation , land use , andclimatic change. The sensitivity of river channels tochange varies between and along rivers.

Principle 5. Fluvial landforms, substrates , and pro-cesses define habitats for biota while vegetation and

:rministic equations

) determine the roleAll rights reserved.

l geomorphology

100d damage , theal degradation as

~s (Hey, 1996), a:hannelized rivertable and aesthet-arson , 1996), andof river channelsvear and Black

vial geomorphol-engineering and

More generally,approach involv-be beneficial to

:tions as a three-

, transverse, andges in morphol-

nt.

:ions in responsem the upstream

mform of a riverthe dynamics of:tween and along

)ility of a rivertivities as river

, land use, and"iver channels toers.

strates , and pro-~ vegetation and

D.J. Gilvear 1 Geomorphology 31 (1999) 229-245 231

ects of fluvial geomorphology with direct relevance to river engineering (number of key areas taken from Brookes, 1995)

Connectivity within the fluvial hydrosystem and environmental impact( Quantitati

field techniques and surveys enabling sediment sources to be traced

( StUdies of the downstream impacts on ri:,er ch nel mo y of ri,:er regulation , channelisation , and river training

( Preliminar equations for catchment sedIment YIeld predIctIOn II relatIOn to land use change

(e.g., agriculture, mining, deforestation and urbanisation), and assessment of the impact of change on the downstream fluvial system

Historical legacy, chronology and channel adjustments( StUdies of channel process (e. , bed and bank erosion and bedload and suspended sediment transport rates)

( Examination of the role of importance of floodplain stratigraphy on channel adjustment( Quantificatio of rates and modes of sediment movement within the fluvial system

( StUdies of past channel adjustment in relation to climatic and anthropogenic change

Landscape sensitivity

( Qualitative and quantitative field techniques and modellng to identify instabilty

( Analysis of river channel cross-sections and planform to predict future change( The influence of large flood events , land use changes and climate changes

Eco-geomorphology( Appraisal and design of mitigation and enhancement measures and restoration projects( Determination of instream flow requirements( Fluvial auditing and river channel typologies

Headings within text.

woody debris play an important role in determningfluvial processes.

Rather than looking back and examining the link-age between geomorphology and engineering overthe last few decades , this paper seeks to identify howfluvial geomorphologists can furter their contrbu-tion to the field of engineering and educate engineersabout the value of geomorphology. Important areaswhere contrbutions can be made are in demonstrat-ing the significance of connectivity within the fluvialsystem in relation to environmental impact; the im-portance of historical legacy and the chronology ofpast river channel changes; landscape sensitivity; andthe interplay between engineering, geomorphology,and ecology. These areas are an inherent feature ofthe fluvial hydro system concept (Amoros et aI. , 1987;Petts and Amoros, 1996), which is increasingly be-ing used as a framework for scientific investigationsand river management and conservation.

2. Connectivity within the fluvial hydrosystemand environmental impact

1. Longitudinal connectivity

Geomorphologists have traditionallycerned with the ways in which rivers

been con-var from

headwaters to mouth via the concept of downstreamhydraulic geometr. This unidirectional approach hasresulted in an understanding of longitudinal connec-tivity in the river system, with upstream impacts

having consequences downstream. These impacts be-gin in a focused area that spreads over time. Engi-

neers have, at best, appreciated short term and localimpacts (e. , scour immediately below a dam over-spil; Fig. 1) and responded accordingly (e. , stilingwells), but have not seen the significance of theabsence of bedload inputs to the regulated riverdownstream. Over the past two decades , howeverthe geomorphic effects of rural land use (Thome,1991; Stott, 1997), impoundments (Petts, 1979),channelization, (Brookes, 1988), and urbanization(Gregory and Whtlow, 1989) on downstream mor-phology and substrate composition has been demon-strated over a range of temporal and spatial scales.

