UBC Undergraduate Research

"Forest bridge design and scour potential" Ebadi-Angorani, Amir 2015-04

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 Amir Ebadi-Angorani University of British Columbia Faculty of Forestry  Department of Forest Resources Management  FRST 497: Graduating Essay “Forest Bridge Design and Scour Potential”     Forest Bridge Design and Scour Potential   1  ABSTRACT Scouring is the removal of sediment materials such as sand or rock which are used to support the foundation of bridge structures through rapidly moving water over time. Scour prevention depends on the proper design, construction and maintenance of a bridge. This process involves the experience and knowledge of professional foresters and professional engineers. When selecting the type of bridge and the site in which it will be built in, professionals will need to consider factors that lead to scouring. The bridge structure influences scour by obstructing a water way channel with sections such as the abutments and pier. The abutments and piers of bridge structures narrow the channel by reducing the cross sectional area in which water will flow. This consequently increases the velocity of water as narrowing occurs to pass through an obstructing bridge structure. High water velocity as well as volume during flooding events leads to scouring through hydraulic forces such as the formation of horseshoe and wake vortex at the foundations of bridge structures, specifically piers. The constant hydraulic forces against the bridge can remove bedding material that the structure’s foundations are set on. Over time, the excess removal of underlying material such as soils and rocks through scouring will weaken the foundations of the bridge, making it lose stability and eventually leading to collapse. To decrease bridge vulnerability to scour, professional can take several countermeasures. The site where the bridge is desired needs extensive and careful assessments of the physical characteristics of the stream and its impacts on the existing bedding material. Using measurement and calculation based methods such as the erodibility index method (EIM), the vulnerability of the bedding material to scour can be found. Technological devices such sonar, the rotating erosion rate apparatus (RETA) and the scour erosion rate flume (SERF) used to assess scour vulnerability can help design a more cost-efficient bridge, but they have limitations such as operability at a large scale.  Periodic inspections of bridges are also necessary to ensure safety and operability. Inspections would be optimized for data collection and observations if done during flood events, but safety precautions require inspections to occur after flood events. Inspections can happen from the bridge or by underwater diving, but underwater diving has limitations such as visibility, hazardous floating debris or aggressive water flow that deem it unsafe.     Forest Bridge Design and Scour Potential   2  TABLE OF CONTENTS Abstract ................................................................................................................................................................. 1 Introduction ........................................................................................................................................................... 3 Factors That Affect Hydrological Behaviors and Induce Scour .................................................................................. 4 Structure ............................................................................................................................................................ 4 Horseshoe and Wake Vortices ............................................................................................................................ 6 Debris................................................................................................................................................................. 8 The type of the underlying rock and soil............................................................................................................ 10 Discussion on Scour Countermeasures .................................................................................................................. 12 Site selection .................................................................................................................................................... 13 Scour Inspections ............................................................................................................................................. 14 Approval........................................................................................................................................................... 15 Work cited ........................................................................................................................................................... 