,. - vasting debris regardless of GIDMDRPHDlDGY he impacts on Geomorphology 31 (1999) 229-245 earch. Equally ELSEVIER s of channels rts to enhance nto considera- bris contribu- wasting as a Fluvial geomorphology and engineering: future roles woody debris utilizing a fluvial hydro systems framework

David 1. Gilvear * Department of Environmental Science, University of Stirling, Scotland, FK94LA, UK ironmental Ethics Received I May 1997; received in revised form 30 June 1997; accepted 15 July 1997 gun. Unpublished metric analysis of . Band 46, 67-77. Abstract , L. , 1982. Con- basins. Sediment River engineering is coming under increasing public scrutiny given failures to prevent flood hazards and economic and ,sins. U. S. Forest environmental 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 . Integrated flood implementation, and post-project appraisal stages of engineering projects. It should be bservation Maga- is, to be integral to the planning, proactive rather than reactive. Areas in which geomorphologists wil increasingly be able to complement engineers in river , 3- management include risk and environmental impact assessment, planning, river audits, determnation of instream curring on a Mc- flow needs, river restoration, and design of ecologically acceptable channels and structures. There are four key contributions d MS thesis, De- that fluvial geomorphology can make to the engineering profession with regard to river and floodplain management: 'ersity. ation to geology. 1. to promote recognition of lateral, vertical, and downstream connectivity in the fluvial system and the inter-relationships xtland, OR, dated between 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 elected landslides significance of both palaeo and active landforms and deposits as indicators of levels of landscape stabilty; 'gon. Unpublished 3. to highlight the sensitivity of geomorphic systems to environmental disturbances and change, especially when close to gon State Univer- geomorphic thesholds, ard the dynamcs of the natural systems; and 4. 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; widespread utilization of new technologies including GPS, GIS , image analysis of satellite and airborne remote sensing data computer-based hydraulic modeling and geophysical technques; dovetailing engineering approaches to the study of river channels which emphasize reach-scale flow resistance, shear stresses, and material strength with catchment scale geomorphic approaches , 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 for prediction of bed and bank stability and bedload transport; and collaboration with aquatic biologists to determineequations 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 In addition , the recent interest in geomorphology stems from the desire to minimize flood damage A few decades ago, the relationship between flu- , the vial geomorphology and river requirement to reduce environmental degradation as engineering was un- a result of river engineering schemes (Hey, clear. Engineering involved the use of straight trape- 1996), a move toward restoring ' sterile ' channelized zoidal channels, impoundments, embankents, and a river range of training Strctures to control and their channel reaches to ecologically valuable and aesthet- flow. Little consideration was given to ically pleasing watercourses (e. , Larson, 1996), and downstream concern with regard to the response of river channels environmental impacts, and when engineering struc- to climate change scenarios (Gilvear and Black tures failed, it was normally explained by ' design flood exceedance rather than that the 1999). dynamics of A number of areas in which fluvial the geomorphic system had not been taken into geomorphol- account. At the same time ogy is directly relevant to river engineering and , geomorphology was gen- management are shown in Table 1. More erally concerned with landscape evolution over generally, timescales that seemed inappropriate to the realm of application of a geomorphological approach involv- ing the following the engineer, and fluvial geomorphology was in its principles would be beneficial to infancy. Over the last few decades river engineering. , however, the disciplines have been on converging paths. Indeed by 1988 a book had been published entitled Fluvial Principle 1. The river channel functions as a three- Processes In River Engineering (Chang, 1988). A dimensional form with longitudinal , transverse more recent ilustration of convergence is that fluvial , and vertical dimensions involving changes in morphol- geomorphologists have recently published a guide- ogy and fluxes of water and sediment. book for the US Ary Corps of Engineers (Thome et aI. , 1997). Perhaps the most important develop- ment over the last decade has been geomorpholo- Principle 2. The river system functions in response to water and sediment gists' move from undertakng relevant or applicable inputs from the upstream catchment. research (Gregory, 1985; Hooke, 1988) to studies in which outcomes are put into practice (e. , Brookes 1992, 1995). An indication that geomorphology can Principle 3. The size, shape, and planform of a river contribute to engineering has been its success in normally varies through time, but the dynamics of assessing the feasibility of using engineering to natural channel adjustment varies between and along tame' the mighty Brahmaputra in Bangladesh (e. rivers. Thome et aI. , 1993). The change in the relationship between fluvial geomorphology and engineering has resulted in par Principle 4. The geomorphic stability of a river from a trend toward process studies, increased pro- system can be upset by such activities as river fessionalism among geomorphologists, greater quan- training, removing riparian vegetation, land use, and tification, adoption of common methodologies and climatic change. The sensitivity of river channels to tools (i. , computer-based hydraulic modeling, re- change varies between and along rivers. mote sensing, GIS, GPS , etc. ) (Cornelius et aI. 1994), and the requirements of geomorphologists to seek funding for their studies from research councils. Principle 5. Fluvial landforms , substrates , and pro- cesses define habitats for biota while vegetation and D.J. Gilvear 1 Geomorphology 31 (1999) 229-245 231

