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Performance of Natural Channel Designsin Southwestern OntarioR. Ness & D.M. JoyPublished online: 23 Jan 2013.

To cite this article: R. Ness & D.M. Joy (2002) Performance of Natural Channel Designs inSouthwestern Ontario , Canadian Water Resources Journal / Revue canadienne des ressourceshydriques, 27:3, 293-315, DOI: 10.4296/cwrj2703293

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Performance of Natural Channel Designs

in Southwestern Ontario

Submitted January 2002; accepted June 2002

Written comments on this paper will be accepted until March 2003

R. Nessl and D.M. Joy'z

ABSTRACT

A significant change in the engineering design approach to streams and rivers has

occurred in many parts of North America in the past decade. Applications of natural

channel design approaches, in which the natural behaviour of the watercourse

including its tendency to meander and change over time are now commonplace.

Although this change has been common for some time with many designs completed

and implemented, little work has been done on the performance of these designs

and how well they achieved their objectives. The work reported in this paper is asummary of the performance of five channels in southwestern Ontario to whichnatural channel design principles were applied.

The selected channels had been in place between four and seven years before

their morphological characteristics were reviewed. The results of the study showed

that there was relatively little channel change in channel morphology during thisperiod. However, this was probably a temporary condition due to the large bed

materials specified and other reinforcing material used in the designs. Althoughbed materials would be expected to be mobile under bankfull conditions in natural

stream systems, analysis suggested that in the present designs they were not

mobile. There was also evidence of misinterpretation in the designs, particularlyin the determination of the bankfull discharge and its use in the selection ofmorphological parameters. As a result, the designs appear incompatible with the

processes acting within their respective watercourses, which may lead to long-termchannel instability and failure.

RESUME

Au cours de la dernidre ddcennie, on a assist6 i un changement consid6rable

dans l'approche de conception technique face aux cours d'eau et rividres dans

de nombreuses parties de lAm6rique du Nord. Les applications d'approches

l Planning and Engineering Initiatives Ltd., Kitchener' ON. Formerly Research

Associate, School of Engineering, University of Guelph' Guelph, ON2 Associate Professor, School of Engineering, University of Guelph, Guelph, ON

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concePtuelles de canaux naturels, qui favorisent le comportement naturel du coursd'eau, y compris sa tendance i serpenter et ) se transformer avec 1e remps, sonrmaintenant monnaie courante. Bien que cette nouvelle orientation soit courantedepuis d6jn un certain temps, de nombreux ouvrages ayant 6t6 r6alis6s et misen cuvre dans ce sens, trds peu de recherches ont 6t6 faites pour d6terminer lerendement de ces ouvrages et dans quelle mesure ils ont respectd les objectifsfix6s. Le travail dont fait 6tat le pr6sent article est un sommaire du rendement decinq canaux du sud-ouest de l'Ontario auxquels ont 6t6 appliquds les principes deconstruction d'un canal naturel.

Les canaux retenus avaient 6t6 fonctionnels entre quatre et sept ans avant qu'onexamine leurs caract6ristiques morphologiques. Les r6sultats de 1'6tude r6vdlentqu'au cours de la p6riode vis6e, la morphologie du canal a subi relativement peu dechangements. Cependant, le tout 6tait probablement attribuable d un 6tat temporairecaus6 par les grands matdriaux du lit sp6cifi6s et les autres matdriaux de renforcementutilis6s dans la construction. Bien qu'on s'attendrait i ce que 1es matdriaux du litsoient mobiles dans des conditions de d6bordement pour des systdmes naturels,l'analyse donne i penser que ces mat6riaux n'6taient pas mobiles pour les projetsi 1'6tude. Des signes d'interpretation erron6e sur 1e plan conceptuel ont pu €tred6gagds, en particulier dans la d6termination du d6bit de d6bordement et ) sonutilisation dans la s6lection des paramdtres morphologiques. Par cons6quent, lesouvrages congus semblent incompatibles avec les processus qui interviennent dansleurs cours d'eau respectifs, ce qui peut entrainer une instabilit6 et une d6faillance ducanal i long terme.