16             Forest Bridge Design and Scour Potential   3  INTRODUCTION Scour refers to the erosion or removal of bank materials from the bridge foundations by flowing water. In addition, scour is also the main cause of bridge damage and failure in forest bridges (Kattell 2008). As forests are crucial water catchment areas, there is always a need to consider weather and rainfall patterns when designing forest bridges. Other design considerations must include hydraulic factors such as discharge patterns, water levels, and ice flow in the water. There are also geotechnical factors such as the boundary materials, soil erodibility and sedimentation that contribute to scouring of the bridge foundations (Richardson & Davis 2001). As a result, designing a quality bridge with a lengthy lifetime requires expertise in scour vulnerability assessment. Also, to reduce future bridge damage through floods and ensure the safety of the public, it is important to implement improved procedure in proper inspections for scour. Bridges must be assessed periodically for vulnerability to scour for determining the safety and operability measures necessary for that bridge and the entire bridge inventory (Richardson & Davis 2001).   Inadequate designing of bridges may cause problems, therefore, it is important that bridges are properly designed and constructed according to the required standards and guidelines of the region. This will always require the application of proven engineering techniques of structural and hydrologic design (Richardson & Davis 2001). This literature review will identify and discuss factors that affect scour of bridge footings as well as techniques for protecting bridge footings from scour.            Forest Bridge Design and Scour Potential   4  FACTORS THAT AFFECT HYDROLOGICAL BEHAVIORS AND INDUCE SCOUR  STRUCTURE The structure type of the bridge is a major influencing factor for determining the scour vulnerability. Structures such as log cribs or drilled shafts are considered to have low vulnerability to scour damage while structures with shallow foundations such as precast cement are considered to have high vulnerability to scour (Kattell 2008). However, this can be argued as anything in the channel is vulnerable to scouring depending on shape, position, velocity and volume of water. The other major factor for determining scour vulnerability is capacity. Capacity refers to the volume of flow able to pass through the bridge opening without the water overtopping over the deck (GNL 1989). The product of the span and rise then the addition of the cross-sectional area indicates the volume of the bridge opening (Figure 1). Span and rise are independent of requirements of the capacity meaning that the bridge dimension needs to be larger than the required by the design flow or else contraction scour could occur. Contraction scour occurs when where there is approaching embankments that cause a reduction in cross-sectional area. This narrowing of the channel due to the bridge structure can result in increased velocity and consequently that increases the chance of scour.  Figure 1: (Nagy et al 1980) showing the variables required for calculating capacity of a bridge, rise (height), span (length) and remaining cross sectional area Forest Bridge Design and Scour Potential   5  Many bridges have abutments from the bridge to the water bed along which aggregate is filled. As a result, these bridges are susceptible to bridge scour, the washing away of fill around structure which compromises the safety of the bridge. As aggregate is washed away during bridge scouring, the foundation becomes exposed and unstable. Consequently the erosive nature of running water results to carrying of material from the bridge piers and abutments (Wu 2006). Scour at bridge foundations is collectively evaluated by professional engineers and geotechnical engineers (Nagy et al 1980). After selection of the site for the structure and having successfully established the height and width requirements of the superstructure, the designer is required to choose the type of bridge to be constructed in the site. To be discussed are the variables in bridge designing, the geometry and length of different approaches, structure type, and location of abutments as well as piers. Figure 2 (Lingelback et al 2009) below provides a general diagram of a bridge with all elements, but it is important to note that elements such as piers may not always be necessary in bridge design.   Figure 2: General diagram of a bridge structure with crucial elements such as piers, abutments and footings.     Forest Bridge Design and Scour Potential   6  It is important to have information on the line, grade and the typical part of the approaching roads to the bridge in order to make good evaluation of the impact of the bridge on flood plain flows. Highway embankments redirect overbank flow, causing it to flow parallel to the embankment and later returning to the main channel in the bridge. In these cases, the designing of the roads should include measures that will limit damage to road fills and abutments of the bridge (Government of Newfoundland & Labrador (GNL) 1989). These measures will include relief bridges, retarding overbank flow velocity through tree planting on the flood plain or slope protection with riprap.   