:rministic equations ) determine the role ects of fluvial geomorphology with direct relevance to river engineering (number of key areas taken from Brookes, 1995)

All rights reserved. Connectivity within the fluvial hydrosystem and environmental impact field techniques and surveys enabling sediment sources to be traced ( Quantitati ( StUdies of the downstream impacts on ri:,er ch nel mo y of ri,:er regulation, channelisation, and river training sedIment YIeld predIctIOn II relatIOn to land use change ( Preliminar equations for catchment (e.g., agriculture, mining, deforestation and urbanisation), and assessment of the impact of change on the downstream fluvial system l geomorphology Historical legacy, chronology and channel adjustments 100d damage, the ( StUdies of channel process (e. , bed and bank erosion and bedload and suspended sediment transport rates) al degradation as ( Examination of the role of importance of floodplain stratigraphy on channel adjustment ~s (Hey, 1996), a ( Quantificatio of rates and modes of sediment movement within the fluvial system :hannelized river ( StUdies of past channel adjustment in relation to climatic and anthropogenic change table and aesthet- Landscape sensitivity arson, 1996), and ( Qualitative and quantitative field techniques and modellng to identify instabilty of river channels ( Analysis of river channel cross-sections and planform to predict future change ( The influence of large flood events, land use changes and climate changes vear and Black Eco-geomorphology vial geomorphol- ( Appraisal and design of mitigation and enhancement measures and restoration projects ( Determination of instream flow requirements engineering and ( Fluvial auditing and river channel typologies More generally, approach involv- Headings within text. be beneficial to woody debris play an important role in determning headwaters to mouth via the concept of downstream fluvial processes. hydraulic geometr. This unidirectional approach has resulted in an understanding of longitudinal connec- Rather than looking back and examining the link- :tions as a three- tivity in the river system, with upstream impacts age between geomorphology and engineering over , transverse, and having consequences downstream. These impacts be- the last few decades, this paper seeks to identify how ges in morphol- gin in a focused area that spreads over time. Engi- fluvial geomorphologists can furter their contrbu- nt. neers have, at best, appreciated short term and local tion to the field of engineering and educate engineers impacts (e. , scour immediately below a dam over- about the value of geomorphology. Important areas spil; Fig. 1) and responded accordingly (e. , stiling :ions in response where contrbutions can be made are in demonstrat- wells), but have not seen the significance of the m the upstream ing the significance of connectivity within the fluvial absence of bedload inputs to the regulated river system in relation to environmental impact; the im- downstream. Over the past two decades , however portance of historical legacy and the chronology of the geomorphic effects of rural land use (Thome, past river channel changes; landscape sensitivity; and mform of a river 1991; Stott , 1979), the interplay between engineering, geomorphology, , 1997), impoundments (Petts the dynamics of and ecology. These areas are an inherent feature of channelization, (Brookes, 1988), and urbanization :tween and along (Gregory and Whtlow, 1989) on downstream mor- the fluvial hydro system concept (Amoros et aI. , 1987; Petts and Amoros, 1996), which is increasingly be- phology and substrate composition has been demon- ing used as a framework for scientific investigations strated over a range of temporal and spatial scales. Appreciation of the geomorphic significance of )ility of a river and river management and conservation. trapping the upstream sediment load by engineering tivities as river structures is now resulting in the use of substrate , land use, and 2. Connectivity within the fluvial hydrosystem replenishment to prevent bed degradation and "iver channels to and environmental impact changes in substrate character downstream. Kondolf ers. 1. Longitudinal connectivity (1995), for example, describes the massive impact that bedload starvation has had on Californian strates, and pro- Geomorphologists have traditionally been con- streams, despite state policies, regulations, and moves cerned with the ways in ~ vegetation and which rivers var from toward replenishment. Elsewhere, there is growing 232 D.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 of riverine recharge or discharge of hyporheic waters. spawning gravels by trapping sediment, and hence Such alterations in flow could be critical to the may be a contrbutory cause of declining numbers of sustainability of groundwater abstractions from allu- salmonids. This further ilustrates that physical dis- vial aquifers and/or river flows. Streambed siltation ruption leads to adjustments in the biotic system. can also alter dissolved oxygen content and Elimination or reduction in frequency and magnitude tempera- tures (Evans , 1997) in spawning gravels , upsetting of floods within regulated rivers also causes siltation spawning success. Conversely, removal of imperme- of ' redds ' (Sear 1993). Consequently, reduced able bed material can lead to loss of in-channel spawning success has led to the introduction of waters to the groundwater store with drying up of flushing' flows to remove excessive accumulations rivers occurring, as occurred on the River Glen of fines (Reiser et ai. , 1989). In the future, river England, following dredging. Lateral exchanges of engineers in consultation with geomorphologists water and sediment are also important. should find new methods to overcome the problem Floodplain inundation, for example, dissipates energy during of discontinuities caused by structures on the longi- floods, and confinement between tudinal continuum. embankments re- sults in higher unit stream powers leading to disas- 2. Lateral and vertical connectivity trous consequences when embankents fail (as was apparent during the high magnitude flood events of Less well appreciated is the importance of vertical 1993 on the rivers Rhine and Mississippi). However and lateral connectivity within the fluvial system despite increases in stream power, the decrease in (Petts and Bradley, 1997). Siltation of the streambed channel width can lead to a reduction in the transport can, for example, reduce permeability and , hence capability (due to a reduced surface area for bedload