INTRODUCTION

Engineering design approaches to rivers and streams have historically focussed onthe conveyance of water with maximum efficiency, in order to transport floodwatersaway from populated areas. However, changes associated with this approach tendto disrupt the natural equilibrium of these fluvial systems, resulting in instabilityand failure of engineered channels in the long term. In recognition of theseoutcomes, current management practices attempt to incorporate knowledge fromfluvial geomorphology of river forms and processes to produce designs that are inbalance with natural processes and sustainable in the long term.

One of the most comprehensive applications of fluvial geomorphology inriver and stream management is termed 'natural' channel design, which involvesthe realignment of rivers and streams with forms that emulate those of naturalwatercourses. The objective ofthis practice is to achieve the self-regulating stabilityof form that characterizes natural rivers and streams, thus reducing damage toproperty and infrastructure, and improving the aesthetic value and ecosystemfunction of the watercourse. Natural channel design has been increasingly applied inOntario over the past decade for the restoration of degraded streams, particularly inurban areas.

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However, beyond short-term anecdotal accounts, there has been little review ofthe performance of natural channel design in Ontario or elsewhere. Consequently,

little is known regarding the success of natural channel design in achieving

stability and preventing the negative outcomes associated with other channel

engineering practices. The research summarized in this paper is intended to examine

the performance of natural channel design from a more quantitative, long-termstandpoint than the current literature contains. In this case, performance of a

number of designed channels was evaluated with respect to success in achieving

geomorphic function, which is the primary aim of natural channel design' Theresults are intended to provide feedback on current design practices and guidance forfuture efforts.

BACKGROUND

There is an ongoing need for engineering intervention in rivers and streams due to

continuing development pressures, which require channel relocation and protectionofindividuals and property from erosion and flooding. Intervention is also required

to replace past works that are failing, and to remediate watercourses that have

become degraded as a result of human activities. Past engineering practices advocated

straightening, hardening, and enlarging natural river and stream channels to convey

discharges as efficiently as possible, to minimize flooding, and to Prevent erosion

(MTO, 1997). However, these changes have been associated with a number ofnegative impacts, including the aggravation, rather than improvement, of floodingand erosion problems. Research suggests that this is the result ofdisturbing, throughchannel modification, the equilibrium between supply and transport of water and

sediment, which is an inherent property of natural fluvial systems.

To address the problems that result when river and stream channels are

engineered into paths or configurations that are incompatible with their formative

processes, there has been a movement to improve channel design practices by

designing 'with nature', rather than against it. The intent of the natural channel

design approach is to replicate the channel form that would naturally occur given

the hydrologic and sediment regimes of the upstream drainage basin, and thus

achieve the stability that is associated with this equilibrium condition. Designs

incorporate the characteristics of natural rivers and streams, including meandering

plan form, bedforms, and channel bed and banks constructed of natural materials.

Morphologic dimensions of the channel are determined either by analog techniques

such as historical channel analysis or reference reaches, or by applying equilibriumchannel geometry relationships taken from geomorphic research on natural rivers

and streams. The empirical nature of these tools introduces significant uncertainty to

the design problem, as natural fluvial systems are highly variable, and the publishedrelationships often used may not necessarily be applicable to local conditions.

Until recently, relationships from a stream classification system developed by

Rosgen (1994) have been used almost exclusively in Ontario designs, in part because

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the Ontario Ministry of Natural Resources strongly advocated the Rosgen systemin the early 1990s (MNR,7994).In addition to the inherent uncertainty of anempirical approach, the basis ofthe Rosgen classification system has been the subjectof significant controversy (Ashmore, 1999). As a result, there is significant potentialfor error in natural channel design, making it a trial-and-error process that dependson experimentation and observation of the results to improve design techniques.However, long-term monitoring of the constructed channels is rarely undertakenbecause of a lack of funding and trained observers, and an absence of establishedmonitoring protocol. Consequently, there are little data avallable regarding theperformance of natural channel design and therefore a lack of feedback with whichto evaluate and improve design practices.

To date, only a limited number of results from applications of natural channeldesign have been published, and report primarily qualitative, short-term (<2 years)observations (Brookes, 1,996a; Iversen et at., 1993; Haltiner et a/., 1996; Gilvearand Bradley, 1997; Hanington, 1999). The absence of comprehensive, quantitativeevaluation studies is due primariiy to a lack of funding for monitoring programs(Brookes, 7996b; Annable, 7999) as well as reluctance on the part of practitioners toreport unfavourable results. Furthermore, no standardized procedures for the long-term monitoring and evaluation of natural channel design currently exist.