It is necessary to provide protection around abutment and piers in case there are signs of any potential scouring. A protective apron of riprap should be installed to the depth below the expected scour level (Adams 2006). The downside to using riprap is that it’s a temporary solution to protect intermediate piers because scour problems could still occur if riprap is improperly placed around piers and against abutments. Abutments should be designed with tapered wing-walls downstream and upstream of the forest bridge. In addition, it is advisable for them to be inclined into the embankment to the vertical axis to increase stability in the structure (Swift 2008). Piers should be designed with ends that are tapered downstream and upstream in the main flow direction. Heaping stones around the abutments is not effective because this kind of protection may require continual replacement (Adams 2006). This makes it important to protect it from the erosive nature of running water and the possibility of increasing water levels. HORSESHOE AND WAKE VORTICES Increasing water levels could cause formations of a horseshoe vortex or wake vortex at the base of piers, therefore inducing the scour (Kwak 2000). This is the most common form of scour that could occur around piers and water embankments. A horseshow vortex occurs when water contacts the upstream end of the pier, flowing downwards and causing the formation of vortices that wrap around the pier. This action of the vortex forming around the bridge structures sinks in to the riverbed, removes bed material and enlarges the scour hole size around the base of the pier. In addition, there are vertical vortices that occur downstream of piers called the wake vortex. Both the horseshoe and wake vortices (figure 3) can remove material from beneath footings and induce scour.    Forest Bridge Design and Scour Potential   7     Figure 3: scouring mechanism from the horseshoe and wake vortex The shape of the piers could influence how much the vortices are an influencing scour factor. For example, at a cylindrical pier, a horseshoe vertex or a wake vortex around the downstream end of a pier can occur. The upstream pier shape can be sharp nosed, round nosed or square nosed. This is something to take account of when designing bridges. An analysis was performed to see the impact of pier shapes on wake and horseshoe vortices (El-Ghorab 2013).  By measuring the scour depths caused by differently shaped piers, this experiment found that piers with sharp nosed ends are least effective in causing scouring vertices. Streamlining the front, upstream end reduces the scouring depth caused by the horseshoe vortex. Similarly, streamlining the downstream end reduces the strengths of wake vortices. Another suggestion this report made for reducing formation of vertices are developing counteracting circulatory flow structures on the river bed to divert the forming vertices. The report also suggests making an armour layer with enough thickness that would inhibit the scouring by vertices at piers.      Forest Bridge Design and Scour Potential   8  DEBRIS The vulnerability to scour depth at piers also depends on the probability of the pier to collect debris. If there is woody debris in the upstream end of a pier during flood situations, the flow will be diverted at a larger scale around the pier, causing larger vertices and further increases scouring (Pendergast 2014). This is because the width of the pier is increased therefore obstructing the water more on the upstream end as shown in figure 4. It’s very important that woody debris against piers is removed or else future floods will further increase the build-up and area of water obstruction.  Figure 4: Debris build-up on the upstream side of the pier causing a decrease in channel width and obstructing the flow of water. Several structural measures can be applied to reduce scour. Abutments should be designed back from the wetted perimeter to keep the natural flow area of the water in order to avoid any contraction of the channel (Swift 2008).  Piers must also be avoided for reasons such as accumulation of debris in the channel passage and the possible formation of hydraulic forces like the horseshoe and wake vortex (Kattell 2008). For sufficient structural support, the width of piers perpendicular to the flow direction should not be designed in excess of what is needed. As stated before, preference for pier shape is sharp nosed ends both on the upstream and downstream sides. All abutment and piers foundations should be well set into the stream bed to ensure solid base for the bridge and the design of the foundation should extend below the estimated lowest level of scour (Lagasse 2007). Forest Bridge Design and Scour Potential   9  When designing bridge width, it is important to assess whether the stream has a single, well-defined channel with a regular width and whether the flood flows are confined to the channel. The bridge