River Bank Erosion Processes Along the Lower Shuswap River

RIVER BANK EROSION PROCESSES ALONG THE LOWER SHUSWAP RIVER

FINAL PROJECT REPORT Submitted to Regional District of North Okanagan

October, 2014

Hilary Cameron and Bernard Bauer University of British Columbia Okanagan

Executive Summary

The Lower Shuswap River is one of many rivers in the interior of British Columbia experiencing chronic bank erosion, which leads to land loss, aquatic habitat degradation, and water quality challenges. Local land owners believe that bank erosion is due to the intense levels of recreational boating traffic during the summer, which is an issue that has been identified as a serious management concern (Shuswap River Watershed Sustainability Plan, 2014). A field study conducted in 2013 (Laderoute and Bauer, 2013; Laderoute, 2014) quantified boat-related erosion along the Lower Shuswap River during the primary boating period (July - August). The overriding objective of the current research was to complement the earlier study by assessing the rate of bank erosion during the spring freshet (May – July) and to assess the near-bank flow mechanisms that may be responsible for erosion. The primary hypothesis upon which this study hinges is that the period of most substantial bank erosion during an annual cycle occurs during the spring freshet rather than the summer boating period. Should the primary hypothesis prove incorrect, then there would be greater reason to assert that boating traffic may be an important cause of bank erosion along the Lower Shuswap River. Boating traffic was monitored at upstream and downstream locations using remotely triggered cameras. During the peak of the spring freshet, when shear stress values acting on the bank are expected to be greatest, velocity profiles were measured and later used to solve for the boundary shear stress acting on the bank. To track erosion rates, erosion pin and bank profile measurements were continued from Laderoute and Bauer (2013). This project report contains a comprehensive literature review (Chapter 2) that describes natural processes associated with river meandering and bank erosion. The remaining chapters describe the methods used (Chapter 3), the results including data tables and figures (Chapter 4), and a brief discussion of the findings (Chapter 5). The report should be ready in combination with the earlier work by Laderoute (2014), which describes the mechanics of boat-wake-induced erosion in greater detail. Overall, the data indicate the following:

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(1) the near-bank shear stresses during the spring freshet (high river stage) are too small to initiate sediment motion by direct hydraulic action and therefore are not sufficiently strong to cause erosion at the study site; (2) the period between early to late July is associated with both rapid decrease in discharge and rapid reduction in river stage as well as a significant increase in boating traffic; (3) during the July drawdown period, there appears to be a discernible tendency for erosion of the upper bank regions and sediment deposition on the lower bank 'aprons' or 'terraces'; (4) during August, the lower bank 'aprons' are scoured free of sediment deposits by repetitive boat-wake waves whereas the upper bank regions are not affected by wave action because the water levels are too low; (5) the influence of boats in late August and early September declines markedly because shallow water precludes the use of the lower reaches of the river for recreational purposes other than kayaking and canoeing, and (6) for the majority of the year (September – May) the river banks are not affected by river flows or boat wakes because the water levels are too low to produce significant stresses on the bank materials, although subaerial processes (e.g., freeze-thaw, runoff, vegetation growth, burrowing animals) may be active and important.

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Acknowledgements

The following landowners generously provided access to their properties:

Hermann and Louise Bruns (Wild Flight Farm) Corinne De Ruiter (Springbend Farms) John and Maryanne (The Old Mara Train Station B&B) Lori and Leo Konge (Viking Farms)

Anna Page and Laura Frank (North Okanagan Regional District) have provided generous time and encouragement for the project. They have played a key role in ensuring that the project went forward.

Financial support for logistics and materials came from the Regional District of North Okanagan. Ms. Cameron was supported by an Undergraduate Student Research Award from the Natural Science and Engineering Research Council, Canada. Additional financial support came from internal grants to Dr. Bauer from the University of Brisith Columbia Okanagan.

Bob Harding (Fisheries and Oceans Canada) is thanked for providing the remote cameras that allowed image capture of boat-wake traffic, and for general advice regarding regional concerns.

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Contents

Chapter 1 Introduction Pages 1-2

Chapter 2 Literature Review Pages 3-22

Chapter 3 Methods Pages 23-40

Chapter 4 Results Pages 41-72

Chapter 5 Discussion and Conclusions Pages 74-81

Chapter 6 Bibliography Pages 82-86

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1. Introduction The Shuswap River is the largest river that flows into Shuswap Lake, with a drainage basin area of 1969 square kilometers and a total length of 195 km (Kramer, 2003). The Shuswap River is important ecologically, economically, and socially. From the ecological standpoint, it hosts many fish species, is an important spawning habitat for salmon and provides habitat to a number of threatened and endangered species. Economically, the Shuswap River is used for hydroelectric power generation, supports timber and agricultural resources and also brings tourists into the region for recreational purposes, including boating, swimming, paddling and fishing. Culturally and historically, the Shuswap River has been used by the Splatsin First Nation peoples, who used the river as a transportation corridor as well as a source of food (Shuswap Nation Tribal Council, 2014). The Shuswap River is generally broken down into three sub-sections : the Upper, Middle and Lower Shuswap River. The Lower Shuswap River is considered to be the 75 km stretch of river that extends from Mabel Lake to Mara Lake. The Lower Shuswap River floodplain largely consists of farmland, although it also hosts the city of Enderby and other small urban areas such as Grindrod and Mara as well as significant transportation infrastructure. The Lower Shuswap River is representative of a number of rivers in the south- central interior of BC that experience chronic bank erosion. Excessive erosion is detrimental in so far as it can cause property damage, lead to water quality issues, and undermine the integrity of aquatic and riparian ecosystems. Many believe that recreational boating is increasing the natural erosion rate of the bank. However, it is well known that rivers naturally sculpt and shape the landscape through which they flow. Although human development is capable of progressively altering the dynamic environment in which rivers perform work, it is important to determine if and how river bank erosion has been altered by these activities. During the summer of 2013, an intensive study was performed on a reach of the Shuswap River near Mara, BC, by Laderoute and Bauer (2013). They assessed the intensity of recreational boating activity and its impact on bank erosion by monitoring boat traffic and tracking erosion rates at nine locations along the Lower Shuswap River. Although their work showed a high volume of boat traffic and

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consequent erosion during the summer season (July-September), the results regarding the impact of boats relative to natural bank erosion are ambiguous because no information was available on the erosive impact of the spring freshet (the annual high- water period during snowmelt). The goal of the current study was to redress this shortcoming by monitoring the near-bank flow conditions and resulting erosion from late March through early September, which includes the spring freshet period, as well as extending the bank erosion time series initiated last year. The primary working hypothesis is that hydraulic action during the high flow period causes substantial bank erosion that dominates the annual cycle of bank change. If the field evidence indicates that erosion is negligible during the spring freshet, then other mechanisms of bank erosion (e.g., boat wakes) must be implicated as relatively more important. The results from this study will have important ramifications for management strategies intended to minimize or mitigate the chronic bank erosion challenge on the Lower Shuswap River.

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2. Literature Review 1. Introduction Riverbank erosion along the Lower Shuswap River is of concern for the local community as it threatens to damage or degrade private property, public roads, fragile ecosystems, and water quality. However, large and uncontrolled channel erosion rates are not just a local problem. In fluvial systems, a very large portion of the sediment in the water is supplied through erosion of the riverbanks (Gardiner, 1983). In fact, as much as 85% of the sediment yield in a watershed can originate from stream-bank erosion alone (Clark and Wynn, 2007). Every year in North America, $16 billion is spent on water pollution damage caused by too much sediment in the water, which ranks as the second biggest pollutant after bacteria (Clark and Wynn, 2007). Large sediment concentrations not only decrease water clarity in streams, but can be damaging to aquatic ecosystems because it inhibits the ability of fish to find food, reduce light availability for aquatic plants, decrease the amount of dissolved oxygen in the water and change the water temperature (Laderoute and Bauer, 2013). In British Columbia, there is large concern for increased sediment concentration in the water because of its impact on salmon. Increased sediment loads of fine-grained material reduce spawning potential and decrease the incubation habitat quality of salmon (Nelitz et al., 2007). Salmon are very important economically, culturally and ecologically to many regions in the Pacific Northwest. In order to improve water quality management, it is necessary to improve channel erosion predictions, which will make it possible to calculate the sediment load in the river (Clark and Wynn, 2007). Bank erosion can also damage riparian habitat. The riparian zone is the transitional zone between dry land and the water channel. The riparian zone is important because it can enhance the water quality by trapping sediments and filtering pollutants. It can also provide aquatic and terrestrial habitat, and produce shade, which keeps water temperatures cool for fish. Damage to the riparian zone can create a positive feedback loop by further increasing erosion rates because the roots of trees and shrubs along the river bank provide internal bank strength (EPA, 2012). Land loss due to bank erosion is of increasing concern for property owners. For example, the Matanuska River in Alaska has eroded private properties over the last few decades as well as a major regional highway and farmland (Curran and

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McTeague, 2011). The financial burden falls on the local landowner whose physical workload can increase by repairing their damaged property. The costs of preventing erosion can also be substantial. The Alberta provincial government recently allocated $116 million to flood erosion control to help repair the damage from the June 2013 floods and to prevent the impacts of future flood events (Water Canada, 2013). Due to the expense involved with installing structures to decrease or reverse erosion damage, perhaps the best strategy is to stop activities that are known to cause or enhance erosion. There are many factors that influence erosion rates but the most important driving factor involves the fluid flow, including the magnitude, frequency and variation in stream discharge. Fluid flow properties also include the shear stress distribution that the flowing water exerts on the bank, the amount of turbulence in the water and the presence of waves. Other factors that impact the degree of river bank erosion is the bank material composition (the texture, sorting, stratification, chemistry and cohesion of the soil), climate (rainfall patterns and freeze thaw cycles), biological influences (root systems and animal activity such as burrowing), subsurface hydrology (pore pressure, soil moisture), channel geometry (width, depth, slope, and degree of bending of the channel) and human influences (urbanization, agriculture, boating and bank protection structures) (Knighton, 1998). The majority of these variables change over the longitudinal profile of the river or from season to season. When and where the most intense erosion is likely to occur can be important for setting up erosion prevention structures or giving warning to people for when it may be dangerous to approach a river. It is important to understand that rivers naturally change their form and structure over time. Much effort has been put forth to derive relationships between flow patterns, bank characteristics, and velocity profiles in order to roughly estimate how river banks will erode. The remainder of this chapter will explain the natural processes of meandering rivers and discuss modern research methods being used to predict riverbank erosion.

2. River Dynamics and Bank Erosion 2.1 River Meandering Rivers naturally erode their banks and change their morphology to reach equilibrium with the ever-changing conditions imposed on the stream. In a rigid 4 channel, such as a bedrock channel, the variation in discharge has to be accommodated within the physical dimensions of the fixed channel geometry, typically by changes in the depth of flow or the speed at which water moves down the channel. However, alluvial channels (i.e., those situated in floodplains with erodible banks) can accommodate changes in the imposed discharge by eroding their banks or the bed, and subsequently deposit the eroded materials in new locations. Meandering rivers, such as the Shuswap River, are very common, typically along reaches where there is a gentle slope and the bed and bank materials are erodible and cohesive.

Figure 2.1 Shear stress distribution along a meandering river (from Knighton 1998, after Dietrich 1987).

The dynamics of river meandering are well understood by geomorphologists based on more than 100 years of empirical and theoretical work. In meandering rivers, the thalweg impinges on the outer bank of the meander bend. This fast flowing water has enough energy to erode material on the outside of the bank and transfer it downstream. Meanwhile, the slow moving water on the inside of the bank has very little energy. This velocity pattern causes an uneven distribution of shear stress on the cross-sectional and longitudinal profile of the river (Figure 2.1). Typically, erosion occurs on the concave bank (outside bank of the meander bend) and accretion occurs on the convex bank (inside of the meander bend). Accretion occurs because the water it is not moving fast enough and lacks the energy to hold up the suspended sediment that the water had carried from upstream. The suspended sediment is deposited on the inside of the meander bend making a point bar. In addition, some sections of the bank are neutral and do not appear to erode at all. As a result of the different distribution of shear stress and therefore pattern of erosion and accretion along rivers, rivers shift their position and evolve in shape over time. Eventually, an oxbow lake is formed because the neck of land that is inside of the meander bend is cut off (Figure 2.2). An oxbow lake represents the end of a

5 meandering cycle because after the neck is cut off, the river is straightened. In straight channels, the thalweg still meanders back and forth, and eventually this leads to a meandering path once again.

Figure 2.2 The formation of an Oxbow Lake from a meandering river (Gamesby, 2013).