Appreciation of the geomorphic significance oftrapping the upstream sediment load by engineeringstructures is now resulting in the use of substratereplenishment to prevent bed degradation andchanges in substrate character downstream. Kondolf(1995), for example, describes the massive impactthat bedload starvation has had on Californianstreams, despite state policies , regulations, and movestoward replenishment. Elsewhere, there is growing

232D.J. Gilvear 1 Geomorphology 31 (1999) 229-245

Fig. 1. Failure of the revetment below Pitlochr Dam on the River Tummel, Scotland due to a flood in Februar 1990.

realization that dams may be impacting the extent ofspawning gravels by trapping sediment, and hencemay be a contrbutory cause of declining numbers ofsalmonids. This further ilustrates that physical dis-ruption leads to adjustments in the biotic system.Elimination or reduction in frequency and magnitudeof floods within regulated rivers also causes siltationof ' redds' (Sear 1993). Consequently, reduced

spawning success has led to the introduction offlushing ' flows to remove excessive accumulations

of fines (Reiser et ai., 1989). In the future, riverengineers in consultation with geomorphologists

should find new methods to overcome the problemof discontinuities caused by structures on the longi-tudinal continuum.

2. Lateral and vertical connectivity

Less well appreciated is the importance of verticaland lateral connectivity within the fluvial system(Petts and Bradley, 1997). Siltation of the streambedcan, for example, reduce permeability and, hence

riverine recharge or discharge of hyporheic waters.Such alterations in flow could be critical to thesustainability of groundwater abstractions from allu-vial aquifers and/or river flows. Streambed siltationcan also alter dissolved oxygen content and tempera-tures (Evans, 1997) in spawning gravels, upsettingspawning success. Conversely, removal of imperme-able bed material can lead to loss of in-channelwaters to the groundwater store with drying up ofrivers occurring, as occurred on the River GlenEngland, following dredging. Lateral exchanges ofwater and sediment are also important. Floodplaininundation, for example, dissipates energy duringfloods, and confinement between embankments re-sults in higher unit stream powers leading to disas-trous consequences when embankents fail (as wasapparent during the high magnitude flood events of1993 on the rivers Rhine and Mississippi). Howeverdespite increases in stream power, the decrease inchannel width can lead to a reduction in the transportcapability (due to a reduced surface area for bedload

Fig. 2. Rates and patterns of ban erosion on the meandering Luangwa River, Zambia. (A) Rates of bank erosion have been observed to bedifferent according to the position around the meander bend and composition of the river banks. (B) Maximum bank erosion rates also varywith meander bend curvatue.


UPSTREAM LIMBOF MEANDER

ENVELOPE CURVEFOR WHERE RIVER IS

ERODING INTOFLOODPLAIN SEDIMENTS

uary 1990.

1yporheic waters. !Ie critical to the

lctions from allu- :reambed siltationtent and tempera-

sravels, upsettingoval of imperme-ISS of in-channel

rith drying up of

the River Glen

ral exchanges of

)rtant. Floodplain:s energy during

embanents re-

leading to disas-lents fail (as was

~ flood events of

ssippi). However, the decrease in

II in the transport: area for bedload

180

z c:wa: 160ID Za: 0

140

:!

:i 120o :!a: c: :!

100CJ :Ez IIc: 0

been observed to be:rosion rates also var

135 180

ANGLE AROUND MEANDER BEND IN DEGREES(90 EQUALS APEX OF BEND)

Maximum erosion rate location

Area of high erosion

135

ANGLE OF MEANDER BEND IN DEGREES(90 EQUALS APEX OF BEND)

160

234 l. Gilvear 1 Geomorphology 31 (1999) 229-245

transport) of the river and thus bed aggradation. Suchaggradation may enhance flood risk despite the floodembankments and increase the risk of channel avul-sion and flood embankment failure (Hoey, personalcommunication). Loss of lateral connectivity also hasenvironmental significance because rivers with func-tioning floodplains are more resilient and have greaterproductivity (e. , fisheries) than those without (forexample , floodplains decoupled from the river byimpermeable flood defense systems).