should be clearing the entire channel abutments at least 0.5 meters from the normal height of the water. In addition, in no case should the abutments decrease the channel width by more than 10% of the estimated flood flow event (GNL 1989). If the stream is irregular with varying widths from one point to another, narrower sections are best preferred for a suitable bridge length. However, it is vital to consider the possibility of overbank flow at narrower channel widths. In flat and low-lying terrain subject to flooding, it is acceptable to allow overtopping of the roadways during extreme floods (Wu 2006). This will reduce the discharge passed through the bridge waterway opening. In case of such designs, provision is necessary to prevent any possible road washout by designing designated overflow sections that are well protected from erosion.  The height of the deck should be designed in a way to ensure it is not endangered by flowing water, floating debris, ice and waves. The issue of selecting design values and safety margins for high level of water and discharge raises many difficult questions (Sheppard & Miller 2006). It is the responsibility of the engineer to set these margins with regard to the reliability of the data on the design values. The engineer should also compare the data with the probability of occurrence of the values, effect of structural failure, type of structure chosen and the economic impact of the bridge (Ritter, 2010). To design an adequate bridge height, it is advisable to include additional height if there is history of ice accumulation and floating debris that poses potential problems to the bridge. Lastly, the length of the bridge should be designed for the opening to pass maximum water that may be expected without the possibility of endangering the bridge structures by scour (Melville & Coleman 2000).   For adequate design of the bridge waterway opening, calculation of volumes, scour, flood discharge, tidal flows, a combination of the following considerations stated by the Government of Newfoundland and Labrador (1989) are important. The first factor is the maximum historical water level recorded in the forest. Record of the historical water level in the forest is important in determining the minimum height of the bridge. This will help in determining the highest volume of water experienced in the site. The second important factor is the frequency analysis. This can be calculated based on recorded water capacity levels in the forest and help create a flood frequency analysis which analyzes the past Forest Bridge Design and Scour Potential   10  floods in the forest stream. If there is inadequate information to give an estimate of the actual highest water discharge over a reasonable historical period, other reasonable methods can be used to estimate the design discharge. These methods include using available regional flood frequencies and maximum probable storm. Estimates are made of the maximum flow rates based on the drainage of the area, rainfall intensity duration and any other appropriate data available that would indicate anticipated flows. In addition, it is advisable to check whether these changes are likely to occur in the future. The last factor is the flow duration. The probable flow duration and the magnitude of large flows are significant especially with reference to scour. THE TYPE OF THE UNDER LYING ROCK AND SOIL  Scouring relies on the ability of the soil to resist erosion and the erosive potential of flowing water; therefore, the underlying materials beneath bridges are a major determining factor of scouring (Kwak 2000).  Generally, soils containing very fine sand particles and silt, poor cohesiveness, low permeability and a low organic matter content are the most erodible (Wall 1997). Since structures such as bridge piers and abutments cause flow pattern changes in the channel, water can cause shear stress on the underlying material. There are cohesive soils where scour occurs at a slower rate than in non-cohesive soils and this has helped derive a method, the Erodibility Index Method (EIM), which predicts scouring at bridges founded on materials such as soil and rock (Annandale 2001).   The EIM uses an equation that incorporates factors such as content of underlying foundation material, available stream power and the required stream power for a scour to occur. The equation is expressed as:  K = (Ms) (Kb) (Kd)( Js) (Ms) is the intact mass strength number which represents a consistently standardized sample of earth material which would serves as relativity to the deficiencies of the material being assessed.  According to the Annandale (2001) criteria, the values of intact mass strength increase as the density and hardiness of the material increases. For example, a “very loose material” that crumbles very easily when scraped with a pick has an Ms Value of 0.02 whereas “hard rock” that cannot be peeled with a knife and required a medium blow with a hammer has an Ms value of 17.7.   