Fig. 2. Rates and patterns of ban erosion on the meandering Luangwa River, . (A) Rates of bank erosion have been observed to be withdifferent according to the position around the meanderbend bend and compositioncurvatue. of the river banks. (B) Maximum bank erosion rates also vary :!

D.J. Gilvear 1 Geomorphology 31 (1999) 229-245 233

UPSTREAM LIMB OF MEANDER

ENVELOPE CURVE FOR WHERE RIVER IS ERODING INTO FLOODPLAIN SEDIMENTS

uary 1990.

1yporheic waters. ! Ie critical to the lctions from allu- :reambed siltation tent and tempera- 135 180 sravels , upsetting ANGLE AROUND MEANDER BEND IN DEGREES oval of imperme- (90 EQUALS APEX OF BEND) ISS of in-channel rith drying up of the River Glen 180 ral exchanges of )rtant. Floodplain zwa: c: :s energy during ID Z 160 embanents re- a: 0 leading to disas- lents fail (as was 140 ~ flood events of ssippi). However :i 120 , the decrease in o :! II in the transport a:c: :! Maximum erosion rate location : area for bedload 100 Area of high erosion CJz II:E c: 0

135 160 been observed to be :rosion rates also var ANGLE OF MEANDER BEND IN DEGREES (90 EQUALS APEX OF BEND) 245 234 l. Gilvear 1 Geomorphology 31 (1999) 229-

, Wer- transport) of the river and thus bed aggradation. Such duced avulsion can occur instantaneously (e. aggradation may enhance flood risk despite the flood rity and Ferguson, 1980). Appreciation of such episodic and rapid embankments and increase the risk of channel avul- changes and the possibility of sion and flood embankment failure (Hoey, personal channel movements should be considered when sit- communication). Loss of lateral connectivity also has ing buildings or routing transport links. meander development and environmental significance because rivers with func- Similarly, modes of meandering rivers tioning are more resilient and have greater mechanisms of bank erosion on , 1995), but have productivity (e. , fisheries) than those without (for are now well documented (Hooke example, floodplains decoupled from the river by not always been incorporated into engineering de- impermeable flood defense systems). signs and plans. Other research has demonstrated that over historical time some reaches are inherently active, and yet others have a tendency to stability (Gilvear and Winterbottom, 1992; Thorne et ai. 3. Historical legacy, chronology, and channel ad- justment 1993). In a study of meander development on the Luangwa River, Zambia, which flows through the , some meander bends Apar from the longitudinal, vertical, and horizon- South Luangwa National Park , and yet tal dimensions described above, another critical com- have shown stability over recent decades , in ponent of what has been termed the fluvial hydrosys- others are actively migrating. Such knowledge tem by Amoros et al. (1987) and Petts and Amoros this case, is important in that there is pressure to develop more safari lodges close to the river, but (1996) is the temporal dimension. In the context of , and river engineering, timescales of channel adjustment currently many are threatened by bank erosion some cases of less than 1000 years might be significant with some have been lost to the river. In on the Luangwa River increasing importance the closer the change occurred however, active to the present day. Such studies have been the focus may be safe locations due to concave bench develop- , 1983) and/or a tendency of attention of fluvial geomorphologists , including ment (Nanson and Page bends those as eminent as Luna Leopold, Ken Gregory, and for rapid downstream migration of meander devel- the late Marie Morisawa. In braided systems, exten- (Fig. 2). An accurate model of meander bend , with a predictive sive research has detailed mechanisms of planform opment on the Luangwa River change via braid bar formation and development and capability, would therefore aid safari lodge site plan- , and indicate ilustrated adjustment over daily, seasonal, and an- ning, minimize the loss of structures speci- nual timescales (e. , Best and Bristow , 1993). In the need for bank erosion protection works in wandering gravel bed rivers, adjustment over annual fied locations (Gilvear et al. , 1999b). atten- decadal, and even longer timescales has been ilus- Many examples of engineers not paying due rates of change are trated, together with the realization that flood-in- tion to fluvial processes and

