EVALUATION OF 'NATURAL' CHANNEL DESIGN
APPLICATIONS IN SOUTHWESTERN ONTARIO
A Thesis
Presented to
The Faculty of Graduate Studies
of
The University of Guelph
by
RYAN NESS
In partial fulfillment of requirements
for the degree of
Master of Science
August, 2001
ORyan Ness, 2001 National Library Bibliothèque nationale 1*1 of du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. me Wellington Ottawa ON K1A ON4 Ottawa ON K1A ON4 Canada Canada Yovr fi@ Votre rBfemce
Our fil8 Notre rëfdrence
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EVALUATION OF 'NATURAL' CHANNEL DESIGN APPLICATIONS IN SOUTHWESTERN ONTARIO
Xyan Ness Advisor: University of Guelph, 2001 Dr. D. Joy
.Natural7 channel design is a practice increasingly used in restoration projects to construct watercourse channels that emulate the physical form and function of natural fluvial systems. However, little is known regarding the success of 'natural' channei design in attaining the stability that characterizes natural systems.
A research study was conducted to evaluate the long-term performance of
'natural' channel design applications in five strearns in Southwestern Ontario. The existing condition of the study channels was characterized through qualitative obse~ation,topogaphic surveys, bed material sampling, and numericd modelling. The results were compared with baseline data to assess channel change since construction.
Findings demonstrate that the study channels had maintained a relatively stable form over a period of 4 to 7 years. However, natural physical function was not achieved at any of the study sites because the boundary matenal used to form the channels was too large to permit channel adjustment. There was also that evidence that inconsistencies and errors in design methods may have resulted in channel designs that were incompatible with local conditions. ACKNOWEDGEMENTS
1 would like to thank to my advisor, Dr. Douglas Joy, for the opportunity to
pursue research in this area and for his guidance and advice throughout this project. 1
would dso like to acknowledpe Dr. Hugh Whiteley and Dr. Ray Kostaschuk for their
involvement in my advisory cornmittee and for their timely and thorough review of this
thesis. Thanks also to Matt Walker for his outstanding work as a field assistant.
1 would especially like to thank my parents for their unwavering support of my post-secondary and post-gaduate education. Thanks also to my friends and fellow graduate students who have made the past two years interesting and enjoyable. Sara, thanks for helping me get through the tough times and for showing me how to have fun no rnatter what.
This research was funded by a National Science and Engineering Résearch
Council Post-Graduate Scholarship. Information provided by the Grand River
Conservation Authority, City of Mississauga, City of Cambridge, City of Waterloo, the
City of Kitchener, Hdngton and Hoyle Ltd., and Cumming-Cockburn Ltd., was invaluable in completing this study. TABLE OF CONTENTS
ACKNO WLEDGEMENTS ...... i
... LIST OF TABLES ...... viri
LIST OF FIGURES ...... x
1. 0 INTRODUCTION ...... I
2.0 BACKGROUND AND LITERATURE REVIEW ...... 4
2.1 The fluvial System...... 5
2.2 Channel Process and Response ...... 6
2.3 The Equilibrium Concept ...... II
2.4 The Channel-Forming Discharge ...... 12
2.5 Channel Geometry of Natural Rivers and Strearns ...... 14
2.5.1 Cross-Sectional Form ...... 15
2.5.2 Bed Configuration ...... 17
2.5.3 Plan Forrn ...... 20
2.5.4 Channel Slope ...... 22
2.5.5 Geometry of Southem Ontario Rivers and Streams ...... 23
2.6 Human-Induced Channel Change ...... 24
2.7 'Natural' Channel Design ...... 27
2.7.1 The Rosgen Classification System ...... 32
2.7.2 Results of Natural Channel Design ...... 34
2.8 Modelling in the River Environment ...... 36 .. II 3 .0 PURPOSE AND OBJECTlVES ...... ,...... 3 9
4.0 METHODOLOGY...... 40
4.1 Site SeIection ...... 40
4.2 Site Characterization ...... 42
4.2.1 Preliminary Inspection ...... 42
4.2.2 Topographie Survey ...... -43
4.2.3 Bed Material Sarnpling ...... 45
4.3 Sumey Data Analysis ...... , ...... 46
4.4 Discharge Data ...... 47
4.5 Numerical Modelling ...... 48
4.5.1 Modelling Software Selection ...... , ...... 48
4.5.2 Mode1 Description ...... 50
4.5.3 Cross-section Data...... 51
4.5.4 Roughness Parameters ...... 52 . . 4.5.5 Boundary Conditions ...... ,.,, ...... 53
3.6 S tabili ty Analysis ...... 54
5.0 RESULTS ...... 56
5.1 Laurel Creek ...... 56
5.1.1 Description of Design ...... 57
5.1.2 Preliminary Inspection ...... 60
5.1.3 Topographie Survey ...... 60
5.1.4 Bed Material Sampling...... 65 5.1.5 Hydraulic Modelling ...... 67
5.1.6 Stability Analysis ...... 69
5.1.7 Discharge Data ...... 70
5.1.8 Interpretation of Results ...... 71
5.2 Little Etobicoke Creek ...... 73
Description of Design ...... 74
Preliminary Inspection ...... 76
Topographie Survey ...... 82
Bed Material Sampling...... 84
Hydraulic Modelling ...... 85
S tability Analysis ...... 86
Discharge Data ...... 88
Interpretation of Results ...... 89
5.3 Groff Mill Creek ...... 91
Description of Design ...... 92
Preliminary Inspection ...... 95
Topographie Survey ...... 96
Bed Material Sampling...... 103
Hydraulic Modelling ...... 104
Stability Analysis ...... 106
Discharge data ...... 107
Interpretation of Results ...... 108
5.4 Henry Sturm Green way ...... 110
6.2 Effectiveness of Study Methodology ...... 151
6.2.1 Preliminary Inspection ...... 151
6.2.2 Topographie Survey ...... 152
6.2.3 Sedirnent sampling ...... 153
6.2.4 Hydraulic Modelling ...... 154
6.2.5 Stability Analysis ...... 155
6.2.6 Discharge data ...... 155
7.0 CONCLUSIONS ...... 157
8-0 RECOMMENDATIONS ...... 160
REFERENCES ...... 162
APPENDIX A .Supplementary Data for Laurel Creek ...... 178
A .1 .Bed Material Sampling ...... 179
A.2 - Cross-Section Survey Results ...... 181
APPENDM B .Supplementary Data for Little Etobicoke Creek ...... 184
B .1 .Bed Material Sarnpling ...... 185
B.2 .Cross-Section Survey Results ...... 187
APPENDIX C .Supplementary Data for Groff Mill Creek ...... 190
C .1 - Bed Material S ampling ...... 191
C.2 - Cross-Section Survey Results ...... 193
APPENDM D .Supplementary Data for Henry Sturm Greenway ...... 196 D .1 .Bed Material Sampling ...... 197
D.2 - Cross-Section Survey Results ...... 198
APPENDIX E - Supp1ement;ur Data for Mill Creek ...... 200
E .1 - Bed Material Sarnpling ...... 201
E.2 - Cross-Section Survey Results ...... 203
vii LIST OF TABLES
Table 4.1. Geomorphic indicators of channel change ...... 43
Table 4.2. Cornparison of numerical mode1 types ...... 49
Table 5.3. Laurel Creek - design morphologic parameters ...... 59
Table 5.4. Laurel Creek - surnrnary of topographie survey results ...... 63
Table 5.5. Laurel Creek - mean cross-section characteristics ...... 65
Table 5.6. Laurel Creek - bed material sampling results ...... 66
Table 5.7. Laurel Creek - numerical rnodelling results (Q = 2.5 m3/s)...... 67
Table 5.8. Laurel Creek - results of stability analysis (Q = 2.5 m3/s)...... 69
Table 5.9. Laurel creek - discharge data ...... 70
Table 5.10. Little Etobicoke Creek - design morphologie parameters ...... 75
Table 5.1 1: Little Etobicoke Creek - sumary of topographie survey results ...... 82
Table 5.12. Little Etobicoke Creek - mean cross-section characteristics ...... 84
Table 5.13. Little Etobicoke Creek - bed material sarnpling results ...... 85
Table 5.14. Little Etobicoke Creek - numencd rnodelling results (Q = 6.7 m3/s)...... 86
Table 5.15. Little Etobicoke Creek - stability analysis results (Q = 6.7 m3/s)...... 88
Table 5.16. Little Etobicoke Creek - discharge data ...... 89
Table 5.17. Groff Mill Creek - design morphologie parameters ...... 95
Table 5.18. Groff Mill Creek - summary of topographie survey results ...... -99
Table 5.19. Groff Mill Creek - mean cross-section characteristics ...... 100
Table 5.20. Groff Mill Creek - bed materid sampling results ...... 103
Table 5.2 1: Groff Mill Creek - numerical modelling results (Q = 0.7 m3 /s) ...... 104
Table 5.22. Groff Mill Creek - results of stability analysis (Q = 0.7 m3/s) ...... 107 . . Vlll Table 5.23. Groff Mill Creek . discharge data ...... 108
Table 5.24. Henry Sturm Greenway - design morphologic parameters ...... 113
Table 5.25. Henry Sturm Greenway - sumrnary of topographie survey results ...... 117
Table 5.26. Henry Sturm Greenway - mean cross-section characteristics ...... 119
Table 5.27. Henry Sturm Greenway - bed material sampling results ...... 120
Table 5.28. Henry Sturm Greenway - numerical modelling results (Q = 0.2 m3/s)...... 122
Table 5.29. Henry Sturm Greenway - results of stability analysis (Q = 0.2 m3 Is) ...... 124
Table 5.30. Mill Creek - Design morphologie parameters ...... 128
Table 5.3 1: Mill Creek - topographie sweyresults ...... 133
Table 5.32. Mill Creek - mean cross-section characteristics ...... 134
Table 5.33. Mill Creek - bed material sampling results ...... 135
Table 5.34. Mill Creek - numerical modelling results (Q = 1.8 m 3/s) ...... 138
Table 5.35. Mill Creek .results of stability analysis (Q = 1.8 m3/s) ...... 139 LIST OF FIGURES
Figure 2.1. Idealized fluvial system ...... 5
Figure 2.2. Interrelationslips in the fluvial system ...... 8
Figure 2.3. Scales of channel adjustrnent ...... 9
Figure 2.4. Definition sketch of effective discharge ...... 13
Figure 2.5. Plan and profile view of the pool-riffle sequence ...... 18
Figure 2.6. Definition sketch for meander geometry ...... 21
Figure 2.7. Rosgen Stream Classification System ...... 33
Figure 5.1 : Laurel Creek - key map ...... 58
Figure 5.2. Laurel Creek - detail of study site ...... 58
Figure 5.3. Laurel Creek - typical design cross-sections...... 59
Figure 5.4. Laurel Creek - looking dsat depositional bar near section 4 ...... 61
Figure 5.5. Laurel Creek - looking dsat erosion on inside of meander bend ...... 61
Figure 5.6. Laurel Creek - looking u/s at bank erosion near section 3 ...... ,...... 62
Figure 5.7. Laurel Creek - plan and profile survey results ...... 64
Figure 5.8. Laurel Creek - water surface profiles ...... 68
Figure 5.9. Little Etobicoke Creek .key map ...... 73
Figure 5.10. Little Etobicoke Creek - typical design cross-sections ...... 74
Figure 5.11 : Little Etobicoke Creek - bank near cross-section 20 (Oct . 2000) ...... 77
Figure 5.12. Little Etobicoke Creek - bank near cross-section 20 (May, 1996) ...... 77
Figure 5.13. Little Etobicoke Creek - looking downsr~earnat section 9 (May, 2000) ..... 78
Figure 5.14. Little Etobicoke Creek - Iooking downstream at section 9 (May, 2000) ..... 78
Figure 5.15. Little Etobicoke Creek - depositional bar near cross-section 15 ...... 79 Figure 5.16. Little Etobicoke Creek .plan and profile survey results ...... 8 1
Figure 5.17. Little Etobicoke Cree1 .water surface profiles ...... 87
Figure 5-18: Groff Mill Creek - key map ...... 93
Figure 5.19. Groff Mill Creek - detail of study site ...... 93
Figure 5.20. Groff Mill Creek - typical design cross-sections ...... 94
Figure 5.21. Groff Mill Creek - Iooking u/s at side channel outlet ...... 97
Figure 5.22. Groff Mill Creek - looking upstream at small island, cross section 12 ...... 97
Figure 5.23. Groff Mill Creek - plan and profile survey results ...... 98
Figure 5.24. Groff Mill Creek - monitoring cross-sections 2 and 3 ...... 101
Figure 5.25. Groff Mill Creek - monitoring cross-sections 12 and 13 ...... 102
Figure 5.26. Groff Mill Creek - water surface profiles ...... 105
Figure 5.27. Henry Stum Greenway - key map ...... 111
Figure 5.28. Henry Stum Greenway - detail of study site ...... Ill
Figure 5.29. Henry Sturm Greenway - typical design cross-sections ...... 112
Figure 5.30. Henry Sturm Greenway - looking upstream at pool (Oct., 2000) ...... 215
Figure 5.3 1: Henry Sturm Greenway - looking upstrearn at pool (Dec., 1996) ...... 115
Figure 5.32. Henry Sturm Greenway - plan and profile survey results ...... 116
Figure 5.33. Henry Stum Greenway - water surface profiles ...... 121
Figure 5.34. Mill Creek - key map ...... 126
Figure 5.35. Mill Creek - detail of study site ...... 126
Figure 5.36. Mill Creek- typical design cross-sections ...... 128
Figure 5.37. hlilill Creek - pool at section 15, looking upstrearn ...... 129
Figure 5.38. Mill Creek - fine sediment on point bar at cross-section 9 ...... 130 Figure 5.39. Mill Creek .plan and profile survey results ...... 132
Figure 5.40. Mill Creek - water surface profiles ...... 137
Figure 5.41. Mill Creek - duration-frequency curve for Shades Mill Dam outflow ...... 140
xii -1.0 INTRODUCTION
Over the past few centuries, the accelerated Pace of human population growth and land developrnent has resulted in widespread alteration and degradation of natural river and stream systems. Physical darnage to river channel structure is of particular concern both because of the direct changes it causes to aquatic and riparian ecosystems and because it is ofien connected with reduced water quality. The construction of works for flood control, flow diversion and channel realignment, as well as land-use changes affecting the riparian zone and surroundhg drainage areas, have dismpted the naturd equilibriurn of water and sediment transport in river systems. The resulting instability has caused rapid physical changes in river and strearn channels, which have had adverse impacts on riverine ecosystems as well as human infrastructure and property.
Recent advances in the field of fluvial geomorphology have improved the understanding of the natural stability of river and strearn channels, as well as the effects of environmental change on channel fonn. It is now known that most natural river and strearn channels develop an equilibrium shape that reflects the hydrology and sediment supply of the watershed, which is relatively stable and resilient under naîural conditions but which can be catastrophically impacted by human activities and development. In recognition of these potential impacts, modem watershed management practices incorporate strategies for the maintenance of the natural balance between channel form and watershed processes, through reguiation of land use, storrn water management, and construction sediment control. The design of channel works has also begun to incorporate principles of fluvial geomorphology. Aspects of the fonn of natural river and stream channels are replicated when realigning channels for engineering, development or
1 restoration purposes. The objective of this practice is to create stable, self-maintaining
channels fom, and is often referred to as '"natural' channel design".
'Natural' channel design requires a departure from conventional engineering
practice, as principles of open channel hydrauiics and sediment transport mechanics are not sufficient to predict the appropriate chamel fom. At present, designs are based pnrnarily on empirical geometric relationships from published geomorpholo,oical research. This empiricism introduces significant uncertainty to the design problem, as natural Stream systems are highly variable and published relationships may not
necessarily be applicable to local conditions. As a result, 'natural' channe1 design in its current form is largely a trial-and-error process that depends on experimentation and observation of the results. However, long-term monitoring of constructed 'natural' channels is rarely undertaken because of a lack of funding, lack of trained observers, and a lack of established protocol. Consequently, there is little data available regarding the behaviour of constmcted 'natural' channels and therefore a lack of feedback with which to evaluate and improve design practices.
This thesis details the activities completed in a research study to address some of the deficiencies in knowledge regarding the results of 'natural' channel design. For the study, a number of locations in Southern Ontario where 'natural' channel design had been applied to strearn channels were revisited to examine their current state. Detailed measurements of the existing channel form were performed, and the results were compared with baseline data to identify changes and interpret channel stability.
Numerical modelling was performed to provide additional insight into the hydraulic and sediment transport characteristics of design channel forrn and existing channel conditions. A thorough literature review is also presented which outlines curent knowledge of forms and processes in natural channels and its application to 'natural' channel design, and highlights the need for irnprovement of design practices and validation of design methods through long-tenn evaluation and monitoring. -2.0 BACKGROUND AND LITERATURE REVIEW
Natural rivers are dynarnic systems whose characteristics Vary on a variety of temporal and spatial scales. The form of a river channel at any time and any location is controlled by the climate, geology, and physiography of the upstream watershed area, as these properties determin hydrologie characteristics and the quantity and type of sediment that is supplied. The resulting interactions between hydraulics and sediment transport within the channel create a complex morphology consisting of three-dimensional forms and structures. The link between the formative processes of water and sediment transport with channel morphology is the focus of the field of fluvial geomorphology. Research in this area has improved the ability to predict channel changes, particularly those that cm potentiaily impact human populations and property. Fiuvial geomorphology has 'also helped to explain the variety of problerns caused by traditional engineering approaches to river management.
'Natural' channel design, although stiIl in early stages of development, is potentially the most comprehensive practical application of lmowledge from fluvial pomorphology. To successfülly emulate natural channel form, the interrelationships between watershed properties, in-channel processes, and channel morphology must be understood. The following discussion is intended to present a review of current understanding in fluvial geomorphology of these processes and their application to
'natural' channel design. -2.1 The Fluvial System
The watershed, or drainage basin, is generally described as the fundamental spatial unit of the fluvial system (Chorley, 1969; Gregory and Walling, 1973). Schumm
(1977) proposed a conceptual mode1 for the drainage basin that is widely accepted in geomorphology. In it, the watershed is divided into three zones: (i) the 'production zone', (ii) the 'transfer zone', and (iii) the 'deposition zone' (Figure 2.1).
WATERSHED PRODUCTION SEDIMENT SOURCE ZONE I AREA
TRANSFER ZONE 2
DOWNSTREAM. CONTROLS OEPQSfTlON (bau )crnl, diortroghnm) ZONE 3 SEDIMENT SSNK DEPOSITIONAL AREA
Figure 2.1: Idealized fluvial system (afier Schumm, 1977) The 'production zone' is the upland area of the drainage basin where the majority of the water and sediment mass that enters the fluvial system originates. Up to 50% to
80% of the total sediment mass has been shown to originate in this zone (Yu and
Wolrnan, 1986; PLUARG, 1977), with sirnilar proportions of surface water runoff
(MacCrae, 1997). In peneral, the 'production zone' bas steeper slopes and close proximity between hillslopes and channels, which facilitates movement of sediment from the Iand surface into the drainage system.
The 'transfer zone' is situated further downstream where the land slope is more gradua1 and the channel floodplain is wider. Much of the sedirnent from the 'production zone' is stored temporarily in this region through deposition on the floodplain or on in- channel bars and is episodically re-mobilized by high flows and by channel migration.
Depending on the characteristics of the drainage basin, 15% to 65% of the material transported to the 'deposition zone' may be re-mobilized material originating from the bed and banks of the charme1 (PLUARG, 1977; Griffidis, 1979).
The 'deposition zone' is near the river mouth, where flow velocities decrease and the ability of the river to transport sediment is reduced, and is generally characterized by the formation of an alluvial fan or delta. This may also occur within a lacustrine environment, such as a reservoir or lake.
-2.2 Channel Process and Response
Rivers are often conceptualized as open thermodynamic systems that exchange matter and energy with their surroundings (e.g. Leopold and Langbein, 1962). As such, nvers respond to changes in extemal, independent controls by adjusting their intemal, dependent channel characteristics. Ultimately, the form of the channel is controlled by the characteristics of the drainage basin: climate, geology, and physiography. The effects
of these variables are reflected in the discharge and sediment load of the system, which
are the dominant controls of channel form. In addition to these dominant controls, valley
slope, which detemiines the overall rate at which energy is dissipated dong the channel,
and vegetation and basin soi1 types, which determine the stability of channel banks, act as
modifying factors on the channel forrn. Land use change as a result of human activities
rnay also modiQ the basin hydrology and sediment loading by changing drainage paths
and increasing imperviousness. The dependent variables of the channel system are the
channel geometry and hydraulic characteristics, which are linked through complex
interrelationships and at any point in time and space reflect the integrated effect of the
independent controls. The relationships between drainage basin characteristics,
independent channel controls, and dependent channel variabIes in the fluvial system are
illustrated in Figure 2.2.