3. Historical legacy, chronology, and channel ad-justment

Apar from the longitudinal, vertical, and horizon-

tal dimensions described above, another critical com-

ponent of what has been termed the fluvial hydrosys-tem by Amoros et al. (1987) and Petts and Amoros

(1996) is the temporal dimension. In the context ofriver engineering, timescales of channel adjustmentof less than 1000 years might be significant withincreasing importance the closer the change occurredto the present day. Such studies have been the focusof attention of fluvial geomorphologists, including

those as eminent as Luna Leopold, Ken Gregory, and

the late Marie Morisawa. In braided systems, exten-

sive research has detailed mechanisms of planformchange via braid bar formation and development andilustrated adjustment over daily, seasonal, and an-

nual timescales (e. , Best and Bristow, 1993). In

wandering gravel bed rivers, adjustment over annualdecadal, and even longer timescales has been ilus-trated, together with the realization that flood-in-

duced avulsion can occur instantaneously (e. , Wer-

rity and Ferguson, 1980). Appreciation of such

changes and the possibility of episodic and rapid

channel movements should be considered when sit-ing buildings or routing transport links.

Similarly, modes of meander development andmechanisms of bank erosion on meandering rivers

are now well documented (Hooke, 1995), but have

not always been incorporated into engineering de-

signs and plans. Other research has demonstratedthat over historical time some reaches are inherently

active, and yet others have a tendency to stability

(Gilvear and Winterbottom, 1992; Thorne et ai.

1993). In a study of meander development on theLuangwa River , Zambia, which flows through the

South Luangwa National Park, some meander bendshave shown stability over recent decades, and yet

others are actively migrating. Such knowledge, in

this case, is important in that there is pressure to

develop more safari lodges close to the river, but

currently many are threatened by bank erosion, and

some have been lost to the river. In some cases

however, active meanders on the Luangwa River

may be safe locations due to concave bench develop-ment (Nanson and Page, 1983) and/or a tendency

for rapid downstream migration of meander bends

(Fig. 2). An accurate model of meander bend devel-

opment on the Luangwa River, with a predictive

capability, would therefore aid safari lodge site plan-ning, minimize the loss of structures, and indicatethe need for bank erosion protection works in speci-

fied locations (Gilvear et al., 1999b).Many examples of engineers not paying due atten-

tion to fluvial processes and rates of change are

Table 2

Inferred dominant mechanisms of flood embankment breaching on the rivers Tay and Earn during extreme t100ding in January 1993

showing the importance of geomorphic varables (indicated by position; see Gilvear et aI. , 1994 for greater detail)

Percentage of breaches

River section Old channel Very old channel

River Tay (wandering coarse gravel bed river)Reach I 34.8 4.Reach 2 8.3 16.River Earn (meandering gravel bed river)Reach I Reach 2

Perpendicular to flow Outside of bend Other

39.3

16. 24.

55. 34.

60. 40.

33.

11.0


eously (e. , Wer-eciation of such

)isodic and rapid

lsidered when sit-inks.development and

neandering rivers, 1995), but have

I engineering de-

1as demonstrated

JeS are inherently

lency to stability; Thorne et ai.elopment on theows through thee meander bends

:lecades, and yeth knowledge, in

e IS pressure to

to the river, butank erosion , andIn some casesLuangwa Riverbench develop-

Ij or a tendency

meander bends

lder bend devel-ith a predictive

lodge site plan-, and indicate

works in speci-

34.40. Fig. 3. Comparson of the position of flood

embankment breaches resulting from historical flood events with the locations of floodembankment failures during the 1990 and 1993

, 1994 and 1997 Tay floods.

1997 Breaches

199 Breches

199 Breche greater than 10m

1993 Breaches les than 10mEdg of flplain

:::::::::

Embankment

aying due atten-of change are

g in January 1993

Other

39.24. 1 km


apparent. For example , a recent response by BritishGas in 1993 to the threat of pipeline failure due toriver bank erosion near Stirling, Scotland was a shortlength of gabions with active bank erosion visible ateither end. By 1994 , the gabions had slumped intothe river as a result of undercutting and were re-placed by a slightly greater length and volume ofgabions; no attempt was made to consider otherapproaches to bank protection or to examine thegeomorphic cause of the erosion or the earlier fail-ure. Knowledge of the cause of instability rather thanthe local symptoms and spatial and temporal aspectsof channel adjustment are central to designing appro-priate mitigation measures (Simon, 1995).