2.2 Types of Bank Erosion Bank erosion occurs through two dominant processes: hydraulic action and mass failure (Posner and Duan, 2012). Hydraulic action entrains particles from the bed and bank by the excess shear force exerted on the boundary by the flowing water. The larger the water velocity, the greater the energy of the flow and the greater the potential for hydraulic action to detach material from the bank. This material is carried down the river until the water velocity slows down enough that it no longer has the energy to suspend the particle. Mass failure occurs when a large slab of material shears away from the bank and slides or slumps to a lower position (Figure 2.3). This typically occurs when the critical height and angle of the bank have been surpassed (Papanicolaou et al., 2007). How prone a river bank is to mass failure depends on the geometry, structure and material properties of the bank (Knighton, 1998). Slumping is a common form of mass wasting in river banks that have cohesive material, which is the situation along the Lower Shuswap River. Slumping usually occurs along concave upwards surfaces when the bank materials are saturated or are experiencing rapid de-watering (Trenhaile, 2010). If the base of the bank is undercut, the stabilizing forces of the bank are decreased. Once the gravitational force becomes stronger than the forces

6 holding the bank together, a portion of the bank breaks off or begins to slip. A bank is most susceptible to slumping when the water levels have declined but the bank is still fully saturated (Trenhaile, 2010), as is the case after a storm event or soon after the peak discharge of the spring freshet has passed (Figure 2.4).

Figure 2.3 Slumping caused by the undercutting of bank material on a river bank (Wikipedia: River bank failure, 2014, retrieved at: http://en.wikipedia.org/wiki/River_bank_failure).

Figure 2.4 Mass wasting event caused by a drop in river stage or a rise of the water table (Wikipedia: River bank failure, 2014, retrieved at: http://en.wikipedia.org/wiki/River_bank_failure).

Hydraulic action and mass failures are often interrelated. The hydraulic action is associated with the shear stress exerted by the flow and is often concentrated at the toe of the bank, resulting in undercutting of the bank. When the bank toe has been

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eroded, and the bank height and angle are changed to the point where the gravitational forces are larger than the forces holding the bank together, failure occurs (Simon et al., 2009). This is depicted in Figure 2.5, where the bank toe is scoured out during high water levels, which are often associated with increased shear stress. When water levels drop, the overhanging material is now unstable due to the removed support. Eventually, the overhanging material drops into the river and protects the lower bank from further erosion until the hydraulic action removes this material once again (Knighton, 1998). The strength of the hydraulic action may also be reduced after a mass failure event because the newly fallen material may shift the flow away from the bank (Kean and Smith, 2006a, 2006b). This can decrease the velocity gradient and therefore reduce the shear stress. The bank retreat process is therefore a cyclical one that alternates between an erosional stage dominated by hydraulic action followed by geotechnical failures and slumping (Clark and Wynn, 2007).

Figure 2.5. Stream-bank retreat via mass failure and hydraulic action (TMDL, 2006).

2.3 Shear Stresses in a Water Channel Erosion occurs when there is an imbalance of forces acting on a bed comprised of erodible materials such as sand and silt. When the hydraulic forces exceed the strength of the material on the bank, erosion takes place. The magnitude and distribution of hydraulic shear stress on the bed of a natural channels has been studied extensively over the years because the rate of bank erosion can be predicted when the distribution of boundary shear stress is known (Kean et al., 2009). The average shear stress on the bottom of an infinitely wide channel is given by the tractive force equation:

τo = ɣRS (1) 8

-2 Where τo is the mean boundary shear stress (given in N m ), ɣ is the specific weight of water (given in N m-3), R is the hydraulic radius of the stream (given in m) and S is the slope of the stream (given in m m-1). However, when studying riverbank erosion, the mean boundary shear stress value for the channel is often not suitable since the distribution of shear stress is not equal along the cross section of the river. Therefore, other methods that take the near bank flow processes into account are often more appropriate. When water travels in a channel, friction is created between the flowing water and the channel boundaries. This causes the water to slow down at the perimeter of a channel (Figure 2.6). The water molecules touching the river bed are considered to not be moving at all (referred to as the 'no-slip condition'). This creates a shear stress on the fluid above and causes the fluid to become strained or deformed. This deformation extends into the interior of the flow domain, creating a velocity profile, and eventually the shear stress acting on the fluid layer at the surface near the centre of the channel becomes negligible (Figure 2.7). The boundary layer is the zone where the flow experiences strain and has been deformed due to the frictional resistance imparted by the bed (Bauer et al., 1992).

Figure 2.6 Typical isovel patterns for different river cross sections (Knighton, 1998).

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Figure 2.7 Boundary layer dynamics of turbulent flow (Knighton, 1998).

The “Law of the Wall” shows how the flow velocity increases as a function of the distance from the bed (Bauer, et al. 1992). For turbulent flow, this relationship can be expressed as:

uz = u*/к ln (z/z0) (2)

-1 Where uz (given in m s ) is the mean flow velocity at elevation z from the bottom -1 (given in m), к is the von Karman constant, u* is the shear velocity (given in m s )

and z0 is the roughness length (given in m). If the mean flow velocity is graphed against ln(z), the relationship should follow a straight line, and with the use of linear

regression the value of u* can be derived. The shear stress (τo) acting on the bed can be calculated with the following equation:

2 τo = ρu* (3)

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-3 where ρ is the density of water (given in kg m ) and τo is the boundary shear stress above the point where the velocity profile was measured (given in N m-2). Fluid shear stress can also be calculated using a Reynolds Stress methodology. Reynolds stresses arise from turbulent fluctuations in the flow field, and in order to calculate them one needs to measure the velocity field at very high frequencies (e.g., faster than 1 Hz). Typically, the vertical and downstream components of the velocity field are of greatest interest because they provide information on how momentum is transferred to the bed by the downstream flow. Specifically, the correlation between the fluctuating components of the velocity field in the downstream (u') and vertical (w') directions are derived from the flow time series by subtracting out the mean flow components ( u , w ), as shown in Figure 2.8. The fluctuating components (or

deviations from the mean flow) are then cross-multiplied (u'w') and averaged ( u'w' ).

This quantity is used to calculate the Reynolds Shear Stress in the downstream direction, as follows:

τ = - ρ u'w' (4)

a) b) Figure 2.8 a) Velocity time series of velocity in the downstream direction b) Correlation between u' and w' over time (Fredsoe, 1990).

Although the method above is useful for deriving the shear stress acting on the bed by the downstream flow, incorporating the lateral flow component is crucial for evaluating momentum transfer on the bank. This is because the lateral (on-offshore) processes that operate in near bank environments are affected by both downstream and lateral eddies. In addition, the vegetation on the banks can alter the water currents and change the direction of momentum transfer, increasing the importance of

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incorporating the lateral Reynolds stress (Hopkinson and Wynn-Thompson, 2012). To solve this issue, rather than correlating only the downstream and vertical velocity components, the downstream and lateral velocity components can be combined and then correlated with the vertical velocity component. Speed (in m s-1) along the X-Y

plane (Suv) can be calculated by taking the square root of the squared sum of the downstream flow component (u) and the lateral flow component (v):

2 2 Suv = √(u + v ) (5)

Now the Reynolds shear stress can be calculated in a similar procedure as above; by

correlating the fluctuating components of the velocity field in the X-Y plane (Suv')

and vertical (w') flow direction. Suv' is derived by subtracting the average speed along the X-Y plane (Suv ) from the measured Suv throughout the time series. The same method is used to calculate w', as explained above. Now the fluctuating components 2 -2 can be cross multiplied (Suv'w') and averaged (Suv'w' given in m s ), to calculate the Reynolds shear stress in the stream-wise direction:

′ τSw = - ρ S w′ (6) ��uv������� Reynolds stresses are generally correlated to the mean primary velocity (Tominaga and Nezu, 1991). Therefore, one would expect a strong relationship between the shear stress values obtained via the Law of the Wall method (which uses the mean primary velocity) and the Reynolds method. However, research has found that the two shear stress values can be quite different (e.g., Sulaiman et al., 2013; Hopkinson and Wynn-Thompson, 2012; Andersen et al., 2007). Although each technique is commonly used in the literature (Andersen et al., 2007), the best method often depends on the environment and purpose. Using the Reynolds method can be useful for analyzing turbulent flows because it factors in the turbulent fluctuations in velocity rather than just the mean flow velocity. The fluctuating turbulence levels can often cause short bursts of intense shear stress that can result in much higher erosion events than if only the mean flow velocity is considered. For the classic Reynolds stress method, these short bursts of net erosion occur when either u' or w' (but not both) is negative and for the revised Reynolds stress method when either Suv' or w'

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(but not both) is large and negative. These erosive events are referred to as sweep and ejection events. Ejection events tend to move sediment upwards into faster flows and sweeping events move the flow and material downwards into slower moving flow (Trenhaile, 2010). The magnitude and frequency of these sweep and ejection events caused from turbulent flow also influences the amount of sediment entrainment that occurs along a stream (Knighton, 1998). Factoring in turbulence can make a large difference in certain environments because it can influence the amount of hydraulic action and therefore the erosion rates (Knighton, 1998). Therefore, when studying erosion rates and the rate of sediment transport, the spatial and temporal variations in turbulent bursting events may be very important due to the high instantaneous shear stresses involved (Trenhaile, 2010).

2.4 Primary and Secondary Circulation in Rivers Although the flow in a channel is primarily in the downstream direction, there can be vertical and horizontal components to the overall flow direction. These variations from the main downstream direction are referred to as 'secondary' flows or 'secondary' circulation patterns. Meandering reaches of rivers, for example, usually have a helicoidal flow pattern which resembles a spiral. Figure 2.9 shows the secondary current pattern formed at the cross section of a river with helicoidal flow. The secondary current is produced by the curvature of the channel and from the vertical velocity gradient of the primary flow (Kitanidis and Kennedy, 1984). When the flow reaches a meander bend in a channel, the water is pushed to the outside bank due to the centrifugal force. This creates a pressure gradient between the concave (outside) and convex (inside) banks. However, due to the friction acting on the water by the river bank, this centrifugal force is smaller near the bottom of the bank than at the top of the bank (Einstein, 1926). In the boundary layer, the pressure gradient is stronger than the centrifugal force. This balance of forces results in the circular flow pattern that becomes superimposed on the primary flow, generating the helicoidal flow and migrating thalweg that is seen in rivers.

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Figure 2.9 Helicoidal flow present in a river bend (Knighton, 1998 after Markham and Thorne, 1992). The tilt of the water surface is grossly exaggerated in this diagram.

Secondary currents influence the formation of meandering rivers because they shift the distribution of shear stresses along the river channel. As discussed earlier, the amount of erosion that takes place depends on the strength of hydraulic shear stress. When the stronger, faster primary flow is pushed to the concave banks of a channel bend, more erosion takes place compared to the inside banks because the velocity gradient is larger (Kitanidis and Kennedy, 1984; Einstein, 1926). Furthermore, the erosion is strongest on the bottom section of the concave bank, which results in an asymmetric cross section (Einstein, 1926). This undercutting of the bank also promotes mass wasting events. As the faster thalweg impinges on the outer banks, the slower and less powerful flow sticks to the inside of the bank and often deposits material, creating a point bar. These forces result in the formation of meandering rivers and eventually oxbow lakes. In order to predict erosion rates, secondary currents need to be accounted for, as it alters the distribution of shear stress along natural channels. Secondary currents are often developed in the junction region between the floodplain and the main channel (Tominaga and Nezu, 1991). However, for simplicity sake, the impacts of secondary currents on shear stress are often not included in scientific studies. Results from the Kean et al. (2009) model, which under-predicted the shear stress distribution by about twenty percent, demonstrate the importance of applying the secondary circulation effects in near bank shear stress analyses. In addition, Papanicolaou et al. (2007) found that the sidewall shear stress value increased by a factor of two to five when secondary currents were accounted for, compared to the classic Reynolds method (equation 4). The influence of lateral flow can be incorporated into the Reynolds method by combining the downstream and lateral flow variations into one vector (via equation 5).

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2.5 Initiation of Particle Motion In locations along the river where the shear stress due to the flowing water becomes strong enough to exceed the tendency for bed particles to sit on the bottom, the initiation of particle motion (or sediment transport) begins (McCuen, 2004). The critical condition occurs when the stabilizing forces (gravity and packing of the material) equal the mobilizing forces (the fluid drag, fluid lift and particle impacts). Theoretically, the critical shear stress can be found using the Shields diagram (Figure 2.10), which relates the dimensionless critical shear stress (θ) to the boundary

Reynolds number (Re*). The dimensionless critical shear stress is defined as:

θ = τcr/g(ρs-ρ)D (7)

-1 where g is the gravity constant (equal to 9.81 m s ), ρs is the density of the sediment (given in kg m-3), ρ is the density of the fluid (given in kg m-3), D is the diameter of

the particle (given in m), and τcr is the actual critical shear stress. The boundary

Reynolds number (Re*) depends on the diameter of the particle (D), the shear velocity 2 -1 (u*) and the kinematic viscosity (v in m s ):

Re* = u*D/v (8)

Figure 2.10 The Shields Diagram (after Shields, 1936).