Kondolf and Micheli (1995) and Annable (1999) both advocate the long-termmonitoring of channel morphology for evaluation of these types of projects, which islogical since the primary objective ofnatural channel design is to re-establish naturalform and function. Design guidelines published by leading proponents of naturalchannel design (FISRWG, 1999; Rutherfvd et a/., 1999; Ontario MNR, 1999)recommend that monitoring costs be included in all project budgets to accommodatemore extensive and comprehensive long-term evaluation of this kind. However, atthe present time, these types of monitoring programs

^ppe^r to be limited to high

profile projects with substantial funding (Kronvang et a|.,7998).The goal of this study was to address the lack of knowledge regarding

the medium- to long-term success of natural channel designs by evaluatingthe performance of a number of applications of this practice in southwesternOntario. There can be a number of measures of success in natural channel design,including channel stability, ecological function, water quality and aesthetic value.However, improvements in these areas are dependent on the successful restoration ofgeomorphic function to the watercourse in question, which is the principal objectiveof natural channel design. Therefore, performance of designs for the purposes ofthis study was evaluated in terms of geomorphic function. This was accomplishedthrough detailed chatacterization and measurement of the existing condition of a

number of natural channel designs that had been in place for some time. The resultswere compared with baseline data to assess whether changes to channel morphologythat had occurred were indicative of natural function. Hydraulic and sedimenttransport conditions within the study channels were also investigated and contrastedwith those of natural systems.

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METHODOLOGY

Locations where natural channel design had been implemented in southwesternOntario were identified through the literature and through contact withconservation authorities, municipal governments and private consultants. Because

the goal of the study was medium- to long-term evaluation, it was necessary

to select study sites where the constructed channel had been in place longenough so that adjustment to the prevailing hydrological and sediment transportconditions would have occurred. Study sites were also required to demonstrateproper application of natural channel design principles. A number of sites were

rejected because fixed engineering controls, such as large riprap, gabion baskets orarmor stone had been employed to 6x the constructed channels in place, restrictingnatural adjustment processes. Given the time that would be required to performsurveys and other field measurements, five study sites were selected. These were

selected from the list of candidate sites, based primarily on the quality of design

documentation and available baseline information.Existing channel morphologywas characterized at the sites through topographic

surveys using a total station surveying instrument. Measured characteristics includedchannel cross-sections, longitudinal profile and plan form, and other features ofinterest where appropriate. Where measurement of the total length of a constructedchannel was not practical, a reach with characteristics representative of the overallsite was surveyed. Cross-section surveys were performed at the apex of meander

bends and on riffles. At each cross-section, measurements were taken to define the

channel and overbank areas where possible, and in addition, the existing bankfullelevation and location.

Longitudinal profile measurements were taken in sufficient detail to define

the channel thalweg, with the deepest points in pools, top and bottom of riffles,and other features of interest along the thalweg recorded. Plan form was surveyed

by measuring points at the bankfull elevation on both channel banks. Additionalfeatures measured during topographic surveys included mid-channel bars and

islands, bioengineering treatments, and channel structures such as bridges and stormsewer outfalls. All measurements were recorded in three-dimensional coordinates,

and were referenced to permanent benchmarks nearby.

The existing size distribution of surface bed material was determined at each

study site using the pebble count areal sampling technique (Wolman, 1954). Wherebed material was too fine to perform areal sampling, bulk sampling was used and

particle size distributions determined by mechanical sieve analysis. At each studysite, bed material was measured at a number of locations including both pools and

riffles, because of the differing hydraulic and sediment transport characteristics ofthese structures (Keller, 1971).