Identification of change and interpretation of indi-cators of change, however, can be problematic due tofossil landforms that are no longer active and landuse change, such as deep ploughing, which can maskformer indicators of a dynamc system. Careful read-ing of the landscape in which the dynamics of pastand present processes are manifested is the realm ofthe geomorphologist. Identification of unstable zonesforms of adjustment in these zones , and their signifi-cance have not been fully appreciated by engineers.Engineers have traditionally sought solutions basedon the assumption of limited adjustment or adjust-ment over short timescales (e. , scour around bridgepiers). Reach scale channel aggradation and degrada-tion over medium timescales (100 to 250 years) inrelation to such changes as the frequency of flood-ing, (e. , Tipping, 1994) can also threaten structuresand should be accounted for in designing engineer-ing structures.

Bank protection on the outside of meander bendshas also been a field where longer-term adjustment

has not been incorporated into designs. The zone ofhighest bank erosion wil translate up or downstreamwith meander development, paricularly if the flow

and sediment dynamics of the reach have been dis-turbed, and beyond the area of bank protection. Priorto engineering projects , geomorphologists should beemployed to accomplish two things: (0 interpretfluvial landforms as indicators of stability and insta-bility; (2) document past channel change using caro-graphic (Hooke and Redmond , 1989) and sedimento-logical evidence in order to predict future change.

A retrospective study of the chronology of chan-nel change over the last 250 years , following severe

flood damage on the River Tay in 1990 and 1993

demonstrates the applicability of such an approach tengineering (Gilvear and Winterbottom, 1992). The

study demonstrated that the low sinuosity, single-

thread channel confined between embankments andagricultural land use practices on the floodplain weremasking ' the true dynamics of the rivers. Carto-graphic evidence revealed that in the 18th century,many reaches had a number of channels , each withhigh levels of instability; these were often separatedby short single-channel stable reaches. The signifi-cance of the findings was made apparent when thespatial distribution of recent flood embankment fail-ures and damage was overlain with historically un-stable reaches suffering the greatest damage ((Table2); Fig. 3). Indeed, many embankment breaches

Fig. 4. A flood embankment failure and associated scour hoJe the River Tay, Scotland in March 1997 revealing a gas pipeline atdepth.

1990 and 1993

h an approach totom, 1992). The

inuosity, single-

nbankments and

floodplain weree rivers. Carto-

1e 18th century,Imels , each withoften separated

les. The signifi-Jarent when thenbankment fail-historically un-

damage ((Tablecment breaches

ted scour hole on

: a gas pipeline at

D.J. Gilvear 1 Geomorphology 31 (1999) 229-245

overlay old river courses with embankments overly-ing younger former courses having a greater ten-

dency for failure (Gilvear et al., 1994). One suchfailure with associated scour during a flood in March1997 revealed a gas pipeline beneath (Fig. 4), ilus-trating a need also for a knowledge of depth of scouron the river system.

Geomorphologists also have a role in predicting

channel adjustment over historic time scales in rela-tion to changes in flow and sediment load inducedby anthropogenic change, rare events, or non-sta-

tionar climate. Work by Graf (1984) and Winterbot-tom and Gi1vear (1999) utilizing past channel changeflood event magnitude, probability functions, and

GIS by the later author, aimed at estimating banerosion under different flood scenaros , may provepromising but only where good archival data onchannel change is available. Similarly, historical in-formation of the number, extent, and cause of em-banent failures during floods of different size isbeing used to estimate the implications of changingflood magnitude and frequency on the rivers Tay andEar, Scotland, on flood embankent stability(Gilvear and Black, 1999). Such information couldbe critical to deciding upon new engineering designsor whether it is economically viable to undertake

continual maintenance.

4. Landscape sensitivity

Landscape sensitivity, in the context of this workis the ability of a river to resist changes in itsmorphological varables resulting from an externalstress such as river engineering. Morisawa andLaflure (1979) demonstrated that channels in the

Binghampton and Pittsburgh areas were unable toresist the increase in flood magnitudes as a result ofurbanization; enlargement was generally a feature ofthe channels , as indicated by altered downstreamhydraulic geometry relationships. Knowledge of howrivers adjust their morphology, or what Hey (1978)called degrees of freedom, is an important facet ofsensitivity and is critical to river engineering. Theproblem is that even when autogenic or allogenicchanges in the flow or sediment regime occur

, or thebed and banks are modified by

engineering, subse-quent adjustment of river channel morphology may

237

or may not take place; channels, in other wordsexhibit different degrees of sensitivity to change.