Forest Bridge Design and Scour Potential   11  The second variable for calculating EIM is Kb which represents the rock block or/and particle size number. This variable takes in to account the role of rock block and soil particle sizes for determining their resistance to scour. Values are derived differently in both rock and granular soil. For rocks, it is a function of rock joint spacing and number of joint sets whereas granular soil’s value is determined by cohesiveness. Generally, resistance to scour increases as rock block and soil particle size increases.       The third variable is Kd which represents the discontinuity/Interparticle bond shear strength number. This value accounts for rock discontinuities and the angle of friction of particles.  Rock discontinuities are measured by the degree of roughness on the rock face as well as the degree at which forces will continue to form discontinuity and roughness on the rock face.  Lastly, the relative ground structure number ( Js) represents the relative ability of the underlying material due to ground structure. This variable incorporates the orientation of possible erosion flow and the shape at which the material is existent to predict the effort required for the stream to dislodge material units. The shape of rock influences erodibiity by being difficult to remove if the rock is elongated and easier to remove if the rock is equil-sided or symmetrical. The orientation is derived from the angle towards the least favourable discontinuity due to erosion while considering the direction of flow.    If rocks are significantly the underlying material of bridge foundations, Keaton et al (2012) propose four factors that result in rock scouring due to rock-water interaction. Dissolution of soluble rocks occurs when rocks such as limestone, gypsum and salt are existent under the footing foundations. To prevent this, bridges should not be founded on these rock types, particularly gypsum and salt. The next scour vulnerability factor with rocks is cavitation during high velocity fluctuations that cause formation of vapor bubbles. As this would require highly turbulent flow, it is not a commonly occurring phenomenon with rock scouring and the solution to it is designing a bridge that would most leave the channel in its natural condition. Quarrying and plucking of durable rocks is the third major concern with rock souring, but this would occur through weathering over long periods of time. Lastly, abrasion of degradable rock or breaking off and flaking of rock surfaces could occur from the consistent impact of suspended sediments over time.  Forest Bridge Design and Scour Potential   12   To determine erosion resistance in underlying material, two devices have been created by the University of Florida in conjunction with the Florida Department of Transportation (Raphael 2010). They are called the rotating erosion rate apparatus (RETA) and the scour erosion rate flume (SERF).  These devices can take soil samples and measure the rate at which the soil erodes as a function of the speed of the water flowing over it. This provides information for predicting what the total scour or erosion is going to be over the life of the bridge. This technology can help aid in bridge construction because engineers can determine whether the bed material will erode or not erode and make design adjustments in the structure. Such design adjustments could not only improve the safety of the bridge, but it could also help save costs in designing much larger or complex structures. The weakness to this technology is the requirement of numerous samples tests in case there is varying bed material and the inability to account for bed material containing large rocks and boulders. DISCUSSION ON SCOUR COUNTERMEASURES The aim of scour countermeasures is to prevent and avoid loss of soil from underneath an abutment. The loss of soil can weaken the capacity of bearing or lead to settlement, hence causing structural failure. The depth of scour is also calculated, at an abutment, as the depth sum of contraction scour as well as long-term degradation (Wu 2006). With regards to the water, soil and structural factors that contribute to scouring, cross-referencing to bridge designs in similar environments and adjacent works is ideal to plan for design guidance or future improvements.   One major factor that was consistently repeated amongst bridge design reports was consideration of adequate clearance allotment in cases of excess water. Ideally, the opening of the bridge has to be as large as the natural waterway with resistance against a flood event. Also, bridges and approach embankments not aligned perpendicular to the approach flow further complicate flow patterns and velocity distributions. Factors that affect stream energy include channelization, changes in the downstream hydraulic control, cut-offs of meander loops, regulation or diversion of stream flow, changes in basin rainfall-runoff characteristics, and climate changes.  