Table 2 t100ding in January 1993 Inferred dominant mechanisms of flood embankment breaching on the rivers Tay and Earn during extreme , 1994 for greater detail) showing the importance of geomorphic varables (indicated by position; see Gilvear et aI. Percentage of breaches Outside of bend Other River section Old channel Very old channel Perpendicular to flow River Tay (wandering coarse gravel bed river) Reach I 34. 39.3 8 4. 16. 24. Reach 2 8.3 16. 33.

River Earn (meandering gravel bed river) Reach I 11.0 55. 34. Reach 2 60. 40. :::::::::

D.J. Gilvear 1 Geomorphology 31 (1999) 229-245 235 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 the ows through the e meander bends :lecades , and yet h knowledge, in e IS pressure to to the river, but ank erosion, and In some cases Luangwa River bench develop-

Ij or a tendency meander bends lder bend devel- ith a predictive lodge site plan- , and indicate works in speci- aying due atten- of change are

1997 Breaches 199 Breches g in January 1993 199 Breche greater than 10m 1993 Breaches les than 10m Edg of flplain Other Embankment

39. 24. 1 km

34. 40. Fig. 3. Comparson of the position of flood embankment breaches resulting from historical flood events with the locations of flood embankment failures during the 1990 and 1993 , 1994 and 1997 Tay floods. 236 l. Gilvear 1 Geomorphology 31 (1999) 229-245 apparent. For example, a recent response by British flood damage on the River Tay in 1990 and 1993 Gas in 1993 to the threat of pipeline failure due to demonstrates the applicability of such an approach t river bank erosion near Stirling, Scotland was a short engineering (Gilvear and Winterbottom , 1992). The length of gabions with active bank erosion visible at study demonstrated that the low sinuosity, single- either end. By 1994, the gabions had slumped into thread channel confined between embankments and the river as a result of undercutting and were re- agricultural land use practices on the floodplain were placed by a slightly greater length and volume of masking' the true dynamics of the rivers. Carto- gabions; no attempt was made to consider other graphic evidence revealed that in the 18th century, approaches to bank protection or to examine the many reaches had a number of channels, each with geomorphic cause of the erosion or the earlier fail- high levels of instability; these were often separated ure. Knowledge of the cause of instability rather than by short single-channel stable reaches. The signifi- the local symptoms and spatial and temporal aspects cance of the findings was made apparent when the of channel adjustment are central to designing appro- spatial distribution of recent flood embankment fail- priate mitigation measures (Simon, 1995). ures and damage was overlain with historically un- Identification of change and interpretation of indi- stable reaches suffering the greatest damage ((Table cators of change, however, can be problematic due to 2); Fig. 3). Indeed, many embankment breaches fossil landforms that are no longer active and land use change, such as deep ploughing, which can mask former indicators of a dynamc system. Careful read- ing of the landscape in which the dynamics of past and present processes are manifested is the realm of the geomorphologist. Identification of unstable zones forms of adjustment in these zones, and their signifi- cance have not been fully appreciated by engineers. Engineers have traditionally sought solutions based on the assumption of limited adjustment or adjust- ment over short timescales (e. , scour around bridge piers). Reach scale channel aggradation and degrada- tion over medium timescales (100 to 250 years) in relation to such changes as the frequency of flood- ing, (e. , Tipping, 1994) can also threaten structures and should be accounted for in designing engineer- ing structures. Bank protection on the outside of meander bends has also been a field where longer-term adjustment has not been incorporated into designs. The zone of highest bank erosion wil translate up or downstream with meander development, paricularly if the flow and sediment dynamics of the reach have been dis- turbed, and beyond the area of bank protection. Prior to engineering projects, geomorphologists should be employed to accomplish two things: (0 interpret fluvial 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. Fig. 4. A flood embankment failure and associated scour hoJe A retrospective study of the chronology of chan- the River Tay, Scotland in March 1997 revealing a gas pipeline at nel change over the last 250 years, following severe depth. D.J. Gilvear 1 Geomorphology 31 (1999) 229-245 237 1990 and 1993 overlay old river courses with embankments overly- or may not take place; channels h an approach to , in other words