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Given the values for a range of system parameters such as grain size, flow velocity, and fluid properties, it will be possible to predict the value of the critical shear stress using the Shields diagram. The critical value can then be compared to the calculated shear stress from Equations 1, 3 or 4 to determine whether sediment entrainment is likely and therefore if erosion is likely to occur. Although the Shields curve appears as a single line, it should be interpreted as a region or zone for which sediment entrainment may happen. This is due to a number of reasons. For one, the derivation of the curve assumes that all the particles are the same size (Knighton, 1998). This is certainly not the case in most riverbanks as there is usually a variety of shapes and sizes for the bank material. Typically, the mean grain size is used and it is assumed that the system behaves as if it were comprised of a uniform size of equivalent diameter. Also, there are varying definitions of when a particle begins to move, making the precise moment of particle movement somewhat subjective (Clark and Wynn, 2007). Many researchers have classified the point of failure based on different observed characteristics such as the cloudiness of the water or when the soil of the surface becomes pitted (Clark and Wynn, 2007). Furthermore, the theory assumes steady flows, which is not the case in the majority of natural rivers. And finally, the turbulence in the water can result in instantaneous stresses that are much greater than the average, resulting in erosion at lower-than-predicted mean shear stress values (Knighton, 1998). Because of this, the fine scales of turbulent forces must be evaluated in order to gain a greater understanding of sediment transport rates even if the threshold limit is not reached (Sulaiman et al., 2013). Finally, it needs to be appreciated that the Shields diagram does not strictly apply to cohesive sediment such as those found in river banks along meandering reaches (Debnath et al., 2007). Even though the boundary shear stress can be quantified for very fine grained materials such as silts and clays, these values may not apply. Therefore it will be difficult to calculate erosion rates due to a lack of understanding of the actual shear strength of the bank material when particle cohesion is at play. There are several models that can be used to estimate the critical shear strength of cohesive materials based on such parameters as the percent clay of the soil, the plasticity index and the average particle size of the material. However, these different methods often yield very different values for critical shear stress (Clark and Wynn, 2007). 16

2.6 Bank Erodibility Stream bank erosion depends on the balance between the erodibility of the material and the hydraulic/geotechnical forces applied to the bank (EPA, 2012). As mentioned above, predicting the rate of erosion becomes very difficult in circumstances where cohesive bank materials are involved because it is challenging to estimate the strength of cohesive sediments. Electrochemical bonds between the clay particles make them more resistant to erosion in comparison to non-cohesive sand sized particles. Therefore, the Shields diagram tends to underestimate the critical shear stress value for cohesive material (Clark and Wynn, 2007). The consequence of this is that the Shields diagram offers only a crude means by which to estimate the critical shear stress for cohesive material. Another confounding factor is due to the varying properties of the bank materials, including the moisture content, mineralogy, packing, porosity, stratification, and bank geometry (Knighton, 1998). Thus, the resistance of cohesive material to shear stress is often site dependent and very localized. It can also change throughout the year based on flooding and storm events interspersed with dry periods or periods of freezing temperatures. Many researchers overcome the challenge by taking large samples of the bank and placing them in flumes to measure the critical shear stress of their studied bank material directly. The presence of roughness elements on the bed (e.g., ripples and dunes) and bank (e.g., slump blocks) can also affect the shear stress distribution and therefore the dislodgement of particles. The total bottom stress partially acts on the particles as skin friction and on the roughness elements (such as vegetation and bed forms) as form drag. Only the skin friction component of the total bottom stress acts on the particles and causes erosion and sediment transport. Therefore, the more shear stress acting on the roughness elements the less shear stress is acting on the soil particles and the less energy is available for erosion. Small topographic changes in the bank, often due to slumping events, can also cause form drag which can significantly alter the flow in the river channel (Kean and Smith, 2006a). This means that large mass wasting events can result in a negative feedback loop through which the overall stability of the bank is increased because the slump blocks form elements that absorb the majority of the shear stress in the flow. The amount and density of vegetation in the flow greatly influences the degree of form drag along the bank and therefore reduces the amount of total bottom shear 17

stress available to erode the soil. Significant reduction in particle shear stress can be seen in surfaces only partially covered in vegetation (Thompson et al., 2004). When studying bank erosion rates in natural channels, factoring in riparian and aquatic vegetation may be necessary because the amount of shear stress partitioning can significantly change stream bank erosion rates (Clark and Wynn, 2007). Furthermore, a biofilm layer covering the perimeter of the channel or sections of the channel can have a pronounced impact on the resistance to erosion (Andersen et al., 2007). A biofilm layer is simply a layer of microorganisms that cover a surface which may act as a sort of armor that can withstand strong shear stresses before being eroded to reveal the soil underneath (Andersen et al., 2007). Another factor affecting erosion rates in meandering channels is the layering of bank material or the stratigraphy. The vertical changes in the physical properties throughout the bank may make certain sections more subject to erosion. Wojda (2008) found that bank material analyzed in a laboratory flume would erode in distinct layers based on the erodibility of each layer. If the underlying layers are more easily eroded, it can promote mass wasting due to the undercutting of the bank. Streambeds may also be more resistant to erosion than stream banks because they are not exposed to sub aerial processes and are always submerged underwater (Clark and Wynn, 2007). This causes rivers to migrate laterally faster than vertically. Due to all the variables that influence bank resistance to erosion, it is often difficult to determine the critical value of shear stress needed to yield bank erosion, especially for cohesive materials. Therefore, when studying natural channels, in situ measurements of critical shear stress are often preferable because it is difficult to transfer the bed or bank material to the flume without disturbing some of the many factors that affect erosion resistance (Clark and Wynn, 2007).

2.7 Spatial and Temporal Aspects of River Erosion The general shape of the channel and depth of water will affect how a stream will erode. In meandering rivers, the outer bank of a meander bend has a steep velocity gradient and therefore high shear stress. As a consequence, the outside bank should experience greater erosion rates in comparison to the inside bank (Knighton, 1998). Bank angle and curvature also affect how stable the bank is and therefore how fast it will retreat. Curran and McTeague (2011) found that aggressive bank erosion along the Matanuska River in South-central Alaska was correlated more to bank 18

height and composition than the flow characteristics. Also, the height of the floodplain relative to the water level can affect the rate of erosion. Tominagan and Nezu (1991) performed a flume experiment and discovered that as the height of the floodplain increases relative to the water level, the bed shear stresses decrease. The height of the floodplain can also alter the structure of the secondary currents, which can further impact the rate of erosion (Tominagan and Nezu, 1991). Therefore, the relative structure and location of the river on the longitudinal profile can impact the erosion rates. There are also a number of climate factors that can reduce the strength of the bank material making it more vulnerable to bank erosion (Clark and Wynn, 2007). In North America, where winters can be harsh and cold, a major factor that reduces the soil strength is freeze-thaw action. This is where the water in the bank continuously freezes and thaws during fluctuating temperatures. This can weaken the soil and therefore decrease the critical shear stress value. When the spring melt-water enters the fluvial system, it can easily erode the top layers of the bank because they have been weakened over winter. Simon et al. (2009) found that the streams that bring sediment to Lake Tahoe transfer the most material during the spring snowmelt period. However, Gardiner (1983) who worked in Northern Ireland, also found strong temporal variations in erosion rates but discovered the opposite phenomenon. He found that it was during the summer period that the subaerial processes weakened the soil and were thereby transferred away during the winter months. He also noticed that especially on the upper portions of a riverbank, the formation of needle ice could be the most prominent factor for determining erosion. These factors resulted in the winter months having the greatest erosion rates throughout the year. The impacts of seasonality can vary greatly. Humans also impact bank erosion on a spatial and temporal scale. Urbanization moves water after a storm faster into the fluvial system then would naturally occur. This causes the discharge in the river to increase faster and stronger than in an untouched environment. The strong shear stresses created from the increased discharge can lead to increased bank erosion rates. The same issue occurs with deforestation because more water is entering the fluvial system because it is not being taken up or slowed down by the surrounding vegetation. In addition, deforestation of the riparian zone can damage the stream bank stability.

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Although there tends to be smaller erosion rates during low discharges, there are so many variables that effect bank erosion rates that there is a rather weak correlation between flow volume and amount the bank has retreated (Knighton, 1998). It is clear that there are many seasonal factors that affect bank erosion as well as spatial. The general morphology of fluvial landforms changes in an attempt to reach equilibrium with the surrounding conditions (Trenhaile, 2010). Thus, depending on the climate, bank structure, vegetation, relief, frequency and magnitude of flooding as well as the presence of boating, peak erosion rates may occur at different times of the year and the geomorphology of rivers can be very diverse. Therefore, erosion rates are often site-specific and year-long study periods or longer may be necessary to capture the nature of bank erosion along a river reach.

3. Measuring and Modelling River Bank Erosion Predicting the amount of erosion that will occur over time can be a challenging undertaking. On a large scale, scientists commonly use aerial photographs (e.g., Curran and McTeague, 2011; Constantine et al., 2009; Pizzuto and Mackelnburg, 1989), which are acquired from platforms on planes or satellites. The displacement of the meandering pattern can be tracked by referencing the changing shape to fixed monuments or markers such as roads, railroad tracks, and fence lines. Bank erosion rates can also be measured on smaller scales using simple technologies such as erosion pins. Erosion pins consist of metal re-bar hammered into the ground, which makes for a cheap, simple, accurate, and very portable methodology. The amount of erosion (or accretion) can be monitored at regular intervals (days to weeks) based on the length of rod that sticks out (or is buried) in the bank. Topographic surveying is another common way to monitor small scale erosion. In topographic surveying, the bank profile can be obtained by calculating the relative elevation of the bank to a known elevation that will not change (referred to as a bench mark). These surveys are accurate to about 0.02m and are useful because the bed morphology of the entire channel can be monitored (Pizzuto and Meckelnburg, 1989). Having reliable field data on bank erosion is essential in calibrating predictive models of natural processes in rivers. Ikeda et al. (1981) showed theoretically that river bank erosion can be linearly related to the excess near bank velocity. This is the difference between the depth averaged velocity and the mean cross sectional velocity of the channel. This simple model can predict river migration patterns reasonably 20

well, although there is often large error associated with model predictions (Posner and Duan, 2012). Modern methods have built off and reformed the Ikeda et al. (1981) model focusing more on the distribution of boundary shear stress along the channel rather than just the variation in velocity. The boundary shear stress value can be used to predict sediment transport and shoreline erosion in fluvial environments (Hopkinson and Wynn-Thompson, 2012). One of the main mechanisms to predict the erosion rate of fine grained material in a stream channel is by the excess shear stress equation (Clark and Wynn, 2007):

a ε = kd (τa − τc ) (9)

-1 where ε is the erosion rate (given in m s ), kd is the erodibility coefficient (given in 3 -1 -1 m N s ), a is an exponent (usually assumed to be 1), τa is the applied shear stress -2 -2 (given in N m ) and τc is the critical shear stress (given in N m ). Current measurement techniques limit τa to be estimated using velocity measurements and then solving for shear stress by using methods such as the Law of the Wall or the Reynolds method as discussed earlier. The most appropriate method depends on the flow characteristics of the water. Since the Ikeda et al. (1981) model was introduced, numerous bank erosion models have been developed in an attempt to predict erosion rates based on bank and flow characteristics. Today, analyses of bank erosion are often performed in laboratory flumes, which are man-made channels intended to represent natural rivers (e.g., Hopkinson and Wynn-Thompson, 2012; Kean et al., 2009; Clark and Wynn, 2007; Czernuszenko and Holley, 2007; Debnath et al, 2007; Papanicolaou et al., 2007; Thompson et al., 2004; Song and Chiew, 2001; Tominaga and Nezu, 1991). Flumes are useful because a number of variables (such as bank material and structure, flow characteristics, and channel geometries) can be controlled. However, diligence must be maintained when applying results from these experiments to natural channels. This is because natural channels do not have the simple, smooth, channel geometries that flumes have and rarely have uniform flow, which is used in laboratory experiments (Papanicolaou et al. 2007). Also, when bank materials are tested in the lab, the properties of the cohesive materials can significantly change when they are transported from the field to the laboratory (Debnath et al, 2007).

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Despite the difficulty in understanding channel migration, there is a growing interest for investors, property owners, and municipal and provincial governments to understand the physical flow properties of water. Modern models try to incorporate many variables, such as bank composition, bank height and flow characteristics to estimate the boundary shear stress that is exerted on the bank by flow. However, little work has been done to test these models in the natural environment. Therefore, there is a need for scientific testing to analyze the accuracy of these predictive models and to gain a greater knowledge on how fluid flow properties change the landscape.

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3. Methods 3.1 Overview Multiple methods were used to investigate erosion processes along the banks of the Lower Shuswap River. Long-term bank erosion was monitored from the spring to the fall with the use of erosion pins and standard surveying methods. Changes in the bank profiles were evaluated in light of boating traffic patterns and shear stress intensity measurements during the spring freshet and subsequent summer season. A detailed topographic analysis was conducted on a portion of the bank to capture characteristic features of the banks along the Lower Shuswap River.