Forest Bridge Design and Scour Potential   13  The Government of Newfoundland and Labrador (1989) and its department of environment and labour identified two essential steps of the hydrologic and hydraulic bridge designing. The first step entails estimation of all forces or quantities that have impact on the installation to ensure appropriate return period. The second step is designing all the structural components in order to accommodate those forces or quantities with some safety margin. This is done by measurements of high water mark and water discharge velocities (GNL 1989). Possible blockages such as woody debris and ice whether present or in the future must also be considered to estimate how water will be passing through the waterway.  SITE SELECTION The complex interaction of the above characteristics will produce a wide variety of stream types. Considering scour and erosion the behavior of streams may fall within a broad range, from stable bedrock channel to mobile alluvial river. Many streams undergo complex behavioural changes from one point to another due to the strong influence of the features associated with the process of rainfall patterns and glaciations. It is important to have detailed investigations of the past behavior at a specific site when designing a forest bridge.   Site selection will greatly affect the expense and difficulties of building the bridge and the long-term performance (Sheppard & Miller 2006). This is especially true if there is a likelihood of changes in the land use in the drainage basin upstream of the bridge. This makes it necessary to conduct field studies during route selection in order to choose the best and effective location for the bridge. The general route selection should minimize the number and length of the bridge required to ensure minimum environmental disruption and cost (Richardson & Davis 2001).   A suitable bridge crossing site should be at a stable reach with good flow alignment.. In case of meandering and shifting streams, the engineer should pay more attention on the past trends to ensure there is no stream shifting at the site selected. Straight lengths are the most preferred for bridges (Shirole & Holt 2005). Bridge construction should be avoided on abrupt bends except when the stream is flowing in erosion resistant materials. Lastly, it is necessary that the approaches of the proposed bridge to meet the requirements of grade and alignment to ensure safety.  Forest Bridge Design and Scour Potential   14  SCOUR INSPECTIONS When carrying out scour inspections or assessing structures on site, the present condition of the structure such as water widths upstream and downstream along with images should be recorded. Conditions that indicate potential problems such as nearby waterway walls or obstructions on bridge abutments and piers also need to be identified. The best time to conduct scour inspections with high certainty is after a major flood event. Due to safety precautions or unpredictability, direct assessments during the flood event could not occur. Otherwise, there are scheduled inspections that also assess scour vulnerability. The downside to scheduled inspections is it could be during low flow times where some scouring factors may be not be present. Inspecting scour by going under water is also possible, but there are a few weaknesses (Grman 2010). Poor visibility contributes to challenging inspections and that could prove difficult for divers measuring and estimating sizes accurately. Possible infilling of material or high water velocity could also be challenges for divers to assess scours.  There are also technological ways to inspect bridges in cases where diver inspections are ineffective or not practicable. The federal and transportation agencies in the United States have developed acoustic imaging sonar systems to evaluate scour vulnerability (Gorman 2010). For example, the Coda Octopus Echoscope is a sonar technology that provides geo-referenced imaging with high resolution. This technology can be on set up on a small boat that would asses areas of scour speculation. While scanning underwater infrastructure, the Echoscope could compare surveys to previous ones to assess changes over time, show damaged bridge structures, build ups of debris and the overall effects of water flow.        Forest Bridge Design and Scour Potential   15  APPROVAL After designing the whole bridge, it is important to get approval before the construction process (Kattell 2008). Approval should be obtained from the relevant authority and it is important to ensure correction of any mistake that may have occurred in the process of designing the bridge. After approval, the last phase is the construction of the bridge (Kattell 2008). Construction procedure should adhere to the set standards and guidelines to prevent adverse effects to the environment. It is obvious that no structures are designed to last forever, however the more important the forest bridge the longer the expected lasting period. If the bridge has a longer lasting period expectation, there is a high chance of being subjected to more floods. Return period is the term used to indicate the probability that particular magnitude flood will occur (GNL 1989). For example, a bridge with A 100 year return period flood means a flood whose flow would exceed average once every 100 years. The selection of an appropriate return period for a forest bridge depends on the value or importance of the bridge. This will also include cost of repair or any replacement if the flows exceed the design flow and causing damages to the bridge. However, it is necessary for a selected return period to reflect the importance and reliability of the bridge and it must take in to account the effects of potential bridge scouring.    