In alluvial channels, channel geometry is essentially constant at short time scdes
( in response to short-term changes in channel discharge (Schumm and Lichty, 1965; Cullingford et al., 1980). At longer tirne scales, the channel geometry adjusts to prevailing conditions of discharge and sediment transport in the drainage basin. This adjustment of channel geometry can be considered in terms of four different aspects of channel form, or degrees of freedom (Knighton, 1998): 1. Cross-sectional fom - the widdi, depth and shape of the channel cross-section. 2. Bed con.guration - characteristic periodic forms found in sand- and gravel-bed streams, such as the pool-riffle sequence. t INDEPENDENT BASIN 11-11 CONTROLS i pq1 f\ v 1 t i 1 1 I INDEPENDENT VALLEY STREAM IX 0ANK MATERIAL COMPQSJTION SLOPE DISCHAROE, O AND STRENOTH, B CHANNEL J----.) CONTROLS MATERIAL ' l-.-- IZE, O 1 SEDIMENT --1--- TRANSPORT RATE DEPENDENT t CHANNEL AND MEANDER WAVELENOTH A RATIO FLOW GEOMElRY Figure 2.2: Interrelationships in the fluvial system (from Knighton, 1998) 3. Plan fom - the form of the channel as seen from above, including characteristics such as meandering or braiding. 4. Bed dope - the gradient of the Stream at a reach level as weil as the overall gradient and shape of the strearn profile. Channel adjustments in response to changes in discharge or sediment load occur over a wide range of temporal and spatial scales (Figure 2.3). In general, the channel cross-sectional form is most sensitive to change, while the longitudinal profile is the most stable. Because of these differences in response tirne, the fom of the different aspects of channel geometry rnay not always reflect the same characteristics of the independent con trols. 10' - PROFILE 10'- IOmi E ui A 2 102- A MEANOER , 4 $ WAVELENGTH * E - Pian a BE0 CONFIGURATION: + '- 2GRAVEL-BU) STREAMS Figure 2.3: Scales of channel adjustrnent (from Knighton, 1998) The nature of the adjustment of channel geometxy in response to changes in discharge and sediment load was initially documented by Lane (1955), who related mean annual discharge (Q,) and channel slope (S), to bed matenal sediment load (es)and median bed sediment size (Dso): Lane's relationsbip suggests that changes in discharge and sediment load are balanced by changes in channel gradient or sediment size. Schumrn (1977) developed the following extended interpretation of channel geometry adjustments to include cross- sectional and plan forrn charactenstics. He suggested that width (w),depth (6) and meander wavelength (Â) are directly proportional, and channel dope is inversely proportional to discharge: Schumrn (1977) aiso suggested that width, meander wavelength, and channel slope are directly proportional, and depth and sinuosity are inverseIy proportional to bed material sediment load: These relationships are often used in fluvial geomorphology and engineering practice to qualitatively predict the direction of channel change in response to increases or decreases in discharge or sediment supply. -2.3 The Equilibrium Concept The concept of equilibrium, as it peïtains to alluvial channels, originated from the observation of imgation canals which evolved a channe1 form, over time, such that imposed discharge and sediment suppIy were conveyed without erosion or aggradation (Lindley, 1919). Such canals were said to be in 'regime', and regime theory was developed, consisting of a set of empirical equations relating the width, depth and slope of such canals to the design discharge (e.2. Lacey, 1929). Unlike imgation canals, discharge and sediment load in natural alluvial strearns is highly variable. As a result, sediment is perpetually stored and remobilized in the channel as it is transported from areas of production to areas of deposition. Changing patterns of erosion and aggradation cause the channel to constantly shifi. Despite this movement, channels have been observed to maintain consistent average geometric characteristics over short to interrnediate time scales if the controlling variables remain relatively unchanged. To explain this aspect of fluvial behaviour, geomorphologists theorized that natural Stream channels also evolve an equilibniim or 'regime' form (Mackin, 1948; Leopold and Maddock, 1953; Leopold and Wolman, 1957). The term 'dynamic equilibrium' was cohed to describe the short-tenn variability that is present within the longer-term stable form (Hack, 1960). The concept of dynarnic equilibrium may be illustrated by exarnining temporal variability in channel width, which has been observed in natural rivers to vary continually in the short-tem with changes in discharge and sediment load. At an intemediate time scale where the effect of short-tem fluctuations in the independent variables is averaged out, the mean channel width remains roughIy constant. This mean state rnaintained by negative feedback mechanisms which regulate the effect of disturbances and rninirnize the geometric adjustment of the channel (Richards, 1982). For exarnple, in response to a short-term increase of frequency of high ma,pitude flows, the width of the channel cross- section will increase to reduce channel velocities at higher flows and limit excessive erosion. Analysis of equilibrium in natural strearn channels requires the distinction between short-term adjustments and long-tenn evolutionary behaviour (Schumrn and Lichty, 1965). This is complicated by the fact that different aspects of channel geornetry adjust at different rates (Figure 2.3), and that potentid for adjustrnent differs between systems, based on the scale and the resistance of the material in which the channel is formed (Knighton, 1998). It is therefore unlikely that the equilibrium forrn is actually achieved for my significant lenad of time, but it remains ~i~pificantas an average condition about which the channel properties fluctuate. -2.4 The Channel-Forming Discharge Because of the short-term variability in discharge and sediment load in natural strearn systems, it is diffxcult to relate these variables to characteristics of the equilibrium channel form. The concept of a channel-forming or dominant discharge, was developed to produce a charactenstic discharge value to which the equilibrium channel geometry could be related, as in regime theory for canals where discharge is constant. The basis of the concept is that the total sediment transport over a time interval with a particular discharge is the product of the magnitude of transport by the discharge and the discharge frequency (Wolman and Miller, 1960). The discharge responsible for the equilibrium channel form is therefore the flow that is effective in transporting the most sediment, or the efféctive discharge. Although this relation was originally developed for sand-bed channels in which suspended sediment load is dominant, it has aIso been shown to apply to gravel-bed channels with primarily bed load transport (Andrews, 1980). Figure 2.4 illustrates the form of the relationship for gravel-bed channels. The discharge that is most effective in transporting sediment is of some intemediate magnitude and frequency, since while larger magnitude flows may transport more sediment per event, they occur too infrequently to significantly affect long-tenn equilibrium channel geometry. 1 Discharge 1 Figure 2.4: Definition sketch for effective discharge (from Wolman and MilIer, 1960) The effective discharge is often equated with the bankfull discharge, which is the discharge that just fills the channel cross-section before overtopping the banks. This is 13 based on the assumption that the channel-forrning discharge should correspond with the shape of the channel. A number of studies have verified that the frequency of the effective discharge in stable natural channels is very similar to the frequency of the bankfull discharge (Wolman and Leopold, 1957, Wolman and Miller 1960, Andrews 1980; Andrews and Nankervis, 1995). Researchers have observed that the recurrence intervd for the effective and bankfull discharges is most often between 1 and 2 years (e.g. Wolman and Miller, 1960; Richards, 1982; Kellerhals and Church, 1989; Hey, 1998). For the majority of rivers and streams in hurnid, temperate areas, the bankfull discharge is a usefbl means of replacing the frequency distribution of channel discharges with a single value that can be used in modelling and analysis. However, it has been argued that the concept of a single channel-forrning discharge or bankfbll discharge is not applicable to certain cases, such as nvers in arid areas that expenence infkequent, high- magnitude flooding (Graf, 1988), incised channels (FISRWG, 1999), and uban strearns (MacCrae, 1997). -2.5 Channel Geometry of Natural Rivers and Streams The channel geometry of natural rivers and strearns is three-dimensional and can be described by four degrees of freedom: cross-sectiond form, bed configuration, plan forrn, and bed slope. Nthough these parameters vary at different spatial and temporal scales (Figure 2.2), they are interrelated in the adjustment of channel form to changes in the independent variables of the fluvial system, as dernonstrated in (2) and (3) above. The following is a discussion of the characteristics of these four aspects of channel forrn as they occur in natural rivers and streams. For the purposes of this thesis, the discussion is limited to single-thread grave1 bed channels, which are characteristic of the streams at the research study sites as well as the majority of rivers and streams in Southern Ontario (Ashrnore and Church, 2001). The geometric characteristics of Southern Ontario streams are also discussed. 2.5.1 Cross-Sectional Form Histoncally, geomorphologists focused on the relationship of natural river slope to discharges and sediment load (e.g Mackin, 1948), although hydraulic engineers were aware that cross-sectional characteristics were critical to the design of irrigation canals. More recently, it ha. been recognized that natural river channels experience cross- sectiond adjustment in response to shorter-term changes in independent variables to maintain continuity in sediment transport. The relationship between cross-section geometry and independent variables has traditionally been explored by statistical analysis, similar to regime theory for canals, which relates cross-sectional width (w) and depth (d) to discharge (Q) by single-variable power functions (e.g. Leopold and Maddock, 1953): Where a and c are coefficients, and b and f are exponents obtained from logarithmic regession analysis of field observations. The value for discharge used is the bankfull value (abf), or the flow of a paaicular return period n as a surrogate (Q,). The results of most studies show that channe1 width and depth are strongly correlated with 15 bankfull discharge over a wide range of conditions (e.g. Bmsh, 1961; Kellerhals, 1967; Bray, 2982; Andrews, 1984; Hey and Thome, 1986). Values of the exponents b and f are constrained in very narrow ranges with average vzlues of about 0.5 and 0.36, respectively. The coefficients a and c are less consistent, which reflects the influence of other independent variables on channel width and depth. There is evidence, however, that a value for the coefficient a of 4.5 for channel bankfull width is consistent across rnany orders of maahtude (Kellerhals and Church, 1989) Hey and Thome (1986) conducted extensive multivariate analysis on a large number of gravel-bed rivers to examine the influence on channel form of independent variables in addition to discharge such as sediment load, bank strenogh, and sediment size. They concluded that while bankfull width was primarily related to discharge, it was also affected by the density of bank vegetation. MuItivariate analyses have also shown that the cohesiveness of bank material, which also affects channel bank strength, influences cross-section width and depth (Schurnm, 197 l), and that depth is also affected by the particle size of bed sediment (Kellerhals, 1967; Hey and Thorne, 1986). The effects of independent variables on cross-section form have also been exarnined through analytical models for channel geornetry (see dso section 2.7). These utilize principles of hydraulics and sediment transport mechanics to predict the cross- section shape and slope of the equilibrium channel. Theoretical models fa11 into two groups: mechanistic approaches that describe the distribution of shear stress on the channel cross-section (e-g. Parker, 1978; Ikeda and Izumi, 1990), and extremal hypotheses approaches based on thermodynamic theory (e.g. Chang, 1988; Millar and Quick, 1992). The results of these models are in general agreement with empirical analysis, showing that cross-sectional dimensions are strongly correlatecl with bankfull discharge, bank strength, and sediment size. Proportionality exponents relating discharge to width and depth in theoretical models are very sirnilar to values of b and f obtained by ernpirical research. 2.5.2 Bed Confiauration Non-uniform flow in natural channels causes complex patterns of erosion 2nd deposition, resulting in bed irreplarities, which in turn affect flow patterns. In gravel-bed rivers, these interactions result in the accumulation of coarse sediment at semi-reglar intervals, creating a self-maintaining bed structure of altemating deeps and shallows known as the pool-riffle sequence. The pool-xiffle sequence is closely related to the plan form of the channel, with pools often occurring at the apex of meanders, and riffles at the inflection points between successive bends, although regular pools and riffies are also observed in straight channels (Richards, 1976). Riffle cross-sections tend to be wider and shallower, while pools are relatively deep and are generally asymmetrical (Fig 2.5). An oft-quoted geomorphological relationship is the occurrence of riffles at a consistent spacing of between Sand 7 bankfull channel widths (Leopold et al, 1964) although more recent studies indicate that a range of 4 to IO channel widths may be more appropriate (Keller and Melhorn, 1978; Hey and T'home, 1986). While the exact mechanism for the formation of rimes and pools is unknown, many researchers believe that consistent patterns of scour and deposition may be created by turbulent flow structures in open channel flow that have a spacing of 2x times the channel width, as theorized by Yalin (1971, 1992). The three-dimensional character of these flow patterns is also thought to be responsible for the asymmetry that is observed at pool cross-sections (Thorne, 1997). From a data set encompassing a large number of gravel-bed rivers in the United Kingdom, Hey and Thorne (1986) observed an average rime spacing of 6.3 1 channel widths. PLAN VlEW Poo/ Rmie secriori Pool section I Figure 2.5: Plan and profile view of the pool-riffle sequence (fiom Chang, 1988) Because the pool-riffle sequence is a stable, regular structure, it believed to be an important component of equilibnum channel geometry (Knighton, 1998). However, the mechanism for the maintenance, once forrned, of a stable pool-riffle structure is not well understood. It has been obser-ved that, at Iower channel discharges, the water over riffles has a higher velocity with a steep water surface slope. Meanwhile, flow in pools is slow and flat, the result of a backwater effect created by the bed elevation increase at the next 18 downstream nffle (Leopold et al., 1964). This creates a depositional environment in pools and as a result sediment particle sizes are generally much smaller than in riffles (Keller, 197 1). At discharges approaching the banlcf~~lllevel, the hydraulic control at the downstream riffle becomes "drowned out", and the slope of the water surface may become nearly uniform. Some researchers have hypothesized that at higher discharges, velocity and sediment transport competence may actually become reversed, which scours the pools and prevent them from being filled with sediment (Keller, 1971; Lisle, 1979). Attempts to verify this phenornenon in the field have shown that properties indicative of sediment transport capacity such as average velocity, near-bed velocity, bed shear stress, and Stream power tend to converge in rlffles and pools at bankfull flow. However, no actual reversal could be observed (Keller and Florsheim, 1993). It has been suggested that sediment in pools is more loosely structured and is more easily mobiiized than on rimes, which may explain the maintenance of the pool shape by erosion (Sear, 1996). Another possible explanation is that velocity and shear stress may be concentrated in the deep part of the asymmetrical pool section because of two- or three-dimensional fiow characteristics. Limited information has been pubIished regarding the physical configuration of individual riffles and pools. Generally, models for channel geometry descnbe the average cross-sectional geometry of the reach, and do not describe differences between pools and riffles. Hey and Thorne (1986) reported data on the variability of width and depth between nffles and pools, while other relationships have been presented for maximum pool depth in meandering alluvial streams based on bend characteristics (e.g. Apmann, 1972). From an analysis of Southern Ontario strearns, Amable (1995) determined that riffle slope was approximately twice that of the overall channel gradient, and that the length of riffles was weakly correlated to channel width. 2.5.3 Plan Form The majority of strearns, particularly those in areas of low physiographic gradient, tend to flow in a winding, sinuous course, referred to as meandering. III general, the meandering plan fom is believed to be an inherent property of free-surface, open ctiannel flow, rather than a passive response to environmental factors such as topographic or geologic conditions (Richards, 1982). This is based on observation of the formation of regular, syrnrnetrical meanders in natural rivers and in other systems, such as laboratory flumes, supraglacial streams and the flow patterns of the Gulf Stream (Leopold et al., 1964). The geometry of meanders is generally described in ternis of their wavelength (4 and bend radius of curvature (Rd as illustrated on Figure 2.6. In nature, river meanders tend to be irreplar because of heterogeneous surfkial geology, which causes the channel banks to be more resistant to erosion in some locations than others (Richards, 1982). However, results from numerous studies (e.g. Leopold and Wolman, 1957; Hey, 1976, Williams, 1986) have shown that average values of meander wavelene@ and radius of curvature are strongly related to bankfull channel width, with dimensions of between 10- 14 and 2-3 channel widths, respectively. Richards (1982) observed that a linear relationship between meander wavelength and channel width could be defined by: where the coefficient (12.34) is nearly equal to 4n. This, in addition to the ofien coincident location of pools and bend apices, has been taken to indicate that the periodic flow structures that may be responsible for the formation of the pool-riffle sequence are also significant in the formation of meanders. Figure 2.6: Definition sketch for meander geometry Meanders are formed and maintained by two- and three-dimensional flow patterns. In meanders, the flow is helical, the result of two opposing lateral forces: centrifuga1 force that acts to push the flow outwards and pressure gradient force acting inwards because of superelevation of the water surface on the outside of the bend (Knighton, 1998). This creates an inward-flowing current at the bed, and an outward- flowing current at the surface, which concentrates the flow velocity and shear stress at the outside of the bend. The location of maximum shear stress and erosive force is located just downstrearn of the bend apex, where the energy gradient is locally steep because of the superelevation of the water surface at the outside of the bend. As a result, erosive 21 forces are concentrated downstream of the bend apex, which causes meanders to "migrate" outwards and downstream (Richards, 1982). Velocity and shear stress are at a minimum at the inside of the bend, often allowing the formation of a point bar through deposition (Figure 2.5). The relationship between channel width and radius of curvature has been related to the experimental work of Bagnold (1960) which indicated that energy losses in curved open-channel flow were minimized at a radius of curvature: width ratio between 2 and 3. The lateral (dom-valley) migration rate of natural meandering rivers is also rnaximized at a RJw ratio of 2 to 3 (Hickin and Nanson, 1975, 1984). It is therefore thought that meanders are formed with this curvature to maxirnize hydraulic and sediment transport efficiency. When the radius of curvature is outside this range in natural channels, the helical flow pattern is underdeveloped or disrupted leading to non-uniform migration and creation of irregularities in pIan forrn (Hooke and Le, 1975; Chang, 1988). 2.5.4 Channel Slope The overall longitudind profile of a naturd river is the generatly the least transient aspect of channel geometry. Changes in slope in response to short-tenn changes in independent variables are accomplished at the reach scale by plan form adjustments. Consecutive reach dopes make up the overall longitudinal profüe of the fluvial system, the shape of which is adjusted to maintain a long-term equilibrium sediment transport rate. The shape of the longitudinal river profile tends to be concave upward, with slope decreasing exponentially from the production zone to the deposition zone. This form has been atûibuted to a combination of increasing discharge and decreasing bed matenal size through hydraulic sorting and/or abrasion in the downstream direction. As a result, the slope required to achieve sediment transport decreases in downstream areas of the system. This theory is supported by empirical analysis which has shown that channel slope is positively correlated with sediment size and negatively correlated with discharge (e.g. Kellerhals, 1967; Bray, 1982; Hey and Thorne, 1986). Changes to the channel gradient often occur in response to watershed changes that affect the continuity of sediment transport. Bedload starvation in upstreâm watershed areas rnay result in downstrearn-progressing degradation of the bed whiIe excess bed load inputs will cause aggradation. Upstream-progressing degradation can also occur in the form of a knickpoint, which is the result of lowering of the base level at the body of water to which the channel outlets. In general, changes in channel gradient are limited to dramatic or long-term changes in the independent variables. Most of the adjustment required to maintain equilibrium is accomplished in other aspects of channel geometry which respond more rapidly (Leopold and Bufl, 1979). 2.5.5 Geornetrv ofSouthem Ontario Rivers and Strearns Annable (1995) performed the most comprehensive investigation of rivers and Stream morphology in Southern Ontario. Ln this study, he measured rnorphological characteristics at representative locations on 47 rivers and streams throughout the region. The data was analyzed statistically to identiQ relationships between geometric parameters and independent variables. In general, Annable (1995) found that the form of traditional empincal relationships (e-g. (4), (3, and (6))could be applied to the results with good statistical sipificance. For example, it was determined that bankm1 channel width could be related to bankfull discharge according to (4), with a coefficient a of 3.71 and m exponent b of 0.49. Other results included a mean radius of curvature to width ratio of approximately 3 and a mean pool-riffle spacing of 4.8 channel widths. It has been argued that classical empirical relationships, which have primarily been developed for alluvial channels, are not applicable in Southern Ontario. Southern Ontario streams have been described as 'serni-alluvial', meaning that the morphology is largely controlled by the nature of glacial deposits through which most streams flow, rather than the characteristics of the discharge regime and the sediment supply from upstream (Ashmore and Church, 2001). Foster (1999) observed that, while the morphology of semi-alluvial streams is superficially like that of alluvial channels, glacial materiai, such as clay till and coarse outwash grave1 in the bed and banks restricts channel adjustment. In a detailed study of three such streams, Foster (1999) observed highly inegular pool-riffie pacing, erratic meandering, and a lack of correspondence between pools and channel bends. Further research is Iikely required to understand the significance of semi-alluvial characteristics on the morphology Southem Ontario streams before empirical relationships can be confidently applied. -2.6 Hurnan-Induced Channel Change Throughout historic times, human activities have had a profound effect on river and Stream systems, particularly in the last few centuries with the onset of urban development on a massive scale. The magnitude of the change that humans have inflicted on rivers and streams has been equated to the effect of a large-scale climatic shift (Knighton, 1998). Human-induced changes cm be classified into two types: direct changes to the channel geometry or flow regime caused by activities within the channel, or indirect changes to the hydrologie and sediment regime brought about by human activities in the surrounding watershed. Direct changes to rivers by humans are the result either of regdation of flow by damming associated wiîh hydropower generation, flood regulation, and water supply, or by modification of the channel geometry by channelization. In North America, over 20% of total water runoff is regulated by dams (Knighton, 1998). Dam construction generally results in accumulation of sediment upstream in the created reservoir and degradation of the channel bed downstream because of bedload starvation. Dams and flow regulation dso tend to reduce the ma- tud de of flood peaks, which results in a reduction of channel width and overall channel capacity. Channelization is undertaken for flood control, improved drainage, maintenance of navigation routes and erosion control. These include straightening and enlarging channels, bank hardening, and levee construction. It is estimated that between 1930 and 1980, 8500 km of channelization works were undertaken in England in Wales, and 26,500 km in the United States (Brookes, 1985). Channel straightening efiminates meanders, which increases the overall gradient of the Stream. This increases flow velocity and sediment transport capacity, which causes degradation that progresses upstream as a knickpoint, and aggradation of the eroded sediment downstream. Degradation may dso increase the height of the banks until they collapse into the channeI, further increasing the downstream sedirnent supply. Structures such as gabion baskets or amour Stone are often used to try to harden the eroding banks, but this tends to hrther concentrate erosive forces on the bottom of the bed, causing the structures to be undermined and to eventually fail (Knighton, 1998). Indirect changes to fluvial systems are the result of land-use change resulting from human activities and developrnent, the most significant which have been deforestation and urbanization. The process of deforestation and land clearing for agriculture and other hurnan development has been occumng for thousands of years with large-scale changes over the last few centuries. The rernoval of vegetative cover causes an increase in hillslope erosion and increases sediment supply, and also increases runoff volumes and peak discharges by decreasing flow resistance, infiltration, and evapotranspiration. In general, the increase in sediment yield is more significant than the i~creasein runoff (Richards, 1982) and channels tend to become wider, shallower and less sinuous. The removal of riparian vegetation further reduces stream bank strength and increases in-channel flow velocities, which fürther promotes channel widening. Although urbanization is a more localized phenornenon than deforestation, its effects are more hydrologically extreme. Urban development is associated with the creation of extensive impervious areas, which significantly reduce overall infiltration and depression storage and increase overiand flow velocities. Artificial drainage systems and stom sewers kealso constructed that transport runoff much more quickly than natural river and stream channels. The overall effect of these changes is to create a flashier discharge regime where flood hydrographs have shorter