Changes to channel morphology were identified by comparison with available

baseline data, which consisted primarily of construction drawings and contractdocuments in which the plan form, longitudinal profile, cross-section shape for pools

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and riffles, and bed and bank materials were specified. Construction drawings werescanned or obtained in electronic format and aligned with existing conditions surveyresults to facilitate comparison with the design channel morphology. Substantialqualitative judgement was required for this exercise, as it became evident that thealignment of constructed channels rarely matched the design specifications. Anyas-constructed data were incorporated into the analysis although these were notgenerally available. As a result, this restricted quantitative estimates of channelchange to general characteristics such as overall bed profile shape and cross-sectionshape. Changes to the composition of the channel bed material were assessed bycomparing the measured existing size distributions with bed material specificationsfrom thc design documentation.

To charactertze the hydrologic regime at the study sites, estimates of dischargesfor typical return periods were obtained. Discharge data were obtained from a varietyof sources. Where continuous streamflow records were available for the subjectwatercourse or for nearby streams, flow frequency analyses were performed usingthe Log-Pearson Type III distribution on the annual maximum series (Kite, 1977).Other estimates of discharge were obtained from hydrological models developed byconsultants, municipalities or conservation authorities. Of particular interest werethe 1- to 2-year return period discharges, which are indicative of the magnitude ofthe formative bankfull channel discharge in natural river and stream channels (Hey,1998). These were compared with design bankfull discharge capacity of the studychannels in order to assess the suitability ofthe channel size.

Numerical modelling was also performed to assess hydraulic characteristicswithin the subject sites and to assess channel stability. The U.S. Army Corps ofEngineers HEC-RAS software (v. 2.2) was used to calculate water surface profilesfor both original and existing conditions at the design bankfull discharge. Hydraulicparameters including depth of flow, velocity, cross-sectional flow area, hydraulicradius, energy slope, and shear stress were obtained for both original and existingconditions simulations.

Calculated shear stress was used to analyze the stability and mobility of thechannel bed material in terms of the Shields criterion. or critical dimensionless shear

stress. defined bv

x

8(p,-p) D,o

where T., is the critical dimensionless shear stress, t is the actual shear stress, 8'is the gravitational constant, p, and p are the densities of water and sediment,respectively and D..o is the median bed sediment diameter. A Shields criterionof 0.045, which is typical of conditions in natural channels (Andrews, 1983)was used to determine bed sediment mobility under bankfull conditions for bothdesign and existing channel morphology.

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RESULTS

Table 1 summarizes the five natural channel sites selected for evaluation in thestudy. Each site contained a length of stream channel realigned according to naturalchannel design. At the time of the study (summer 2000), the constructed sectionshad been in place for between four and seven years. At each site, the channel designhad incorporated pool-riffle bed structures, meandering plan form and bed andbanks of natural materials, with few or no fixed engineering controls. The designs

were documented by a minimum of a set of construction drawings, with some

additional information available for some sites consisting of limited as-built surveydata or photographs. Drainage areas for study sites ranged from < 2 kmz to> 70 km2. A11 of the sites were locate d in urban environments with widely varyingimperviousness in the contributing watersheds. A description of the individual sites

is provided beloq followed by a summary of the study results.

Table 1.5ummary of study sites.

Stream

Name

Mill Creek

Laurel Creek

Little Etobicoke Creek

Groff Mi11 Creek

Henry Sturm Drain

Year Drainage

Constructed Area

(km')

Design Design

Basis QBr

(m3/s)

Rosgen Type'C' 1.3

Rosgen Type'C' *2.5

Rosp;en Type'Bc' 6.7

Rosgen Type'C' .0.7

Rosgen Type'E' 0.15

(hannel

Length

(m)

Cambridge

Waterloo

Mississauga

Cambridge

Kitchener

100 600

70 300

22.3 1200

7.6 300

1..7 450

1995

1.996

7996

7993

1.993

*Estimated from hydraulic modelling.

MillCreek

The Mill Creek study site is located in Soper Park, Cambridge, approximately2.5 km upstream of the confluence with the Grand River. In 1996, the erodingengineered channel through the park was realigned based on natural channel designto restore aquatic and riparian habitat and to improve aesthetic value.