However, in many cases, channels are sensitive tochange in that they are in dynamic equilibrium withfluvial processes. For example, periodic dredgingoperations on the River Allen, Scotland temporarilycreates a homogenous bed, but following the nextmajor flood point bars and pool-riffe sequencesform. Sensitivity analysis could thus be used toassess the extent to which bed forms need to beengineered in river restoration projects; in somecases , they wil develop naturally and rapidly, whilein other cases, they wil not.

Geomorphologists have traditionally been con-cerned with the subject of sensitivity, and the con-cept is implicit in, for example, Schumm s work onriver metamorphosis and thresholds (Schumm, 1969,

1973). In paricular, channels wil be sensitive tochange if they lie near a threshold. Gilvear (unpublished; Fig. 5a) used proximity to the meandering/braided threshold (Leopold and Wolman, 1957) todemonstrate how the current sinuous single-threadplanform of the River Tay, modified by man over thecenturies and constrained by embankents, is sensi-tive to change back to a braided planform while theRiver Ear is well-within the meandering domain.On a much larger river system, Rutherford andBishop (1996) showed the River Mekong to lie closeto the meandering/braided threshold (Fig. 5a) andsingle/multi-channel forms (van den Berg, 1995;Fig. 5b). However, geomorphological investigationson the Mekong demonstrated that present rates ofsand and gravel extraction were small compared tothe bedload transport rate of the river and wereunlikely to have measurable impacts on rates of bankand bed erosion and, hence, channel p1anform. In thesame study, it was shown that revetments on eitherbank were unlikely to cause erosion on the oppositebank (a cause for concern given the Mekong formsthe border between Thailand and Laos PDR); how-ever, erosion downstream of each length of revet-ment may be enhanced - a feature often observedby geomorphologists. Hence, slight alterations in ariver s sediment budget or discharge induced by flowregulation, land use change, or bank modificationmayor may not result in geomorphic adjustmentdepending upon the sensitivity of the system. An-other important fact is that differing reaches along

238

005

001

0005

0001

00005

(W/m

D.J Gilvear Geomorphology 31 (1999) 229-245

Zone of bankfull stream powers

. .

within alluvial channels on theRiver Tay, Scotland UK

. (y' . .

Q,:O . Braided

...

I! ..

.. , ~~~;,,

;r.

Straight

.. Meandering

Braided

.. Zone of bankfull stream Meanderingpowers within alluvial channelson the River Earn , Scotland UK

Position of bankfull

stream powers on theMekong River, Thailand

50 100 500 1000BANKFULL DISCHARGE, m

5000 10,000 50 000 100,000

. ..

C .

!: !:

. Multi-thread channels (braided)

o 1.3.: P:S 1.1.5.: P :s 1.

Single-threadchannels

!: P::1.

100

MEDIAN GRAIN SIZE OF RIVERBED (mm)


the length of a river wil have varing sensitivity to

change, and response to a disturbance (e., by the

effect of engineering strctures) can var along the

length of the river both in spacand time. T

importance of sediment waves movmg through sedI-ment systems inducing short-

term and varable re-

sponse is key to this concept (e., Hoey, 1989).

Further understanding of the sensitivity of river

chanels to change is an area for future development.

For a range of environmental conditions, sensitivitycould be quantified by determning the ratio

of the

response to external forces. Such an approach couldform the basis of risk assessment in relation to riverengineering. The growth of robust river channel

typologies (e. , Rosgen, 1994) may also be helpful

(Downs, 1995) in the development of rapid evalua-tion techniques for assessing sensitivity. Certainly,

the Rosgen classification is increasingly being usedin engineering impact studies in the western USA.Coupling of computer-based modeling techniques

with high spatial resolution data using remote sens-ing (Bates et al. , 1999; Gilvear et ai., 1999a) wil

also allow the sensitivity of rivers to be explored.