lag times and the bases, and higher overall peaks. Flood flows with shorter return periods are more affected than extreme events. Hollis (1975) showed that after 20% urbanization in a watershed, the 1- year event was increased by a factor of ten and the 2-year event by a factor of 2 or 3, while the effect on extreme events was minimal. This is likely due to the more pronounced difference in infiltration capacity between developed and undeveloped surfaces during frequent events, compared with Iarger events where the infiltration capacity of undeveloped soi1 surfaces is reduced because of saturation. Watershed sediment yield is reported to be elevated by 2 to 200 times dunng construction (Richards, 1982). but may decIine to below natural levels once urban development is complete (Wolrnan, 1967). In urban river channels, this results in a penod of fine sedirnent deposition on both the channel bed and the flood plain, followed by enlargement of the channel cross-section to accommodate the increased flood flows (Leopold, 1973). Cross-sectional channel area in urbanized watersheds has been found to be up to about 6 times greater than that of comparable rural channels (Hammer, 1972; Roberts, 1989). The individual contributions of width and depth changes to increased cross-sectional area tend to Vary between channels depending on the relative resistance to erosion of the bed and banks. However, deep incision of the channel, ofien through the fine sediment deposited during the construction phase, is a commonly observed lori,- terrn adjustment pattern in urban strearns (Graf, 1975; Booth, 1990). -2.7 'Natural' Channel Design In the past few decades, there has been an increasing recognition of the importance of natural fom on river channel stability. This has resulted from observation of the results of channelization practices, as well as from failure of river and Stream rehabilitation projects that were not consistent with natural Stream processes (e.g. Holmes, 1991; Fnssel and Nawa, 1992). In addition, it has been acknowledged that the morphology and physical function of natural channeis is fundamentally linked with aquatic and riparian habitat and ecosystern health (Statzner et al., 1988; Gregory et al,, 199 1). Consequently, principles of fluvial geomorphology are increasingly incorporated into the design of contemporary river engineering and rehabilitation projects. One of the most comprehensive applications of geomorphic principles is the construction or reconstruction of channels with natural forrn, ofien terrned 'naturd' channe1 design. The intent of 'natural' channel design is to establish geomorpholo$c function in the constnicted channel, so that it will display the stability and resilience of a natural system. It is prirnarily used in the restoration of disturbed rivers or streams (cg- Harrington-Hoyle, 1996; Rosgen, 1997), but is also applied in construction and developrnent projects requiring channel relocation, and in channel realignment to protect property and infrastructure from erosion damage (e.g. Newbury and Gaboury, 1993, Cumming-Cockbum, 1993). The most significant challenge in 'natural' channel design is the determination of appropriate channel morphology, including cross-section geometry, bed configuration, plan form, and overall longitudinal slope. The overall objective is to replicate as closely as possible the equilibrium state which is representative of the prevailing conditions within the watershed. Currently, there is no standardized procedure for detemiinhg appropriate morphologie characteristics in natural channel design. The methods that are used are dependent on the designer, regdatory requirements, and the data that is available. In general, the following four methods are used in design: 1. Reproduction of historical channel morphology (e.g. Brookes, 1987, 1993, 2. Replication of morphological characteristics from reference reaches elsewhere in the Stream of interest or in nearby rivers or strems (e-g.Newbury and Gaboury, 1993), 3. Application of empirical models of equilibrium channel geometry (e-g. Hey and Thorne, 1986; Hey, 1997), and 4. Use of analytical channel geometry models based on equilibriurn transport of sediment through the channel (e.g Chang, 1988; Copeland, 1994) Replication of historical morphoIogy as detemined from maps, air photo analysis, geomorphologic evidence or other data, if available, is the most straightforward approach for design. The channel form predicted rnay be very stable if watershed conditions have not changed significantly. However, the hydrologic and sedirnent regime associated with the historical geornetry may change as a result of development, and the current conditions may be incompatible with the old channel foim (Kondolf and Larson, 1995). Therefore, use of this method generally requires land use to have remained relatively constant over the period of record from which histoncal information is used. The use of reference reaches is based on the assumption that undisturbed reaches of a Stream or in other strearns nearby exhibit geornetric characteristics that reflect the, prevailing independent variables. Morphologie characteristics are measured on the reference reach and are scaled and applied to the design for the disturbed channef. This technique requires a high degree of similarity in hydrologic and sedirnent inputs, physiography, and surficial geology between the project reach of interest and the reference reach. The use of reference reaches in the same river or Stream can reduce uncertainty with the use of this technique (Newbury and Gaboury, 1993). However, appropriate reference reaches are often difficult to locate, particularly in disturbed urban environments. Empirical relationships describing channel geometry (section 2.6), are often applied to estimate the equilibrium geometry in natural channel design. Cross-sectional forrn and channel slope are calculated using relationships (e.g. Bray, 1982; Hey and Thome, 1986) that relate these quantities to independent variables. Pool-rime spacing and meander geometry, for example, are determined based on established proportionality to channel width. The use of empincal relationships involves uncertainty because the regression parameters in the published results are highly variable. The use of relationships developed for regions with similar physiographic and hydrologic conditions is recommended to improve the quality of the results (Shields, 1996; FXSRWG, 1999) aIthough these are not always available, particularly for urbanized areas. Relationships can be developed for the region of a study Stream, but such an undertaking is usually beyond the scope of most natural channel design projects. Another technique using empiricd relationships is based on the work of Rosgen (1994, 1996), and is described in more detail below. Analytical models for equilibriurn channel geometry are based on traditional tractive stress (zero bed movement) charme1 design techniques (Glover and Florey, 1951 ; Lane, 1955) with refinements to account for sediment transport and other characteristics of natural streams (see also Section 2.5.1). Analytical methods account for the additional degree of freedom presented by sediment transport by assuming characteristics of the shear stress distribution (Parker, 1978; Ikeda and Izumi, 1990), or extrema1 hypotheses postulating some condition of energy dissipation in the channel (Bettess and White, 1987; Chang, 1988; Millar and Quick, 1998). Analytical models have produced some inuiguing results, but they are not generally considered any more reliable in their current form than purely empirical approaches because of assumptions and simplifications that are made in their formulation (Shields, 1996; Hey 1997). In particular, there are significant difficulties in accounting for bank vegetation and heterogeneous and cohesive bank materiais. As well, because there is currently no analytical formulation for equilibrium channel bed configuration or plan form, rmpirical relationships must still be used to specify these aspects of the morphology. Both empirical relationships and analytical models relate charinel geometry to a single dominant or effective discharge. The detexmination of an appropriate design discharge is therefore a critical part of the NCD process. The most appropriate measure is the bankfull discharge as determined by field measurement (Hey, 1998). However, in disturbed strearn systems, particularly in urban in environments, a stable bankfuil channel rnay not exist (Doyle, 1998). Calculation of the effective discharge can also be perforrned if a continuous historical record of flow and sediment information is available (Goodwin et al, 1998), although this is rare. A discharge with retum penod between 1 and 2 years can be used, although this practice is controversial because these discharges may differ by more than an order of maa~tude(Hey, 1998). The potential for error increases when the discharge of a specified return period is calculated using a hydrological model, particularly if it is uncalibrated. In general, the literature acknowledges the limitations of current natural channel design methods, and recognizes that the level of confidence afforded by them individually is not suffkient for restoration or engineering projects. Two approaches are advocated to reduce the uncertainty in determinhg an appropriate natural channel geometry. The first is the application of multiple approaches and the use of results from each method to supplement and verify results from the others (Newbury and Gaboury, 1993; Rutherfurd et al, 1999). For example, a preliminary design rnay be developed from historical information and a reference reach, and verified with empirical relationships. The other approach involves comprehensive stability checks to verify that velocity, shear stress, and sediment transport are within acceptable ranges (Shields, 1996; RSRWG, 1999) 2.7.1 île Rosgen Classification Systern Based on observation and field measurement of 450 nvers in the US., New Zealand, and Canada, Rosgen (1994) proposed that namal rivers could be classified into 7 distinct strearn types, each of which was delineated by unique morphological characteristics (Fig. 2.7). Rosgen (1994) found an association of certain physiographic and valley charactenstics with particular stream types, which were assumed to represent the equilibrium channe1 geometry resulting from those conditions. A methodology for stream restoration was developed based on the classification system that has since becorne widely used in North America, including Southern Ontario, a popularity which has been attributed to the widespread availability of training courses and workshops (Kondolf, 1995). The basis of the restoration procedure is the identification of the appropriate stable strearn type for the study reach, based on local physiography and other sections of the channel. An appropnate bankfull discharge and bankfull width are determined by field measurement, and the channel morphology is specified based on published charactenstics of the stream type. These charactenstics are given as typicd ranges of slope, width to depth ratio, channel to floodpIain width ratio, and sinuosity, rather than relationships to independent variable such as discharge. 6 SLTIMV ~1.4 1.4-22 >22 N/A >22 > 22 t 1.4 < 1.4 ratio siflu-W c 1.2 > 1.2 > ?A c 1.t 1.1 - 1.6 > 1.5 > 1.4 w 12 -:- cl2 > 12 > 12 >40 <40 c 12 1 >12 c 12 rata W*wiface 0.M - 0.099 0.02 - 0.m c 0.02 < 0.02 c 0.005 c 0.m c 0.02 0.02 0.039 dope - Figure 2.7: Rosgen Stream Classification System (from Rosgen, 1994) The validity of the Rosgen classification system has been questioned because of its lack of consideration of the effects of equilibrium processes and hydrology on channel form. In particular, a lack of geomorphic significance in the boundaries between stream types (Miller and Ritter, 1996), and omission of the effects of bed materid size on the stable channel geometry (Ashmore, 1999) have been cited. There is also significant uncertainty in the application of the system to restoration because of the wide ranges of geometric parameters given for each stream type (Rosgen, 1996). Rosgen (1994) recornmends the creation of more specific regional data sets to reduce uncertainty, but these are not available for most areas and are rarely used in practice. More recently, Rosgen (1998) has begun to advocate the use of reference reaches in design. The meîhodology for applying the Rosgen Systern to restoration is ambiguous and creates significant potential for misapplication by unskilled practitioners who are not aware of its limitations. For example, Rosgen (1997), emphasizes that restoration requires the identification of the single stable strearn type that corresponds with vaUey characteristics, then proceeds to describe three different stream types that may be constmcted to restore the same hypothetical incised stream, based on budget and land use constraints. The Rosgen system is no longer presented as an approach to 'natural' channel design in most modem river and strearn restoration Iiterature and guidelines (Brookes and Shields, 1996; FISRWG, 1999; Rutherfurd et al, 1999, Ontario MNR, 1999). 2.7.2 Results of Na tural Channel Desinn To date, only a limited number of results from applications of natural channel design have been published, and report primarily qualitative, short-tem (4 year) observatioiis (e.g. Brookes, 1990, 1996a; Iversen et al., 1993; Hdtiner et al., 1996; Gilvear and Bradley, 1997; Hanington, 1999). In most cases, only-successful results are reported, and generally consist of qualitative descriptions of physical channel condition and change, water quality, and aquatic habitat. Little or no attempt has been made to relate observed changes to geomorphological processes or to characteristics of the original design. In general, the long-term results of al1 river and stream restoration projects, including naturd channel design are poorly docurnented, which can be attcibuted to a lack of funding for monitoring prograrns (Kondolf and Micheli, 1995; Brookes, 1996b, Annable, 1999). Financing for most projects tends to be for design and constmction aspects, while monitoring activities are not pubTicly visible and 'are not perceived to add value to a project. Furthemore, there is reluctance on the part of restoration practitioners to report unfavorable results. It is widely recognized that the lack of systematic post- project evaluation and reporting is hindenng the improvement of strea& restoration and natural channel design practices (Gore, 1985; Brookes, 1990; Brookes, 1996a, Osborne et al., 1993, Annable, 1999). Traditionally, monitoring of river and Stream restoration projects has focussed on biological or water quality indicators, rather than rnorphological measurements (Kondolf and Micheli, 1995). However, because the pnmary objective of modem restoration and natural channel design projects is to recreate geomorphological form and function, overall success should be measured by the long-term physical stability (Amable, 1999). To date, no stândardized procedures for the long-term monitoring and evaluation of natural channel design projects have been developed. Kondolf and Micheli (1995) and Annable (1999) have suggested guidelines for developing monitoring programs based on physical channel characteristics. General recornrnendations are for repeatable measurements of channel cross-section, bed configuration, plan form, longitudinal profile and bed material size beginning imediately after construction, at regular intervals over five to ten years and after major stom events. Currently, cornprehensive monitoring prograrns of this type are very rare, and are limited to recent large projects with substantial funding (Sear et al., 1998; Kronvang et al., 1998). Restoration guidelines published by govemment agencies (e.g. FISRWG, 1999; Rutherfurd et al, 1999, Ontario MNR, 1999) now recommended that monitoring costs be included in all project budgets to accommodate more extensive long-term evduation. -2.8 Modellin~in the River Environment Numerical models are often used in river engineering design to sirnulate behaviour and impacts of channe1 works prior to implementation. Although natural channel design is presently a largely ernpirical process, numerical model studies have the potential to verify some of the hydraulic and hydrodynamic characteristics of proposed channel configurations (Shields, 1996). Capabilities of contemporary modelling software that are potentially applicable to natural channel design include: calculation of water surface profiles in irregular natural channels, simulation of two- and three-dimensional flow structure, and prediction of changes in aspects of channel morphology. The most simple and widely used numerical hydraulic models are those that calculate one-dimensiond water surface profiles in channels and floodplains, such as HEC-RAS (U.S. Amy Corps of Engineers, 1998a), and WS-PRO (Shearman, 1990). These use a steady flow solution of the one-dimensional energy equation for open charme1 flow with friction losses deterrnined using Manning's equation. Water surface profXe models are generaLly used for simple hydraulic analysis of in-stream structures ar in floodplain mapping to estimate water depth and channel or floodplain water surface width at specified cross-sections. Cross-section averaged flow characteristics such as velocity, shear stress, and Stream power may also be calculated fiom the model output. Some software packages can calculate horizontal distributions of these parameters across cross-sections by localized solution of the energy equation, but resuits are not accurate in the presence of secondary flow patterns. Step-backwater models may also require extensive calibration of roughness coefficients when applied to natural streams because of additional flow resistance created by irregularities in channel form and by secondary flow characteristics. More recently, computational routines have been developed to simulate free- surface flow in two or three dimensions using finite-element, finite-volume or finite- difference techniques for the solution of the goveming equations of continuity and momentum. Two-dimensional flow models, such as FESWMS (Froehlich, 1989), and RMA2 (U.S. Anny Corps of Engineers, 1999), are increasingly used in detailed analysis of in-channel structures and modifications where the spatial distribution of depth and velocity and shear stress is of interest. These models require a more detailed description of bed topography than step-backwater models, but provide more accurate solutions because fewer assumptions are made regarding channel roughness and energy losses. Two-dimensional models generally assume a hydrostatic pressure distribution and constant vertical velocity distribution, which lirnits the computational accuracy in areas of steep or rapidly changing slope, and does not allow for simulation of secondary flow characteristics (Steffler, 2000). Three-dimensional modelling software packages that include treatment of vertical flow properties are being developed, but these require much more detail in specifying mode1 topography and boundary conditions, and software appropriate to river modelling is not widely available at present (Lane et al., 1999). Other hydrodynamic models calculate the transport of sediment in the channel to predict changes in channel shape over time. Additional governing equations for sediment transport, sediment continuity, and conservation of momentum for a sediment-water mixture are incorporated in modelling algorithms. Early models of this type such as HEC-6 (US Axmy Corps of Engineers, 1993a) were developed by incorporating a sediment transport routine into water surface profile models to simulate one-dimensional changes in average channel depth dong the longitudinal profile. Sediment transport routines have since been combined with more advanced two-and three dimensional computation techniques to predict the spatial distribution of bed level changes. A shortcorning of most of these models is the neglect of aspects of channel adjustment other than depth, such as width and plan form, which are often significant in both short-term and long-term channel response to change (Thorne, 1998b). A limited number of models have been developed that incorporate algorithms for width adjustment based on either physical sub-models of bank mechanics (e.g. Li and Wang, 1994) or extrema1 hypotheses (e-g. Chang, 1988). In general, these rnodeIs are considered to be lirnited in their representation of bank erosion processes, particularly in the presence of complex flow patterns, and are considered suitable only for tentative predictions of channel width changes (Darby, 1998). -3.0 PURPOSE AND OBJECTIVES 'Natural' channel design is an experimental practice for which no standard methodology has been established, although a variety of techniques have been proposed. Data regarding the long-term success or failure of 'natural' charnel design is essential to the advancernent of the practice, by validating techniques or eIiminating them from consideration. However, because of a lack of funding for monitoring studies, and probable reluctance to report negative outcomes, there is a dearth of published results. The overall goal of the research is to address this issue by examining the long-term performance of a nurnber of applications of 'natural' channel design in Southern Ontario. To meet this goal the following objectives were set: IdentiQ applications of 'natural' channel design in Southern Ontario, review available documentation and select approximately five sites that were suitable for the purposes of the study. Characterize the existing condition of the channels within the study sites including detailed topographie surveys and sediment sampling. Assess channel changes since construction by cornparhg the existing conditions with avaifable baseline data. Determine the utility of numerical hydraulic modelling for design and evaluating 'naniral' channel design projects. Evaluate the performance of 'natural' channel design at each site and make recornmendations for improvements for design procedures. -4.0 METHODOLOGY During the study, detailed survey measurernents and other observations were made to characterize the existing condition of a number of constructed 'natural' channels. The results were analyzed and compared with baseline data to evaluate the long-term performance of the channel designs. Estimates of the return penod of various discharges were obtained to provide information on the hydrologic re,oime of the each study site. Numencal modelling of water surface profiles using HEC-RAS was performed to determine whether data on hydraulic characteristics could be obtained and related to channel stability or change. The folowing chapter describes the methods used in the characterization, modelling, and anal ysis of data for each channel that was investigated. The rationale for the selection of study sites is also presented. -4.1 Site SeIection In order to achieve the objectives of the study, it was necessary to select sites where a constructed 'natura17 channel had been in place for a significant length of time so that long-term results could be evaluated. Onginally, it was proposed that a minimum of five years in place be required for each study channel, to provide time for adjustrnent to the characteristic fluvial forces and processes of the Stream in which it was located. However, it was necessary to reduce this critenon to four years because of a lack of suitable sites. Because multiple visits to study sites would be required for the research, the target region was constrained to areas within 100 km of Guelph. 'Natural ' channel design projects based on natural channel principles meeting these criteria were identified through contact with conservation authorities, municipal governments, and consultants. Design information and other relevant data was obtained from these sources to review the history and design basis of each project. A number of sites were rejected because fixed engineering controls, such as large rip-rap, gabion baskets, or arrnor Stone had been employed to fix the alignment of the constnicted channels in place, restricting natural adjustment processes. Other sites were elirninated after site visits revealed that the local topography or vegetative cover would rnake measurement with survey instruments impractical. Given the time that would be required to perforrn survey measurements and otiler field activities, it was estimated that approximately five study sites could be characterized during one field season. These were selected from the list of remaining candidate sites, based pnmarily on the quality of baseline information available with which to compare current observations. Where there was no advantage regarding baseline information, sites were selected to achieve variety in fom, scale, and location. The following five sites were determined to best satisfy the sîudy requirements: O Groff Mill Creek from Coronation Boulevard to the CNEP railway, Cambridge. O Henry Sturm Greenway, from Resurrection court to the CN railway, Kitcherier. O Mill Creek through Soper Park north of Dundas Street, Cambridge. O Little Etobicoke Creek from Bumarnthorpe Drive to Bloor Street, Mississauga. Laurel Creek from the Conestoga Parkway through Bechtel Park, Waterloo. The channels at these sites were constructed no later than 1996, and incorporate all of the elements of 'natural' channel design, including pool-riffle bed profile and meandering planform, with no fixed engineering controls within the reach of study. The channel design for each study site was documented by a minimum of a set of detailed 41 construction drawings, with additional baseline information consisting of survey data or photographs for rnost sites. Drainage areas for study channels range from less than 2 km2 to greater than 70 km2. Al1 of the sites are located in urban environments but have contributing drainage areas of widely varying imperviousness. -4.2 Site Characterization At each study site, present channel fom and condition was characterized through qualitative observations, topographic surveys, and sarnpling of bed material. The results of these activities provided a basis for cornparison with baseline information and for overall evaluation of channel performance. 