The basis of the design was the classification of the creek as a Type 'C' channelaccording to the Rosgen Classification system, with morphological dimensionsselected in accordance with the published ranges for this type. A design bankfulldischarge of 1.3 m3/s was determined based on the capacity of the existing channel.Round stone 100-150 mm in size was imported for lining the channel at rifflesections, and 150-200 mm round stone was placed in pools only on the point bars at

the inside of meander bends. Bioengineering treatments including brush mattresses

and iive fascines were applied throughout the channel with the intent of stabllizing

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the banks until vegetation could become established. Approximately 600 m of thecreek was realigned but only the uppermost 450 m were measured in the studybecause portions at the downstream end were constrained by armour stone and notconsidered to be indicative ofnatural channel design.

Observation of the site and topographic survey results indicated that the planform alignment of the channel was more or less intact (Figure 1). However, profilesurvey results showed that at the bottom of the study reach the bed elevation haddecreased up to 1 m since construction. Confidence in the observation was high as

an as-constructed survey ofthe deepest points in pools had been performed. Cross-section surveys did not suggest major changes although some increase in depth hadoccurred particularly at the downstream end ofthe channel.

Laurel Creek

The Laurel Creek study site is located in Bechtel Park, Waterloo, approximately3 km upstream of the confluence with the Grand River. The channel through thesite was realigned in 1996 using natural channel design to resolve erosion problemsthat were threatening the integrity of an underground sewage effluent pipe andexposing solid waste from an old municipal landfill in the floodplain.

The study reach, which had been previously straightened during the installationof the effluent pipe, was redesigned with a meandering form with the objectiveof reducing channel slope and dissipating excess energy. The design was based ona 'C'-type channel geometry as given in the Rosgen Classification System. Nobankfull discharge estimate was documented in the design, but a bankfull dischargeof 2.5 m3/s was estimated based on hydraulic modelling results. Graded round stone

25-150 mm in diameter was imported to create the bottom material for the newchannel. Bioengineering treatments, including brush mattresses, live fascines, andvegetated 1og crib walls were applied for the initial stabilization of meander bendswhile vegetation became established. The entire 300 m length of reconstructedchannel was examined during the study.

Observations and survey results suggested that the plan form of the channelhad remained relatively unchanged since construction (Figure 2). However, therewas evidence of significant erosion outside the channel on the inside point barof two meanders. Side-channel gravel bars were observed to have formed on theriffles at these locations where the channel had widened as a result. Aggradationat the upstream end of the reach was suggested by the profile results, primarilyin the first two pools and on the first riffle. Changes to cross-section shape were

generally minor.

Little Etobicoke Creek

The Little Etobicoke Creek study site is located in Applewood Hills Park in the Cityof Mississauga. The site included al,.2kmlength of designed natural channel which

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r'.1

I

bgao

: a d'-" *

EEF;g"t=;E3cF'1'4;3; { E;otH6

i l.t,;ilu) N0rlvn:t:

Figure 1. Plan and Profile Survey Results, Mill Creek.

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1z

(!o Lt-l

"T

OzoJ

(JaZ!3o=

t9rE: 5!:*o

; oI:i q q: +

! -r i --o.' a; i

r.! ;.: d Fqoql!

uou()6

| :-I

Figure 2. Plan and Profile Survey Results, [aurel Creek.

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was constructed tn 1996 following the catastrophic failure of an earlier attemptednatural channel design (Alexander, 1999). The objective of the new design was toprovide a solution that would not fail during high-magnitude flood events as did theprevious one, but one that would maintain natural characteristics.

The basis for the design was a Rosgen type 'Bc' channel, with a design bankfulldischarge of 6.7 m3/s estimated from a field measurement of bankfull capacityupstream ofthe study site. The'Bc'channel is a laterally confined and entrenchedstream type and was selected in part because it would allow construction of thenew channel within the over-large channel excavated during the previous attempt.Graded round stone remaining from the previous channel with an estimatedmedian diameter of 60 mm was used to form the channel boundary. Numerousbioengineering treatments were used, including rootwad revetments at the outsideof channel bends. Rock vortex weirs and rock deflector vanes were also used tomaintain the alignment of the channel thalweg and to protect the banks of thechannel. Because ofthe long constructed channel length, a representative segmentapproximateiy 450 m long in the middle of the reach was selected for detailed study.