3.2 Long-Term Bank Erosion Monitoring Laderoute and Bauer (2013) installed a network of erosion pins at several sites along the Lower Shuswap River in May 2013, which were reoccupied and continually monitored during 2014. Erosion pins are long pieces of re-bar that are installed either vertically (V) or horizontally (H) along the bank. They are hammered into the bank so that the tip of the re-bar is flush with the ground surface. The progressive exposure or burial of the re-bar during a given time increment correlates to the rate of erosion or accretion, respectively. Typically, horizontal pins show progressive erosion only unless a major bank slumping event occurs that buries the pin. In contrast, vertical pins show erosion during periods of bank scour, but also sediment accumulation on top of the pin when eroded material from higher on the bank settles on the lower bank apron. A detailed explanation of the erosion-pin installation procedure can be found in Chapter 3 of Laderoute (2014). Five sites were selected to conduct the erosion pin experiments between Enderby and Mara, British Columbia. These five sites were located on the Cox, De Ruiter, Stewart, Konge and Bruns properties. The location of these sites along the Lower Shuswap River is shown in Figure 3.1.

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Figure 3.1 The five erosion pin sites containing nine erosion pin profile lines along the Lower Shuswap River as it flows into Mara Lake (from Laderoute, 2014).

The Cox Site was used as the control site because it is protected from boat wakes by a long mid-channel island. The bank is further protected by thick vegetation on the bank and is composed of mud and silt deposits. These features reflect the main characteristics of banks along the Lower Shuswap River. Because the site is not strongly influenced by boats, it should represent accurately the seasonal bank changes that take place due to natural processes, including the spring freshet. The site is located immediately downstream of the Mara Bridge on river left. The De Ruiter Site is the most upstream site, located on the entrance to a large meander bend on river left. The banks are steep and there is abundant vegetation, including a number of large cottonwood trees. Compared to the rest of the sites, it experiences the least boating traffic due to its distance from Mara Lake. The Stewart property is located upstream of the Mara Bridge and next to River Side Road, on river right of a straight reach. It has gently sloping banks and vegetation. There are many shallow sand bars adjacent to the Stewart Site that pose a hazard to boaters during low flows. These can be seen in Figure 3.2 in the middle of the channel at low flow when the tops of the bars are exposed.

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Figure 3.2 Mid-channel sand bars at the Stewart Site during low flow (September 5, 2014).

The Konge Site is downstream of the Cox Site and is on the outer (left) bank of a gentle meander bend. The Konge Site has steep banks, some vegetation, and signs of chronic erosion. This is likely because the thalweg impinges on the outer bank of the meander bend and causes increased shear stress during floods. The local landowner has lived there for approximately 45 years, and he notes that the total amount of bank erosion in front of his house has been on the order of about 6 metres. Figure 3.3 shows a drainage tube that was installed during house construction 45 years ago, when only about 0.5 m of the tube was exposed. The long-term rate of erosion is therefore about 0.13 m per year according to this anecdotal information.

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Figure 3.3 Erosion at the Konge Site can be tracked by this drainage tube that was installed 45 years ago with only 0.5 m exposed.

The Bruns property is located downstream of a very tight meander bend and is situated on a relatively straight reach of the river. It is located at the most downstream position of all the sites and is influenced by proximity to Mara Lake in respect of both the intensity of boat traffic and by backwater effects. The banks are vegetated with grasses and some trees, and the soil consists mainly of mud and clay. There are evident signs of slumping along the banks of the entire property. Detailed descriptions of all the sites can be found in Laderoute (2014). In total, nine erosion-pin profile lines were set up across the five sites. Specifically, the Bruns site has five profile lines whereas all the other sites only have one profile line. Every profile line consists of five or six pieces of rebar that are inserted along the bank. Erosion data from Laderoute and Bauer (2013) were acquired for this study, which consist of measurements throughout the summer of 2013, except during May and June when the water levels were too high to safely access the pins. Laderoute and Bauer (2013) monitored the pins until September 16, 2013. During the fall and winter months, very little erosion by river processes is

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expected because of the low water levels. In addition, there was substantial snow and ice cover, and therefore erosion rates could not be monitored. The erosion pins were measured again on March 21, 2014 to assess the degree of erosion that took place over the winter months, if at all. These data prior to the spring freshet provide a baseline against which to assess the amount of erosion during the subsequent flooding season. Because of high water levels during April, May, and June, further access to the pins was not possible until July 25, 2014, when only the upper erosion pins could be accessed safely. The erosion pins were then monitored on a regular basis to check for changes in bank elevation during the remainder of the summer.

3.3 Long-Term Boat Traffic Monitoring Many residents along the Lower Shuswap River believe that bank erosion is due primarily to the wakes produced by passing watercraft. The vessels responsible for producing wakes along the Lower Shuswap River include personal watercrafts (or PWCs) (Figure 3.4), pontoon boats (Figure 3.5) and speedboats (Figure 3.6). To assess whether the banks are eroding due to the waves induced by passing boats, boat traffic needs to be monitored and correlated with bank erosion.

Figure 3.4 Personal Watercraft (or PWC) travelling downstream along the Lower Shuswap River. Image obtained using PlotWatcherTM Pro camera installed at the Bruns site.

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Figure 3.5 Pontoon boat travelling upstream along the Lower Shuswap River. Image obtained using PlotWatcherTM Pro camera installed at the Bruns site.

Figure 3.6 A speed boat travelling upstream along the Lower Shuswap River. Image obtained using PlotWatcherTM Pro camera installed at the DeRuiter Site.

Two PlotWatcherTM Pro cameras were installed on May 23, 2014 to monitor boat traffic along the Lower Shuswap River throughout the primary boating season. The cameras were installed in upstream (De Ruiter site) and downstream (Bruns Downstream site) locations. These are the same sites used by Laderoute and Bauer (2013) during the 2013 boating season. Images captured at the Bruns site of a passing

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PWC and pontoon boat can be seen in Figure 3.4 and Figure 3.5 respectively and an image of a speedboat passing at the DeRuiter site can be seen in Figure 3.6. Laderoute and Bauer (2013) observed many more boats at the Bruns site than the DeRuiter site, and they suggested that tourists and lake residents often travel up the Shuswap River to explore or to use the calmer waters that are ideal for water skiing and other recreational water sports. The DeRuiter property is much farther upstream and likely does not receive as much traffic originating from Mara Lake, although the boat launch located in Enderby provides an additional source of boats. The cameras ran from 5:00 to 22:00 every day and took a still picture every 3 seconds. At this capture rate, a 32 GB memory card filled in about 10 days. Initially, the memory cards were switched every one and a half weeks. However, during mid- June we were unable to service the cameras in time, and some of the images from June 18-21 were over-written. In September, the camera at the Bruns site failed to trigger, so there is another brief period for which boat images are not available. Fortunately, the boating intensity was minimal during these periods; in June because the freshet was at its peak and the boating season had not really begun yet, whereas in September because the water was too shallow for safe boating to take place in the river. The lack of data for these days will not have a significant impact on the total traffic intensity estimates. The photos captured with the PlotWatcherTM Pro cameras were viewed in fast succession on a program called GameFinderTM. An image was saved for every passing boat. In addition, the time the boat passed the camera location, direction the vessel was travelling and type of watercraft were recorded in a spreadsheet. This allows the intensity of boat traffic to be related to erosion rates as well as to assess where the boats originate from (presuming round trips).

3.4 Short-Term Water Velocity and Turbulence Monitoring Natural rivers are often dominated by turbulent and fluctuating flow. The degree of turbulence can impact the rate of bank erosion because it can increase the amount of shear stress. The amount of shear stress acting on the bed or bank can be calculated using the Law of the Wall method (Equations 2 and 3) or using the Reynolds Stress method (Equation 4 and 6). Both of these methods require velocity measurements. The Bruns property was chosen to monitor the flow velocity of the Lower Shuswap River during the spring freshet to correlate the calculated shear stress 29

values with erosion rates. The Bruns site was easy to access by road and the property owners were willing to accommodate us during the experiments. The site also has characteristic bank features for the lower reaches of the river as it shows significant signs of bank slumping and the property consists of sparsely spaced trees with thick grass vegetation. Figure 3.7 shows a picture of the Bruns Middle Site with the two middle erosion pin profiles during installation (May 2, 2013).

Figure 3.7 Bruns Middle Site. Note substantial slumping features on the bank.

In March 2014, a measurement station was set up near the Bruns Middle Site to record velocity profiles along the bank in order to derive values for shear stress. The measuring station was composed of two sets of scaffolding stacked on each other. Sets of concrete bricks were emplaced into the bank where the corners of the scaffolding were to be located. Velocity profiles were measured using a Velocimeter at locations between the left and right sets of bricks seen in Figure 3.8. Figure 3.9 shows the measurement station sitting on top of the bricks during higher water flow. The purpose of using the concrete bricks was to prevent the scaffolding from sinking into the bank over time and to keep the structure relatively level. Plywood was laid down and tied to the scaffolding to act as a platform.

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Figure 3.8 Velocity profile measurements were taken between the left and right cement blocks to monitor shear stress acting on the river bank along the Bruns Property. The photo shows the upstream direction and the two upstream brick pairs.

Figure 3.9 Scaffolding platform used to support the instruments during collection of velocity data on May 22nd, 2014. View is in the downstream direction.

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A SonTek Field Acoustic Doppler Velocimeter (ADV) was installed on a rack that was attached to the scaffolding. The rack had several sliding components that enabled accurate positioning of the ADV, and these sliding components were accurately levelled during the installation process. The ADV probe has a transmitter and three receivers. The transmitter sends out an acoustic signal and the velocity is measured based on the Doppler effect. Because turbulent flow is so complicated, very precise and accurate instruments are needed to analyse the water flow. An ADV is an ideal instrument to use to analyse flow characteristics because it can collect data at very high frequencies (25 Hz) and uses a small sample volume. This is useful because it allows turbulent eddies in the water to be analysed at high resolution (Voulgaris and Trowbridge, 1997). The ADV is capable of collecting the velocity data of a turbulent flow along the three Cartesian coordinates (X, Y, Z) as well as calculating the distance from the sample to the boundary surface. This is useful for establishing the velocity profile. The rack attached to the scaffolding allowed the ADV to be moved left, right, up and down on level angle iron. A picture of the set up can be seen in Figure 3.9. Extra care had to be taken in mounting the instrument because if it is not aligned properly, it can impact the calculated shear stress results (Andersen et al., 2007). The ADV was oriented so that the positive x-axis aligned with the downstream direction, the positive y-axis was in the transverse direction pointing towards the bank and the z-axis was in the vertical direction with the positive axis pointed upwards. The Bruns Middle Site offers the characteristic bank features displayed throughout the Lower Shuswap River. Bank slumping is very prominent at the Bruns property and although the section of the bank used to set up the measuring station had few distinctive characteristics relative to the rest of the property, the chosen location resembled the remnants of slumping events. As seen in Figure 3.8, a large portion of the bank was missing and has been eroded away over time, resulting in a section with fairly flat topography. This location was chosen because the scaffolding could be easily installed relative to other sections of the bank. However, it was also at a slightly lower elevation compared to just upstream of the scaffolding. There is about a 0.3 m step in bed elevation between where the measurements took place and about 0.5 m upstream.

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Figure 3.10 High flow conditions (June 2, 2014). Note that one-half of the scaffolding platform is submerged and that the water line was quite some distance onshore.

On June 2nd the water levels were very close to peak stage for the spring freshet of 2014. The highest discharge for the Shuswap River took place on June 18th. According to Environment Canada (Real-time Hydrometric Data for the Shuswap River near Enderby) the average river discharge on June 2nd was 327 m3s-1 and 348 m3s-1 on June 18th. These values are sufficiently similar to indicate that the level of shear stress acting on the bank on June 2nd is approximately equivalent to the maximum shear stress acting on the bank during the peak of the spring freshet.

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Figure 3.10 Seven velocity profiles were taken from just beyond the farthest brick and to 1.08m to the right (the water flow in this picture is coming out of the page). Note that the topography just upstream of where the measurements were taken is at a higher elevation.

On June 2nd, seven detailed velocity profiles were taken off of the bank during high water levels. The velocity profiles were spread over a 1.08 m horizontal span of bank that was expected to be most affected by the spring freshet. The ADV could be moved up or down in 0.05 m increments. For the profiles closest to the shore, the ADV could not be fully lowered on the rack because the instrument came in contact with the bottom of the bed. For the profiles farthest from the shore, the ADV was able to be lowered fully, and a maximum of 17 measurement points were collected. However, due to the 0.3 m step in bed elevation just upstream of where measurements took place, a recirculation zone was created in the lee of the step. This made the bottom points unsuitable for use in calculating shear stress because they did not represent a characteristic portion of the flow field. The bottom few points were omitted as outliers for purposes of the velocity profile analysis. The ADV was also used in the locations between the velocity profiles to detect the topography of the bed. The ADV can calculate the distance of the nearest boundary from the acoustic sensor. Since the position of the sensor was known relative to the water surface, the elevation of the bed could be calculated relative to

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the water surface as well. Figure 3.11 illustrates the sampling points in relation to the bottom of the bed and to the June 2nd high water level mark.