4.2.1 Prelirninary Inspection Prior to more detaiIed measurements, each site was visually inspected and channei condition was assessed. Observations included the condition of channel bed and banks, presence and type of riparian vegetation, extent of the floodplain, and conditions in the surrounding zrea. Geomorphic characteristics that have been identified as indicators of channel change or instability (Table 4.1) were also examined. Observations were documented with written descriptions, photographs, and sketches as appropriate. Locations of particular interest were noted so that they could be revisited during topographic surveys. Table 4.1- Geomorphic indicaiors of channel change* Type of Change Indicators Aggradation -Mid-channel bars -Fine sediment deposits within the channel -Embedded coarse material on riffles -Deposition on point bars -Deposition on floodplain surfaces -Poor longitudinal sorting of bed material Degradation -Knickpoints -Exposed bedrock or overburden materid -Unusual scour locations Widening -B ank failure -Basal scour -Exposed tree roots -FaIlen or leaning trees Plan Form Changes -Formation of cutoff chutes -Thalweg out of phase with meander form -Bend erosion unbalanced between inside and outside * - after Gaiii (1996), MacCrae (1997)and Thorne (1998b) 4.2.2 Topographic Survev Topographic surveys were conducted using a Nikon@ DTM-3 10 total station with on-board data storage capability and a 2.1-m prism rod with a single optical prism. Systematic instrument error was Iess than 0-5cm plus 0.3 cm per 1000 m horizontally, and 0.9 cm per 1ûûO m vertically. Measurements were recorded in three-dimensional (XYZ) coordinates, and were referenced to nearby municipal survey benchmarks to maintain the vertical mean sea level (MSL) datum to which construction drawings were referenced. Latitude and longitude were arbidly established unless construction drawings were referenced to a particular horizontal coordinate system. Permanent features such as bridges, Storm sewer outfalls, and culverts were measured to facifitate horizontal alignment of survey results with baseline data. At most sites dense vegetation and steep slopes made it impossible to survey the entire length of channel fmm one location. To maintain the established frme of reference when the total station was moved, additional temporary benchmarks were established at various locations down the reach using lm lengths of steel rebar. Elevation and x-y coordinates of these benchmarks were established from a known position, and were used for free-station orientation of the total station after it was moved. The accuracy of each new station set-up was verified by measuring back to other temporary benchmarks of known position. Channel characteristics measured during the topographie surveys included cross- sections, longitudinal profile, and plan form. Other features of interest were rneasured and recorded where appropriate. Where a survey of the entire channe1 within the study site was not practical, a reach with characteristics representative of the site was selected. Cross-section surveys were performed at the apex of meander bends and on riffles. One or two cross-sections were surveyed at each nffle depending on the length of the nffle and the size of the channel. Additional cross-sections were surveyed where required to capture variation in channel fom. At each cross-section, an average of about 20 points (depending on channel width) were measured within the banks while additional measurements were taken, where the vegetative cover allowed, to describe overbank areas. The existing bankfull elevation was also identified and measured at each cross- section, based on visual observation of the bank profile and the lower lirnit of perennial vegetation species (e.g. Harrelson et al., 1994). Longitudinal surveys were performed by taking measurements at points approximately 1 to 2 m apart dong the thalweg over entire Iength of the survey reach. The deepest points in pools, top and bottom of riffles, and other features of interest dong the thalweg were measured, using additional points if necessary. Plan fom was surveyed by rneasuring points at the bankfull elevation on both channel banks at sidar intervals (1 to 2 m) as the longitudinal survey. Additional features that were measured included rnid-channel bars and islands, and rernnants of bioengineering structures that may have delineated the as-constructed bankfull channel. 4.2.3 Bed Material Sarnpling The size distribution of surface (pavement) bed matenal was determined at a number of pools and riffles within each study site using the pebble count technique developed by Wolman (1954) which has been shown to characterize bed materid with the sarne accuracy as large bulk samples (Church et al., 1987). At each pool or riffle measured, a unifom sarnpling grid was visually established dong which single particles were randornly sampled, measured and returned to the Stream. A gravel-sizing template with square holes at half-phi intervals from 2 to 180 mm was used to rneasure the length of the intermediate axis of each particle. A minimum of 100 particles was sampled at each location, which has been shown to generate reproducible results and to be free of operator error at the 95% significance level (Hey and Thorne, 1986). The pebble count data was used to generate frequency-by-number particle size distributions from which characteristic sizes of the bed sediment could be deterrnined (e-g. DI^, DSO,DM). Bulk sampling was used at some sites to charactenze material in pools where the material was too fine or where water was too deep to perform a pebble count. Up to two sarnples were taken from a representative area in each pool. Particle size distributions of bulk samples were analyzed in the laboratory using a mechanical sieve test. -4.3 Survev Data Analvsis Survey data was downloaded from the total station in ASCII text form and imported into AutoCADB (v. 2000) as three-dimensional points. Points were connected with polylines to show attributes such as the channel outline, thalweg alignment, and Iocation of cross-sections. Cross-section and longitudinal profile views were created by sampling the surface elevation dong specified horizontal alignrnents. Measurement of geometnc parameters was performed using AutoCAD dimensioning and inquiry tools. Cross-section width, depth, wetted perimeter, and area were measured directly on cross-section views that were created from the three- dimensional surface. Longitudinal characteristics of dope and pool-riffle spacing were calculated dong the centerline of the channel. The radius of curvature of meander bends was determined by fitting a circular arc to the channel centerline through each bend, as suggested by Annable (1996), and measuring the arc radius with the AutoCAD dimensioning tools. Bend angle was determined by measuring the angle between the straight riffle sections that preceded and followed each bend (Zelazo and Popek, 1995). For cornparison of the existing and original channel form, construciion drawings were scanned and imported into AutoCAD. Information from the drawings was digitized, exactly scaled, and superimposed on the survey results. Vertical alignment was based on the comrnon MSL datum, and horizontal alignment was performed using fvred reference objects that had been measured. Substantial qualitative judgement was required because horizontal alignment of the constructed channels often deviated significantly from the design specifications. This lirnited the resolution of comparative measurernents, and restricted quantitative estimates of change to general characteristics such as overall slope, riffle and pool length, and cross-section width, depth and area. -4.4 Discharge Data Discharge data for the study sites was obtained from a variety of sources. Where continuous strearnflow records were availabie for the study channel or for nearby streams, flow frequency analyses were performed using the Log-Pearson Type III distribution on the annual maximum series (Kite, 1977). The LogPearson distribution has been recornmended for use in fiow frequency analysis because the inclusion of skew allows non-normal samples to be fined to a distribution (U.S. Army Corps of Engineers, 1993b). Analysis and adjustment of the annual maximum series to account for urban developrnent over the period of record was not performed as this was beyond the scope of the study. Where necessary, flows were adjusted for different drainage areas according to: where Q and (3, are the ungauged and gauged discharges and A and Ad are the drainage areas of the ungauged and gauged channel reacbes (Ontario Ministry of Transportation, 1997). Other estimates of flow data were obtained from hydrological models that had been developed by consultants, municipalities, or conservation authorities. -4.5 Numerical Modelling The purpose of the numerical modelling component of the study was the identification of hydraulic and hydrodynarnic characteristics of the study channels that could help explain observed behaviour. A number of types of numencal models were examiried for this purpose, including one- and two-dimensional hydraulic modeIIing software packages, and software with mobile-bed subroutines for simulation of changes in channel morphology. After comparing the available software, the U.S. Amy Corps of Engineers HEC-RAS (V2.2) software package was seIected for use in the study. Details on model selection and determination of input parameters for the use of HEC-RAS in the study are described in the following sections. 4.5.1 Modellin~Sofnvare Selection Cornparison of the available types of model software (Table 4.2) showed that a one-dimensional water surface profile model, such as HEC-RAS, was most appropnate for use in the study. The topographic data requirements of one-dimensional models were well suited to the study as cross-section data had already been compiled for both the survey results. Although only one-dimensional hydraulic characteristics can be calculated, empirical relationships for predicting velocity and shear stress distribution in natural and curved channels are available. Relationships are also available for predicting roughness coefficients to account for aspects of non-uniform flow in natural channels (refer to subsection 4-54 below). Table 4.2- Cornparison of numerical mode1 types (e-g. HEC-RAS, WS-PRO) - cross-section input 3-D flow effects ~OPOPP~Y- requires calibration of - widely used roughness parameters - readily available 2-D full solution - some simulation of spatial - requires detailed (e.g. RMA2, RiverZD, variation in veiocity and topogaphic data FESWMS) shear stress - 3-D flow in channel - more rational calibration bends is not simulated of roughness parameters 1-D Mobile bed - simulation of erosion and - requires detailed (e.g. Mobed,HEC-6) deposition on channel bed streamflow data - width and plan fom adjustments not simulated - requires extensive calibration Mobile Bed with width - simulation of erosion and - requires detailed adjustment deposition on channel bed streamflow data (e-g. Fluvial-12, GSTmS) - simulation of channel - requires extensive width changes validation and calibration - bank property data The abiiity of two-dimensional hydraulic models to simulate the spatial variation in hydraulic characteristics can be useful in characterizing local erosion potential in a channel. However, two-dimensional models require more detailed topographic data input than one-dimensional models, which would have significantly increased the time required for data collection. As well, two-dimensional computational routines do not account for three-dimensional flow effects, which would limit the usefulness of these models in simdating flow in meander bends. The use of three-dimensional modelfing routines was briefly investigated as an alternative, but appropnate software was not readily available. One-dimensional mobile-bed models were judged inappropriate for the study, as observations suggested that changes to channel width were as significant as bed level changes at most sites. Mobile-bed models with width adjustment capability were also rejected, as accurate estimates of channel width changes are difficult to obtain, particularly when there is local or longitudinal heterogeneity in bank properties (Darby, 1998). Mobile-bed models also require histoncal discharge and morphology data for calibration and validation. Detailed discharge records were only available for one site, and generation of data for other sites through techniques such as continuous hydrologie modelling was beyond the scope of the study. Historical morphology data was also insufficient for al1 of the study sites. 4.5.2 Model Description HEC-RAS calculates one-dimensional water surface profiles for steady, gradually varied flow by solution of the energy equation between cross-sections using the standard step rnethod. Energy losses between cross-sections are calculated as friction losses using Manning's equation and as expansion and contraction losses. The momentum equation is used to continue the calculation of the water surface profile under conditions of rapidly varied flow. Two- and three-dimensional flow characteristics are not simulated. HEC-RAS was selected for the modelling component of the snidy after reviewing a number of different types of hydrauiic and hydrodynarnic modelling software. To evaluate the significance of hydraulic charactenstics at the study sites, HEC-RAS was used to calculate water surface profiles for both original and existing conditions. Cross- section data files for original conditions were generated from design drawings, and from the topographic survey results for existing conditions. Manning's n values were estimated using empirical relationships derived for natural river channels. For comparative purposes, water surface profiles for both original and existing conditions were calculated at the original bal1discharge. This value was estimated by calculation of water surface profiles for a number of discharges and visual cornparison of the results with the original top-of-bank stage. Hydraulic data including depth, velocity, cross-sectional flow area, hydraulic radius, energy slope, and shear stress was obtained from the HEC-RAS output and used to compare the results of original and existing conditions simulations. Calculated shear stress was also used to analyze the stability and mobility of the channel bed material. 4.5.3 Cross-section Data For modelIing of the hydraulic conditions in the design channel, the HEC-RAS geometry input files were based on specifications from design drawings. Cross-sections were constructed, based on dimensions and shapes given in the design, at suitable locations to represent the major changes in channel. slope and hydraulic properties. In general, these locations consisted of the upstrearn and downstrearn end positions of riffles and the deepest points in pools. Input files representing the existing conditions were generated using the cross- sections measured during the topographic surveys. If required, additional cross-sections were estimated to fit locations of important changes in channel slope as shown from longitudinal profile surveys. 4.5.4 Roughness Parameters Channel bed roughness was estimated using resistance equations for natural rivers proposed by (8) Limerinos (1970), (9) Hey (1979), and (10) Bray and Davar (1987): where f is the Darcy-Weisbach friction factor, R is the hydraulic radius, Da4 is the characteristic bed matenal size, and a, in (9), is a coefficient that varies from 11.2 to 13.46 to account for changes in width to depth ratio. Predicted roughness values using (8)-(10) were generally within 20% of one another, and an average value was selected for each simulation. Friction factor was converted to Manning's n for HEC-RAS input by equating the Darcy-Weisbach and Manning flow resistance equations, giving the following relations hip: The relationships in equations (8)-f 10) were developed for straight, relatively unifonn sections of naturd channels, such as riffles. Therefore, flow resistance effects in addition to grain roughness, such as non-uniform bed material and rninor bed variations, are accounted for, although the effects of pool-riffle variation are not. Also, the relationships account for the relative roughness, which was important because the relative size of bed particles to depth of flow varied widely among the study sites. Additional rvsistance losses from flow curvature in meander bends were estimated after to Zelazo and Popek (1995): where n, and nSl are Manning coefficients for curved and straight sections, a is the bend angle of curvature in degrees, Rc is the bend radius of curvature, w is channel top width, d is mean channef depth, and& is the Darcy-Weisbach friction factor for straight sections. The relationship predicted increases in Manning's n of 5% to 26% in bends at the study sites, which is consistent with observations by Chow (1959) which have show that natural channel curvature may increase resistance by up to 30 percent. Contraction and expansion loss coefficients of 0.1 and 0.3 respectively were selected for all cross-sections. These values are typicd of gradua1 transitions in natural channels (US Army Corps of Engineers, 1998b), which is consistent with the study sites. 4.5.5 Boundarv Conditions Al1 of the simulations with HEC-RAS resulted in sub-critical flow conditions, requiring the selection of appropriate downstream boundary conditions. In most cases, the use nomal or critical depth as the downstream boundary condition was inappropnate because of the variable bed topography. Boundary conditions were specified as depth rating curves based on the calculated flow depths at locations with similar dope and cross-section shape fuIzher upstrearn. Observed bankfull elevations were also used to improve boundary condition estimates. Sensitivity analyses in al1 cases showed that changes in downstream boundary conditions, within a range of reasonable values, had negligible effects on the shape of the calculated water surface profiles. -4.6 Stabiliî-y Analvsis Shear stress data from HEC-RAS output was used to evaluate the stability and potential for movement of the channel bed under both the design conditions and the existing conditions. Anaiysis was performed in terms of the Shields criterion, or critical dimensionless shear stress, defined by where t~,is the critical dirnensionless shear stress, t is the actual shear stress, g is the gravitational constant, p, and p are the densities of water and sediment respectively, and is the median bed sediment diameter. When the actual dirnensionless shear stress (given by the right side of the equation) equals or exceeds the critical value the bed rnaterial is considered to be mobile. For natural channels with coarse (>2 mm) bed sediment, the citical dirnensionless shear stress for entrainment of bed material has been found to have a median value 0.045 to 0.06 (Andrews, 1983; US Arrny Corps of Engineers, 1994). Stable natural channels with bedoad may have dimensionless shear stress values of up to 0.08 (Andrews, 1984). The stability of bed material at the study sites was evduated by deterrnining the size of particles that could be entrained given actual average and local maximum shear stresses in the charnels. A Shields criterion of 0.045 was used to determine particle sizes, as larger values are generally associated with irregular bedfoxms which were not present in the study channels (Andrews, 1983). Average cross-sectional shear stress was taken from the HEC-RAS output. Maximum local shear stress in straight nffle sections was assumed to occur at the deepest point of the channel, and was estimated by y&, where d is the maximum depth of the channel (Lundgren and Jonssen, 1964). Maximum shear stress in bends was estimated after the U.S. Federd Highway Administration (1988): where Kb is the ratio of maximum shear stress in the bend to shear stress in an equivalent straight section, and w* is the characteristic width of the channel defined as the flow area divided by the maximum depth. The following chapter presents the results of the observations and data analysis performed during the study. The results for each study site, including prelirninary inspection, toposaphic survey, hydraulic modelling, discharge data analysis, and channel stability analysis, are presented in separate sections. The implications of the results and possible explanations for observed channel behaviour are included for each site. -5.1 Laurel Creek The Laurel Creek study site is Iocated in Bechtel Park in the City of Waterloo. The section of interest is a constructed 'natural' channel reach approximately 300 metres long imrnediately downstream of the culvert that conveys the creek beneath the Conestoga Parkway (Figure S. 1). Laurel Creek drains an area of approxirnately 74 km2, including the majority of land within the City of Waterloo and additional lands to the West and northwest of the city. Bechtel Park is about 3km upstream of the confluence of Laurel Creek and the Grand River at Bridge Street in Kitchener. Approximately 90 - 95% of the total watershed discharge is contained in the channel at the study site. Through the park, the floodplain of the creek is extensive and very flat, with lateral slopes generally less than 0.5%. A large constmcted wetland is located adjacent to the creek on the south side immediately downstream of the study site. A smaller wetland pond was constructed on the north side of the creek as a part of the realignment project. 5.1.1 Description of Desian The natural channel works in the study site were part of an initiative to resolve erosion problems in lower Bechtel Park that were threatening the integrity of an underground sewage effluent pipe md exposing solid waste from an old municipal Iandfill in the floodplain. The study reach, which had been straightened during the installation of the effluent pipe, was redesigned with a meandering form with the objective of reducing channel slope and dissipating excess energy. Figure 5.2 illustrates the locations of the original and realigned channels within the study site. The design was based on a "C-type channel geornetry as given in the Rosgen Classification System (refer to section 2.7.1). The rationde for this selection is not docurnented, but was likely based on observation of the locd physiography andor nearby reaches, which is typical of the Rosgen technique. The cross-sectional shape of the channel was based on the bankfull width of the existing channel (pers. cornm., Harrington, 2001). Slope, meander wavelen,oth, radius of bend curvature, and pool-nffle spacing were determined according to typical relationships for "C" type channels developed by Rosgen (1994). The morphologie parameters for the design are surnmarized in Table 5.1 and typical cross-sections for riffles and pools are shown in Figure 5.3. Graded round Stone 25mm to 1SOm.m in diameter was imported to create the bottom material for the new channel. Bioengineering treatments, including brush mattresses and live fascines, were applied to stabilize the outside of meander bends while vegetation became established. Vegetated log crib walls were established at the outside Figure 5.2: Detail of Laurel Creek study site 58 of two meander bends that were close to the path of the effluent pipe, for an additional factor of safety against erosion and bank movement at these locations. Table 5.1: Laurel Creek - design morphologie parameters - -- Parameter Range Mean channe1 slope Sinuosity Bankfrrll width (m) - riffle - pool Maximum depth (m) - riffle - pool Bankfull capacity (m2)- riffle - pool Pool-riffle spacing (m) Pool-rifle spacing : width ratio1 Bend radius of curvature (m) Bend radius of curvature: width ratio1 1 - mean channel widrh taken to be 8.3 m Figure 5.3: Laurel Creek - typical design cross-sections 59 5.1.2 Prelirninarv Inspection Preliminary inspection of the site suggested that the original alignment of the channel was intact. However, several signs of channel change were observed, including depositional bars at two nffles (Figure 5.4), si,onificant erosion downstream of one bend causing failure of a vegetated log cnb wall (Figure 5.3, and locdized bank erosion at a nurnber of riffles (Figure 5.6). Indicators of the existing bankfùll level were sornewhat lower on the banks than highest break in slope, which was taken as the ori,oinal bankfull elevation. It was also observed that vegetation from plantings had failed to become established on meander bends through the bottom half of the study site, and that the water in the channel was highly turbid at al1 times. 5.1.3 Topographie Survev The results of the topographie survey for the Laurel Creek study site are sumrnarized in Table 5.2. Figure 5.7 shows the plan view of the existing bankfull channel and the view of the longitudinal profile dong the thalweg. The bank lines shown in the plan view represent the existing bankfidl stage as observed in the field. Plan and profile information fiom the design is superimposed on the survey results for cornparison. The results of cross-section surveys are included in Appendix A. It cm be seen on the plan view that there are significant differences between the design channel form and the existing alignment. This was attributed mostly to modifications during construction rather than channe1 migration, as there was no evidence that the channel had moved to the extent suggested by Figure 5.7. The results show that the existing bmkfull channel is narrower than designed except near cross- Figure 5.4: Laurel Creek - looking u/s at depositional bar near section 4 (Nov., 2000) Figure 5.5: Laurel Creek - looking u/s at erosion on inside of meander bend (Nov., 2000) Figure 5.6: Laurel Creek - looking u/s at bank erosion nearsection 3 (Nov. 2000) sections 4 and 10, the locations where depositional bars were observed. The bars have caused an irregular alignrnent of the thalweg at these locations. Table 5.2: Laurel Creek - sumrnary of topographie survey results Parameter Range Mean m if fer en ce^ Channel slope - 0.0034 +42% S inuosity - 1-12 -1.7% Bankfull width (m) - riffle 5.8 - 9.7 7.58 -5.3% - pool 5.8 - 8.7 6.98 -22% Maximum depth (m) - riHe 0.35 - 0.69 0-49 -30% - pool 0.72 - 1.07 0.84 -36% Bankfull capacity (mZ)- riffle 1.8 - 3.6 2.3 8 -32% - pool 2.7 - 4.6 3.80 -37% Pool-riffle spacing (m) 34 - 57 49 -2.0% Pool-riffle spacing : width ratio' 4.6 - 7.8 6.7 +12% Bend radius of curvature (m) 19.2 - 50.4 28.2 -0.7 % Bend radius of curvature: width ratio' 2.6 - 6.9 3.9 +15% 1 - mean channel wicith taken to be 7.3 m 2 - from design The longitudinal profile results indicate that the crests of riffles are somewhat higher that designed, while rime slope is greater, resulting in less abrupt transitions between nffles and pools. This suggests that material has deposited at the upstream riffles and eroded from the bottom. The mean slope of the existing riffles was measured at 0.012, compared to the value of 0.005 specified in the design. The pools at cross- sections 6 and 11 are somewhat shallower than designed. The lack of ali=pment between existing and designed pools and rimes shown in Figure 5.7 was attnbuted to modifications during construction. Cross-section survey results suggest that the channel shape through nffles is similar to the design, although sections 4, 10 and 15 show signifiant widening. The majority of the results for pools suggest an accumulation of material on the point bar, resulting in narrowing and a reduction of asyrnrnetry of the cross-section. In general, pool depth is less than specified in the design. Table 5.3 shows a cornparison of mean cross-section characteristics of the existing channel and of the design. When compared at the design bankfull elevation, pools are narrower and shallower for the existing conditions, with smaller cross-section capacity. Riffles are wider on average for the existing conditions with larger cross- section capacity and approximately equal depth. When compared at the existing bankfull elevation, the width, depth and capacity of cross-sections were smaller than designed with the most marked difference measured in pools. Table 5.3: Laurel Creek - mean cross-section characteristics Design Existing Top Exlsting Bankfuil of ~ank' ~ankfull' Pools Width (m) 10.2 9 .O 7.0 Max Depth (m) 1.3 1 1.O6 0.84 Capacity (m2) 6.0 5.8 3 -4 Riffles Width (m) 8.0 9.1 7 -6 Max Depth (m) 0.70 0.7 1 0.49 Capacity (m2) 3.5 4.1 2.4 I - existing conditions with respect to design bankfull elevation 2 - existing conditions with respect to existing estimated bankfull elevation -5.1.4 Bed MaterialSamplina Bed material at the study site was sarnpled at three different riffles and two pools, as shown on Figure 5.7. Sampling on riffles was performed using the pebble count technique while bulk sampiing and mechanical sieve analysis were used to characterize the sediment in pools which were too deep to allow spatial sarnpling. The results of the sarnpling are summarized in Table 5.4 and detailed data including particle size distribution analyses are included in Appendix A. Table 5.4: Laurel Creek - bed material sarnpling resutts Location Dl6 (mm) Dso (mm) DM(mm) 1. Rime 10 30 60 4. Pool 5. Pool Mean Pool The sarnpling results suggest that the distribution of bed surface sediment in the Stream has shified substantidly towards smaller particle sizes compared to the 25 to 150 mm material used for the original channel bed. The presence of finer particles at location 1 compared to other nffles is due to the presence of the depositiond bar at this location, which was comprised primarily of sediment in the 10 to 50 mm size range. At al1 three riffles it was observed that the majonty of larger stones (>64 mm) were firrnly embedded in the channel bottom, while the smaller materid was loose and more easily moved. The bed matenal of pools was typically sand and silt-size particles which overlay larger stones to a estimated depth of between 100 to 200 m. Manning's n coefficients for the design conditions were estimated at 0.042 for riffles, and between 0.041 and 0.051 for pools, based on an estimated Dm of 105 mm for the original bed material. For the existing conditions, roughness estimates of 0.041 for nffles and from 0.033 to 0.042 for pools were determined using Dg4 values cbtained from bed material sampling. Floodplain roughness was assigned a value of 0.05 which is typical for surfaces covered in light bmsh (Chow, 1959). For the design channel geometry, a discharge of 2.5 m3/s generated a water surface profile that corresponded closely to the design banldull elevation. The design bankfull discharge for the channel was therefore estirnated as 2.5 rn3/s. A discharge of approximately 1.8 m3/s was found to correspond with existing bankfull elevation of the channel. For hydrauIic cornparison of the design geometry and the existing conditions, HEC-RAS simulations were performed at the design bankfull discharge of 2.5 m3/s. Figure 5.8 shows the resulting water surface profiles and the simulated hydraufic conditions are surnrnarized Table 5.5. Table 5.5: Laurel Creek - numencal modelling resuIts (Q = 2.5 m3/s) RiMes Pools Parameter Oesign Existirtg Design Existing Water surface width (m) 7.9 9.2 13.5 7.7 Maximum water depth (m) 0.70 0.60 1.30 0.93 Hydraulic radius (m) 0.43 0.36 0.42 0.50 Energy grade slope (x lo3) 2.8 4.7 1.4 2.0 Velocity (m/s) 0.72 0.80 0.43 0.64 Mean shear stress (Mn2) 11.9 14.8 5.5 9.4 Maximum shear stress m/m2) 19.2 25.6 9.9 17.0 As can be seen in Figure 5.8, the simulated depth of flow in the upper hdf of the channel is nearly identical to that predicted for the existing and design conditions while a substantially lower depth of flow is predicted for the lower half. Calculated velocity, energy grade slope and shear stress were higher in the lower section of the channel for existing conditions. Mean and maximum shear stress in pools is significantly higher for existing conditions, while mean shear stress in nffles is somewhat higher than for the design geometry. 