Observations and survey measurements indicated that the channel continued tofollow its original course with no major changes in alignment (Figure 3). However,there was evidence of significant erosion and bank failure at the outside of manychannel bends, with partial or complete failure of the rootwad revetments in some

locations. Cross-section surveys showed that the channel appeared to be moreincised and to have steeper banks than specified in the design. The results of thelongitudinal profile survey indicated that while the overall slope ofthe channel hadremained relatively unchanged, significant bed erosion had occurred in pools, inexcess of 0.5 m in some locations.

Groff MillCreek

The Groff Mill Creek study site contained a 300 m long reach that had been

realigned in 1993 using natural channel design. The site is located betweenCoronation Boulevard and the CN Railway in Cambridge, Ontario. Rehabilitationwas initiated to address erosion problems within the study area that had resulted froma combination of urbanization in the upstream drainage area and channelizationwithin the creek. The natural channel works were intended to restore stability to thestudy reach by reducing velocity and sediment transport.

The basis for the design was primarily a Rosgen type 'C' channel. Designbankfull capacity of the channel was not documented but a hydraulic analysisperformed indicated it was approximately 0.7 m3/s. Graded round stone witha median particle size of approximately 125 mm was imported to create rifflebedforms within the channel. Bioengineering treatments were applied to stabilizethe banks while vegetation became established. Bed material for pool sections was

not documented but is assumed to have consisted of the native material exooseddurins excavation.

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O

NteE

a,IJJ

v{NI

oOzJ

?ur'lz:{

ao

oIa

OO+O

O()+O

O-O+O

i:,*:-n

il -i-iij-l

E; R! EE ! !i do;n;:o"hdi

,;o{;iai:iiu-u()o \

%,

nR

(ur) Norrvn!1:

-i'4//

//-': l,' -

Figure 3. Plan and Profile Survey Results, Little Etobicoke Creek.

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Observations and survey results indicated that the overail alignment of the channelthrough the upper 75o/o of the study site had remained essentially unchanged (Figure

4). However, at the downstream end of the site a side channel was observed tohave formed to the south of the main channel, as shown. Other changes includedsignificant channel widening throughout and the formation of mid-channei islands

at two locations. No design specifications for the profiie of the stream were

prepared but comparison of the longitudinal profile survey results with limitedsurvey measurements taken in 1994 suggested the channel bed had aggraded at the

upstream end ofthe reach while one pool near the downstream end had increased indepth.

Henry Sturm Greenway

The Henry Sturm Greenway study site is a 450 m reach of a small creek thatflows south of Resurrection Crescent in Kitchener. The creek was realigned in1994 to accommodate the development of a residential subdivision. Natural channel

design was utilized for the realignment to provide greater ecological and aesthetic

function than traditional channelizatron apptoaches. It was also anticipated thatmaintenance costs for natural channel design would be lower compared to traditionalalternatives.

The design was based on the Rosgen'E' channel type with a design bankfulldischarge of 0.15 m3ls based on field measurements in the channel prior to relocation.Channel boundaries were constructed of graded round stone with an estimated

median diameter of 50 mm. Bioengineering techniques were not extensively used

but the bed material was sized to withstand a 100-year return period flood event.

Because of the highly repetitive meander pattern of the channel, a representative

section 150 m in length was selected for detailed study.Results of the site investigation indicated that the channel alignment was

essentially unchanged from the design plan form (Figure 5). In many locations, the

existing creek was observed to consist of a narrow, relatively deep, inset channel witha wider ufper section. The channel bed was also observed to be nearly covered withfine sediment with the original bed material visible in only a few areas. Longitudinalprofile survey results did not align with the construction drawings, in spite of the use

of MSL referenced benchmarks for the survey. However, the results as well as visualobservations suggested that sediment accumulation has fil1ed in some pools in the

upper section of the channel while pools further downstream are less affected. Some

pools at the downstream end of the study section also showed significant widening.

Summory of Results

Existing channel morphology measurements from the survey results are compared

with design specifications for the study channels in Table 2. For purposes ofcomparison, the existing dimensions were calculated at the original bankfull

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elevation ofthe stream as specified in the design documents. In general, the results

indicated that some degree of morphological change had occurred at all of the studysites. The results suggested increases in bankfull channel width (up to 100%o), depth(up to 34o/o) and cross-sectional area at most locations. An exception to this was the

decrease in pool depth at the Laurel Creek (-19%) and Henry Sturm (-180lo) sites,

which appeared to be due to deposition of fine sediment.