ing younger former courses having a greater ten- exhibit different degrees of sensitivity to change. tom , 1992). The dency for failure (Gilvear et al. , 1994). One such However, in many cases inuosity, single- , channels are sensitive to failure with associated scour during a flood in March change in that they are in dynamic equilibrium with nbankments and 1997 revealed a gas pipeline beneath (Fig. 4), ilus- fluvial processes. For example floodplain were , periodic dredging trating a need also for a knowledge of depth of scour operations on the River Allen e rivers. Carto- , Scotland temporarily on the river system. creates a homogenous bed 1e 18th century, , but following the next Geomorphologists also have a role in predicting major flood point bars and pool- Imels, each with riffe sequences channel adjustment over historic time scales in rela- form. Sensitivity analysis could thus be used to often separated tion to changes in flow and sediment load induced assess the extent to which bed les. The signifi- forms need to be by anthropogenic change, rare events , or non-sta- engineered in river Jarent when the restoration projects; in some tionar climate. Work by Graf (1984) and Winterbot- cases, they wil nbankment fail- develop naturally and rapidly, while tom and Gi1vear (1999) utilizing past channel change in other cases, they wil not. historically un- flood event magnitude, probability functions, and Geomorphologists have traditionally been damage ((Table con- GIS by the later author, aimed at estimating ban cerned with the subject of sensitivity, and the con- cment breaches erosion under different flood scenaros, may prove cept is implicit in, for example, Schumm s work on promising but only where good archival data on river metamorphosis and thresholds (Schumm, 1969, channel change is available. Similarly, historical in- 1973). In paricular, channels wil be sensitive to formation of the number, extent, and cause of em- change if they lie near a threshold. Gilvear (unpub- banent failures during floods of different size is lished; Fig. 5a) used proximity to the meandering/ being used to estimate the implications of changing braided threshold (Leopold and Wolman, 1957) to flood magnitude and frequency on the rivers Tay and demonstrate how the current sinuous single-thread Ear, Scotland, on flood embankent stability planform of the River Tay, modified by man over the (Gilvear and Black, 1999). Such information could centuries and constrained by embankents, is sensi- be critical to deciding upon new engineering designs tive to change back to a braided planform while the or whether it is economically viable to undertake River Ear is well-within the meandering domain. continual maintenance. On a much larger river system, Rutherford and Bishop (1996) showed the River Mekong to lie close to the meandering/braided threshold (Fig. 5a) and 4. Landscape sensitivity single/multi-channel forms (van den Berg, 1995; Fig. 5b). However, geomorphological investigations Landscape sensitivity, in the context of this work on the Mekong demonstrated that present rates of is the ability of a river to resist changes in its sand and gravel extraction were small compared to morphological varables resulting from an external the bedload transport rate of the river and were stress such as river engineering. Morisawa and unlikely to have measurable impacts on rates of bank Laflure (1979) demonstrated that channels in the and bed erosion and, hence Binghampton and Pittsburgh areas were unable to , channel p1anform. In the same study, it was shown that revetments on either resist the increase in flood magnitudes as a result of bank were unlikely to cause erosion on the opposite urbanization; enlargement was generally a feature of bank (a cause for concern given the Mekong forms the channels , as indicated by altered downstream the border between Thailand and Laos PDR); how- hydraulic geometry relationships. Knowledge of how ever, erosion rivers adjust their morphology, or what Hey (1978) downstream of each length of revet- ment may be enhanced - a feature often observed called degrees of freedom, is an important facet of by geomorphologists. Hence, slight alterations in a sensitivity and is critical to river engineering. The river s sediment budget or discharge induced by flow problem is that even when autogenic or allogenic regulation, land use change changes in the flow or sediment regime occur , or bank modification ted scour hole on , or the mayor may bed and banks are modified by not result in geomorphic adjustment : a gas pipeline at engineering, subse- depending upon the sensitivity of the quent adjustment of river channel morphology may system. An- other important fact is that differing reaches along .... ,(y' ~~~!:... ;,,!:......