5.1.6 Stabili~Analvsis Stability analysis results for the channel are summarized in Table 5.6. The results suggest that mean and maximum shear stresses under design conditions were well below critical values for entrainment of the original 25 ta 150 mm bed material, in both riffles and pools. For the existi~gconditions, the estimated mean shear stress in nffles is sufficient to entrain about 30% of the existing bed material while the maximum shear stress is sufficient to entrain particles up to about Dso. The shear stress in pools at 2.5 m3/s is theoretically sufficient to entrain al1 of the fine sediment therein. Table 5.6: Laurel Creek - results of stability analysis (Q = 2.5 m3/s) Rimes Pools Parameter Design Existirtg Design Existing Mean shear stress (lV/rn2) 11.9 14.8 5.5 9.4 - theoretical particle size en trained (mm) 12 20 6 13 Maximum shear stress (Wm2) 19.2 25.6 9.9 17.0 - theoretical particle size entrained (mm) 26 35 17 23 Median particle size - Dso (mm) Fraction of bed material mobilized 5.1.7 Discharge Data Continuous discharge data from 1960 to 1998 was available for Laurel Creek from a gauge located approximately 3 km upstream of the study site. The drainage area gauged at this location is 59.6 km2. Log-Pearson Type III frequency analysis was performed on the annual maximum series and the resulting retum period discharges were scaied to an estimated drainage area of 68 kmz at the study site. Table 5.7 shows the predicted frequency distribution of discharges at the study site. Table 5.7: Laurel creek - discharge data Retum Period Discharge (mJ/s) Previously, a bankfull discharge of 9.04m3/s with a retum penod of 1.17 years was predicted for Laurel Creek 2km upstream of the study site (Grand River Conservation Authority, 1990). Gauge data from the penod after the charme1 was constructed (1996 - 1998) indicates that the rnean daily discharge exceeded the design bankfull value of 2.5m3/sthithirty times during ten different events over a 2.5 year penod. 5.1.8 Zntervretation ofResults The results indicate that the constnicted 'natural channel' at the Laurel Creek study site has maintained its general form and alignment over four years. However, the data suggests that some significant changes have occurred, including: formation of depositiond bars at two riffles establishment of an inset 'bankfull' Channel within the original channe1 channel wideninz at riffles narrowing of pools and a decrease in pool depth. increased nffk slope and less marked transitions from riffles to pools an overall increase in channel slope The discharge data shows that the design bankfull capacity of approxirnately 2.5rn3/s is significantly lower than the 1- to 2-year retum period discharges (6-17 rn3/s), as weli as the previously predicted bankfuI1 discharge upstream. Although the dominant discharge in urban strearns may be more frequent than in natural systems (refer to section 3.6)' the design bankfiill discharge appears to be very low- Some of the observed changes, particularly erosion outside the cbannel banks, rnay indicate the effects of excessive overbank flows. While the formation of an inset 'bankfull' channel appears contradictory, this rnay be the result of the increased fiequency of low-mabonitude discharges and bimodal distribution of effective work that can be associated with urban hydrologie regirnes (MacCrae, 1997). In addition, the channel may still be in the process of adjustment making it impossible to observe an equilibrium bankfull level at this time. The results of stâbility analysis suggest that the banldull discharge under design conditions was not competent to entrain or transport any of the large material onginally 71 used to fom the channel bed. As a result, the ability of the channel to adjust to its own bankfull discharge is almost nil, which is contrary to the principles of natural channel behaviour. As a result, erosive forces may have been directed to areas outside the channel, which could explain the observed widening and erosion beyond the channel banks. The depositional bars near cross-sections 3 and 10 suggest that the sediment transport competence of the channel is insufficient to transport the bedload from upstream. Stability analysis indicates that the 10-50mm material that comprises the bars would have been too large to be transported in the design geometry at the bankfbIl discharge of 2.5 m3/s. Channel widening near cross-sections 3 and 10 may have further reduced transport competence at these locations.. Hydraulic modeliing with HEC-RAS suggests that the bars have induced a negative feedback response by increasing the overall dope of the reach, which has steepened the overall water surface gradient and increased shear stress. Deposition of material on the thalweg and point bar of pools suggested by the survey results is likely the result of low flow transport competence in pools relative to riffles. The resulting narrowing and reduction in depth in the pools have acted to increase shear stress by increasing hydraulic radius and energy slope. Modelling and stability andysis results suggest that sedirnent transport competence in pools has increased si,onificantly and that the difference in competence between riffles and pools has been reduced. -5.2 LittIe Etobicoke Creek The Little Etobicoke Creek study site is located in Applewood Hills Park in the City of Mississauga. The reach of interest is a 1.2 km constructed 'natural channe!' between Burnamthorpe Drive to the north and Bloor Street to the south. Because of the lenk& of the channel. a representative se*ment approximately 450 m long was selected for detailed study and topographie survey, as shown on Fi,mure 5.9. Figure 5.9: Little Etobicoke Creek - key map of study site Little Etobicoke Creek is a uibutary of Etobicoke Creek and has drainage area at the study site of 22.3 km'. The entire watershed is highly urbanized with a variety of industrial. commercial and residential land uses. Applewood Hills Park occupies a natural floodplain 100 to 300 m wide with mild to medium lateral slopes. 5.2-1 Descriprion of Design The current channel at the study site is a redesign of a 1993 'natural channel' project that was designed to alleviate erosion problems but which failed catastrophically within the fxst year. The design banWu11 discharge used for the original design in 1993 was the 1.5-year retum period discharge as predicted by a hydrological model. This discharge was Iater detemiined to be sigificantly greater than the actual bankfull discharge for the creek and was identified the pnnciple cause of failure (Rosgen, 1994(a); Alexander. 1999). The redesigned channel was constructed in 1996 within the over-large channel from the onpinal construction. The objective of the project was to produce a stable channel with a natural configuration diat would avoid the failure experienced by the original design. l Figure 5.10: Little Etobicoke Creek - typical design cross-sections The geometry for the design was based on a field-estimated bankfull discharge of 6.7 m3h in a reach upstream of the study site, which corresponded with a remperiod of approximately one year (Rosgen, 1994(a)). A 'B' type channel, which is a slightly entrenched and laterally confined strearn type, was recommended bzcause this that could be constmcted within the previous channel to reduce costs. A 'C' channe! with a wide floodplain was identified as the ideal type for the study site but was rejected because it did not meet cost criteria. Morphologie parameters of bankfull width, depth, slope, sinuosity, and pool-riffle spacing were determined based on typical values for 'B' Stream types. Table 5.8 sumarizes the morphologic parameters of the channel design. Typical cross-sections for pools and rimes from the design are illustrated in Figure 5.10. Table 5.8: LittIe Etobicoke Creek - design morphologic parameters Parameter Range Mean Channel dope - 0.0027 Sinuosity Bankfûll width (m) - riffle - pool rvlaximum depth (rn) - riffle - pool Bankfull capacity (m2) - riffle - pool Pool-riffle spacing (m) Pool-riffle spacing : width ratio' Bend radius of curvature (rn) Bend radius of curvature: width ratio' 1 - mean channel width taken to be -11 rn Rock vortex weirs were constructed at the top and bottom of the straight sections between pools to provide grade control and to concentrate the flow in the middle of the channel. Upstream-oriented rock vanes were also constnicted in a nuinber of locations with the intention of reducing near-bank flow velocities to prevent unwanted erosion. In addition to typical bioengineerïng treatrnents, root wad revetrnents were installed on the outside of meander bends to stabilize the banks. The location of the above structures within study reach is illustrated on Figure 5.16, below. Remnant roundstone material 25 to 150rnm in diarneter from the previous project was recovercd and used to form the bed of the new channel. In addition to design drawings, a set of photographs docurnenting the condition of the charme1 a few months after construction was available. -5.2.2 Prelirninarv inspection Overall, the inspection suggested that the charme1 followed its original course with no major changes in alignment. However, significant erosion and bank failure at the outside of meander bends was visible, in spite of the protection afforded by the root wad revetments. In some locations the boles of root wads that had been buried in the bank were completely exposed. Figure 5.11 shows the existing condition the bank at the outside of one bend while Figure 5.12 shows the condition of the same banks shortly after construction. The bends that appeared most highly eroded had the least amount of bank vegetation and plants with minimal root depth, apparently the result of park maintenance practices. Along riffles, the channel appeared to be more incised with steeper banks than suggested by photographs taken shortly after construction (Figures 5.13 and 5.14). The top of the vortex rock weirs was measured with a scale to be 0.2 to 0.3m above the surrounding channel bed, although the channel design called for a maximum exposure of O.lrn. Other indications of channel change were localized and included a depositionzl Figure 5.11: Little Etobicoke Creek - bank near cross-section 20 (Oct. 2000) - - Figure 5.12: Little Etobicoke Creek - bank near cross-section 20 (May, 1996) Figure 5.13: Little Etobicoke Creek - looking downstream at cross-section 9 (May 2000) Figure 5.14: Little Etobicoke Creek - looking downstream at cross-section 9 (May 1996) 78 bar downstream of one bend as shown in Figure 5.15, and a number of small leaning trees on the banks of the channel, indicative of bank erosion, 5.2.3 TopographicSurvev Survey results from the study reach are sumrnarized in Table 5.9. Plan and profile views are presented in Figure 5.16, with the plan and profde information from the design drawings superimposed. Complete results of the cross-section surveys are included in Appendix B. Table 5.9: Little Etobicoke Creek - sumrnary of topographic survey results Parameter Range- Mean Difference" Channel slope - 0.0028 +3.7% Sinuosity Bankfull width (rn) - nffle - pool Maximum depth (m) - nffle - pool Bankfull capacity (m2) - nffle - pool Pool-riffle spacing (m) Pool-riffle spacing : width ratio1 Bend radius of curvature (m) Bend radius of curvature: width ratio1 -. 1 - mean chknel width taken to be 7.3 m 2 - from design As shown in Figure 5.16, the existing plan form of the channel was sornewhat different than specified in the design. This was largely attributed to modifications that had occurred during channel construction, although the observed erosion at the outside of bends had also affected the plan form. The extent of bend erosion is illustrated on Figure - Exlstlng channel - Design channd -- Exbtlng thahmg 14 Cross-sectbn iocalbn @ Sedlment sampling locellon N fi Rock voitex welr / Rock defiector vane - 126 VE z 2 1 125 .--. 124 W 1 e 123 0t000 0t040 0t080 0t120 0t160 0t200 0t240 0t280 Of320 0t360 Of400 0+440 DISTANCE ALONG THALWEG (m) Figure 5.16: Little Etobicoke Creek - plan and profile survey results bar dcwnstream of one bend as shown in Figure 5.15, and a number of small leaning trees on the banks of the channel, indicative of bank erosion. 5.2.3 Topographie Sunrev Survey results from the snidy reach are summanzed in Table 5.9. Plan and profile views are presented in Figure 5.16, with the plan and profile information fiom the design drawings superimposed. Complete results of the cross-section surveys are included in Appendix B. Table 5.9: Little Etobicoke Creek - sumrnary of topographie survey results Parameter Range Mean m if fer en ce' Channel slope - 0.0028 +3.7% Sinuosity Bankfidl width (m) - riffle - pool 6.9 - 8.9 8.0 +43 % Maximum depth (m) - riffle - pool Bankfull capacity (m2) - riffie 2.4 - 7.0 4.3 -6.5% - pool Pool-riffle spacing (rn) Pool-riffle spacing : width ratio' 4.1 - 8.9 6.6 44% %endradius of curvature (m) 14.5 -35.6 24.2 +16% Bend radius of curvature: width ratio1 2.0 - 4.9 3 -3 +74% 1 - mean channel width taken tu be 7.3 rn 2 - from design As show in Figure 5.16, the existing plan form of the channel was somewhat different than specified in the design. This was largely attributed to modifications that had occumed dunng channel construction, although the observed erosion at the outside of bends had also affected the plan form. The extent of bend erosion is illustrated on Figure 5.18 by the difference between the existing bank and the measured position of the root wad revetments, which represent the approximate location of the original bank. On this basis, up to 2 m of bank recession was estimated at some locations. The longitudinal profile results suggest significant erosion of the bed, primarily in pools. The difference between channel invert elevation for the design geometry and the existing conditions is greater than 0.6 m in some pools, while the bed elevation over riffles is nearly unchanged. As a result, the approach and departure slopes from pools have significantiy increased, wtiich bas created more variability in the bed confi,guration of the channel. Cross-section surveys at rimes support the preliminary observation that the channel had become incised (Figures 5.13 md 5.14). In general, cross-sections at rimes showed that the existing channel had steep banks and trapezoidal shape, compared to the parabolic shape of the design. Pool cross-sections reflected the observed bank erosion at the outside of the bends and the decrease in invert elevation dong the thalweg. Existing cross-section dimensions are compared with the design geometry in Table 5.10. A comparison of the results at the design bmll stage suggests a net increase in channel width, depth and capacity at both pools and riffles. The existing 'bankfull channel', as estimated from field indicators of bankfull elevation, is considerably smaller than the design bankfull channel. Table 5.10: Little Etobicoke Creek - mean cross-section characteristics Design Existing Top Existing Bankfuil of ~ank' ~ankfu11~ Pools Width (rn) 14.9 15.8 8 .O Max Depth (m) 1.35 1.69 1.O8 Capacity (m2) 10.5 11.6 5 -5 ~imes Width (m) 8.8 9.2 7 .O Max Depth (m) 0.9 1 1-00 0.84 Capacity (rn2) 4.6 5.5 5.3 1 - existing conditions with respect to design bankfdl eIevation 2 - existing conditions with respect to existing estirnated bankfull elevation 5.2.4 Bed Material Sarnplinx Bed surface materid was sampled at three riffles and two pools using the pebble count technique, as shown on Figure 5.16. Bdk sampling was not required for pools because the bed sediment was relatively coarse and depths were sufficiently low to permit wading on some occasions. The resuits of the sampling are shown in Table 5.1 1. Detailed data including grain size distribution curves are incl~dedin Appendix B. The rnedian particle size of the existing bed material was significantly smaller than specified in the design, although the distribution showed that some larger matenal was still present. h appreciable fraction of fine material was also present, which may have originated upstream or in the eroding channel banks. The material in pools was somewhat larger than that in riffles, which is atypical of the sorting that occurs in natural Table 5.11: Little Etobicoke Creek - bed material sampling results Location D16 (mm) D5o (mm) Da (ml 1. Rime 11 41 80 2. Rifle 3. Riffle Mean Rifle 4. Pool 5. Pool Mean Pool For the design conditions, Manning's n estimates of 0.037 for nffles and between 0.035 and 0.046 for pools were obtained, assuming an original Dgq of 90 mm. Estimates for existing conditions were 0.034 for nffles and between 0.035 and 0.048 for pools. For both design and existing conditions, a higher roughness factor was applied to a narrow section at the outside of meander bends to represent the ineffective flow area around the root wad revetrnents. A Manning's n of 0.10 was used to represent the roughness in this part of the channel, based on typical values for sluggish, wooded reaches as given by Chow (1959). Floodplain areas were assigned an coefficient of 0.05. For the design geometry, simulation of the design bankfull discharge of 6.7 m3/s generated a water surface profile almost exactIy matched the design bankfull elevation of pools. However, the simulated water depth in riffles was much greater than bankfull because the bankfull elevation in riffles was designed to be significantly lower than in pools. The design bankfull elevation in niTies corresponded with a simulated discharge of approximately 3 m3/s. For hydraulic cornparison of the design geornetry and the existing conditions, water surface profiles were computed for the original design 85 discharge of 6.7 m3/s. Water surface profiles for the design and existing conditions are shown in Figure 5.17. A sumrnary of the ïesuIts is given in Table 5.12. Table 5.12: Little Etobicoke Creek - numerical modelling results (Q = 6.7 m3/s) ~imes Pools Parameter Design Existing Design Existing Water surface width (m) 13.6 11.1 15.4 12-3 Maximum water depth (m) 1.20 1-18 1.35 1-48 Hydraulic radius (m) 0.54 0.57 0.62 0.70 Energy grade slope (x 10'~) 2.1 2.3 2.0 1.8 Velocity (ds) 1.01 1.16 0.69 0.8 1 Mean shear stress (N/m2) 10.9 12.2 12.2 12.2 Maximum shear stress (N/m2) 23.0 24.6 23.5 21.2 On average, the HEC-RAS results predict lower water surface widths and higher velocities for the existing conditions than for the design. The predicted difference in energy grade slope between rimes and pools is greater under the existing conditions. Mean shear stress is correspondingly greater in nffle sections and has rernained unchanged in pools. As çhown on Figure 5.17, the water surface elevation at 6.7 m3/s is somewhat lower under the existing conditions compared to the design geometry. The existing bankfull elevation estimated during survey measurements corresponds with a discharge of approximately 3 m3/s. -5.2.6 Stabilitv Analvsis The results of stability analysis for the Little Etobicoke Creek study site are presented in Table 5.13. For the design geometry, the predicted flow cornpetence of within-bank flows was generally too low to transport the 25 to 150 mm rnatenal that was used to construct the bed. The smaller material of the existing bed should be more mobile, with an estimated 40% of the material entrained on riffles and 30% of the material entrained in pools at the design bankfull discharge of 6.7rn3/s. Maximum shear stress values suggest that matenal up to about the Dso of bed material may be entrained at the baaldull discharge of 6.7 m3/s. Table 5.13: Little Etobicoke Creek - stability analysis results (Q = 6.7 m3/s) Rimes Pools Parameter Design Existing Design Existing Mean shear stress (N/m2) 10.9 12.2 12.2 12.2 - theoretical particle size entrained (mm) 15 17 17 17 Maximum shear stress (Mn2) 23 .O 24.6 23.5 21.2 - theoretical particle size entrained (mm) 32 34 33 29 Median particle size - Dso (mm) Fraction of bed material rnobilïzed 5.2.7 Discharne Data Discharge data for Little Etobicoke Creek was available from a hydrological model maintained by the Metro Toronto Region Conservation Authority. The model is event-based and is calibrated at Etobicoke Creek downstream of the confluence with Little Etobicoke Creek. The model has been used to develop 2, 5, 10, 25, 50, and 100- year remperiod flood discharge estimates as well as an estimate for the regional event. Continuous discharge records were available for two nearby streams, Black Creek and Mimico Creek, which also drain highly urbanized land. For Black Creek, data from 1967 to 1996 for was available for a station gauging a 58 km2 area. Data for Mimico Creek from 1966 to 1996 was available fiom a station gauging a 70.6 km2 drainage area. 88 Log-Pearson discharge-frequency analysis was performed for both discharge records and the results were scaled to the drainage area of Little Etobicoke Creek at the study site. The results are shown compared with estimates generated by the hydrological rnodel in Table 5.14. Table 5.14: Little Etobicoke Creek - discharge data Basis for Discharge Estimate Return Period Mode1 Black Creek Mixnico Creek (m3/s) Data (m3/s) Data (m3/s) 1.1 - 14.9 8.1 1.2 - 16.9 9.3 1.5 - 20.3 11.5 2 16.1 24.0 13.7 5 27.6 34.1 19.5 10 36.4 41.9 23 -7 25 47.6 53.0 29.3 50 57.4 62.4 33.9 5.2.8 Znterpretation ofResults The results for the Little Etobicoke Creek study site suggest that after four years in place, the charme1 continued to follow its original alignment although changes to the longitudinal and cross-sectional shape had occurred. These changes include: O bank erosion and recession at the outside of meander bends significant increase in pool depth O formation of an inset 'bankfill' channel O more marked transitions between pools and nffles The outer bank erosion at meander bends rnay be partly due to the relatively srnall radius of bend curvature in some cases, which rnay cause the flow to impinge on the outside bank rather than flowing smoothly around the curve. However, bank erosion appears to be just as extensive in some mild bends, such as cross-section 5 (refer to Figure 5.18). HEC-RAS modeIlhg results indicate that for the design geometry, the mean shear stress in pools was higher than in nffles at the bankfull discharge, suggesting greater erosive energy in pools. Pools may also have been more prone to erosion because the bank soi1 structure was disturbed during the installation of the root wads revetments, and deeply rooted vegetation was absent on the outside of most meander bends. In addition, much of the erosive energy rnay have been directed to the channe1 banks because of the Iarge size of the original bed material. The original design did not include any significant changes in slope to demarcate the transition fiom riffles to pools. h general, pools in natural channels have an approach slope that is steeper than that of the preceding riffle (Annable, 1995; Hartley, 1999). The omission of this feature rnay have resulted in insufficient expansion energy losses at pool entrances and excess erosive energy in the pools, as suggested by the modelling resuIts. The longitudinal profile that has evolved in the study reach has a more similar shape to the classicai pool-rime sequence, which rnay indicate a negaùve feedback response to the inappropriate geometry. However, the vortex rock weirs, which are located at the top and bonom of nffles, rnay have limited the bed erosion from progressing further upstream and downstream of pools. Bed material in the study reach exhibits poor longitudinal sorthg, with approximately equal size distributions in pools and riffles. Typically, nffles tend to have coarser bed matenal than adjacent pools, which is thought to be directly related to the mechanisms responsible for maintaining the pool-xiffle sequence (Keller, 197 1; Lisle, 1979; Clifford, 1993). The lack of sorting suggests that the pool-riffle sequence of the study reach is not functioning as in a natural Stream. Additional evolution, such as further deepening of pools, may be required to establish proper function. The inset 'bankfull' channel that was measured during the survey corresponds wiih a discharge of 3 m3/s, which the discharge data suggests should occur multiple times per year. This may represent the lower of two modal effective discharges in the urban discharge regime of Little Etobicoke Creek. -5.3 Groff Miil Creek The Groff Mill Creek study site is located between Coronation Boulevard and the CNRKPR railway in the City of Cambridge as shown on Figure 5.18. The site contains a 300 m constnicted 'naturd' channel that spans the distance between the railway and Coronation Boulevard. It is situated approximately 500 m upstrearn of the confluence of Groff Mill Creek with the Grand River. The floodplain of the creek through the site is flat and extensive in some areas but is codmed by the railway embankment to the north and by other steep embankments in the lower sections (Figure 5.19 below). Groff Mill Creek is a tributary of the Grand River and drains an area of approximately 7.6 kd within the City of Cambridge. The watershed is highly impervious with a large amount of primarily commercial and industrial development, although there are localized areas of residential and open space land use (Paragon Engineering Ltd., 1993). 