Channel slope was also observed to have increased significantly at most sites

(+37 to +53o/o), and appeared to have been the result of either aggradation at the

upstream end of the measured reaches or degradation at the downstream end. There

was litt1e change in overall slope measured at the Little Etobicoke Creek site (.0027

to .0028), where the large boulders in the vortex weirs appeared to have controlled

the overall grade of the channel in spite of the localized erosion in pools. The size

distribution of bed material had shifted towards smaller particles at all of the study

sites, in general with significantly more fines in pools than in riffles'Hydraulic modelling and stability analysis results for the study sites are

summarized in Table 3. When the design bankfull discharge was modelled, theresults for both existing and design conditions suggest a large difference in shear

stress between riffles and pools at all of the study sites except for Little Etobicoke

Creek. This characteristic appeared to correlate with the fining of pool bed materialrelative to riffles and accumulation of fine sediment in pools, particularly at the

Laurel Creek and Mill Creek sites. Mean shear stress calculated to result fromthe design bankfull discharge was insufficient to transport any of the original bed

material at all of the sites. Existing conditions modelling suggested that some

fraction ofthe existing finer bed material is entrained at all five study channels at the

same discharge, but that the original bed material continues to remain immobile.Discharge data analyses for each study site are summarized in Table 4. Where

discharge data were available, the results indicate that the bankfull discharge values

usedindesignweresignificantlylowerthanthe T.Tro2yearreturnperiodvalues,with the exception of Little Etobicoke Creek. In the case of Mill Creek, no gauge-recorded discharge data were available but data from an upstream reservoir showed

that the dailv average discharge from the reservoir exceeds the bankfull capacity ofthe channel L30/o of the time, indicating a high frequency of overbank flooding.

DISCUSSION

Overall, it was difficult to quantify any channel changes that had occurred at the

study sites because of the poor quality baseline data. In most cases, it was evidentthat the as-constructed form ofthe channels did not match the design specifications,

although these were generally the only baseline information available with whichto compare the existing conditions measurements. In general, it did appear that the

channels had been more or less stable since construction, with little indication ofrapid or catastrophic change at any ofthe study sites.

Hydraulic modelling and stability analysis results indicated that original bed

material at all five sites would have been immobile at the bankfull discharse,

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Iable 4. 5ummary of discharge data analysis.

5tudy Site

Discharge (mr/s)

Design Q,, Q,., Q,., Q,

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and suggested that it would require a major flow event to mobilize the bed. Thischaracteristic is atypical of natural gravel- or cobble-bed watercourses in which bed

particles ranging from the median size to the 90th percentile fraction are entrainedby the bankfull discharge (Andrews, 1983). This property allows the channelto change form in response to the prevailing hydrologic and sediment transPortregimes to achieve a state of dynamic equilibrium, which is inherently stable. Thestability observed at the study sites may therefore be the result of a lack of channelmobility from the large bed material used in construction, rather than success inreplicating the equilibrium channei form. There is evidence at many of the studysites that the immobile main channel is being circumvented suggesting that failuremay occur in the future. The most extreme example is the Groff Mill Creek site

where an entirely new channel has formed at the downstream end of the site.

Other results also cast doubt as to whether the equilibrium geometry had been

successfully designed in the subject channels. Changes to channel cross-sectionalshape, bed configuration, plan form and overall slope were measured at all of the

study sites, suggesting that the channels were undergoing adjustment in spite oftheir partial immobility. Discharge data indicated that at four of the five study sites

the return period of the bankfull discharge was significantly less than one year,

suggesting that the channels were undersized compared to typical natural channels.

MacCrae (1997) provtdes some evidence that the return period of the bankfulldischarge is lower in urban environments than rural ones. However, erosion in out-of-bank areas such as occurred at the Groff Mill Creek and Laurel Creek studysites is likely the result of frequent, aggressive overbank flows. Because the channels

themselves are limited in their ability to adjust and enlarge to these conditions due

to the immobility of the channel bed, additional capacity to accommodate large

flows is being created in the floodplain. This process is similar to that of naturalanastomosing rivers where anabranching occurs because the bed and bank materialare resistant to erosion (Knighton, 1998).