Measurement Points Used for Shear Stress Profiles

-1.4

-1.6

-1.8

-2.0

-2.2

-2.4

Elevation Relative to June 2nd Water2nd to June Relative Elevation Surface (m) -2.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Distance From Profile 0 (m) Figure 3.11 Sample and boundary elevations relative to June 2nd water level using ADV boundary detector for measurements. In this figure, the river water is flowing out of the page and the shore of the bank is located on the right.

Buffin-Bélanger and Roy (2005) found that the optimal record length to capture river turbulence characteristics is between 60-90 seconds. Therefore, a three minute record was taken at each sample location, which should provide enough information to capture the low frequency velocity variations. The data-acquisition system was programmed to collect velocity measurements at 25 Hz (the maximum speed of the ADV) resulting in 4500 data points for each sample location. To guarantee river conditions did not change throughout the ADV sampling collection, a current meter and two optical backscatter sensors (OBSs) monitored the flow velocity in the X and Y (downstream and transverse) directions and the turbidity of the water, respectively, at a fixed position throughout the measurement period. The two OBS sensors were separated by 0.3 m to collect data at the bottom of the bed and 0.3 m above the bottom of the bed. The instruments were installed a few metres upstream from where the ADV data were collected.

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Figure 3.12 ADV rack set up prior to being installed on July 18, 2014

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Figure 3.12 Velocity profile measurements being taken on the lee of the spring freshet (July 18, 2014).

On July 18th, when water levels had dropped significantly, a single velocity profile was taken farther offshore as compared to the June 2nd measurement location. The scaffolding platform was no longer of utility because the water level was at the level of the bricks. The period around July 18th is a critical one because the water level was declining very rapidly at the end of the spring freshet and boating traffic was ramping up rapidly because of the hot weather. It was believed that the additional data obtained from this singular deployment would be valuable in assessing the relative importance of boat traffic versus shear stress due to the natural flow of the river. The set up for this experiment was slightly different than it was for June 2nd (see Figures 3.11 and 3.12), but both sets of data were collected using an ADV and processed the same way. All measurements were taken where the shear stress acting on the bank was expected to be the most intense. During the July 18th experiment, seven velocity measurements, each spaced 0.05 m apart were taken along a single profile. The measurements were taken during periods when watercraft were absent on the river. Unfortunately there were occasions when boats did travel past the site during the recording. In these instances, the data points influenced by the passing wakes were cut out and set aside for separate 37

analysis. A sampling duration of three minutes was used and data were collected at 25 Hz, as before. All ADV data were analyzed using a program called WinADV, provided by SonTek. Each file associated with a single sampling run had 4500 data points. The raw data were filtered to remove any measurements that had a signal-to-noise ratio smaller than 5 or a correlation under 70. Less than 0.53% of the original data were filtered out using these criteria, which shows that the data are of very high quality. The filtered data were exported into an Excel format for manipulation, and eventually imported into Sigma Plot for graphing. Velocity profiles are necessary to solve for shear stress (Equations 2 and 3) and to understand the flow characteristics of the river. Various components of the velocity field can be used in such an analysis, but for our purposes it was decided that

flow speed (Suvw) would be the truest representation of the flow field. Flow speed is the magnitude of three-dimensional flow vector, and it can be calculated as follows: 2 2 2 Suvw = √(u + v + w ) (10) where u, v and w are the instantaneous velocities in the x, y and z directions, respectively. In order to produce a speed profile, the mean (S ) was calculated for

every sample file by summing all the instantaneous Suvw values��uvw��� �and dividing by n, the number of samples in the file (typically 4500). The speed profile is generated by comparing the mean speed against the natural logarithm of the distance from the bottom measurement (Equation 2). The slope of line is proportional to the shear stress acting on the bed (Equation 3). The bed shear stress values for each of the seven profiles on June 2nd and the single profile collected on July 18th were calculated using this technique. Shear stress was also calculated using a Reynolds Stress method (Equation 6). In this instance, the flow speed along the horizontal (X-Y) plane was used (Equation

5), which is referred to as Suv. The Reynolds Stress was then calculated using the following equations:

S = 1/n ∑ Suv (11) ��uv��

S'uv = Suv - S (12) ��uv��

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′ τSw = - ρ S w′ (13) ��uv������� for which any term with an overbar refers to an average (mean) quantity, a prime (') indicates a fluctuating component, and an unprimed quantity refers to the

instantaneous variable in the time series. For Equation 13, τSw is the shear stress and ρ is the density of the fluid. Note that the averaging is done on the cross-multiplied terms, not on the separate terms prior to averaging. In statistics, terms such as these are equivalent to cross-correlations, and this indicates that the Reynolds Stress is a measure of the degree to which the fluctuating components in the velocity field (in the downstream and vertical directions) are correlated. Large correlations are expected for strongly sheared flows such as those close to fixed boundaries. The Reynolds Stress method provides a shear stress value for each measurement location rather than an overall average stress on the bank. However, a shear stress profile can be plotted and extrapolated to the surface to provide an estimate of shear stress on the bed or bank. As stated earlier, there was a recirculation zone at the base of our velocity profiles, so the lowermost points in the profiles were removed during filtering. The upper points in the profile were used to estimate the bed shear stress.

3.5 Topographic Surveying A number of topographic surveys were conducted on the Bruns property between May 27, 2014 and August 2, 2014, including detailed surveys of the five erosion-pin profile lines and of the topography immediately upstream of the scaffolding. The profile data from Laderoute and Bauer (2013) were placed in a common reference framework and combined with the 2014 data relative to a standard high water stage (June 2, 2014). In addition, a detailed survey of a 6 m by 5 m grid was undertaken to characterise the topography of the river bank. The hope had been to survey this grid prior to the arrival of the spring freshet and then to re-occupy the site after the freshet to assess bank change. However only a portion of the grid was completed on April 27, 2014 and soon thereafter the water stage rose to a level that precluded access to the grid. The remainder of the grid was completed between July 9 and July 25, 2014 as the water levels declined, and the data were used to produce a

39

Digital Elevation Model. Corner pins were left in the field so that the grid can be re- occupied in future years. On June 10, 2014 water surface slope was measured in order to drive calculations such as tractive force. Unfortunately because of backwater effects from the lake at this downstream site (close to the mouth of the river), the measured water surface slope was essentially zero, taking into account measurement uncertainty. In fact, a regression line through the water surface elevation data taken along the 385 m reach suggests that the water surface slope was actually negative with a mean change of -0.01 mm.

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4. Results 4.1 Boat Traffic Survey Still images obtained with the PlotWatcherTM Pro cameras were used to monitor boat traffic at the Bruns and De Ruiter properties and to classify watercraft into three categories: speedboats (SB); personal watercraft (PWC); or pontoon boats (P). The cameras were installed on May 31, 2014 and ran until September 5, 2014. Tables 4.1 and 4.2 present the weekly boat counts for the Bruns and De Ruiter sites, respectively. Weeks begin on Tuesday and end on the following Monday in order that the influence of long weekends can be included in a single weekly cycle. Boating traffic significantly increases on weekends relative to weekdays, and this is particularly true for long weekends, which reinforces the significance of using a Tuesday to Monday definition for these data. Figures 4.1 and 4.2 are graphical representation of boating intensities along the Bruns and De Ruiter properties throughout the 2014 boating season. Also shown is the distribution of different watercraft at each site. The same scaling is used on both graphs to reveal the relative differences in boat traffic intensity at the two sites. Although there are many more boat passages recorded at the Bruns site, both sites display the same overall boating patterns. Specifically, boating traffic increases significantly during the weekend just before Canada Day, comes to a peak over the August long weekend and then declines during mid-August and September. The Bruns site experiences greater boating intensity due to its proximity to Mara Lake. Boaters travelling up and down the river often do not make it very far upstream and tend to head back well before reaching the De Ruiter property. The greater proportion of PWCs at the De Ruiter site is likely because PWCs can travel in much shallower water and move much faster than traditional watercraft, and therefore PWCs tend to make it farther up the river. The shallow water levels during the mid to late summer are less hazardous to PWC operators, and there is a significant drop in boats towing water skiers and wake boarders toward the end of the boating season in late August. Although the De Ruiter site is located closer to the Enderby boat launch, the boat launch is a significant distance from Mara Lake and is therefore not a popular launch option for boaters travelling to Mara Lake for the day.

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Table 4.1 Weekly watercraft count for the Bruns property during the 2014 boating season, sorted by day of the week and type of vessel Week Type T W T F S S M Sum 1 May 22-26 SB 0 5 1 4 0 10 PWC 0 0 0 0 0 0 P 0 0 0 0 0 0 2 May 27-June 2 SB 1 0 0 0 0 3 2 6 PWC 0 0 0 0 0 0 0 0 P 0 0 0 0 0 2 0 2 3 June 3-9 SB 0 0 2 3 8 3 0 16 PWC 0 0 0 0 0 0 0 0 P 0 0 0 0 0 0 0 0 4 June 10-16 SB 0 7 0 0 0 0 0 7 PWC 0 0 0 0 0 0 0 0 P 0 2 1 0 2 0 0 5 5 June 17-23 SB 4 0 6 10 4 0 24 PWC 0 0 0 10 8 0 18 P 0 0 0 5 1 0 6 6 June 24-30 SB 2 1 6 2 17 4 43 75 PWC 0 5 4 0 13 10 33 65 P 0 2 0 0 3 0 14 19 7 July 1-7 SB 60 20 26 29 43 20 25 223 PWC 33 18 30 20 22 2 8 133 P 6 6 4 2 2 1 0 21 8 July 8-14 SB 21 28 41 25 56 53 31 258 PWC 12 10 21 10 24 24 6 107 P 5 12 4 4 7 14 0 46 9 July 15-21 SB 42 34 43 40 15 79 66 319 PWC 29 18 10 18 12 29 19 135 P 4 4 1 12 1 5 2 29 10 July 22-28 SB 37 30 0 48 88 90 92 385 PWC 23 15 0 15 58 34 33 178 P 3 2 0 2 4 4 7 22 11 July 29-Aug 4 SB 69 79 74 108 92 114 94 630 PWC 67 31 39 44 53 56 48 338 P 8 12 2 7 7 12 10 58 12 Aug 5-11 SB 54 45 61 44 57 62 36 359 PWC 18 28 27 10 29 35 22 169 P 12 11 8 2 7 8 4 52 13 Aug 12-18 SB 14 14 4 4 6 14 12 68 PWC 12 10 26 11 0 6 8 73 P 8 2 4 0 0 4 0 18 14 Aug 19-25 SB 6 9 8 2 9 34 PWC 0 31 20 5 13 69 P 0 0 0 0 0 0 15 Aug 26-Sept 1 SB 0 4 1 0 5 PWC 2 0 2 0 4 P 0 0 2 0 2 16 Sept 2-8 SB 2 0 0 1 3 PWC 0 0 0 0 0 P 0 0 0 0 0

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Table 4.2 Weekly watercraft count for the De Ruiter property during the 2014 boating season, sorted by day of the week and type of vessel Week Type T W T F S S M Sum 1 May 22-26 SB 0 0 6 1 0 7 PWC 0 0 0 0 0 0 P 0 0 0 0 0 0 2 May 27-June 2 SB 0 0 0 0 2 2 2 4 PWC 1 0 0 0 2 0 0 0 P 0 0 0 0 0 0 0 0 3 June 3-9 SB 0 0 0 1 5 4 4 14 PWC 0 0 0 0 0 0 0 0 P 0 0 0 0 0 0 0 0 4 June 10-16 SB 0 2 3 0 0 0 0 5 PWC 0 0 0 0 0 0 0 0 P 0 0 1 0 0 0 0 1 5 June 17-23 SB 2 0 4 0 6 PWC 0 0 0 0 0 P 2 0 3 0 5 6 June 24-30 SB 0 2 0 0 8 12 6 28 PWC 0 0 0 0 0 2 6 8 P 0 0 0 0 0 0 3 3 7 July 1-7 SB 5 7 4 2 8 1 9 36 PWC 5 4 2 0 0 0 6 17 P 0 1 0 0 0 1 1 3 8 July 8-14 SB 2 8 2 10 8 13 2 45 PWC 0 8 8 0 7 5 0 28 P 0 1 0 0 0 7 0 8 9 July 15-21 SB 0 3 6 2 0 13 2 26 PWC 4 4 4 3 4 6 0 25 P 0 0 0 0 0 0 0 0 10 July 22-28 SB 6 0 0 3 4 13 4 30 PWC 3 0 0 0 2 4 6 15 P 0 0 0 3 2 7 0 12 11 July 29-Aug 4 SB 8 0 4 10 13 15 10 60 PWC 14 4 10 25 22 31 38 144 P 0 0 0 2 2 3 0 7 12 Aug 5-11 SB 0 6 9 6 4 7 4 36 PWC 20 34 26 18 8 6 4 116 P 0 0 0 2 0 2 0 4 13 Aug 12-18 SB 2 2 0 0 2 7 2 15 PWC 0 2 0 4 0 0 4 10 P 0 0 0 0 0 0 0 0 14 Aug 19-25 SB 0 5 4 0 7 2 0 18 PWC 4 2 6 0 0 2 0 14 P 0 0 0 0 0 1 0 1 15 Aug 26-Sept 1 SB 3 2 0 0 7 0 0 12 PWC 0 2 0 0 0 0 0 2 P 0 0 0 0 0 0 2 2 16 Sept 2-8 SB 0 0 0 0 0 PWC 0 0 0 0 0 P 0 0 0 0 0

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Figure 4.1 Weekly watercraft count by speedboats (SB), personal watercraft (PWC) and pontoon boats (P) at the Bruns property during the 2014 boating season.