5.3.1 Description of Design Rehabilitation of Groff Mill Creek was initiated in 1992 to address erosion problems in the study reach resulting fiom a combination of urbanization effects and channelization of the creek upstream. The natural channel works were intended to reduce velocity and sediment transport in the reach and to restore stability. The design was also intended to improve the quality and variability of aquatic habitat within the reach. Realignment of the channel with the 'natural' channel design approach took place in May of 1993. The design involved the addition of a number of meander bends to the histoncally straight channel, as shown in Figure 5.19. Because the existing channel had become confined through incision, additional floodplain area was excavated where possible. A pool-riffle sequence was also incorporated on the channel bed. Typical cross-sections for pools and nffles from the design are shown in Figure 5.20. Riffles were constructed with 100 to 150 mm round Stone, while pools were excavated in the existing material. Channel banks were initially stabilized using bioengineennp techniques including bmsh mattresses and live fascines. A live log-crib wall was constructed at the outside of the large bend in the rniddle of the reach to provide additional bank protection, as shown on Figure 5.2., below. Two rock vortex weirs were constnicted near the downstream end of the reach to provide grade control and to focus high velocity flow in the center of the channel at the entrance to the culvert under Coronation Boulevard. A deep 'plunge' pool was constructed at the outlet of the railway culvert at the top of the reach ta provide energy dissipation for high velocity flows exiting the culvert during flood events. Figure 5.18: Groff Mil1 Creek - key map - CONÇTRUCTED CHANNEL AUGNMENT - HlSTORlCAL CHANNEL ALIGNMENT --- EKiENT OF STUDY AREA Figure 5.19: Groff Mill Creek - detail of study site The charme1 was designed to be a Rosgen 'C' type for most of the len,oth of the reach, based on the characteristics of the existing channel and of the floodplain (Harringon and Hoyle Ltd., 1996). The final 50 m upstream of the Coronation Boulevard culvert was designed as a 'B' channel because of the relatively high gradient r..'3.5m Figure 5.20: Groff Mill Creek - typical design cross-sections through this section. The bankfull discharge used as the bais for the design was not documented. It is assumed that morphological parameters were selected fiom typical values for 'B' and 'C' Stream types. For the study, only the section of 'C' channel was used because the 'B' channe1 was essentially fmed in place by the vortex weirs and the use of very large bed material. Geometric characteristics of the design for the 'C' study reach are summarized in Table 5.15. Groff Mill Creek is unique among the study sites in that a monitoring program was undertaken for three years after channel construction, from April 1994 to May, 1996. Monitoring activities included repeated cross-section surveys at two riffles and two pools, bed materid measurement through pebble counts, water temperature monitoring, and fish population monitoring (Hanington and HoyIe Ltd,, 1996). No cross-section surveys or other monitoring measurements were performed in the year of construction (1993). A limited set of post-construction photographs taken in the sumer of 1993 was dso available. Table 5.15: Groff Mill Creek - design morphologic parameters Parameter Range Mean Channel slope - 0.0060 Sinuosity - 1.O8 Bankfull width (m) - nffle - pool Maximum depth (m) - rifle - pool Bankfull capacity (m2) - riffle - pool Pool-rime spacing (m) Pool-nffle spacing : width ratio' Bend radius of curvature (m) Bend radius of curvature: width ratio' 4.7 - 11.4 6.3 1 - mean channel width taken to be 3.1 rn 5.3.2 Preliminary Inspection Preliminary inspection showed that the general alignment of the channel was intact with no signs of widespread erosion. Localized bank erosion was observed at a number of locations throughout the reach where it had appeared that the channel had 95 become wider. As a result of the erosion, the rniddle of the channel was often shallower than the sides. At one location, this had caused the formation of a srnaIl island, as shown in Figure 5.2 1. At the downstrearn end of the channel, a small side channel was observed to have formed approximately 10m to the south of the main channel, as shown in Figure 5.22. The location of this side channel relative to the main channel is shown in Figure 5.23, below. Additional visits to the site in the spring of 2001 showed that the extent of the bank erosion at the island, and the size of the side channel, had increased ctramatically. Visual comparison of the existing condition with post-construction photographs indicated that vegetation had become very well established dong the entire reach. The vegetated 'live' crib wall in the middle of the reach on the north bank of the strearn no longer contained vegetation but the structure of the logs was still intact. 5.3.3 Topographic Survev Topographic swey results for the Groff Mill Creek study site are sumrnarized in Table 5.16. Plan and profile results are shown in Figure 5.23 and complete cross-section survey results are included in Appendix C. The plan survey results did not align well with the design geometry suggesting that some changes to the design had occurred during constmction. No data describing the proposed longitudinal foxm of the channel was included in the design, although the results of a limited longitudinal survey performed in April 1994 are shown for comparison as the shaded lines on the profile view of Figure 5.23. For the comparison of cross-section swey results, the original bankfull elevation was estimated according to the existing elevation of the floodplain. Figure 5.21: Groff Mill Creek -1ooking u/s at side channel outlet (Nov. 2000) Figure 5.22: Groff Mill Creek - looking upstream at small island near cross section 12 (Nov. 2000) Plan survey results suggest that the existing channel has the sarne general alignment as given by the design, with some rninor variations. The channel thalweg follows a regular path except in the large bend between cross-sections 3 and 8 which contains two pools. Cornparison of the existing longitudinal profile to the 1994 survey indicates that material has accumulated on the bed near cross-sections 1 and 2, while the pool-riffie sequence between sections 10 and 12 has remained stable and the pool at section 13 has become deeper. Table 5.16: Groff Mill Creek - surnrnary of topographie survey results Parameter Range Mean m if fer en ce^ Channel slope - 0.0068 +13% Sinuosity - 1.O8 0% Bankfull width (m) - riffle 3.2 - 6.1 4.0 +38% - pool 3.3 - 5.3 4.3 +23% Maximum depth (m) - riffle 0.24 - 0.35 0.30 -35% - pool 0.48 - 0.74 0.60 -24% Banldull capacity (m2) - riffle 0.6 - 1.O 0.85 -13% - pool 1.0 - 2.7 1.45 +12% Pool-riffle spacing (m) 22 - 37 27 0% Pool-riffle spacing : width ratio' 5.0 - 7.2 6.6 -23% Bend radius of curvature (m) 13.0 - 55.0 26.0 +25% Bend radius of curvature: width ratio1 3.1 - 13.2 5.9 -6.3% 1 - mean channel width raken to be 4.2 rn 2 - from design Table 5.17 summarizes the cross-sectional characteristics of the channel as measured at the existing bankfull elevation and the estimated design bankfull elevation. The results suggest that the existing channel is wider and shallower than the design at both pools and riffles, whiIe the overall capacity of the channel has increased. The obsened existing bankfull level is somewhat lower than the estimated design bankfXl elevation- Cross-sections 2, 3, 12, and 13 repeated the cross-section surveys from the 1994- 1996 monitoring program, using the permanent monuments that marked the sections as reference, The results of the current surveys are compared with the monitoring data in Figures 5.24 and 5.25. For al1 of the cross-sections, the results suggest that considerable widening had occurred by the time the first surveys were perforrned, although this cannot be verified because of the absence of as-constructed measurements. However, anecdotal reports in the monitoring documentation indicated ihat the channel had widened significantly between 1993 and 1994, pior to the commencement of the monitoring program (Harrington and Hoyle Ltd., 1996). Little change is indicated by the results between April 1994 and July 2000 except at cross-section 2 where the pool is shown to have become progressively wider and shallower. Table 5.17: Groff Mill Creek - mean cross-section characteristics Design Existing Top Existing Bankfull of ~ank' ~ank€uIl~ POOIS Width (m) 3.5 6.3 4.3 Max Depth (m) 0.79 0.68 0.60 Capacity (mL) 1.5 1.9 1.6 Riffles Width (m) 2.9 5.8 4.1 Max Depth (m) 0.45 0.37 0.30 Capacity (mL) 0.9 1.2 0.7 1 - existing conditions with respect to design bank!Üll elevation 2 - existing conditions with respect to existing estimated bankfull elevation SECTION 2 282.5 - Distance (m) SECTION 3 Distance (m) July 2000 ------May 1996 Figure 5.24: Groff Mill Creek - monitoring cross-sections (left to right looking dls) SECTION 12 Distance (m) SECTION 13 -2.5 0 2.5 Distance (m) July 2000 ------May 1996 Figure 5.25: Groff Mill Creek - monitoring cross-sections (left to right looking d/sd) 5.3.4 Bed Material Sarnplinx Bed material was sampled at three riffles and two pools using the pebble count technique, at the locations shown on Figure 5.23. The results of the sampling and particle size analysis are summarized in Table 5.18. Sarnpling locations 1 and 2 were selected to correspond with locetions where sediment had been sampled during the previous monitoring program. Table 5.18: Groff Mill Creek - bed matenal sampling results Location Dl6 (111111) Dso (mm) Dw (111111) 1. Riffle - near cross-section 3 2. Riffle - near cross-section 8 3. Rime - near cross-section 12 Mean Rifle 4. Pool - near cross-section 2 5. Pool - near cross-section 13 Mean Pool At riffles, the sampling results indicated that the bed included a significant proportion of sediment that was smaller than the original 100 to 150 mm material. At riffles, most of the larger stones were embedded and thickly covered with algae, suggesting that much of the original material had remained in place. The median size of pool material was slightly smaller than in riffles, although a some large round stones were observed. Sarnpling performed in October 1994 for the monitoring program found a DM of 75 mm at location 1 and 35 mm at location 3. Results from June 1995 showed DM of 64 and 70 mm at locations 1 and 3 respectively (Harrington and Hoyle Ltd., 1996). Manning's n for the design conditions was estimated at 0.047 for riffles, and between 0.038 and 0.041 in pools. Calculations were based on a Da of 135 mm in riffles based on design data and DM of 80 mm for pools assuming that the original bed material size was similar to the existing distribution. For the existing conditions, estimates of n for riffles were between 0.043 and 0-053, and between 0.035 and 0.041 for pools were obtained based on the results of particle size analysis. Floodplain roughness was assigned an n value of 0.05. Because of the lack of design information, longitudinal characteristics for the design conditions were estimated based on the slope and configuration of the short sections surveyed in Apd 1994. HEC-RAS results showed that the design bankfull elevation corresponded with a discharge of approximately 0.7m3/s. This discharge was taken as the design value and was used for hydraulic cornparison of the design geometry and the existing conditions. The results of the design and existing conditions simulations are surnmarized in Table 5.19. Water surface profiles for both cases are shown in Figure 5.26. Table 5.19: Groff MiII Creek - numerical modelling results (Q = 0.7 m3/s) a Rimes Pools Parameter Design Existing Design Existing Water surface width (m) 2.9 5-4 3.5 5 -3 Maximum water depth (m) 0.42 0.35 0.75 0.70 Hydraulic radius (m) 0.28 O. 19 0.25 0.35 Energy grade slope (x 10'~) 9.5 15 1.O 1.6 Velocity (rn/s) 0.86 1.16 0.46 0.40 Mean shear stress (N/m2) 26.0 25.2 3.4 4.3 Maximum shear stress (bJ/m2) 39.1 48.6 7.0 6.5 The results suggest a smalier depth of flow with greater water surface width for the existing conditions at the design bankfill discharge. Mean velocity has increased over riffles but is nearly the sarne in pools. Shear stress charactenstics are sirnilar for both cases although maximum shear stress over riffles is somewhat greater for the existing conditions. The water surface elrvation at 0.7 m3/s for the existing conditions was approximately equal to the observed, existing bankfull elevation. Major differences in calculated water surface elevation and water surface slope, and energy slope were observed at the downstream end of the channel (Figure 5.26). These may be the result of assumptions made in estimating the design longitudinal profile, which suggest that the bed elevation at downstream end of the channel has decreased by approximately 0.3 m. The flow is also divided into two channels at this Iocation under existing conditions, which may contribute to the reduced depth of flow. 5.3.6 Stabilitv Analvsis Stability analysis results are presented in Table 5.20. Under design conditions, the predicted flow competence at the banldùll discharge of 0.7 m3/s was not sufficient to transport the large rnaterial used to constmct the riffles. For the existing conditions, a discharge of 0.7 m3/s may potentially entrain about 35% of the smaller bed material that is currently found at rifles. Only a small fraction of the bed rnaterial in pools could be transponed at 9.7 m3/s under existing conditions. Table 5.20: Groff Mill Creek - results of stability analysis (Q = 0.7 m3/s) - Riffles Pools Parameter Design Existing Design Existing Mean shear stress (N/m2) 26.0 25-2 3 -4 4.3- - theoreticai pparticle size entrained (mm) 36 35 5 6 Maximum shear stress (N/m2) 35.8 48.6 7.O 6.5 - theoretical particle size entrained (mm) 54 67 10 9 Median particle size - DS0 (mm) Fraction of bed material mobilized 5.3.7 Dischur~edata Estimates for the 2, 5 and 100-year return penod discharge at the study site were available from an event-based, uncalibrated hydrological mode1 that had been developed for Groff Mill Creek through the shidy site (Paragon Engineering Ltd., 1993). Discharge was aIso predicted using gauge data from nearby Schneider Creek in Kitchener from data which were recorded between 1971 and 1992. The 25.1 km2 drainage area upstream of the gauge station is also highly urbanized, and experiences similar precipitation patterns to Groff Mill Creek because of its close proximity. Log-Pearson discharge-frequency analysis was conducted on the data and the resulting return period flows were scaled to the drainage area of the study site. The results are shown compared with the mode1 predictions in Table 5.2 1. Table 5.21: Groff Mill Creek - discharge data Basis for Discharge Estimate Return Period Mode1 Schneider Creek (m3/s) Data (m3/s) 1.1 - 3 -4 1.2 - 4.3 1.5 - 6.0 2 8-2 7.9 5 12.1 14.3 10 - 19.7 25 - 28.2 100 18.8 44.6 Differences between the 100-year rehun penod discharges predicted by the model and by andysis of the Schneider Creek data may be partly due to the large storage volume created by the railway embankment upstrearn of the study site during large flood events. The hydrological model accounts for storage effects and routing of flow through the culvert uncier the embankrnent and into the study site, whereas estimation of discharge using the Schneider Creek data does not. The results indicate that, over the past 7 years, the alignment of the reconstnicted section of Groff Mill Creek has remained largely unchanged. However, a major change has occurred in the formation of a major secondary channel at the downstream end of the study reach. Other changes suggested by the results include: Widening of the channel over the entire reach O Aggradation of the bed at the top of the reach Formation of an inset 'bankfull' channel The discharge data suggests that the design banldull discharge of 0.7 m3/s occurs much more frequently than the 1- or 2-year return period discharges. Therefore, the channel widening suggested by the results rnay be the result of over-frequent discharges around the bankfull level. Widening has been the primary mode of adjustment in the Stream because the 100 to 150 mm material used to constmct the riffles is practically immobile, while the design documentation suggests that the banks were largely unprotected. However, the excessive frequency of flows around the bankfull level is reflected in the widening of the channel. Widening has been the primary mode of adjustment in the Stream, particularly on riffles because the 100 to 150 mm Stones useci to on the bed are too large to be transported under rnost conditions while the original banks appear to have been unhed. The formation of the side channel at the downstream end of the reach rnay be another indication that the initial capacity of the channel was insufficient to transport the discharge and bed material. The formation of the inset 'bankfull' channel rnay be result of a birnodal distribution of effective discharges resulting from the urban hydrologie regime of Groff Mi11 Creek. The increase in bed elevation in the upstrearn end of the reach rnay indicate the inability of the channel to transport bedload from upstream. This is particularly significant in pooIs, where modelling results suggest that sediment transport cornpetence is very low at the design bankm discharge. Discharges greater than the bankfüll value rnay not be sufficient to transport the bedload because the energy of the flow is dissipated over the fioodplain. Modelling results suggest that the increased slope resulting from aggradation at the top of the reach has increased shear stress, which rnay indicate a negative feedback rnechanism for balancing sediment transport. -5.4 Henrv Sturm Greenway The Henry Sturm Greenway study site is located south of Resurrection Drive, near the western terminus of University Avenue in the City of Waterloo (Figure 5.27)- The watercourse that flows through the Greenway is a small tributary to Schneider Creek, which drains the majority of the land within the City of Kitchener. At the smdy site, the watercourse drains an area of approxirnately 1.7 km2 including two srnall subdivisions, an Ontario Hydro transformer station, and agricultural land. Through the site, the watercourse consists of a 450-m constructed 'natural' channel, which conveys flow through the site from the western boundary of the Springfield subdivision dong Resurrection Drive to a culvert beneath the CNR railway. The Greenway valley has been reconstnicted within the study site and provides a floodplain approximately 25 m wide with lateral slopes of approximately 5%. 5.4.1 Description of Design In order to accommodate the development of the Springfield subdivision, the channel within the study site was realigned in 1994 to shift the watercourse 30 to 35 m to the south. 'Natural' channel design was sdected over a previously proposed design for a straight gassed waterway with concrete drop structures, in order to provide a channel with greater ecological and aesthetic fùnction. It was also expected that the natural channel design would be more economical and require less long-term maintenance than other alternatives (Cumming-Cockbum Ltd., 1993). Figure 5.28 shows the historical alignment of the channel and the 'natural' channel design for the study site. The design for the realigned channel incorporated a highly sinuous plan fom, compared with the original channel, which was relatively .In I l--y STUDY -/I SITE Figure 5.27: Henry Sturm Greenway - key map = CONSIRUCTED CHANNEL WGNMWT - HISTORICAL CHANNEL ALIGNMEJUT --- DCTEHT OF STUDY AREA ~iiiii:lIlli;;;;;;;;,;;;;; ::!!!!!!$ CNR RAILWAY I Figure 5.28: Henry Sturm Greenway - detail of shidy site straight. The design also included a pool-riffle configuration with shallow affles and deeper asyrnrnetrical pools located at the apices of meander bends. The size of the channel bed material was 20 to 150 mm, based on zero movernent during the 100-year flood event. The area where the channel was relocated was excavated when necessary to provide a minimum 25 m wide floodplain area. A pond at the upstream end of the study reach and a wetland at the downstream end were created to act as energy dissipation features and to provide additional habitat diversity. Channel banks and floodplain areas were planted with various types of vegetation to stabilize the surface of the floodplain after construction. A Rosgen 'E' type channel was used as a template for the design based on valley slope and other characteristics of the existing channel. A bankhi11 discharge of 0.15 m3/s was selected based on a combination of field measurement and results from previous investigations of channels with similar drainage areas (Cumming-Cockburn Ltd., 1993). The geometric parameters for the channel were selected based on typical values for 'E7- type streams. As the meander pattern of the strearn was more or less uniforrn and repetitive, a representative section 150 m long in the rniddle of the site was selected for the study. The design parameters for this section are summarized in Table 5.22. Typicd riffle and pool cross-sections from the design are shown in Figure 5.29. RIFFLE Figure 5.29: Henry Sturm Greenway - typicd design cross-sections Table 5.22: Henry Sturm Greenway - design morphologic parameters Channel slope - 0.0082 Sinuosity - 1.36 Bankfull width (m) - riffle - pool Maximum depth (rn) - riffle - 0.3 - pool 0.6 Bankfull capacity (m2) - riffle - pooI Pool-riffle spacing (m) 7-9 8 Pool-riffle spacing : width ratio1 Bend radius of curvature (m) Bend radius of curvature: width ratio' 2.4 - 2.7 2.6 1 - mean channel width taken to be 1.3 m 5.4.2 Preliminary Inspection Preliminary inspection of the site suggested that the channel was considerabiy narrower and shallower than the design. During the investigation of the site, primarily baseflow was observed which was contained primarily in a narrow, deep channel, between 0.3 and 0Sm wide and 0.1 to 0.2m deep. Grass and other herbaceous vegetation thickly lined the channel banks to below the observed water level. In most locations, the channel bonom material consisted of fine sediment with no evidence of the original Stone lining. Figure 5.30 shows a typical pool in the channel in September 2000, during the study. Figure 5.31 shows a typicd pool il? December 1996, suggesting that the channel was previously much wider. The original bed material can be clearly seen in Figure 5.30 while it appears to be completely overgrown in Figure 5.3 1. The condition of the channel varied throughout the study reach. Most pools had become much narrower, as shown in Figure 5.31, whiIe some appeared to have become wider with a larger vo!ume of water. In some areas, the original bed material codd be seen on the channel bottom, while in others the bed consisted entirely of fine sediment. The energy dissipation pool at the upstream end of the channel had become filled with fine sediment, elirninating most of the storage volume. 5.4.3 Topoaraphic Survev The results of the topographie survey at the Henry Sturm Greenway study site are summarized in Tzble 5.23. Plan and longitudinal profile results are shown in Figure 5.32 and complete results of cross-section surveys are included in Appendix D. 1 Figure 5.30: Henry Stum Greenway - looking upstream at pool (Oct., 2000) Figure 5.31: Henry Sturm Greenway - looking upstream at pool (Dec.. 1996) Table 5.23: Henry Sturm Greenway - summary of topographic survey results Parameter Range Mean m if fer en ce' Channel slope - 0.01 1 +34% Sinuosity - 1.4 +2.9% Bankfull width (m) - riffle 1.5 - 4.1 2-3 +92% - pool 1.8 - 5.5 2.5 +67% Maximum depth (m) - riffle 0.27 - 0.40 0.32 +6.7% - pool 0.38 - 0.72 0.49 -18% Bankfdl capacity (rn2) - rime O. 13 - 0.76 0.32 +60% - pool 0.32 - 1.56 OS5 +25% Pool-riffle spacing (m) 7-9 8 0% Pool-riffle spacing : width ratio' 3.1 - 3-5 3 -3 -47% Bend radius of curvature (m) 1.8 - 3.6 2.6 -24% Bend radius of curvature: width ratio1 0.7 - 1.6 1.1 -58% 1 - mean channel width taken to be 2.4 m (see below) 2 - frorn design The plm survey results indicate that the existing alignment of the channel was very similar to the form set out in the design. It was difficult to identiQ an existing bankfull stage during the survey because of conflicting bankfüll indicators and heavy overgrowth of the channel with vegetation. As a result, an arbitrary boundary approximately 0.2m outside the observed water edge was used for plan fonn measurements during the survey. This boundary is illustrated on the plan view results in Figure 5.32. Cross-section survey results showed that there was a consistent break in bank slope at a distance between 0.75 and 2 m from the channel centerline. The bankfull charactenstics of the existing channel, as shown in Table 5.23, were calculated at this topographic break- The elevation of the channel invert measured during the study was significantly higher than the design longitudinal profile, with an average difference of 0.5m. Re- measurement of the temporary benchmarks used in the swey indicated that the results were accurate. It was assumed that this discrepancy was mostly the result of undocumented changes to the design prior to or during construction rather aggradation of the bed. As a result, no quantitative estimates of the changes to the bed level could be confidently obtained. However, it can be seen from the results that the bed configuration throughout most of the study reach is much less variable than the design, which suggests accumulation of material in pools. Cornparison with design dimensions suggested that the cross-sectional width and capacity of the channel had increased. Many of the cross-sections, particularly riffles, showed the presence of a small, deep inset channel approximately 0.3 m wide (refer to Appendix D). It is interesting to note that in the investigation of the existing condition of the Stream prior to the realignment, the engineering consultant noted the presence of a similar foxm. An inset channel 0.3 m deep and 0.3 m wide was observed with a larger channel an additional 0.1 - 0.15 m deep extending 0.6 m on either side (Curnming- Cockbum Ltd., 1993). Because it was not possible to align cross-section survey results with the design information using the elevation of the longitudinal profile, the data was aligned by rnatching the dopes of the floodplain, which were of a known gradient (refer to Appendix D). The results of the cornparison suggested that considerable aggradation had occurred in pools in the upstream end of the surveyed section. Cross-sections further downstream suggest significant widening and enlargement of the channel, particularly at pools. Table 5.24 surnrnarizes the bankfull mean cross-section characteristics of the channel for the design and for existing conditions. Table 5.24: Henry Stunn Greenway - mean cross-section charactenstics Design Existing Bankfull Bankfull Pools Width (m) 1.5 Max Depth (m) 0.60 Capacity (m2) 0.44 Riffles Width (m) 1.2 2.3 Max Depth (m) 0-30 0.32 Capacity (m2) 0.20 0.22 5.4.4 Bed Material Sarnplinz . Bed material throughout the channel consisted primarily of a uniforrn mixture of fine silt and sand with some gravel. Bulk samples of this material were taken in two locations and were analyzed using the mechanical sieve technique. In some areas, cobble-sized matenal was exposed at the bottom of the narrow inset channel. The material at one of these locations was sarnpled using the pebble count technique, although the large size of the sediment relative to the size of the channel ciid not allow the ideal sarnple size of 100 particles to be obtained. The locations where bed material was sampied are indicated in Figure 5.33. The results of particle size analysis are summarized in Table 5.25. DJ6values are not given for the fine sediment as ap2roximately 20% of the material passed through the finest sieve (~0.075mm). The remainder of the material consisted entirely of sand-size particles (0.075 - 4.75 mm). The pool materid was somewhat finer than riffle material, which was expected given the hydraulic charactenstics of the channel (refer to section 5.4.5, below). The size distribution of coarse sediment from sampling location 3 is consistent with the size of the ori,oinal bed material. Table 5.25: Henry Sturm Greenway - bed material sampling resuIts ------Fine Sediment 1. Pool 2. Riffle Coarse Sediment 3. Riffle 5.4.5 Hvdraulic Modelling Manning's n for the design conditions was estirnated at 0.037 for riffles and 0.045 for pools, based on an estunated Dw of 50mm for the bed material. It should be noted that the size of the study channel does not fdl within the range of channel sizes for which equations (8), (9) and (10) were developed, which increases the uncertainty of the estimate. Manning's n could not be estimated using grain size correlations for the existing conditions because of the small size of the bed material and the amount of vegetation present in the channel. An n value of 0.030 was used for riffles, which is typical of winding earth channels lined with gras and weeds (Chow, 1959). From this, a value of 0.036 was estimated for pools using equation (12). Floodplain roughness for the original and existing conditions was estimated at 0.05 (Chow, 1959). The water surface profiles calculated for design and existing conditions are shown on Figure 5.33. For the design geometry, modelling showed that the channel capacity was approximately 0.2 m3/s, which is slightly higher than the bankfidl discharge of 0.15 m3/s given in the design documentation. The calculated value of 0.2 m3/s was used as the bais for hydraulic cornparison of the existing conditions and the design geometry. The results are summarized in Table 5.26. Table 5.26: Henry Sturm Greenway - numerical modelling results (Q = 0.2 m3/s) mes Pools Parameter Design Existing Design Existing Water surface width (m) 1.80 2.56 2.02 3.28 Maximum water depth (m) 0.3 1 0.27 0.6 1 0.44 Hydraulic radius (m) O. 13 0.09 0.21 0.17 Energy grade slope 0.0 11 0.0 18 0.0020 0.0035 VeIocity (ds) 0.85 0.92 0.40 0.39 Mean shear stress (N/rn2) 14.2 16.0 4.2 4.0 Maximum shear stress (N/rn2) 31.5 48.2 7.0 7 -2 The calculated water surface profile for 0.2m3/s provided cross-sections that were wider and shaiIower for the existing conditions than for the design geometry, which was expected given the change in cross-sectional shape indicated by the survey results. The greater energy grade slope for existing conditions over