Forest resource managers have the task of delivering services that relate to planning, design, construction, inspection, and maintenance of forest road crossings. To deliver these services they depend on qualified and certified engineers. A relationship based on effective principles should be established to ensure that professionals act professionally and that they are supported to improve on their professional work.          Forest Bridge Design and Scour Potential   16  WORK CITED Adams, Paul W. 2006.Estimating streamflows on small forested watersheds for culvert and bridge design in Oregon. Corvallis, Oregon Forest Research Laboratory, College of Forestry, Oregon State University. Annandale, G., & Smith, S. 2001. Calculation of bridge pier scour using the erodibility index method. Golden Associates. U.S. Department of Transportation. Crowley, Raphael, and Bloomquist David. 2010. Enhancement of FDOT’s SERF Device and Study of Erosion Rates of Rock, Sand, and Clay Mixtures Using FDOT’s RETA and SERF Equipment. Washington, D.C. National Academy. Florida Department of Transportation Research. Transportation Research Record. EL-Ghorab, Entesar A.S. 2013. Reduction of Scour Around Bridge Piers Using A Modified Method For Vortex Reduction. Alexandria Engineering Journal 52.3 467-478.  Gorma, Chris. 2010. Improving bridge inspection, scour monitoring and infrastructure management for DOTs using the echoscope - A true 3D, high-definition and real-time imaging tool. Coda Octopus. Edinburgh, UK. Keaton, R.,Jeffrey. Su, K. Mishra. Clopper, E., Paul. 2012. Scour at Bridge Foundations on Rock. National Research Council (U.S.) National Cooperative Highway Research Program., & American Association of State Highway and Transportation Officials. Transportation Research Board. Washington, D.C Kattell, John. 2008. Bridge Scour Evaluation: Screening, Analysis, & Countermeasures. San Dimas, Calif: USDA Forest Service, San Dimas Technology and Development Center. Kwak, Kiseok. 2000. Prediction of Scour Depth Versus Time for Bridge Piers in Cohesive Soils in the Case of Muli-flood and Multi-layer soil systems. Texas A&M University.  Labrador, Government of Newfoundland and. 1989. Department Of Environment and Labour. Environmental Guidelines for Bridges. Water Resources Management Division. Water Investigations Section 4  Lagasse, Peter F. 2007.Countermeasures to Protect Bridge Piers from Scour. Transportation Research Board. Washington, D.C Lingelback, T. A., Hilliard, J., Pebworth, J. 2009. Johnson’s Ford Early 19th Century Bridge Remains on Osage Creek, Carroll County, Arkansas. Arkansas Archeological Survey. Forest Bridge Design and Scour Potential   17  McCullah, J., Gray, D. H. 2005. United States National Research Council (U.S.)., Environmentally sensitive channel- and bank-protection measures. National Cooperative Highway Research Program & American Association of State Highway and Transportation Officials. Transportation Research Board. Washington, D.C Ministry of Forests, Lands and Natural Resource Operations. 2015. Lands and Natural Resource Operations. Water Act. Section 9.BC  Melville, Bruce W, and Stephen E. Coleman. 2000. Bridge Scour. Water Resources Publication. Highlands Ranch, Colorado. Nagy, M.M. et al.  1980.  Log Bridge Construction Handbook.  Forest Engineering Research Institute of Canada, Vancouver Prendergast, L.J., & Gavin, K. (2014). A Review of Bridge Scour monitoring Techniques . Journal of Rock Mechanics and Geotechnical Engineering, 6(2), 138. Richardson, E.V. and Davis, S.R. 2001. Evaluating Scour at Bridges, in Hydraulic Engineering Circular No. 18. Federal Highway Administration. Ritter, Michael A. 2010.Timber bridges: Design, construction, inspection, and maintenance.  Shirole, A.M. and Holt, R.C. 2005. Planning for a Comprehensive Bridge Safety Assurance Program. Transportation Research Record. Denver, Colorado.   Sheppard, D.M. and Miller, W. 2006. Live-bed Local Pier Scour Experiments. Journal of Hydraulic Engineering-ASCE.  Swift Jr, Lloyd W. 2008.Forest access roads: design, maintenance, and soil loss. Forest hydrology and ecology at Coweeta. Springer New York.  Wall, G. J., Coote, D. R., Pringle, E. A., & Sheltn, I. J. 1997. Revised Universal Soil Loss Equation for Application in Canada.Agriculture and Agri-Food Canada. Ottawa, On. WU, Jonathan T. H. 2006. Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing. Transportation Research Board Washington, D.C.  

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