Figure 4.2 Weekly watercraft count by speedboats (SB), personal watercraft (PWC) and pontoon boats (P) at the De Ruiter property during the 2014 boating season.

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Table 4.3 Monthly watercraft count at the DeRuiter and Bruns properties during the 2013 boating season (from Laderoute and Bauer, 2013) Month Boat Type Bruns De Ruiter SB 18 11 May 17-31 PWC 14 2 P 5 6 SB 58 43 June PWC 37 8 P 25 4 SB 1140 165 July PWC 586 99 P 120 21 SB 566 81 August PWC 520 213 P 40 9 SB 9 17 September 1-23 PWC 12 0 P 2 1 SB 1791 317 Total PWC 1169 322 P 192 41 All 3152 680

Table 4.4 Monthly watercraft count at the De Ruiter and Bruns properties during the 2014 boating season Month Type Bruns De Ruiter

May 22-31 SB 11 9 PWC 0 3 P 0 0 June SB 127 57 PWC 83 8 P 32 9 July SB 1407 149 PWC 690 113 P 140 23 August SB 874 129 PWC 516 258 P 108 12 September 1-5 SB 3 0 PWC 0 0 P 0 2 Total SB 2422 344 PWC 1289 382 P 280 46 All 3991 772 45

Tables 4.3 and 4.4 compare the monthly boat counts for the 2013 and 2014 boating season, respectively. As shown, despite the longer monitoring period throughout the summer of 2013, the 2014 boating season experienced 839 and 92 more vessel passages at the Bruns and De Ruiter sites respectively. Although there is not enough data to conclude any long term trends, it is clear that boating traffic will vary from year to year. If bank erosion is linearly related to boat wakes then more bank erosion is to be expected in 2014 compared to 2013.

4.2 Short-Term Water Velocity and Turbulence Monitoring On June 2, 2014, seven velocity profiles were taken next to the river bank at the Bruns property during a high flow period. These vertical profiles were named Profile 0, Profile 1, Profile 3, Profile 5, Profile 7, Profile 9 and Profile 11 and spanned a distance of 1.08 m perpendicular to the bank. Profile 0 was located farthest from the bank (in deep water) and Profile 11 was closest to shore (in shallow water). The original intent was to measure profiles at 0.05 m spacings in the horizontal, but this proved to be too time consuming and the spacing too small leading to profile redundancy. Profile 0 and Profile 1 were separated by 0.08 m and all other profiles were separated by 0.1 m. The general layout of the profile lines relative to the bed were shown in Figure 3.11, in the Methods chapter. Figures 4.3 to 4.9 show the speed profiles for the seven vertical profiles taken on the 1.08 m horizontal span of river bank. Each speed profile was generated using the mean speed at each sample location during the three-minute sampling period (Equation 10). Note that the bottom samples of each profile have been removed because they were located in the recirculation zone at the base of the scaffolding platform. These points had minimal or negative velocities, which skewed the profiles unrealistically. The number of points removed depended on the height of the bed immediately upstream of the profile, which varied from profile to profile. All profiles except Profile 11 have at least 10 points in the profile, which makes for robust regression analysis. Profile 11, which is closest to the bank, only has five points because the profile was positioned immediately above the large bricks that supported the scaffolding and the probe could not be lowered to greater depths.

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Profile 0, Speed Profile Using Average of V-Mag, Bottom 6 Samples Omitted (7up-17up)

0.12 Curve 1: 0.10 Avg V-Mag (m/s) column 9: Coefficients: b[0] 0.1022764293 0.08 b[1] 0.0719462819 r ² 0.75692202

0.06

0.04 Speed(m/s)

0.02

0.00

-0.02 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Ln(Distance From Bottom Measurement (m)) Figure 4.3 Speed profile for Profile 0

Profile 1, Speed Profile Using Average of V-Mag, Bottom 6 Samples Omitted (7up-17up)

0.12 Curve 1: 0.10 Avg V-Mag (m/s) column 9: Coefficients: b[0] 0.0868218109 0.08 b[1] 0.0554418296 r ² 0.6767248177 0.06

0.04

Speed(m/s) 0.02

0.00

-0.02

-0.04 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Ln(Distance From Bottom Measurement (m)) Figure 4.4 Speed profile for Profile 1

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Profile 3, Speed Profile Using Average of V-Mag, Bottom 6 Samples Omitted (8up-17up)

0.12 Curve 1: 0.10 Avg V-Mag (m/s) column 9: Coefficients: 0.08 b[0] 0.0799993714 b[1] 0.0538370196 r ² 0.6908729197 0.06

0.04

Speed(m/s) 0.02

0.00

-0.02

-0.04 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Ln(Distance From Bottom Measurement (m)) Figure 4.5 Speed profile for Profile 3

Profile 5, Speed Profile Using Average of V-Mag, Bottom 7 Samples Omitted (8up-17up)

0.12 Curve 1: 0.10 Avg V-Mag (m/s) column 9: Coefficients: b[0] 0.0689317456 0.08 b[1] 0.0456295845 r ² 0.8212972168 0.06

0.04

Speed(m/s) 0.02

0.00

-0.02

-0.04 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Ln(Distance From Bottom Measurement (m)) Figure 4.6 Speed profile for Profile 5

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Profile 7, Speed Profile Using Average of V-Mag, Bottom 3 Samples Omitted (8up-17up)

0.12 Curve 1: 0.10 Avg V-Mag (m/s) column 9: Coefficients: b[0] 0.0578699819 0.08 b[1] 0.037514161 r ² 0.6118735093 0.06

0.04

Speed(m/s) 0.02

0.00

-0.02

-0.04 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Ln(Distance From Bottom Measurement (m)) Figure 4.7 Speed profile for Profile 7

Profile 9, Speed Profile Using Average of V-Mag, Bottom 5 Samples Omitted (9up-17up)

0.12 Curve 1: 0.10 Avg V-Mag (m/s) column 9: Coefficients: b[0] 0.0542036308 0.08 b[1] 0.0350808881 r ² 0.5419838606 0.06

0.04

Speed(m/s) 0.02

0.00

-0.02

-0.04 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Ln(Distance From Bottom Measurement (m)) Figure 4.8 Speed profile for Profile 8

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Profile 11, Speed Profile Using Average of V-Mag, No Samples Omitted (13up-17up)

0.12

0.10 Avg V-Mag (m/s) column 9: Coefficients: b[0] 0.0538919953 0.08 b[1] 0.0558180686 r ² 0.8089648625 0.06

0.04

Speed(m/s) 0.02

0.00

-0.02

-0.04 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Ln(Distance From Bottom Measurement (m)) Figure 4.9 Speed profile for Profile 11

From these speed profiles, u* was derived from the regression coefficients because u* is equal to the slope multiplied by the von Karman constant (equal to approximately 0.4). The value of shear stress acting on the bed was then determined using Equation 3. The calculated boundary shear stress values for each profile using the Law of the Wall method are given in Table 4.5.

Table 4.5 Boundary shear stress values obtained using the Law of the Wall method and the Reynolds Stress method

Boundary Shear Stress Calculated Average of Upper 5 Boundary -2 Using Avg. V-Mag (N m ) Shear Stress Using Suv’w’ as Reynolds Stress (N m-2)

Profile 0 0.827 0.423

Profile 1 0.491 0.308

Profile 3 0.463 0.307

Profile 5 0.332 0.176

Profile 7 0.225 0.186

Profile 9 0.196 0.074

Profile 11 0.498 0.062

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Shear stress was also calculated using the Reynolds Stress method and these values also appear in Table 4.5. Figures 4.10 to 4.16 show the Reynolds Stress profiles that were calculated by correlating the vertical turbulent fluctuations and the speed fluctuations along the X-Y plane (Equation 13). Shear stress values obtained using the Reynolds method do not give the boundary shear stress directly, although the lowermost sampling point could be used as a reasonable estimator. Shear stress is expected to be greatest at the bottom of the bed and will decrease linearly as the distance from the bed increases. However, as seen in Figures 4.10 to 4.16, this was not the case for our profiles. In fact, the opposite phenomenon is displayed; shear stress increased as distance from the bed increased. In addition, the correlation between shear stress values and distance from the bottom measurement are quite poor for all velocity profiles. Also note that all calculated shear stress values are all very small and are occasionally negative, which means that the bank erosion is extremely unlikely. Because of these factors it was difficult to choose a shear stress value that accurately reflects the true force caused by the turbulent water on the bank.

Profile 0, Shear Stress Profile Using Suv'w', Omitting Bottom 6 Points (7up to 17up)

1

Distance From Bottom Measurement (m) Bottom From Measurement Distance 0.1 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-2 Shear Stress (N m )

Figure 4.10 Shear stress profile generated using the Reynolds method for Profile 0

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Profile 1, Shear Stress Profile Using Suv'w', Omitting Bottom 6 Points (7up to 17up)

1

Distance From Bottom Measurement (m) Bottom From Measurement Distance 0.1 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Shear Stress (N m-2)

Figure 4.11 Shear stress profile generated using the Reynolds method for Profile 1

Profile 3, Shear Stress Profile Using Suv'w', Omitting Bottom 6 Points (8up to 17up)

1

Distance From Bottom Measurement (m) Bottom From Measurement Distance 0.1 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-2 Shear Stress (N m )

Figure 4.12 Shear stress profile generated using the Reynolds method for Profile 3

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Profile 5, Shear Stress Profile Using Suv'w', Omitting Bottom 7 Points (8up to 17up)

1

Distance From Bottom Measurement (m) Bottom From Measurement Distance 0.1 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-2 Shear Stress (N m )

Figure 4.13 Shear stress profile generated using the Reynolds method for Profile 5

Profile 7, Shear Stress Profile Using Suv'w', Omitting Bottom 3 Points (8up to 17up)

1

Distance From Bottom Measurement (m) Bottom From Measurement Distance 0.1 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-2 Shear Stress (N m )

Figure 4.14 Shear stress profile generated using the Reynolds method for Profile 7

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Profile 9, Shear Stress Profile Using Suv'w', Omitting Bottom 5 Points (9up to 17up)

1

Distance From Bottom Measurement (m) Bottom From Measurement Distance 0.1 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-2 Shear Stress (N m ) Figure 4.15 Shear stress profile generated using the Reynolds method for Profile 9

Profile 11, Shear Stress Profile Using Suv'w', No Samples Omitted (13up-17up)

1

Distance From Bottom Measurement (m) Bottom From Measurement Distance 0.1 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-2 Shear Stress (N m )

Figure 4.11 Shear stress profile generated using the Reynolds method for Profile 11

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Reynolds Shear Stresses Across Bank

0.5 ) 2 - 0.4 Bottom Suv'w' 0.3

0.2 Bottom 5 Suv'w' 0.1 0 Top 5 Shear Stress (N m (NStress Shear 0 0.5 1 -0.1 Suv'w' Distance From Profile 0 (m)

Figure 4.12 Shear stress values generated using the Suv’w’ Reynolds Stress method. Note that Profile 0 is farthest from the bank and experiences the greatest flow velocities.

Figure 4.12 shows the results from three alternative approaches for estimating the value of boundary shear stress following the Reynolds Stress method for each of the seven profiles. Three alternative values were plotted for each of the seven profiles:(1) the shear stress value from the bottom-most sample location in each profile, (2) the average of the shear stress values from the five bottom sample locations in each profile, and (3) the average of the shear stress values from the top five sampling locations in each profile. Figure 4.12 indicates that the shear stress values obtained using the bottom-most points are very small, as are the values obtained by averaging the bottom five shear stress values. In contrast, averaging the top five shear stress values for each profile produced more realistic results that are more in line with the values obtained by the Law of the Wall method. Table 4.5 and Figure 4.13 compare the boundary shear stress values obtained with the Law of the Wall method and the Reynolds Stress method using the upper points in the profile. The Law of the Wall method gives significantly larger estimates for all profiles, especially close to the bank and in deep water.

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Bottom Shear Stress Values Across Bank 0.9 0.8

) 0.7 Avg V-Mag 2 - 0.6 0.5 0.4 0.3

Shear Stress (Nm 0.2 Top 5 Suv'w' 0.1 0 0 0.2 0.4 0.6 0.8 1 1.2 Distance From Profile 0 (m) Figure 4.13 Shear Stress values obtained from 7 profile measurements along the river bank at the Lower Shuwap River. Profile 0 is farthest from the bank in deepest water.