both riffles and pools is due to a greater topographic slope than specified in the design. As a result, the velocity over nffles has increased, while velocity in pools has decreased because of the increased cross-sectional area and volume in most pools. Relative difierences in water surface elevation between the design conditions and existing conditions could not be estirnated because of the discrepancies in the elevation data. For the existing conditions, the calculated water surface elevation for 0.2 m3/s was lower than the second break in bank flow, which was taken as the existing bankfull elevation. A water surface profile for 0.25 m3/s corresponded well with this level. Other simulations showed that the capacity of the small inset channel was approximately 0.03 rn3/s. However, +As is only a very rough estimate as the effects of vegetation and channel roughness elements would significantly alter the roughness at low discharges, making accurate numencal simulation difficuk 5.4.6 Stabilitv Analvsis Results of stability analysis for the study site is surnmarized in Table 5.27. For the design geomeuy, the estimated tractive force in the channel at the bankfulI discharge of 0.2 m3/s suggests that the original bed matenal would be compietely immobile. The maximum shear stress in the rniddle of the chamel may have been sufficient to mobilize some of the smaller particles from the original material, although larger materiai was generally applied in these areas. For the existing conditions, the results suggest that maximum shear stress in the channel has increased such that more of the original be material could be transported. However, the majority of that material is fixed in place by the overlying fine sediment and vegetation. Stability relationships indicate that the tractive force in the existing channel at 9.2 m3/s is suficient to entrain al1 particle sizes of the fine sediment that made up the existing bed (Soi]. Conservation Service, 1977). The dimensionless Shield' s stress of 0.045 was not used for analysis because this parameter is not constant for particles sizes smaller than 2rnm. Table 5.27: Henry Sturm Greenway - results of stability analysis (Q = 0.2 rn3/s) Rimes Pools Parameter Design Existing Design Existing Mean shear stress (N/m2) 14.2 16.0 4.2 4.0 - theoretical particle size entrained (mm) 20 22 6 6 Maximum shear stress (~/rn~) 31.5 48.2 7 .O 7-2 - theoretka1 particle size entrained (mm) 43 66 10 10 Median particle size - D50(mm) *50 0.6 *50 0.2 Fraction of bed material rnobilized 0% 100% 0% 100% * - estimated 5.4.7 Discharne Data Discharge data was given by an event-based, uncalibrated hydrological mode1 developed during the channel relocation project (Cumming-Cockbum, 1993). Estimated peak flows for the 2, 5, and 25 and 100-year storm events were 5.9 rn3/s, 10.0 m3/s, 19.0 m3/s and 27.8 m3/s respectively. The study also estimated channel base flow at approximately 0.03 m3/s. -5.4.8 Interpretation of Results The results of the study suggest that while the realigned channel in the Henry Sturm Greenway still foIlows its constnicted alignment, major physical changes have occurred since 1994. It appears that a large amount of fine sediment has been deposited throughout the reach, causing changes in cross-sectional and longitudinal form. As a result, the channel has evolved a form that is sirnilar to that of the watercourse before realignment, a narrow inset channel surrounded by a wider bankf'ull channel. The deposition of fine sediment throughout the study site suggests that flow in the channel is not competent to transport the sediment load from upstream. Although most sizes of particles of sediment that comprise the channel bed rnaterial should theoretically be entrained at the bankfùll discharge, the supply of sediment from upstrearn may exceed the transport capacity of the flow. Fine sediment supply may be very large because the agriculturd land upstream drains directly to the watercourse with few buffer areas. The high sinuosity of the constructed channel may have increased flow resistance so much that this fine sediment cannot be transported. The widening of the channel would have also reduced sediment transport cornpetence. The sirnilarity of the existing cross- sectional shape to that of the channel pnor to realignment suggests that the channel has narrowed towards its original form to improved sediment transport at medium flows. -5.5 Mill Creek The Mi11 Creek study site is Iocated in Soper Park between Elgin Street and Dundas Street in the City of Cambridge (Figure 5.34). Mill Creek is a tributary of the Grand River and drains an area of approximately 104 km2, prirnarily mal land to the West of Cambridge as far as the Town of Milton (CH2M Gore and Storrie Ltd., 1996). The study site is located approximately 2.5 km upstrearn of the confluence of Mill Creek with the Grand River, and the creek at this location contains an estimated 95% or more of the total watershed discharge. Fiow in the creek through the City of Cambridge, including the study site, is largely controlled by the dam at the Shade's Mill Reservoir upstrearn. Soper Park occupies a natural floodplain approximately 100 to 200 m in width, 7with very rnild laterd slopes. The entire length of the creek through the park, approximately 600 m, has been realigned as shown on Figure 5.35. Only the uppermost 450 rn were used for the study because portions at the downstream end were constrained by amour Stone and were not considered to be indicative of 'natural' channel design. Figure 5.34: Mill Creek - key map Figure 5.35: MiIl Creek - detail of study site 5.5.1 Description of Desi~n The objective of the 'natural' channel design at the site was to restore aquatic and riparian habitat, primarily for brown trout, by replacing the historical stone-lined channel with a meandering channel (Figure 5.35). The design included a pool-riffle sequence with shallow nffles and deep asymmetrical pools at the apices of meander bends. Figure 5.37 shows the ternplate pool and riffie cross-sections fiom the design. Round stone 100 to 150 mm in size was imported for lining of the channel at riffle sections and 150 to 200 mm round stone was placed in pools only on the point bars at the inside of rneander bends. Bioengineering treatments including brush mattresses and live fascines were applied throughout the channel to stabiiize the banks until vegetation could become established. Two small ponds were constnicted adjacent to the channel in the lower section to provide additional variety of habitat. Construction of the channel took place in autumn of 1995. The selection of design morphological characteristics is not documented although the basis of the design was a field estimate for bankfull discharge of 1.3 m3/s, which was measured in the reach within the park (Harrington and Hoyle Ltd., 1995). The dimensions of the channel appear to be based on typical values for the Rosgen 'C' Stream type. The morphologic parameters of the channel design within the study reach are surnmarized in Table 5.28. In addition to the design drawings, the results of an as-built survey of pools, taken in December 1995, was also available. The results uidicated the elevation of the deepest point in each pool at that tirne. No photographie documentation of the original condition of the channel was available. RIFFLE POOL Figure 5.36: Mill Creek - typical design cross-sections Table 5.28: Mill Creek - Design morphologie parameters Parameter Range Mean Channel slope - 0.0032 Sinuosity BWllwidth (m) - riffle - pool Maximum depth (m) - riffle - pool Bankfull capacity (m2) - rime - pool Pool-riffle spacing (m) Pool-riMe spacing : width ratio' Bend radius of curvature (m) Bend radius of curvature: width ratio' 1 - mean channel width taken to be 8.7 m 5.5.2 Preli17zinar-vInspecrioiz Initiai observations suggested that the onginai alignment of the channel was intact. At the pools in meander bends, the water surface width was observed to be approximately twice as wide as in riffles, with high veiocity fiow concentrated near the outside bank. Some signs of rninor erosion were noted at the outside banks of most meander bends, as seen below in Figure 5.37. The banks of the straight nffle sections appeared to be stable. Bank vegetation had become very well established throughout the reach with significant root depth. Figure 5.37: Mill Creek - pool at cross-section 15, looking upstream (Sept., 2001) The dominant bottom material of the channel appeared to comprise coarse material of the size specified in the design. However, significant quantities of fine sediment were observed in pools throughout the reach. This sediment appeared to consist 129 Figure 5.38: Mill Creek - fine sediment on point bar at cross-section 9 (Sept., 2000) of silt- to fine gravel-sized particles and was found primarily on the inside of meander bends although small deposits were also observed in the thalweg. On the inside of bends, the fine material covered the original large Stones up to a depth of approximately 20 cm. Figure 5.38 shows the accumulation of sediment on the point bar of one pool. Accumulations of coarse, cobble-sized materid were also observed at the top of sorne riffles. Numerous brown trout were observed at the study site throughout the investigation, pnmarily in shaded pool sections near ziccumulations of brush or debris. 5-5.3 ToponraphicSurvey The results of the topographie survey are summarized in Table 5.29 below. Plan and profile sumey results are presented graphically in Figure 5.39, and cross-section survey results are included in Appendix E. Plan survey results show that the existing channel alignment is very similar to the design form, with no major deviations. Channel bends appear to be more abrupt and less rounded than the design. At the estirnated existing bankfull elevation, the channel width is significantly wider in pools than in riffles. The baseline longitudinal profile information show in Figure 5.39 was adjusted to include the results of the as-built survey of pools. With this data it can confidenîly concluded that the majority of the pools in the surveyed reach have become deeper since construction. In the upper section of the reach, the bed profile is similar to the design with only minor differences. Beginning at station 2+50, the elevation of the existing channel invert is significantly lower than designed/constnicted, with the difference increasing in the downstream direction. By the end of the study reach the difference in elevation is slightly greater than 1 m. The results also show the localized accumulation of coarse matenal at the top of riffles, particularly near cross-sections 10, 13, and 19. Table 5.29: Mill Creek - topographie survey results Parameter Range Mean m if fer en ce^ Channel slope Sinuosity Bankfull width (m) - nffle - 6.0 -29 % - pool Maximum depth (m) - rime - pooI Banldrrll capacity (m2) - riffle - pool - 4.9 +17% Pool-riffle spacing (m) 41 - 82 61 - 1.6% Pool-nffle spacing : width ratio' 5.5 - 10.9 8.2 +15% Bend radius of curvature (m) 12.1 - 37.0 26.0 -29 % Bend radius of curvature: width ratio1 1.7 - 5.3 2.6 -3.3% 1 - mean channel width taken to be 7.0 m 2 - from design Cross-section surveys at riffles showed only rninor differences in shape between the existing channel and the design template, with the exception of cross-sections near the downstream end of the channel which were significantly deeper and somewhat wider than designed (refer to Appendix E). At most riffles, an inset 'bankfûll' channel appeared to have formed within the original channel banks. Pool cross-sections were generally narrower and deeper than the design template, although the existing banknill elevation was close to that of the design. The accumulation of material on the point bar of pools was clearly illustrated by the cross-section survey results (refer to Appendix E). Table 5.30: Mill Creek - mean cross-section characteristics -- Design Existing Design ~ankfuil' ~ankfull~ Pools Width (m) 9 -5 9.2 8 -4 Max Depth (m) 1.O 1.12 1.10 Capacity (m2) 4.2 5-6 4.9 Riffles Width (m) 8 7.2 6.0 Max Depth (m) 0.5 0.67 0.57 Capacity (m2) 2.9 3.1 2.1 1 - existing conditions with respect to design bankfull elevation 2 - existing conditions with respect to existing estimated banldull elevation A cornparison of the existing channel cross-section form with the design shape, is presented in Table 5.30. When compared at the design bankfull elevation, the results suggest that there has been little net erosion or deposition at rifles. The increase in mean depth is the result of the lowered bed elevation at the downstream end of the surveyed section. At the design bankfull elevation, pools are narrower, deeper, and have greater cross-sectional capacity than designed. Cornparison at the existing, estimated banidulI elevation suggests that the existing bankfbll capacity of nffles is significantly less than designed while the capacity of pools is sornewhat greater. 5.5.4 Bed Material Sarnplinz- Bed matenal was sampled at three riffles using the pebble count technique, and at three pools by bulk sarnpling and sieve analysis because of the predominantly fine sediment. Some large material that was assumed to be part of the original channel lining was also observed in pools but this were too large and embedded to be extracted with bu& samples. Table 5.3 1 sumrnarizes the results of the particle size analysis. Detailed results are included in Appendix E. Table 5.31: Mill Creek - bed matenal sampling results Location Dl6 Dso (mm) D84 (mm) * 1. Riffie - near cross-sections 1,2 - 18 100 2. Riffle - near cross-sections 13, 14 3 90 102 3. Riffle - near cross-sections 19, 20 2 65 105 Mean Rifle 3 58 102 4. Pool - near cross-section 6 5. Pool - near cross-section 15 6. Pool - near cross-section 18 Mean Pool * no DI6calcdated because >16% of matenal was smaller than 2mm The results indicate that the existing bed matenal of riffles consisted pnmarily of larger stones of the original size used in the construction of the channel, although a significant portion of sand and fine grave1 was present. The large stones tended to be embedded in the channel bottom but loose accumulations were observed in some locations such as the top of riffles. The higher proportion of smaller matenal at the top of the reach suggests that sediment aniving from upstream may be accumulating at the upstream end. Particle size analysis of pool bulk samples showed that the bottom material consisted primarily of sand and fine-grave1 sized particles. 5.5.5 Hydraulic Modellin~ Manning's n for the design geometry was estimated at 0.046 for riffles, and between 0.040 and 0.049 for pools. For riffles, roughness was calculated based on an estimated original Dm of 135mm. Pool roughness was specified in two parts, a calculation for the point bar based on an estimated original Dm of 185mrn, and an estimate of 0.25 for the thalweg assuming the rernainder of the channel was unlined (Chow, 1959). The composite roughness value for pools was calculated by HEC-RAS. For the existing conditions, the roughness coefficient was estimated to be 0.041 for riffles and between 0.030 and 0.039 for pools. The estimate for riffles was based on the mean Dm of 58mm from the sarnpling results. Poo1 roughness was calculated usinga base estimate of 0.030 which is typical of excavated earth channels with some cobbles (Chow, 1959), with additional curvature effects deterrnined using (11). An estimate based on particle size in pools was considered inappropriate due to the observed variability in the bed material. Floodplain areas were assigned Manning's rz of 0.050. Simulation of the design geometry with HEC-RAS showed that a discharge of approximately 1.8 m3/s generated a water surface profile that corresponded with the bankfull elevation compared with the design bankfull discharge of 1.3 m3/s. To perform a comparative simulation for the existing conditions, a discharge of 1.8 m3/s was used. Table 5.32 summarizes the results of rnodelling for the design geometry and the existing conditions. The resulting water surface profiles for both cases are shown in Figure 5.40 For the existing conditions, the discharge of 1.8 m3/s produces a water surface profile that is slightly higher than the estimated bankfilll elevation- Larger calculated values of velocity, energy grade dope and shear stress for the existing conditions are associated pnmady with the increase in topographie dope in the downstrearn section of the channei. Relative to the hydraulic conditions for the design geometry, the existing water surface is predicted to be slightly higher in the upstream section and significantly lower downstream. Table 5.32: Mill Creek - numerical modelling results (Q = 1.8 m3/s) Riffles Pools Parameter Design Existing Design Eksting Water surface width (m) 7.7 6.8 9.5 8.1 Maximum water depth (m) 0.46 0.55 0.97 1 .O7 Hydraulic radius (m) 0.33 0.30 0.54 0.35 Energy grade slope 0.0052 0.083 0.0005 0.0006 Velocity (mk) 0.7 1 0.9 1 0.46 0.40 Mean shear stress (Wm2) 15.8 23 -4 1.8 2.9 Maximum shear stress @lm2) 22.1 41 -4 3.4 4.7 The difference in shear stress between riffles and pools is very pronounced for both the design and existing conditions. This is the result of a significant backwater effect created by the top of rimes downstream of each pool, which can be seen in the water surface profües. The significant increase in calculated shear stress at riffles between the design geometry and the existing conditions is due to both the increase in energy grade slope, and an increase in flow depth. 5.5.6 StabiLiry Analvsis Table 5.26 surnmarizes the stability analysis for the Mill Creek snidy site. The results suggest that, for both the design geometry and the existing conditions, the sediment transport cornpetence of the channel at the bankfull discharge of 1.8 m3/s was too low to entrain any of the large (100 to 200 mm) original bed material. For the existing conditions, the results suggest that a significant fraction of the fine portion of the bed material is transported at 1.8 m3/s. The tractive force in pools is theoretically sufficient to transport the majority of the sampled fine sediment up to a particle size of approximately 6 mm. Table 5.33: Mill Creek - results of stability analysis (Q = 1.8 m3/s) Rimes Pools Parameter Design Existing Design Existing Mean shear stress (N/m2) 15.8 23.4 1.8 2.9 - theoretical particle size entrained (mm) 21 32 2.5 4 Maximum shear stress (N/m2) 22.1 41.4 3 -4 4.7 - theoretical particle size entrained (mm) 30 57 4.7 6 Median particle size - Dso (mm) 125 58 '185 O. 8 Fraction of bed materid mobilized 0% 35% 0% 295% 1 - materid on point bar 2 - fine materiai only 5.5.7 Discharge Data No long-term continuous discharge data was available for the study site, as the onIy permanent gauging station on Mill Creek is above the Shade's Mil1 Reservoir. Daiiy discharge data from the reservoir, which should be characteristic of low and average Cows through Soper Park was availabie fkom July 1985 to the present. A flow duration curve sumrnarizing the reservoir discharge data is shown in Figure 5.37. The data indicates that the daily mean discharge from the reservoir exceeds the design banldull discharge of 1.8 m3/s for the study channel 13% of the time. Storm flows through Soper Park are largely governed by drainage from storm sewers and roads that enter Mi11 Creek between the park and the reservoir. Therefore, the reservoir discharge data cannot be used to characterize the discharge regime at the study site during storms. A short period of discharge data was collected at Kerr Street, approximately 1 km downstrearn of the study site, from March 1994 to February 1995. Peak instantaneous flows of 8.0 to 9.0 m3/s were recorded during the spruig of 1994, and average surnmertime flows were between 0.3 and 0.5 m3/s (CHZM Gore & Stome Ltd., 1996). Daily mean discharge at Kerr Street exceeded reservoir discharges by a factor of approxirnately 1.25 to 2 dunng low flows and by 2 to 4 dunng flood events (zlm3/s)). This suggests that the bankfull discharge at the study site is exceeded more frequently than the 13% indicated by the reservoir discharge data. I % Total Duration -- -- Figure 5.41: Mill Creek - duration-frequency curve for Shades Mil1 Dam outflow 5.5.8 Interpretation of Results The results indicate that the existing alignment of the channel follows the course set out in the design. Some significant changes to the channel morphology are suggested by the results, including: Erosion of the channel bed in pools dong the thalweg Deposition of fine sediment in pools on the inside of bends O The formation of an inset channel smaller than &e original bankfull charnel Significant erosion of the channel bed in the downstream part of the snidy site Deposition on point bars at the inside of bends may be partly explained by the very low tractive force in pools compared to riffles, as predicted by the HEC-RAS modelling results (Table 5.32). The low velocity, energy grade slope and shear stress predicted for pools are largely due to the backwater effects of downstream riffles, which were designed with top inverts higher than the preceding upstrearn. As a result, the backwater effects of riffles are not 'drowned out' at discharges approaching bankfull. As well, the current shape of the bends appears to create an area of entirely quiescent flow over the point bar, even at higher discharges. Erosion in pools along the thalweg may have occurred in spite of the low average tractive force because of result of two- or three-dimensional bend flow characteristics that concentrate shear stress at the outside of bends. This may have been the result of the relatively small radii of bend curvaîure and the abrupt trimsitions suggested by the plan survey results. In addition, the design documentation suggests that the thalweg and the outside of the bends were not Iined with Stone, which increased their susceptibility to erosion compared with other parts of the channel. The discharge data indicates that the original bankfull discharge of l.8m3/s is exceeded at least of 13% of the time, and is like!y exceeded even more often. This suggests a return penod for the bankfull discharge of much less ehan one year. Although it is expected that the creek would become enlarged in response to frequent discharges close to bankfull, the ability of the channel to adjust within the study site is lirnited because of the large size of the bed material. Because of the nearly flat floodplain within the park, the flow energy should be dissipated over a wide area upon exceedence of the bankfull discharge. It is therefore udikely that conditions capable of mobilizing the original bed material exist except under extreme conditions. The only evidence of enlargement was the erosion of the unlined section of pools. The formation of an inset 'bankfull' channel suggested by the results is Iikely due to the effects of the urbanized and regulated flow regime in Mill Creek. The bed erosion at the downstream end the study reach indicated by the results is not explained by the hydraulic modelling, which predicts a decrease in velocity and shear stress in the downstream direction for both the design geometry and the existing conditions. The erosion may be associated with the energy dissipation properties of pools and nffles, as the length between subsequent nffles and pools increases in the downs trearn direction. The distance between nffles increases fiom 40m (5.5 channel widths) at the top of the reach, to nearly 80 m at the bottom (1 1 channel widths), which may indicate an insufilcient frequency of pool-riffle energy dissipation. However, a pool-nffle spacing of 11 channel widths is within the range observed in natural strearns. An altemate explanation is that degradation may have been initiated in the unlined sections of pools and progressed into the nffle sections by underminhg the bed. -6.0 DISCUSSION The following chapter presents a discussion of the overall findings of the study. First, the overall performance of natural channel design at the five study sites is evaluated. The frarnework for evaluating natural channel design is presented, as well as a discussion of the causes of success or lack of success at the study sites, and recornmendations for improvement. The chapter is concluded with a discussion of the effectiveness of the study methodology in obtaining data that can be used to evaluate natural channel design. -6.1 Evaluation of 'Natural' Channel Design The prima~objective for perfodng 'natural' channel design and realigning the channels was to create a stable condition, Le. that changes in the shape and location of the channels would be slow, and the rapid or catastrophic changes that are associated with unstable channels would be avoided. This was intended to provide an advantage over other types of channelization, which can create instability and require extensive maintenance. Other design objectives included the creation of natural habitat and the improvement of aesthetic values, although these were generally secondary to channel stability. By definition, the use of the tem "'natural' channel design" by the proponents of the channel realignment undertaken at the study sites implies additional objectives. One is that the designed morphology should replicate as closely as possible the equilibrium fom of the channel, given the local hycirologic and sediment transport conditions. Another is that the physical function of the channel should be maintained or restored so that it can respond in a controlled manner to changes in independent variables through negative feedback mechanisms. In order to evduate the performance of natural channel design at the study sites, the designs were judged on their success in meeting the objectives stated above. A discussion of each objective is presented in the following sections. 6.1. 1 Channel stability From construction until the time of the study, the realigned Stream reaches within the study sites were largely successful in preventing the types of changes that characterize unstable channels, such as rapidly eroding and failing banks, major plan form changes, or widespread bed erosion or deposition. At al1 of the sites, the results indicated that the charme1 alignment had remained relatively unchanged. Some changes to channel morphology were suggested, but the effects were not catastrophic and were most often localized. Although stability was achie~idat the sites for periods of between 4 to 7 years, the long-tem channe1 stability at some sites is uncertain. In many instances, reinforcing material or structures that have maintained the channel form have failed or are in a state of impending failure. Because the form of the channels was often maintained in large part by these fixed materials or structures, it is questionable whether natural adjustment processes will act to maintain or to alter the current morphology once the control elements are removed or lose effectiveness. It is possible that undesirable and uncontrolled changes rnay occur which may be propagated within the study sites as well as upstream and downstream. Examples of this type of failure or irnpending failure include: Failure of root wad revetrnents due to bank erosion in Little Etobicoke Creek. Erosion near vortex rock weirs in Little Etobicoke Creek from pool deepening. Flanking of a live log-cnb wall in Laurel Creek through bank erosion Formation of a secondary channel in the lower section of Groff Mill Creek Underrnining of Iarge sediment on a riffle at the downstrearn end of Mill Creek 6.1.2 Habitat Improvernent and Aesthetic Values Detailed evaluation of habitat quality and ecosystem health was beyond the scope of the study. However, the results suggest that, with the exception of the Henry Sturm Greenway site, the designs were successful in producing topographic and hydraulic variability sirnilar to natural strearns with diverse aquatic ecosystems. Aesthetic values were almost certainZy irnproved, when compared to the unstable conditions that characterized most of the study sites prïor to the implementation of natuml channel design. In general, the banks of the channels appeared to be stable, with well-established riparian vegetation. The regular, sinuous course of the channels within the study sites was also visually appealing in most cases. 