The manner in which the Rosgen classification system was applied in thedesigns for the subject channels suggested further potential for misinterpretation ofthe equilibrium channel form. For example, channel slope was determined indirectlythrough selection ofa sinuosityvalue typical ofthe chosen channel type. Flowever, it

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is widely accepted in fluvial geomorphology that channel slope is an integral aspectof channel sediment transport and equilibrium geometry (Knighton, 1998), whichimplies that it requires significant consideration in design. This oversimplificationin the specification of channel slope 1ike1y contributed to the bed aggradation anderosion observed at many of the sites as the channels adjusted to a more appropriategradient. In addition, morphological parameters were often selected arbitrarily fromwide ranges of values given by the classification system.

The longitudinal configuration of the constructed pool-riffle sequences alsoreceived only superficial treatment in the designs although the geometry of thesestructures is also believed to have major significance in regard to the equilibriumchannel form (Keller and Florsheim ,7993). Hydraulic modelling indicated that thedesign bed profile resulted in significantly greater shear stress in riffles comparedto pools at the bankfull discharge for four of the study sites. These conditions areatypical ofnatural stream channels, where velocity and shear stress tend to becomeequalized over pools and riffles at the bankfull discharge, which prevents them frombecoming fil1ed with fine sediment (Ke11er, 1971,; Lisle, 1979). Cursory treatmentof this aspect of channel morphology may have resulted in the filling, to variousdegrees, of pools with fine sediment at the Laurel Creek, Mill Creek, and HenrySturm Greenway sites.

For all of the study sites, there was no documentation of the use of alternatechannel geometry relationships or analog techniques such as reference reaches toverify the design specifications determined using the Rosgen Classification System.At the time the designs were prepared, literature related to other channel geometryrelationships (Bray, 7982) and analog techniques (Newbury and Gaboury, 7993) hadpreviously been published.

CON(LUSIONS

The objective of the research was to evaluate the performance of a number of naturalchannel designs in southwestern Ontario by examining success in re-establishinggeomorphic function. The results showed that natural channel designs at five studysites had generaily maintained their form for periods of four to seven years, whichsuggests channel stability. However, hydraulic analysis indicated that this mighthave been due in large part to the immobility of the bed and bank material usedin construction. Such channel changes observed to have taken place were generallyatypical ofnatural fluvial process, and often occurred outside the channel itself, suchas the formation of a major side channel at Groff Mill Creek, erosion and failure ofrootwads at Little Etobicoke Creek and channel development within the floodplainat Laurel Creek. Such changes are likely a reflection of both the inability of thechannel to adjust to prevailing conditions because of the large substrate material,as well as unsuitable morphological characteristics. The long-term stability of thedesigns is questionable as ongoing changes circumvent the channel substrate andreinforcing structures and erode much less resistant areas.

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There appeared to be misinterpretation in the selection of channel geometrydesign parameters at all five of the study sites, particularly in the selectionof design bankfull discharge values. Analysis indicated that bankfull dischargewas significantly underestimated for four of the five streams, and significantlyoverestimated in the other, causing the designs to have inappropriate channelcapacity. Furthermore, the application of the Rosgen classification system in designintroduced additional possibility for error in the selection of other morphologicalparameters, which was exacerbated by the use of immobile substrate materials thatrestricted the channels from adjusting to a more suitable capacity and shape. Theseresults suggest that natural channel design in urban watercourses needs to take intoaccount site-specific conditions, rather than using generalized measurements andrelationships that are more applicable to rural rivers and streams.

An important feature of the work was the difficulty in quantifying specificaspects of channel change due to a lack of baseline data. Given the largelyexperimental nature of natural channel design practice, it is necessary to generateappropriate as-constructed data, such as cross-section and longitudinal surveys, onwhich monitoring and evaluation studies can be based. The results of such studiesare critical in evaluating and improving channel design techniques.

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Alexander, LJ.D. 1999. "Little Etobicoke Creek - A Natural Channel DesignCase Study for Urban Streams." In Stream Corridors, Adaptive Management andDesign: Proceedings of tbe Second International Conference on Natural Channel Systems.

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