Although there is a significant difference between bottom shear stress values calculated by the two methods, the trends are very similar. Specifically, the shear stress is at its maximum for the farthest offshore profile (in deepest water) and gradually decreases toward the bank. The single exception to this trend is the shear stress estimate from the Law of the Wall for Profile 11, which is much larger than the other estimates. The likely explanation for this is that the vertical speed profile is not robust (see Figure 4.9) because it only has five points that are relatively high in the flow domain. The other aspect of these shear stress values that is of note is that the magnitudes are very small in comparison to what might be expected for near-bottom stresses in the middle of a river of this size (typically 3 Nm-2 or greater). The largest boundary shear stress value was obtained for Profile 0 using the Law of the Wall method, and it was only 0.83 Nm-2. Examination of the Shields diagram (Figure 2.10) shows that these small values are insufficient to promote any sort of erosion. On July 18, 2014, another single velocity profile was taken using a different deployment scheme farther offshore than the June 2 measurements. The water level (and discharge) was greatly reduced, and the reason for measuring this profile was to draw a comparison with the profiles during the height of the spring freshet. Seven sampling points were taken along the vertical profile, starting close to the bottom and increasing at 0.05 m intervals to reach an elevation of about 0.3 m above the bed. The majority of samples were collected at 25 Hz for 180 seconds. However, due to boats

56

passing by during data collection and some equipment failure, the time spans for the second measurement location from the bottom and highest elevated measurement location are slightly shorter than the rest of the samples. The data were filtered using

the same methods as for the June 2 data. The speed profile is shown in Figure 4.14, and it demonstrates that the near bank velocities on July 18 were smaller than the

outer speed profile (Profile 0) measured on June 2. The primary reason for this is that the spring freshet was almost over and the mean downstream flow velocities in the river were reduced with declining discharge. This was anticipated but the data confirm the expectation that the shear stresses during the low-flow period are insufficient to mobilize sediment in the near-bank region.

July 18th Speed Profile Using Average of V-Mag

0.06 V-Avg 0.05 Coefficients: b[0] 0.0571673947 b[1] 8.3459764405e-3 0.04 r ² 0.8133914611 0.03

0.02 Speed(m/s)

0.01

0.00 -5 -4 -3 -2 -1 0 Ln(Distance From Bed (m))

Figure 4.14 Speed profile for July 18 measurements

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July 18th, Shear Stress Profile Using Suv'w'

1

0.1

0.01 Distance From Bed (m) From Distance

0.001 -0.10 -0.05 0.00 0.05 0.10 0.15

-2 Shear Stress (N m ) Figure 4.15 Shear stress profile for July 18th measurements

The shear stress profile generated using the Reynolds method for the July 18 measurements can be seen in Figure 4.15. The general trend is similar to those seen for the June 2 data; the shear stress intensity increases with distance from the bed. The most elevated sample on the shear stress profile has a much higher shear stress value, but is still not nearly strong enough to induce any erosion. The rest of the sample measurements have a shear stress value very close to zero or even negative. This indicates that the turbulent forces in the water are too small to initiate sweep or ejection events capable of altering the bank structure.

4.3 Erosion Pin Measurements The erosion pin profile lines installed by Laderoute and Bauer (2103) were measured on March 21, 2014, before the rise in the spring hydrograph. Figure 4.16 shows the water stage at the Enderby gauging station for the spring and summer period. During the peak of the spring freshet, all erosion pins at all locations were under water. It was too dangerous to access the erosion pins until July 25 because the water levels were too high. Even at this time, only the upper pins for the majority of the sites could be accessed.

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Figure 4.16 Water level recorded for the Shuswap River at Enderby during the summer of 2014 (Environment Canada Water Office, 2014)

Figure 4.17 Water level recorded for the Shuswap River at Enderby during the summer of 2013 (Environment Canada Water Office, 2014)

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Comparing the 2013 hydrograph (Figure 4.17) and the 2014 hydrograph, it is clear that the two spring freshets show similar characteristics, but are not identical. Both hydrographs show that the rise of the spring freshet takes place in early April, and reaches base level (around 2 m at the Enderby gauging station) by early September. However, the 2013 hydrograph is much more irregular, due to a series of rainstorms as well as alternating hot and cold periods that influenced the rate of snowmelt runoff. In addition, the drawdown period in the 2013 spring freshet (between late June and early August) occurred at a much faster rate than the 2014 drawdown period (between mid-June to early September). This rapid decrease in stage would cause the upper and middle bank portions to be exposed to boat wakes for a shorter period compared to the 2014 measurement period. Data from the nine erosion pin profiles established along the Lower Shuswap River were organized in a time series format in Figures 4.18 to 4.26 in order to capture the seasonal change in bank structure. The time series extend from May 2013 through to September 2014, and include the data from Laderoute and Bauer (2013) as well as the data collected in this study. The spring freshet period is highlighted on the figures because this is a period of special interest for this study. The erosion pin measurements were processed to represent the average upper horizontal bank change, average lower horizontal bank change, average upper vertical bank change and average lower vertical bank change. The rate of bank change is also depicted in Figures 4.18 to 4.26. The data in these figures demonstrate how fast the sequence of accretion and erosion can occur during and shortly after the spring freshet, which reinforces how critical this period is in the overall annual cycle of bank adjustment. The De Ruiter site is the farthest upstream of the nine erosion pin profiles along the Lower Shuswap River (Figure 4.18). Five pins (two horizontal pins on the upper portion of the bank, one horizontal pin on the lower portion of the bank, one vertical pin on the upper portion of the bank, and one vertical pin on the lower portion of the bank) were installed along this line. It is clear that there was little bank change during the summer of 2013. However, more significant changes were seen in the summer of 2014. A few centimeters of erosion took place at the upper horizontal and upper vertical pins. More substantial changes are evident at the lower vertical pin; accretion took place over the spring freshet followed by rapid erosion rates during the later summer months as water levels declined and boating traffic intensified. It is possible that this also occurred in the summer of 2013, but the critical period on the 60 declining limb of the hydrograph was not adequately measured to discern whether accumulation occurred on the lower pins. In contrast, the measurement interval in 2014 was able to capture the sequence of accretion and erosion events. The Stewart site profile consists of six pins; one upper horizontal pin, one lower horizontal pin, three upper vertical pins and one lower vertical pin (Figure 4.19). Minimal changes were seen in the top pins during the summer of 2013 and the summer of 2014, indicating the small effects the spring freshet has at eroding bank material at this heavily vegetated site. Rapid erosion rates are depicted at the lower horizontal pin, which is beneath a small cut-bank that is perennially submerged. The lower vertical pin shows that accretion took place over the spring freshet of 2014 followed by a short but intense erosion period. Once again, this could have taken place during the summer of 2013, but was not captured since the first pin measurement after the spring freshet of 2013 was not taken until July 26th. At this date the water level was at a slightly lower elevation than it was at August 1, 2014. By August 1, 2014, the accreted material over the pin had been quickly eroded to the point that the pin was almost flush with the bank material. This would have been interpreted as no bank change if the earlier measurements were not taken. The Cox site (Figure 4.20), consists of one lower horizontal pin, three upper vertical pins and one lower vertical pin. A small amount of erosion was apparent at the uppermost pins during the summer of 2013 and summer of 2014. The lower vertical pin indicates that a small amount of accretion occurred during the spring freshet for both years, which was then eroded during the late summer. The lower horizontal pin showed no bank change during the summer of 2013 and only slight erosion during the summer of 2014, with no accretion during the spring freshet. Overall, there was minimal net change for all pins throughout the monitoring period at the Cox site, which is expected given that this site was chosen as a 'control' site. At the Konge site (Figure 4.21) six erosion pins were installed (two upper horizontal pins, two upper vertical pins and two lower vertical pins). Erosion of the upper horizontal pins during the summer of 2013 and summer of 2014 is evident. The apparent accretion that took place over the course of the winter is likely due to a measurement error rather than actual accretion because we can think of no obvious mechanism by which accretion can occur on a steep bank such as this. The uppermost vertical pins, which sit at the base of the cut bank, experienced net accretion over the year. This is likely due to material being deposited over the spring 61 freshet and not being eroded because of the quick drop in water level to the point where boat wakes can no longer influence them. The lower vertical erosion pins show some erosion during the summer of 2013 and the familiar pattern of accretion followed by rapid erosion as water levels fall in the summer of 2014. At the Bruns Upstream site, upstream profile (Figure 4.22), there are six erosion pins; two upper horizontal pins, two upper vertical pins and two lower vertical pins. However, when averaging the erosion rates the lowest vertical erosion pin was not included because it was completely scoured out during the 2014 spring freshet and could not be found. The upper horizontal pins and upper vertical pins saw some erosion during the course of the 2013 and 2014 spring freshets, but nothing substantial. The lower vertical pin shows significant erosion during the summer of 2013. During the 2014 monitoring period, accretion takes place once again during the spring freshet. This material is quickly stripped away during a few weeks of intense erosion, and correlated with lower water levels and high boating intensity. The Bruns Upstream site, downstream profile (Figure 4.23) shows a similar pattern as the Bruns Upstream site, upstream profile. This profile consists of three upper horizontal pins, one upper vertical pin and two lower vertical pins. Unfortunately, one of the lower pins could not be measured because a very large drift log was emplaced overtop the pin during the declining stage of the 2013 spring freshet. This pin was eliminated from further analysis. The log stayed in place until the 2014 spring freshet, at which time it was remobilized and transported downstream. At this site, slight erosion took place at the upper horizontal pins during the course of the spring freshets. The upper vertical pin saw slight accumulation during the summer of 2013 and accretion followed by aggressive erosion in the 2014 measurement period. Because the lowest vertical pin is at a much lower elevation, during both the 2013 and 2014 measurement periods, the accretion can be seen during the higher water levels, followed by erosion as the water levels drop and the energy from the boat wakes can reach the pin. The Bruns Middle site, upstream profile (Figure 4.24), has one upper horizontal pin, one lower horizontal pin, one upper vertical pin and two lower vertical pins. Only slight erosion takes place at the horizontal pins and is initiated as the water levels drop to where the energy from the boat wakes can reach the pins. The upper vertical pin was not measured until late in the season for the 2013 measurement period, where it shows slight erosion. The changes for the upper vertical pin in the 62

2014 measurement period are much more dramatic, showing substantial accretion over the spring freshet. As water levels drop, boat wakes quickly erode the bank material. The lower vertical pin shows the same trend, although on an even greater scale. The pattern for this pin is also captured during the 2013 season as well as the 2014 measurement period. The Bruns Middle site, downstream profile (Figure 4.25) has one upper horizontal pin, one lower horizontal pin, one upper vertical pin and two lower vertical pins. The upper horizontal pin shows a slumping event that took place between July 25 and August 1, 2014. This time range started with the water level at a similar elevation to the pin, but then dropped quickly throughout the week. This slumping event was likely caused by the saturated bank material no longer being supported by the high water level. When the weight became too much, a large portion of the bank broke off. At the lower horizontal pin, significant erosion took place over the 2013 spring freshet. However, throughout the winter, material was redeposited. During the 2014 measurement period, accretion followed by erosion occurred, although over the entire measurement period there was no net bank change. The upper vertical pin shows that in 2013, only slight accretion occurred. Over the winter, significant erosion scoured the bank, but the spring freshet of 2014 deposited a healthy amount of material, reburying the pin. As the summer wore on and water levels dropped, this material was eroded out and net erosion occurred. Finally, the lower vertical pins show the familiar pattern of accreted material when water levels are high, and erosion of the bank as the water levels fall. Overall, the lower vertical pins show minimal net change. And lastly, the Bruns Downstream site (Figure 4.26), containing two upper horizontal pins, two upper vertical pin, one middle vertical pin and one lower vertical pin also had dramatic bank changes throughout the measurement period. At the upper horizontal pins, slight erosion occurred during the 2013 spring freshet, followed by accretion over the winter months. During the summer of 2014, a slumping event at one of the pins was a source of major erosion. The upper vertical pins show minimal changes over the summer of 2013, but once again, a pattern may have been missed since measurements took place at a lower water elevation. In 2014, it is clear that accretion took place over the spring freshet followed by a quick burst of erosion that brought the bank back to the original elevation. The middle vertical pin shows in both the 2013 and 2014 season that accretion occurs when water levels are high, 63 followed by erosion when water levels drop. This pattern is also seen in the lower vertical pin, but the effects are delayed since it is at a lower elevation.

Figure 4.18 Cumulative bank change and rate of bank change at the DeRuiter Site. Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets

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Figure 4.19 Cumulative bank change and rate of bank change at the Stewart Site. Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets

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Figure 4.20 Cumulative bank change and rate of bank change at the Cox Site. Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets.

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Figure 4.21 Cumulative bank change and rate of bank change at the Konge Site. Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets.