. bankfulllIers on theiver, Thailand

000

5. Eco-geomorphology

1. Geomorphic habitats and biotopes

River engineers have traditionally viewed riverchannels as a conduit for flow and sediment. Increas-ingly, they have to examine the environmental im-pact of their activities, and this is naturally focusedon the effect of biota. The importance of geomorpho-logical control on habitat type and diversity has yetto be appreciated by the engineering profession at

large. In the same way that the importance of con-serving botanically defined terrestral habitats forconservation of endangered animal species is nowwell established, so the importance of geomorpho-logically defined habitats such as pools, riffes , un-

dercut banks, and backwaters to riverine species

needs to be fully appreciated.In addition, the subject of hydraulic stream ecol-

ogy is of growing importance; it is based upon thefact that velocity influences all major groups oforganisms in running waters. Engineering designs

and flow management strategies should therefore bedesigned to maintain instream velocities within eco-logically acceptable values (Gore et al. , 1994). Jung-

wirh et al. (1992), working on an Austrian river

system, demonstrated a correlation between varanceof water depth (geomorphologic ally determned) as ameasure of habitat structure with the number anddiversity of fish species. Similarly, Oswood andBarber (1982) correlated fish numbers with hydrauli-cally and geomorphologically defined habitats on

Alaskan streams. Flow conditions, area of undercutbanks, extent of spawning material, and gradient

together with overhanging vegetation and forest de-bris were the key variables.

With respect to macroinvertebrates, Bickerton(1995) showed that hydrogeomorphic varables werethe dominant control on populations with vegetationof secondar importance vegetation itself, in parbeing controlled by substrate type and stability andflow conditions (Townsend et ai. , 1997). Thus , in the

UK, methodologies to quantify or predict river con-servation status (e. , SERCON), fish caring capac-

ity (e.

g.,

HABSCORE), and invertebrate assem-blages (e. , RIVPACS) (Johnson and Law , 1995) all

give considerable importance to geomorphic var-

ables. In addition, the importance of riparan vegeta-tion and coarse woody debris in controllng anddefining geomorphological habitat and stream

ecosystem functioning is being realized (Gurnell

1995); thus; the traditional engineering approach ofremoving 'deadwood' and cutting back riparan veg-etation to allow free passage of floodwaters needs tobe rethought. The ability to demonstrate that riverengineering schemes wil maintain or enhance geo-morphically defined biotopes wil be of increasing

importance. With the appreciation of such knowl-

Fig. 5. Examning the susceptibility of channel planform change type according to empircal formulae. (A) The Rivers Tay and EarnScotland and Mekong River, Thailand (both according to the work of Leopold and Wolman , 1957). (B) The Mekong River, Thailand

(according to the work of van den Berg, 1995).

240 D.J. Gilvear 1 Geomorphology 31 (1999) 229-245

edge comes the necessity to gain panoptic informa-tion on morphological variability and monitor changeat and downstream of engineered reaches, and in thisrespect, advances in the use of remotely sensed datamay prove fruitful (Fig. 6; Hardy et aI. , 1994; Miltonet al. , 1995; Gilvear et aI., 1999a).

2. Eco-engineering and river restoration

Future river engineering activity wil simultane-ously be required to achieve traditional objectives

such as maintenance of bank stability, reduction offlood levels, and prevention of sediment incursioninto water intakes , but also maintenance of instreamand floodplain habitats (e. , McDonald and Rickard1993). This, together with geomorphological riverrestoration (Sear, 1994), presents an enormous chal-lenge to engineers. Geomorphological approachesand input wil need to be the major component ofsuch challenges. Incorporation of morphological andsedimentological varability (Fig. 7) into river engi-

neering to benefit salmonids has recently been under-taken on the Evan Water, Scotland. Here, river di-versions were built to allow room for upgrading tomotorway status because the proposed motorway layalong the course of the natural river. Initially, theriver diversion was to be contained within straightgabion-lined channels. Engineering concerns werefor stability, given the proximity of the diversions toroad embankments , but incorporation of morphologi-cal features, including a sinuous course, did not

jeopardize overall stability (Gilvear and Bradley,1997). In fact, these measures probably enhancedlong-term stability within and downstream of di-verted sections by not increasing water velocitiesabove those in natural sections of the river. Thesuccess of these diversions , which were completed in1995 , is being monitored, and early signs indicate

that they are meeting their ecological objectives.River channel restoration is now well advanced.