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

Zone of bankfull stream powers Straight within alluvial channels on the River Tay, Scotland UK .. Meandering Braided

Q,: 005 O . Braided

I! .. ;r. 001

0005 Position of bankfull stream powers on the Mekong River, Thailand .. Zone of bankfull stream Meandering 0001 powers within alluvial channels on the River Earn, Scotland UK 00005 50 100 500 1000 5000 10, 000 50 000 100,000 BANKFULL DISCHARGE, m

(W/m C .

. Multi-thread channels (braided)

o 1. 3.: P:S 1. Single-thread 1.5.: P :s 1. channels !: P::1.

100 MEDIAN GRAIN SIZE OF RIVERBED (mm) g.,

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

species varing sensitivity to dercut banks, and backwaters to riverine the length of a river wil have , by the needs to be fully appreciated. change, and response to a disturbance (e. var along the In addition, the subject of hydraulic stream ecol- effect of engineering strctures) can and time. T ogy is of growing importance; it is based upon the length of the river both in spac fact that velocity influences all major groups of importance of sediment waves movmg through sedI- term and varable re- organisms in running waters. Engineering designs ment systems inducing short- , Hoey, 1989). and flow management strategies should therefore be sponse is key to this concept (e. river designed to maintain instream velocities within eco- Further understanding of the sensitivity of area for future development. logically acceptable values (Gore et al., 1994). Jung- chanels to change is an wirh et al. (1992), working on an Austrian river For a range of environmental conditions, sensitivity of the system, demonstrated a correlation between varance could be quantified by determning the ratio of water depth (geomorphologic ally determned) as a response to external forces. Such an approach could measure of habitat structure with the number and form the basis of risk assessment in relation to river species. Similarly, Oswood and engineering. The growth of robust river channel diversity of fish Barber (1982) correlated fish numbers with hydrauli- . bankfull typologies (e. , Rosgen, 1994) may also be helpful cally and geomorphologically defined habitats on lIers on the (Downs, 1995) in the development of rapid evalua- Alaskan streams. Flow conditions, area of undercut iver, Thailand tion techniques for assessing sensitivity. Certainly, , and gradient the Rosgen classification is increasingly being used banks, extent of spawning material in engineering impact studies in the western USA. together with overhanging vegetation and forest de- Coupling of computer-based modeling techniques bris were the key variables. With respect to macroinvertebrates, Bickerton 000 with high spatial resolution data using remote sens- ing (Bates et al., 1999; Gilvear et ai. , 1999a) wil (1995) showed that hydrogeomorphic varables were also allow the sensitivity of rivers to be explored. the dominant control on populations with vegetation of secondar importance vegetation itself, in par being controlled by substrate type and stability and flow conditions (Townsend et ai., 1997). Thus, in the 5. Eco-geomorphology UK, methodologies to quantify or predict river con- servation status (e. , SERCON), fish caring capac- 1. Geomorphic habitats and biotopes ity (e. HABSCORE), and invertebrate assem- blages (e. , RIVPACS) (Johnson and Law, 1995) all River engineers have traditionally viewed river give considerable importance to geomorphic var- channels as a conduit for flow and sediment. Increas- ables. In addition, the importance of riparan vegeta- ingly, they have to examine the environmental im- tion and coarse woody debris in controllng and stream pact of their activities, and this is naturally focused defining geomorphological habitat and on the effect of biota. The importance of geomorpho- ecosystem functioning is being realized (Gurnell logical control on habitat type and diversity has yet 1995); thus; the traditional engineering approach of to be appreciated by the engineering profession at removing ' deadwood' and cutting back riparan veg- large. In the same way that the importance of con- etation to allow free passage of floodwaters needs to serving botanically defined terrestral habitats for be rethought. The ability to demonstrate that river conservation of endangered animal species is now engineering schemes wil maintain or enhance geo- be of increasing well established, so the importance of geomorpho- morphically defined biotopes wil logically defined habitats such as pools, riffes, un- 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 Earn Scotland 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- Environmental Agency (Brookes , 1992). Straighten- tion on morphological variability and monitor change ing of the channel between 1978 and 1982 resulted at and downstream of engineered reaches, and in this in a doubling of the natural slope and an unstable respect, advances in the use of remotely sensed data channel bed. Indeed, attempts by fisheries staff of may prove fruitful (Fig. 6; Hardy et aI., 1994; Milton Thames Water Authority to install pools and riffes et al. , 1995; Gilvear et aI. , 1999a). and break-up the uniform trapezoidal cross-section immediately following the straightening works had 2. Eco-engineering and river restoration been unsuccessful! To compensate for the uniform cross-section, deflectors on alternate sides of the Future river engineering activity wil simultane- channel were constructed at intervals appropriate to ously be required to achieve traditional objectives the channel width (five to seven times channel width) such as maintenance of bank stability, reduction of in order to produce a more sinuous flow path. The flood levels , and prevention of sediment incursion deflectors were made of large limestone blocks , the into water intakes , but also maintenance of instream dimensions of which were based on bank full shear and floodplain habitats (e. , McDonald and Rickard stresses (47 N m ). Subsequently, these deflectors 1993). This , together with geomorphological river encouraged scour and the creation of pools but main- restoration (Sear, 1994), presents an enormous chal- tained a stable bed morphology. Limestone cobbles lenge to engineers. Geomorphological approaches were also placed in the channel behind the deflectorS' and input wil need to be the major component of to form stable riffle areas. Without knowledge of the such challenges. Incorporation of morphological and morphology of meandering channels and quantifica- sedimentological varability (Fig. 7) into river engi- tion of the stream power, however, the project would neering to benefit salmonids has recently been under- most likely have been unsuccessful. taken on the Evan Water, Scotland. Here, river di- The emphasis , to date , within the field of river versions were built to allow room for upgrading to restoration has stil been on stable forms. The role of motorway status because the proposed motorway lay channel adjustments, including migrating bar and along the course of the natural river. Initially, the bed forms and bank erosion in allowing pioneer river diversion was to be contained within straight species to colonize new substrates (Bravard et aI. gabion-lined channels. Engineering concerns were 1986; Marston et aI. , 1995) and disruption of armor for stability, given the proximity of the diversions to layers to scour fines from spawning beds needs to be road embankments, but incorporation of morphologi- conveyed. Brookes (1990) attempted to evaluate the cal features , including a sinuous course, did not success of restoration projects in terms of the energy jeopardize overall stability (Gilvear and Bradley, or stream power of the river occupying the channel. 1997). In fact, these measures probably enhanced At stream powers of less than 15 W m - , failure long-term stability within and downstream of di- resulted from the deposition of sediment, while at the verted sections by not increasing water velocities highest stream power, instream features were de- above those in natural sections of the river. The stroyed by erosion, ilustrating the necessity for a success of these diversions, which were completed in geomorphological basis to projects. A major task for 1995 , is being monitored, and early signs indicate engineers and geomorphologists should be to pro- that they are meeting their ecological objectives. duce river restoration projects and river diversions in River channel restoration is now well advanced. which channel mobility is integral to the design, but An example of a stream restoration project is that on does not threaten important structures. For example the Scotsgrove Brook, England described by Andrew instead of bank protection, construction of resistant Brookes, a fluvial geomorphologist employed by the bariers 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 Winterbottom and Gilvear, in press). D.J. Gilvear 1 Geomorphology 31 (1999) 229-245 241