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Figure 4.22 Cumulative bank change and rate of bank change at the Bruns Upstream Site (Upstream Profile). Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets

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Figure 4.23 Cumulative bank change and rate of bank change at the Bruns Upstream Site (Downstream Profile). Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets

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Figure 4.24 Cumulative bank change and rate of bank change at the Bruns Middle Site (Upstream Profile). Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets.

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Figure 4.25 Cumulative bank change and rate of bank change at the Bruns Middle Site (Downstream Profile). Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets

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Figure 4.26 Cumulative bank change and rate of bank change at the Bruns Downstream Site. Note that the upper portions of the bank are differentiated from the lower portions of the bank and the pins measurements that have been averaged are in brackets

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5. Discussion and Conclusion Laderoute and Bauer (2013) conducted an in-depth experiment that assessed the impact of boat wakes on bank erosion along the Lower Shuswap River during the summer of 2013. Their methodology included tracking the number of boats travelling along the river at upstream and downstream locations using a camera, and monitoring bank change using a network of erosion pins. Although bank erosion was observed in that study, the data were inconclusive as regards to the overall impact of boating traffic within the annual cycle of bank change because their methodology was not developed for the purpose of quantifying the potential impact of the spring freshet on bank erosion. Although the erosion pin network was installed prior to the spring freshet of 2013, by the time the pins could be accessed safely to measure bank change (late July), the boating season was already well under way. Consequently, any bank change that was measured in late July could have been due to the spring freshet or to boat wake activity (or potentially a combination of both). The project described in this report was therefore designed to quantify the amount of shear stress exerted on the banks during the spring freshet. At the same time, bank erosion and boat traffic monitoring was conducted to extend the time series initiated by Laderoute and Bauer (2013). Boat traffic was monitored from May 23 to September 5, 2014. Overall, speedboats were the most common vessels observed on the river, followed by PWCs and then pontoon boats. The general trend shows boating traffic beginning to increase around late June, coming to a peak at the beginning of August, and quickly declining throughout the month of August. Despite ideal boating weather in August, boating intensity dropped in accord with declining water levels in the river. The low water levels pose a threat to watercraft and water skiers, which explains the decrease in the number of speed boat passages. PWCs, in contrast, sit much higher in the water and are able to operate in shallow reaches, which explains the increasing ratio of PWCs to other watercraft near the end of the summer. By early September, the boating season had effectively ceased. These trends resemble those of 2013, which suggests that the traffic intensity patterns are approximately the same from year to year. The Bruns site, which receives a large number of boats from Mara Lake, had significantly more watercraft passages than the De Ruiter site, which was located

73 much farther upstream. Boaters often travel from Mara Lake and up the Shuswap River to explore or to use the calm waters for water skiing, but tend not venture as far as Grindrod or Enderby. The vessel passages at the De Ruiter site are likely associated with local landowners who maintain riverside docks or with boaters who launched at the Enderby boat launch. Despite an overall increase in total watercraft passages during the 2014 monitoring period relative to 2013 (increases of 839 and 92 vessel passages at the Bruns and DeRuiter site respectively), there was slightly more bank erosion during the 2013 boating season. The implication is that there is not a straightforward, linear relationship between boat traffic intensity and river bank erosion, and that other factors must be at play. Previous studies (e.g., Houser, 2010; Osborne et al., 2007; Bauer et al., 2002; Parnell and Kofed-Hansen, 2001) indicate that bank erosion depends on boat length, boat speed, hull displacement, and sailing distance from the bank, which are difficult parameters to quantify for every boat passage. In addition, there may be additional natural processes that cause bank erosion, such as slumping and mass wasting. Bank structure, soil moisture content, material strength, vegetation cover, and the river flow patterns must also be accounted for. For example, more erosion may have occurred in the 2013 because the spring freshet drawdown period was more rapid than in 2014. The saturated bank material would have been left unsupported by the quick decrease in water stage, resulting in mass wasting events. Estimating riverbank erosion can focus on many different aspects but this experiment focused on only two primary factors: boating intensity and shear stress. It is important to realize that there are numerous other factors that may impact the change of bank geomorphology along the Lower Shuswap River. Frequently, flume studies are preformed to minimize the variables affecting the fluid flow properties (e.g., Hopkinson and Wynn-Thompson, 2012; Kean et al., 2009; Czernuszenko and Holley, 2007; Thompson et al., 2004; Song and Chiew, 2001; Tominaga and Nezu, 1991). The purpose of this is to collect data that is easy to analyse. However, the results from flume experiments may not apply directly to natural channels. Natural channels lack the simple, smooth geometries commonly used in flumes, and almost never have the uniform flow used in lab experiments (Papanicolaou et al. 2007). Frequently, the theories on fluid flow characteristics produced in lab experiments do not align with what is seen in the natural environment, as in the case with our collected data. Typically, shear stress is greatest 74

at the bed and decreases linearly with distance from the bottom, but the Reynolds method in this analysis produced a shear stress profile showing the opposite results (shear stress increasing with distance from the bed). Our interpretation of this trend is that the particular location in which measurements were taken was a relatively slack- water environment with minimal downstream flow velocities. As a consequence, the velocity gradients were rather minimal away from the bank and there was relatively little shear stress to be expended. The increasing Reynolds stress values away from the bank are due to the greater correlation between the horizontal and vertical flow components in the main body of the flow (i.e., farther away from the bank) whereas closer to the bank the flow components are randomly distributed with little correlation. These observations, which are contrary to established theoretical ideas about how boundary layers are structured, emphasizes the importance of collecting data in situ for assessing river bank erosion processes. Even though the measured shear stress profiles do not line up with theoretical predictions, the data are sound, reliable and accurately reflect the flow characteristics at the measuring site. There is some disagreement in the scientific community about how best to process velocity data in order to make boundary shear stress estimates. Some researchers say that the Law of the Wall method produces the best results (e.g. Hopkinson and Wynn-Thompson, 2012; Andersen et al., 2007) and others claim that the Reynolds Stress method should be used when working in the natural environment (e.g. Sulaiman et al., 2013). In this study, both techniques were used and we conclude that the Law of the Wall method produced more reliable results as regards shear stress trends on the banks. There is good reason to conclude that the impact of high flows during the spring freshet are not effective in eroding the banks along the Lower Shuswap River. The ADV measurements on June 2, 2014 at the Bruns Middle site during the height of the freshet indicate that the shear stresses are remarkably small along the river bank. In fact, both the Law of the Wall method and the Reynolds Stress method yield shear stress values that are far too small to initiate sediment erosion at these locations. The measurements on July 18, during the declining limb of the spring freshet, produced similar results. As a consequence, it is reasonable to conclude that relatively little bank change takes place during the high flow portions of the spring freshet, presuming there are no bank slumping events during the height of the freshet. The

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same may not be true for other sites along the Lower Shuswap River, particularly those on the outer banks of extreme meander bends. Figure 5.1 shows the trends in river stage and boating traffic at the Bruns site from the height of the spring freshet to the conclusion of the boating season during 2014. It is readily apparent that there is considerable overlap between the declining limb of the spring freshet and the ramp-up period of the boating season (June through mid-July). Until about mid-July, the impact of boat-wake waves is restricted to the upper portions of the bank exclusively by virtue of the high water levels (above 3 m relative stage). However, from mid-July to mid-August, which corresponds to the peak of the boating season, the water level drops rapidly from the mid-upper bank position to the lower bank position (about 2 m relative stage), and this implies that a one metre vertical swath of bank can be impacted by boat waves in intensely repetitive fashion over about three weeks. It is important to note that measurement of the erosion pins was not possible until the river stage dropped to a safe level (below 2.8 m relative stage), and therefore the first measurements of 2014 were not possible until July 25, at which time several of the lowermost pins on the profiles were still inaccessible. Subsequent pin measurements were taken on August 1, 9, 17, 24, and September 5 (as marked on Figure 5.1), which coincides with low stage and declining boat traffic intensity. The relative timing of these trends reveals that there is a period from mid-June through mid-July for which there are no pin erosion measurements but when the impact of the spring freshet and enhanced boating traffic work in concert. This appears to be a critical period as regards bank erosion during the annual cycle of bank change for which we still lack detailed data. Figures 5.2 and 5.3 are conceptualized renditions of the processes taking place along the Bruns Middle Site (Upstream Profile) during and after the freshet, which also reflects what was observed at other profiles. Based on the ADV measurements and shear stress calculations, it was assumed that no erosion takes place during the high flow period in late June and early July. Boat wakes are relatively unimportant during high flows because there are very few boats (see Tables 4.1 and 4.2) and because the impact of any boat-wake waves is restricted to the uppermost portions of the bank. The influence of these waves does not extend to the bottom of the profile because they are of short wavelength and the orbital velocity field decreases rapidly with depth. Therefore, it is assumed that the bank morphology remained the same during the high discharge period. 76

Figure 5.1 River stage at the Enderby gauging station, recorded by the Environment Canada, Water Office (2014), compared to weekly averaged boat counts at the Bruns Site during the summer of 2014

As water levels begin to fall in mid-June to mid-July, boating intensity begins to become more significant (Figure 5.1). During this period, the moderate intensity of boat traffic yields bank erosion along the upper and middle portions of the river bank (Figure 5.2). As before, the lower portions of the bank and the river bed remain relatively undisturbed because the waves from boats aren't of sufficient magnitude to influence the bottom. However, the material that is eroded from the upper and middle parts of the profile is deposited on the lower apron (horizontal portions of the bank and bed). Even though the discharge levels are still high, the velocity of the flow near the banks is too small to entrain sediment. As a consequence, a thin layer of accretion (approximately 0.02 – 0.08 m in thickness) evolves on the apron during July, whereas localized erosion takes place in the non-vegetated portions of the upper and middle bank (usually in association with previous slumping scarps.

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Figure 5.2 Conceptual diagram of erosion and deposition at the Bruns Middle Site (Upstream Profile) on the declining limb of the spring freshet hydrograph. Note that elevation values are relative to high water mark on June 2, 2014 (i.e., fully 1.2 m above the July 25 level shown)

Figure 5.3 Conceptual diagram of erosion and deposition at the Bruns Middle Site (Upstream Profile) at low water stage when boating traffic is at its most intense. The layer of accretion on the apron is progressively eroded by the agitation from boat wakes.

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The water levels drop very quickly during July and coincidentally the boating traffic increases. By early August only the lower portion of the bank is submerged, and boat-generated waves can no longer reach the middle and upper portions of the bank, which are effectively isolated from further hydrodynamic impacts from the river. However, high energy waves produced by intense boating traffic associated with water skiers and wake boarders are able to erode the newly accreted material at the base of the bank and cause net erosion (Figure 5.3). By the end of the summer, when the river reaches its lowest stage, even the bottommost pins on our profiles are no longer submerged. Measurements indicate that the exposed surface of the lower bank apron may still erode due to the swash motion from larger boat wakes that is able to shoal up the apron. However, the material beneath the accretion layer is much more cohesive and resistant to erosion, and often a layer of algae grows on it which prevents further erosion. Thus there is typically only minimal lowering of this apron surface during low flow conditions, mostly because the river stage is too low to sustain recreational boating traffic. The overall annual cycle of bank change therefore seems to involve the following stages: 1. An extensive low-flow period between July and April when the river banks are exposed mostly to subaerial processes such as wind, rainsplash impact, surface runoff from rain and snowmelt, bioturbation (insects, animals, vegetation), freeze- thaw cycling, dessication, weathering, trampling, and downslope movement driven by gravity, 2. The ramp-up period of the spring freshet from April through early-June when the river stage increases and bank materials are progressively submerged. 3. A high-flow period, roughly from early June to late June, during which the near- bank shear stresses are minimal and the likelihood of bank erosion due to tractive forces is small. 4. The draw-down period of the spring freshet (during July) when river stage declines quickly and boating traffic intensifies. The data indicate that erosion takes place along the upper and middle portions of the bank, most likely due to boat wake activity, and consequent deposition occurs on the lower bank and apron.

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5. A low-flow period during most of August when water stage is at the lower bank and when boat traffic is at its most intense. During this state, the layer of deposition on the apron is progressively eroded and the sediment is moved downstream and into the deeper sections of the main channel.

The cumulative effect of this annual cycle of bank change is that there is net erosion of the bank, mostly through horizontal retreat of the upper and middle portions of the bank. There may also be progressive reduction of the apron in years when the boating traffic is particularly intense during August. And there is always the potential for slumping events to occur, typically during the draw-down period when the bank materials are fully saturated. Bank slumping is particularly pronounced in locations where there has been erosion at the base of the bank by wave action in the previous boating seasons, which leads to greater instability of the bank. The data collected during this study make it quite evident that the high discharge flows during the spring freshet are not capable of significantly altering bank structure at sites that are comparable to the Bruns site. The implication is that boat- generated waves may be of relatively greater importance in forcing bank change throughout the annual cycle than had previously been anticipated. Our data regarding the timing and location of erosion and deposition relative to the intensification of the boating season provide valuable information in regards to management strategies directed at mitigating potential damage to the riparian zones along the Lower Shuswap River.

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