An example of a stream restoration project is that onthe Scotsgrove Brook, England described by AndrewBrookes , a fluvial geomorphologist employed by the

Environmental Agency (Brookes , 1992). Straighten-ing of the channel between 1978 and 1982 resultedin a doubling of the natural slope and an unstablechannel bed. Indeed, attempts by fisheries staff ofThames Water Authority to install pools and riffesand break-up the uniform trapezoidal cross-sectionimmediately following the straightening works hadbeen unsuccessful! To compensate for the uniformcross-section, deflectors on alternate sides of thechannel were constructed at intervals appropriate tothe channel width (five to seven times channel width)in order to produce a more sinuous flow path. Thedeflectors were made of large limestone blocks , the

dimensions of which were based on bank full shearstresses (47 N m ). Subsequently, these deflectorsencouraged scour and the creation of pools but main-tained a stable bed morphology. Limestone cobbleswere also placed in the channel behind the deflectorS'to form stable riffle areas. Without knowledge of themorphology of meandering channels and quantifica-tion of the stream power, however, the project wouldmost likely have been unsuccessful.

The emphasis, to date, within the field of river

restoration has stil been on stable forms. The role ofchannel adjustments, including migrating bar and

bed forms and bank erosion in allowing pioneerspecies to colonize new substrates (Bravard et aI.1986; Marston et aI. , 1995) and disruption of armorlayers to scour fines from spawning beds needs to beconveyed. Brookes (1990) attempted to evaluate thesuccess of restoration projects in terms of the energyor stream power of the river occupying the channel.At stream powers of less than 15 W m - , failure

resulted from the deposition of sediment, while at thehighest stream power, instream features were de-stroyed by erosion, ilustrating the necessity for a

geomorphological basis to projects. A major task forengineers and geomorphologists should be to pro-duce river restoration projects and river diversions inwhich channel mobility is integral to the design, butdoes not threaten important structures. For exampleinstead of bank protection, construction of resistantbariers within the floodplain sediments at some

Fig. 6. Bathymetric map of the River Tummel, Scotland derived from image analysis of airborne thematic mapper data (from Winterbottomand Gilvear, in press).


)2). Straighten-l 1982 resultedtld an unstable

:heries staff of

)ols and riffes1 cross-section

ing works had

)r the uniform

: sides of the

appropriate tochannel width)low path. Theme blocks, the

Jank full shear

lese deflectors

,ools but main-~stone cobbles

l the deflectors)wledge of themd quantifica-project would

Less than 20em.

Pool

Water Depth.

20 to 40em..

40 to 60em.

60 to aOem.

Greater than aOem.

Poolfield of river

lS. The role ofIting bar and

)wing pioneer!ravard et aI.)tion of armords needs to beo evaluate the

of the energy

g the channel.

m - 2 , failure

, while at theIres were de-cessity for a

major task forld be to pro-

. diversions inle design, butFor example

,n of resistant

nts at some

Riffe

Pool

Pool)m Winterbottom


Edge of channel bedChannel bed areaunder 1m 01 waterr- Channel bed areaL- under ..300mm 01 water

Water level underk: normal11ow conditions

Rittle, gravelled area

Rlp-rap toprevent bank

erosion10m

Pools

102

80m 60m 60m 6(Jm

Z' 100

New profile

Fig. 7. Environmentally sensitive river engineering; (A) Geomorphic layout of a river diversion to be constructed on the Evan Water

Scotland, (B) The design plan for a meandering pool and riffe reach constructed on the Pine River Manitoba to increase adult trout (after

Newbury, 1996).

distance from an eroding bank might be preferable.