)2). Straighten- l 1982 resulted tld an unstable :heries staff of Water Depth. )ols and riffes 1 cross-section Less than 20em. ing works had )r the uniform 20 to 40em.. : sides of the appropriate to 40 to 60em. channel width) low path. The 60 to aOem. Pool me blocks , the Jank full shear Greater than aOem. lese deflectors ,ools but main- ~stone cobbles l the deflectors )wledge of the md quantifica- project would Pool field of river lS. The role of Iting bar and )wing pioneer !ravard et aI. )tion of armor Riffe ds needs to be o evaluate the of the energy g the channel. m - 2 , failure , while at the Ires were de- cessity for a major task for Pool ld be to pro- . diversions in le design, but For example ,n of resistant nts at some

Pool )m Winterbottom 245 242 l. Gilvear 1 Geomorphology 31 (1999) 229-

Edge of channel bed Channel bed area under 1m 01 water

Channel bed area L-r- under ..300mm 01 water Water level under k: normal11ow conditions Rittle, gravelled area

Rlp-rap to prevent bank erosion 10m

Pools

6(Jm 80m 60m 60m

102

Z' 100

New profile

constructed on the Evan Water Fig. 7. Environmentally sensitive river engineering; (A) Geomorphic layout of a river diversion to be increase adult trout (after Scotland, (B) The design plan for a meandering pool and riffe reach constructed on the Pine River Manitoba to Newbury, 1996).