6.1.3 Success in Replicatirr~Equilibriurn Morpholo,q The results for ail of the study sites suggested that significant changes to channel morphology had occurred, primarily to cross-sectional shape, and bed configuration, but also to channel slope and plan form in some cases. These changes may indicate that the channels were undergoing adjustment in response to the prevailing hydrological and sediment supply conditions. However, because the scope of the study did not aUow for repeated measurements over tirne, it was not possible tcj detennine whether the changes were a reflection of short-term, dynamic response or long-tem adjustment towards a different equilibrium form. In addition, the large boundary material and other reinforcement techniques moderated the rate and magnitude of change. As a result, the processes of adjustment may still be ongoing at the study sites and the degree to which the designs were successful in replicating the equilibrium form could not be fully determined. Although the results did not provide conclusive evidence, a number of inconsistencies and assumptions in the design documentation for the study sites suggested that there may have been significant misinterpretation of the equilibrium geometry. In particular, the methods for selecting a design bel1discharge were often suspect, such as the estimation of bankfùll discharge within unstable sections of channel. In some cases, hydraulic modelling results indicated that the bankfull capacity for the design geometry was significantly different than the value used as the bais for the design (e-g. Mill Creek, Little Etobicoke Creek). As well, the frequency of the design bankfull discharge was rarely verified with other discharge data, although in many cases this would have suggested that design values were excessively low when compared with the 1- to 2-year return penod discharges. Additional potential for error was discovered in the methodologies used to select the morphological characteristics of the reaiigned channels. AU five designs used the Rosgen classification system, in which typical values for width to depth ratio, sinuosity, and meander wavelength were selected for the desired Stream type. Channel slope was specified only indirectly through the sinuosity, although it is widely accepted in fluvial geomorphology that slope is an integral aspect the equilibrium state of natural alluvial strearns. Slope has been shown to be a fûnction of bankfull discharge, size of the bed material, and bed-load transport rate (Bray, 1982; Hey and Thorne, 1986; Chang, 1988), but none of these factors were taken into account in selecting design slope. As a result, the sediment transport characteristics (and the discharge capacity) of the designed channels may not have been consistent with the rernainder of the stream. This was thought to have caused the aggradation that was observed some of the study sites (e-g. Laurel Creek, Groff Mill Creek). Another inconsistency in the use of the Rosgen classification system was that it was developed based on observation and analysis of mal streams, whereas al1 of the study sites were located in urban areas. The formation of inset 'bankfull' channels at most of the sites suggested the influence of urban hydrological characteristics. Because Little is known of the equilibrium morphology of urban streams, it may not be appropnate to base 'natural' channel design on the Rosgen system or other rnorphological relationships derived from rural rivers and streams. These methods are generally based on a single value for bankfull discharge, while there may be more than one geomorphically significant discharge in urban channels (MacCrae, 1997). Furthermore, the 'semi-alluvial' properties of strearns in Southern Ontario (Section 2.5.5) may also preclude the use of these techniques. There was dso no documentation for any of the designs suggesting the use of reference reaches, in the sarne stream or in nearby streams, to verim morphological dimensions. In the case of the Henry Sturm Greenway site, the properties of the original channel were documented, but not used in the design. The form of the constructed channel reverted to that of the original channel, suggesting that the reference reach technique could have provided valuable information regarding the equilibrium morphology at this and other sites. For al1 of the study sites, the design of the pool-rime sequence involved only the placement of pooIs in meander bends, the inclusion of point bars and the use of appropriate spacing (between 4 and 11 bankfull widths). The selection of bed configuration and other aspects of cross-sectional shape appeared to have been completely arbitrary. Pools at al1 of the sites were designed to be wider than rimes at the bankfull stage. The longitudind confiaauration of pool-rime sequences, except at the Little Etobicoke Creek site, was such that a si,gnificant backwater existed over pools at the bankfull discharge. Hydraulic modelling results suggested that this would cause significantly lower velocity and shear stress in pools compared to riffles for most of the designs at the bankfull discharge. However, in natural gravel-bed streams, riffles are typically wider and shallower than pools at al1 stages of flow, while water surface slope, velocity, and shear stress show a tendency to converge in riffles and pools at the bankfull stage (Richards, 1976; Carling, 1991, Keller and Florsheim, 1993). This discrepancy may have resuhed in the widening of nffles and narrowing and deposition in pools observed at Laurel Creek, Groff Mill Creek, and Laurel Creek. -6.1.4 Restoration of Phvsical Function Zn general, the coarse rnaterial used to form the channel boundaries li-mited the physical function of the designed channels. Stability analysis results for all five sites indicated that little or none of this boundary rnaterial could be transportai at the design banMull discharge. This suggests physical dysfunction because the bankfull discharge is by definition the discharge that should do the rnost work in transporting sediment and modifying the channel, Because the boundaries are largely fixed, the channels are limited in their ability to adjust their cross-section, plan form, and longitudinal profile. This suggests that the energy of the flowing water could not be properly used to shape the chamels in response to hydrological and sedirnent transport conditions to maintain a state of dynamic equilibrium. Channel change at the study sites was therefore concentrated in areas of low resistance which were not reinforced, which may potentially result in failure in the long terrn. The irnmobility of the boundary material in riffles mây have also had implications to the process of downstream sediment transport within the study sites. At discharges approaching bankfbll in naturd grave1 bed streams, particles are transported downstream from one riHe to the next while the position of the riffles remains essentially constant (Keller and Melhome, 1978). At the study sites, the material used to constmct the riffles would not have been mobile at the bankfull discharge, which may have rekicted the exchange and downstream transport of particles from the upstream channel bedload. This appeared to have caused erratic patterns of aggradation and deposition of coarse bedload material, particularly at the Laurel Creek study site. Typically, natural gravel-bed streams will form an arrnour or pavement layer of coarse material approximately one grain diameter in thickness on the surface of the bed, which restricts the rate of sediment transport at low fiows. However, at discharges approaching bankfull, the armour Iayer is entrained by the flow, causing the bed to become mobile (Parker and Klingeman, 1982). Andrews (1983; 1984) observed that bed particles ranging from the median size to the 90' percentile fraction are enuained by the bankfull discharge in natural nvers. In contrast, the material used to form the channel boundaries at the snidy sites does not allow for mobilization of the bed during discharges approaching bankfüll. If natural channel design is the objective, channel boundaries should be constmcted of material that can realistically be mobilized so that the channel can fieely adjust and the continuity of sediment transport throughout the strearn system can be maintained. The degree of mobilization would Vary depending on the sediment supply to the channel frorn upstream areas. -6.2 Effectiveness of Studv Methodoiom The study was successful in obtaining data that could be used to evaluate some aspects of long-term performance of natural channel design. However, a number of areas were identified where additional information could have significantly improved the quality of the evaluation. The effectiveness of each of the tasks performed in the investigation of each study site is discussed in the following sub-sections. 6.2.1 Preliminary Inspection The preliminary inspection was generally useful in identiQing visual indicators of major channel change or instability. In some cases, the changes observed would not have been captured by the topographic survey, and the results were often used to improve the interpretation of other data. The inspection was more successful when photographs documenting the condition of the channel shortly after construction were avaitable for cornparison with the existing conditions. Repeated senes of photographs (Le.. on an annual basis) from monumented locations were not available for any of the study sites. These would require little effort to obtain and would have been very useful for documenting the process of channel change. 6.2.2 Topographic Su rvev Topographic survey measurements provided detaïled depictions of the existing cross-sectional, longitudinal, and plan forn geometry of the chamels within the study sites. However, because little or no as-constructed baseline data was available for cornparison, it was difficult to quuititatively assess channel change as design drawings did not necessarily represent the original condition of the channel. Cornparison of existing and original conditions was therefore limited to more general characteristics and obvious signs of change. More confident predictions of change could be made where as- constructed data was available (eg.Groff Mill Creek, Mill Creek). Survey measurements would be much more effective in evaluating the performance of natural channel design if comprehensive baseline data was availabie. As- constructed surveys of cross-sections at pools and the top and bottom of nffles and the longitudinal profile of the channel along the thalweg would dramatically improve the quality of results from evaluation studies. Repeated measwernents on an annual or biannual basis, would allow the association of channel change with particular conditions or events. Kondolf and Micheli (1995) and Annable (1999) have previously recornmended that the above practices be undertaken for monitoring of natural channel design and Stream rehabilitation projects. Modem surveying and GIS technology could improve the utility of topographic survey measurements in channel evaluation through the use of three-dimensional, digital topographic modelling @TM) techniques. DTM representation of the channel and its surroundings in three-dimensional space wouid dow cross-sectional or longitudinal profile data to be extracted at any location. Contour maps could be crated to show the topogaphy of the channel in detail. Lane and Chandler (1994) and Keim et al. (1999) have had success in monitoring natural channel changes using DTM through calculation of the volume of eroded material rather than through one- or two-dimensional analysis. DTM data could also improve the resolution of channel geometry for modelling with water surface profile software such as HEC-RAS, or for two- and three-dimensional modelling routines which require three-dimensional geometry data. However, the time and effort required to collect DTM data may not be justifiable for many studies, particularly when the quality of baseline information with which to compare the results is poor. 6-2.3 Sediment samplin~ Sediment sampling using the pebble count technique was usefd in detennining the distribution of sediment particle sizes over relatively large areas without requiring a large number of bulk samples. The results were useful in deterrnining changes in bed composition when the approxirnate original particle size distribution was known. In cases where there was an appreciable quantity of fine sedirnent on the bed, the grain size distribution curve was somewhat truncated as it was not possible to meaure particle sizes e smaller than approximately 2 mm using this method. The use of the technique was generally not appropriate for pools, because of the depth of water and small bed material. Where it was used, bulk sarnpling was effective in detennining the particle size distribution of fine material in pools. Determination of the relative size distribution in pools compared to riffles was useful, in addition to the resuits of hydraulic modelling, for assessing the variation in sediment transport over the pool-nffle sequence. However, particle sizes were often heterogeneously distributed throughout pools and sampling at only one location was not necessarily representative of the entire pool. 6.2-4 Hvdraulic Model1in.q Modelling with HEC-RAS was useful for comparing the one-dimensional hydraulic characteristics of the design geometries and the existing channels. In many cases the modelling results of energy grade slope, velocity, and shear stress were used to explain obsemed changes. The mode1 results were particularly useful in identifying the variability in hydraulic characteristics between pools and riffles. However, because the mode1 parameters were not calibrated to known water surface elevations, the numerical values obtained for various parameters could not be considered highly accurate. In addition to hydraulic charactenstics, HEC-RAS was also usefbl for estimating the bankfull discharge capacity for the design geometries. In many cases, the results suggested that channel capacity of the designs had not been thoroughly verified pnor to implementation. At most of the study sites it was apparent from the observations that two- or three- dimensional flow characteristics had significantly affected the magnitude and direction of channel change, particularly in bends. In these cases the one-dimensional results from HEC-RAS could not be used to explain the cause or the location of channel changes. More advanced two- or three-dimensional modelling and detailed field measurements wouId be required to identiQ the flow characteristics responsible for such change. However, such advanced modeiling is beyond the scope of most small channel redignment projects and evaluation studies. 6-2.5 Stability Analysis Stability analysis was most useful in determining that the size of the material used to construct the channel boundaries, at al1 of the study sites, was too large to be transported by discharges approaching bankfull. The results dso suggested that the differences in particle size distribution between pools and riffles were associated with variability in sediment transport cornpetence- Calculated maximum shear stress suggested that, considerable larger particles could potentially be transported at dlschvges approaching bankfull, although in most cases this was still insuffkient to transport the original bed materiai. The validity of the stability analysis results is questionable because of the uncertainty in the use of critical dimensionless shear stress. While the r*of 0.045 used in the study is a reasonable average value for channels, values of between 0.020 and 0.25 have been observed in practice (Andrews, 1983). Other research suggests that critical shear stress for entraiment is affected by the size distribution of the bed material and particle shape (Andrews, 1983) and may differ significantly between pools and riffles (Sear, 1996). As a result, estimates of particle sizes entrained cannot be considered highly accurate. However, it can still be concluded that the majority of the original bed material used in the designs would be immobile during discharges at or close to bankfull. 6.2.6 Dischar~edata The most cornmon form of discharge data available for the study sites consisted of hydrologie mode1 predictions for flood events, generally with a retum period of two years or more. Continuous discharge data was not available for most sites, but extrapolation of data fiom nearby streams allowed the tentative prediction of the magnitude of discharges down to a 1-yr return period. For most of the sites, the results were usefui in sbowing that the return period of the design bankfull discharges was si,onificantly less than one year, suggesting that most of the channels had not been designed with sufficient capacity. However, the me bankfull discharge of the study streams could not be estimated using the typical 1 to 2 year retum penod because the bankfuil return period of urban streams is highly variable (MacCrae, 1997). Continuous discharge data for the period the designs had been in place would have improved the quality of the evaluation. Channel stability and sediment transport within the study sites could have been more comprehensively analyzed according to discharge frequency and duration. Because the majority of the study sites were not gauged, continuous hydrological modelling would have been required to generate appropriate discharge data (e-g. Badelt, 1999). However, the necessary data collection and cdibration for continuous modelling of five different watercourses was beyond the scope of the study. -7.0 CONCLUSIONS The primary objective of this research was to evaluate the long-term performance of 'natural' channel design at different locations. This was accomplished by comparing existing channel conditions with baseline data for fîve sites in Southwestern Ontario where 'natural' channel design had been implemented. The existing channel morphology was charactenzed through detailed topographie surveys and bed sediment sampling and was compared with the original channel condition using available baseline data. The original and existing hydraulic characteristics of the study channels were compared by calculation of water surface profiles at equal discharges using HEC-RAS. The results of the HEC-RAS modelling were used to analyze the stability of channel boundary materiai and to generate hypotheses for the causes of observed channel changes. The following conciusions were drawn from the results of the study: 'Natural' channel design, as applied at the study sites, was successful in maintainhg overall stability of the strearn channels for periods ranging fiorn 4 to 7 y. The long-term stability of the designs is questionable given a number of indications of recent or impending failure at some locations. Most aspects of charme1 change could not be accurately quantified at the study sites because of a lack of accurate baseline data describing the original, as-constructed condition of the channel. It was not possible to associate channel change with particuiar flow events or time penods because of limited discharge data and an absence of data to describe the fom of the channels between the time of construction and the time of study. At all of the study sites, most of the charme1 boundary surfaces were constructed with material that could not be transported at the design bankfull discharge. Natural physicd function was not achieved at the study sites because of the large size of the boundary material. The ability of the channels to adjust to the prevailing hydrologie and sedirnent transport conditions through channel enlargement or plan form change was highly restricted. It was not possible to determine the success of the designs in creating appropriate equilibriurn geometry because channel change was prevented or limited by the fixed boundary material. The bankfiI1 discharges used as the basis for the 'natural' channel designs generally had retum periods of siojpïficantIy less than one year, compared with typical bankfull return periods of 1 to 2 years for natural, rural streams. The use of smaller flows for design may result in insufficient sediment transport capacity and out-of-channe1 erosion. The methods used to predict the design bankfull discharge are inconsistent and suggest significmt potential for error. The use of the Rosgen Classification System for 'natural' channel design is arnbiguous and allows for significant subjectivity on the behaif of the designer. Channel dope is not incorporated in the design methodology although it is a criticd characteristic of the equilibrium form of natural streams. The use of the Rosgen system or other empirical relationships that have been developed for rural streams may not be appropriate for urban settings where streams have different morphological characteristics. 13. The longitudinal configuration of the pool-riffle sequence was arbitrarily selected in the designs, and modeIling indicates that the pool-riffle hydraulics were different from those of natural streams. The results suggest that inappropriate pool-riffle form may result in unusual channel changes, in pa~ticularirregular sediment deposition. -8.0 RECOMMENDATIONS Recornmendations ensuing from the study results are presented in two parts. The first relates to the methodology for 'naturai' channel design and the second relates to monitoring and evaluation of channels subject to 'natural' channel design. The following improvements are suggested for the application of naturd channel design techniques, based on the designs evaluated in the study: 1. Bankfull discharge estimates to be used as a basis for naturd channel design should be performed in stable channel reaches, upstream or downstream of the project area if necessary. Results shouId be cornpared with other discharge data to assess whether the return period of the estimated bankfull value is realistic. 2. One-dimensional hydraulic modelling should be used in natural channel design to veri@ the bankfull capacity of the channel, the hydraulic characteristics of the pool- riffle sequence, and the mobility of the bed material. 3. Reference reaches should be used to determine whether the hydrologic and sediment supply conditions of the Stream are associated with unique rnorphological characteristics. Rese characteristics should be incorporated into the design geornetry to achieve the best possible replication of the equiIibrium form. 4. 'Natural' channels should be constmcted with bed and bank materid that can be mobilized and transported by discharges around bankfull. If a reasonable prediction of the equilibrium geometry has been made, the channel should adjust in a controlled manner to the prevailing conditions. If the channel is meant to be static, there may be little or no advantage, from a stability viewpoint, to using pseudo-'naîural' channel design techniques with large boundary material over traditional engineering practices. The following recornrnendations are made regarding monitoring and evaluation activities related to natural channel design: 5. The as-constnicted condition of channels realigned with 'natural' charnel design should be thoroughly documented, including surveys and photographs. The results are essential for evaluating long-term channel change, and are also important because the most significant changes may occur shortly after construction, 6. Survey measurements and photographs should be repeated at least once per year so that rates of channel change can be calculated and related to temporal changes in watershed conditions. 7. 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APPENDIX A .Sup~lementaw data for Laurel Creek Bed matenal smpling particle size distribution curves ...... 179 Cross-section survey results ...... 181 Sarnpling location 1 1O 100 Partick Süe (mm) Sampling location 2 10 100 Particie Size (mm) Sarnpling location 3 1 10 100 Partick Sue (mm) Sampling location 4 1 10 Partick Sue (mm) Sampling- location 5 0.01 0.1 1 1 O 100 Particie Size (mm) Sam~linolocation 6 Particle Size (mm) 5 1O 15 20 Dishna (m) SECTION 3 SECTION 4 SECTION 6 310.00 309.50 - g 309.00 3- -f 308.53 LU 308.00 307.50 O 5 1O 15 20 üistancc (m) SECTION 8 5 10 15 20 ûi$tancs (m) EXISTING CHANNEL -- EXISTING BANKFULL LEVEL DESIGN CHANNEL -- DESIGN BANKFULL LNEL SECTION 9 O 5 10 1s M Dstanca (m) SECTION 11 SECTION 12 309. - 309. 308. 5 O -: W. W 307. 307. O 5 10 15 20 DuUt~e(m) SECTION 13 SECTION 14 EXISTING CHANNEL -- EXISTING BANKFULL LNEL DESIGN CHANNEL -- DESIGN BANKFULL LEVEL SECTION 17 O S 1O 15 20 Distanat (m) EXISTING CHANNEL -- WlSTlNG BANWULi LEVEL - -- DESIGN CHANNEL -- DESIGN BANKFULL LEVEL APPENDIX B .Supplementam data for Little Etobicoke Creek Bed material sarnpling particle size distribution curves ...... 185 Cross-section survey results ...... 187 Sampling location 1 1 1O 100 Particie Sue (mm) Sampling location 2 10 100 Particle Sue (mm) Sarnpling location 3 Sarnpling location 4 Particle Sue (mm) Sampling location 5 4 1O 100 Particie Sue (mm) SECTION 2 O 5 1O 15 20 Distance (n) Distance (m) SECTlON 3 O 5 1O 15 20 25 SECTION 5 Distance (m) Distance (m) SECTION 8 Distana (m) Ditance (m) EXISTING CHANNEL ---- EXlSTlNG BANWULL LEVEL . -- .- -. DESIGN CHANNEL -- - - DESIGN BANKFU LL LEVEL SECTION 70 O 5 10 15 M 25 Ohtance (m) Distance (rn) SECTION 11 SECnON 12 O 5 1O 15 20 25 O 5 10 15 2! Dhtanat (m) Dittance (m) SECiION 13 SECTION 14 O 5 1O 15 20 25 O 5 10 15 20 21 Dktanœ (m) Distance (m) SECTION 16 O 5 1O 15 20 Distanca (m) Ostance (m) - EXISTING CHANNEL - - DISllNG BANWULL LEVEL .. - DESIGN CHANNEL .. - DESIGN BANWULL LEWEL SECIION 17 O 5 10 15 20 25 Distance (m) O 5 1O 15 20 E Distance (m) SECTION 21 O 10 15 20 25 Distance (m) - EXlSTlNG CHANNEL -- EXISTING BANKFüU LEVEL DESIGN CHANNEL --- - - DESIGN BANKFUU LEVEL APPENDIX C .Siipplernentar~ data for Groff Mill Creek Bed material sampling particle size distribution curves ...... 191 Cross-section survey results ...... 193 Sampling location 1 1 10 100 looo Partick Sue (mm) SampIing location 2 1 O 100 Particie Sue (mm) Sarnpling location 3 10 100 Particle Sue (mm) Sarnpling location 4 Particie Sue (mm) Sampling location 5 Partieie Size (mm) [)rrturi (m) SECTION 3 - EXlSTlNG CHANNEL -- EXlSTlNG BANKFULL LEML - . DESIGN CHANNEL -- DESiGN BANWULL LEVEL SECTION 11 1 5.00 10.00 15.W Ikiancm (rn) 281 6 - 281 .O j 2=5 2 zm.0 279.5 0.00 5.00 10.00 15-00 3 mana (m) Obmnm (m) EXISTING CHANNEL -- EXISTING BANWULL LEVEL -. - DESIGN CHANNEL .--- -- DESIGN BANKFULL LWEL EXlSTlNG CHANNEL -- EXISTING BANKFULL LEVEL DESIGN CHANNEL . - DESIGN BANKFULL LEVEL APPENDIX D .Supplementarv data for Henw Sturm Greenway Bed material sampling particle size distribution curves ...... 197 Cross-section survey results ...... 198 Sampling location 1 0.01 o. 1 1 10 Particle Size (mm) Sarnplin~Iocation 2 1 10 Partick Sire (mm) Sampling location 3 Partick Size (mm) SECTION 2 SECTION 3 SECTION 4 362.0 361.5 361 .O 360.5 O 2 4 6 8 SECTION 5 SECTION 6 SECTION 7 SECTION 8 EXlSTING CHANNEL -- EXISTING BANKFtlLL LEVEL - - DESIGN CHANNEL .- - DESIGN BANWULL LEVEL SECTION 9 SECTION 10 SECTION 11 SECTlON 12 361 -5 . 361.O 360.5 360.0 SECTION 13 SECTION 14 361 -5 361.5 361.O 361.O 360.5 360.5 360.0 360.0 O 2 4 6 8 O 2 4 6 8 = SECTION 16 EXlSTlNG CHANNEL -- EXlSllNG BANWULL LEVEL -- - - DESIGN CHANNEL DESIGN BANWULL LNEL SECTION 18 SECTION 19 SECTION 21 EXISTING CHANNEL -- EXISTING BANKFULL LEVEL - DESIGN CHANNEL --- . - .-. DESIGN BANKFULL LEVEL APPENDIX E .Supplementarv data for MiIl Creek Bed material sampling particle size distribution curves ...... 202 Cross-section survey results ...... 204 Sarnpling location 1 IO 100 Partick Sue (mm) 100 - A Aw 80 -. 60 -- Sampling location 2 40 - 20 -. O i 1 10 100 Io00 Partick Sue (mm) Samplin~location 3 Particle Size (mm) Sampling location 4 0.01 0.1 1 10 100 Partick Size (mm) SarripIiw location 5 0.01 0.1 1 1O 100 Partick Sue (mm) Sarnpling location 6 0.01 O. 1 1 10 Partick Size (mm) SECTION 2 m.o = 276.5 u 6 276.0 2 " 2755 275.0 O 5 10 15 20 oDtanœ (rn) SECTION 3 SECTION 4 m.o -Ê 276.5 276.0 P LU 2755 275.0 O 5 10 15 20 Disena, (m) DirPnce (ml SECTION 5 SECTION 6 m.o 277.0 Ê 278.5 = 276-5 u - 5 276.0 276.0 : P "j 275.5 275.5 n5.o 275.0 O 5 1 O 15 O 5 10 15 20 oirtanes (m) SECTION 7 O 5 10 Di- (m) EXlSTlNG CHANNEL -- EX1STING BANKFU LiLEVEL DESIGN CHANNEL . -- DESIGN BANWULL LEVEL SECTION 9 Ohnue(m) SECnON 12 Wnca(m) SECnON 14 O 5 1O 15 M O 5 10 15 Distance (m) SECTION 16 2765 -g 276.0 C nss -> 275.0 2745 O 5 10 15 20 oisance (m) - EXISTING CHANNEL -- EXISTING BANKFLJLL LEVEL DESIGN CHANNEL . -- DESIGN BANKFULL LEVEL 276.0 -= 275.5 2 275.0 iZ 2745 274.0 O 5 1O 2l Distanoc (m) Distance (rn) EECIlON 19 SECTION M Distance (m) Datance (m) EXISTING CHANNEL -- EXISTING BANKFULL LEVEL - DESIGN CHANNEL - .- DESIGN BANKFULL LNEL