Such strctures might bring benefits in being easierto construct and ecologically less disruptive.

their respective preoccupation with available habitatfor species and water abstraction, regulation, and

diversion. There is, however, a greater need for ageomorphological input beyond that of defining in-stream morphology. The application of the instream

flow methodology has traditionally been based onthe assumption of stationarty of form and flows. Theoptimum flows prescribed for habitat may, however,

3. Instream flow requirements

Instream flow requirements have traditionally beenthe realm of hydro-ecologists and engineers due to

Isanea

Rlp-rap toIrevent bank

erosion

,n the Evan Waterse adult trout (after

vailable habitat

regulation, anditer need for aof defining in-of the instream

been based on

and flows. Themay, however,

D.J Gilvear 1 Geomorphology 31 (1999) 229-245 243

not be suitable for maintaining the channel morphol-ogy and substrate condition in the long-

term, on

which the allocation of flows is based. Hence, there

is the need for prescribing channel maintenance flows(e. , as was recently successfully undertaken on theColorado River below Glen Canyon Dam; Hecht1997) and flushing flows (calculated with referenceto bedload algorithms and geomorphological theoryand observation). Flows pertaining to 8% exceedancewere specified for the Thomson River

, Australia for

channel maintenance (Gippel and Stewardson, 1995).

Similarly, Andrews and Nankervis (1995) devel-oped a method for determning channel maintenanceflows based on application of an appropriate bedloadtransport function and channel morphology to com-pute the quantity of bed material in each size fractiontransported by increments of discharge. However

selection of appropriate bedload functions and appre-ciation of their reliability is problematic, and applica-tion of such an approach falls firy within therealm of geomorphology, although the outcomes wilbe utilized by a water resource engineer and hydroe-cologists to determine an ecologically acceptableinstream flow regime. This is an area where im-

proved scientific understanding of the concept dominant discharge, the importance of both high andlow flows on stream morphology, and bedloadmovement within mixed gravel bed situations couldlead to improved river management and engineering.

6. Conclusion: the way ahead

This chapter has demonstrated that geomorphol-ogy can make a substantial contribution in the nextmillennium to river engineering and management bypromoting the notion of 'designing with nature

Such an approach should lead to more sustainableuse of rivers and environmentally sensitive engineer-ing. Designing within the context of the riverinegeomorphic system implies understanding the naturalpathways and rates of movement of water and sedi-ment, the role of active and fossil landforms being ofgreat importance. The geomorphological approachshould promote proactive planning and managementat river catchment, segment and reach scales , mor-hologica1 varabilty in maintaining channel stabil-

Ity and resilience, channel dynamics, and the sensi-

tivity of systems to change. The use of geomorpholo-gists in the planning phase of engineering wil alsohopefully become established with hazard identifica-tion and interpretation of reach scales. Of increasingimportance wil also be an understanding of thecontrol that morphological diversity and fluvial dy-namics have in supporting biotic populations andecosystem resilience.

The greatest contrbution that geomorphology canmake to future river engineering and management isfor geomorphologists to continue to strive for im-

proved understanding of the geomorphic behaviour

of river systems. Wolman (1995) has termed thisplay , as opposed to ' work' . He stresses, howeverthat the two are not totally separate, and that al-though applied studies seek answers to very practicalquestions , they can generate new knowledge. Hefurther states that within applied studies theplayground' of geomorphologists " must be broad

enough to allow pursuits of new directions, direc-tions often unforeseen in the original formulation ofthe problem" (Wolman, 1995). The problem withplay , however, is that it has primarly been directedat small river systems, whereas some of the majorriver engineering issues pertain to the world' s major

river systems. There has also been an absence ofresearch into tropical river systems which have

unique features and process dynamics (Gupta, 1995).

Large and tropical rivers should become a play-ground for geomorphologists as well as small or

medium-size temperate rivers; here, remote sensingand modeling is likely to make a contribution. Alledto fundamental scientific research into the geomor-

phology of river systems should be collaborative

studies that examne interrelationships between geo-morphic and biotic behaviour and response to change.With improved scientific understanding of connectiv-ity and sensitivity within the fluvial hydrosystem

wil come the ability to undertake engineering withgreater reliabilty, reduced environmental impact, and

less uncertainty.

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