distance from an eroding bank might be preferable. their respective preoccupation with available habitat , regulation, and Such strctures might bring benefits in being easier for species and water abstraction to construct and ecologically less disruptive. diversion. There is, however, a greater need for a geomorphological input beyond that of defining in- instream 3. Instream flow requirements stream morphology. The application of the flow methodology has traditionally been based on Instream flow requirements have traditionally been the assumption of stationarty of form and flows. The the realm of hydro-ecologists and engineers due to optimum flows prescribed for habitat may, however, 243 D.J Gilvear 1 Geomorphology 31 (1999) 229-245

tivity of systems to change. The use of geomorpholo- not be suitable for maintaining the channel morphol-term, on gists in the planning phase of engineering wil also substrate condition in the long- ogy and , there hopefully become established with hazard identifica- which the allocation of flows is based. Hence tion and interpretation of reach scales. Of increasing is the need for prescribing channel maintenance flows importance wil also be an understanding of the , as was recently successfully undertaken on the (e. control that morphological diversity and fluvial dy- Isa Colorado River below Glen Canyon Dam; Hecht nea namics have in supporting biotic populations and 1997) and flushing flows (calculated with reference ecosystem resilience. to bedload algorithms and geomorphological theory The greatest contrbution that geomorphology can and observation). Flows pertaining to 8% exceedance , Australia for make to future river engineering and management is were specified for the Thomson River for geomorphologists to continue to strive for im- channel maintenance (Gippel and Stewardson, 1995). proved understanding of the geomorphic behaviour Rlp-rap to Similarly, Andrews and Nankervis (1995) devel- of river systems. Wolman (1995) has termed this Irevent bank oped a method for determning channel maintenance erosion play , as opposed to ' work' . He stresses, however flows based on application of an appropriate bedload , and that al- transport function and channel morphology to com- that the two are not totally separate though applied studies seek answers to very practical pute the quantity of bed material in each size fraction , they can generate new knowledge. He transported by increments of discharge. However questions applied studies the selection of appropriate bedload functions and appre- further states that within must be broad ciation of their reliability is problematic, and applica- playground' of geomorphologists " pursuits of new directions, direc- tion of such an approach falls firy within the enough to allow realm of geomorphology, although the outcomes wil tions often unforeseen in the original formulation of " (Wolman, 1995). The problem with be utilized by a water resource engineer and hydroe- the problem cologists to determine an ecologically acceptable play , however, is that it has primarly been directed at small river systems, whereas some of the major instream flow regime. This is an area where im- s major proved scientific understanding of the concept river engineering issues pertain to the world' absence of dominant discharge, the importance of both high and river systems. There has also been an have low flows on stream morphology, and bedload research into tropical river systems which , 1995). movement within mixed gravel bed situations could unique features and process dynamics (Gupta lead to improved river management and engineering. Large and tropical rivers should become a play- ground for geomorphologists as well as small or medium-size temperate rivers; here, remote sensing 6. Conclusion: the way ahead and modeling is likely to make a contribution. Alled to fundamental scientific research into the geomor- This chapter has demonstrated that geomorphol- phology of river systems should be collaborative ogy can make a substantial contribution in the next studies that examne interrelationships between geo- millennium to river engineering and management by morphic and biotic behaviour and response to change. ,n the Evan Water promoting the notion of ' designing with nature With improved scientific understanding of connectiv- se adult trout (after Such an approach should lead to more sustainable ity and sensitivity within the fluvial hydrosystem use of rivers and environmentally sensitive engineer- wil come the ability to undertake engineering with ing. Designing within the context of the riverine greater reliabilty, reduced environmental impact, and vailable habitat geomorphic system implies understanding the natural less uncertainty. regulation, and pathways and rates of movement of water and sedi- iter need for a ment, the role of active and fossil landforms being of of defining in- great importance. The geomorphological approach References of the instream should promote proactive planning and management been based on at river catchment, segment and reach scales, mor- Amoros, C., Roux, A. , Reygrobellet, J. , 1987. A method for and flows. The hologica1 varabilty in maintaining channel stabil- applied ecological studies of fluvial hydrosystems. Regulated may, however, Ity and resilience, channel dynamics, and the sensi- Rivers 1 , 17-36. 244 D.J Gilvear 1 Geomorphology 31 (1999) 229-245

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:cts of river bed er stream, Melk, en! in regulated atural and Afl- logy. Geophysi-

German experi-