River Bank Erosion. Case Study: The Trans Canada Highway Bridge at Beaver River, Glacier National Park

Transcription

1 River Bank Erosion Case Study: The Trans Canada Highway Bridge at Beaver River, Glacier National Park

2 Introduction The Beaver River is incising the bank near the eastern abutment of the bridge of the Trans Canada Highway The implications of this potentially include a wash out of the TCH, which would be devastating to transportation, tourism and the BC economy

3 Background: River Bank Erosion What causes river bank erosion? 2 main mechanisms: Bank scour Mass failure

4 What is bank scour? The direct removal of bank materials by the action of flowing water and the sediment it carries Flow rate is a major factor

5 What is mass failure? A section of the bank slides or falls into the river (collapse or slumping) Common with undermining of the toe of the bank

6 Contributing factors to erosion: Flooding Land use and stream management Clearing of river bank vegetation River straightening Rapid flow drop after flooding Saturation of banks from non-river sources Redirection and acceleration around infrastructure or debris in the channel Intense rainfall events Bank soil characteristics (easily erodible, poor drainage)

7 Affects of erosion at a bridge

8 Case Study: River bank erosion of Beaver River at the Trans Canada Highway Bridge

9 What we did Why did we do this? History of the area Observations and Methodology Assessment Flow measurements Discharge measurements Pebble count Sediment collection and sieve analysis Aerial photo review Historical climate and discharge trends Results Field results Lab results Conclusions Implications

10 Our purpose Why is the river eroding the bank? How fast is the bank being eroded? What are the implications of this bank erosion?

11 Background history CPR first built railway through here in 1885 The Rogers Pass section of highway was completed in 1962 Highway dips into the Rocky Mountain Trench (east of Rogers Pass) Trench created by a major fault, limestone of the Rockies to the east and metamorphic rocks of the Selkirks to the west

12 More background TCH is a major transportation corridor Through traffic in GNP increases by about 1-2% annually TCH thru traffic 1960 to 2001 (Parks Canada)

13 The Beaver River A tributary of the Columbia River Main source is the Beaver Glacier in GNP Mouth is at the Kinbasket Lake Total drainage basin = 1,150 km 2 Max discharge in 1985 (429m 3 /s on May 20 th ) Major flood in July 1983

14 Drainage area of the Beaver River

15 Bridge History Bridge length = 42 metres Single abutment mid-span Concrete Age unknown, possibly original (1962) but has more recent characteristics (adapted for snowplows) Some armouring on east side

16 Major Field Observations

17 Site Diagram

18 Major Field Observations

19 Assessment

20 Field Methods: Flow measurements Pooh sticks Large error associated with method More accurate methods: Weir Flow meter Dye testing

21 Field Methods: Discharge estimates Measurement of channel width and depth to get a cross-section Channel width - tying a rock to the end of the measuring tape and throwing it across the channel Channel depth wading in where possible, otherwise guessing Large error associated with these methods Need waders, measuring tape and ruler take depth measurements at intervals to get an idea of bed morphology

22 Field Methods: Pebble count Established transects along point bars upstream and near the bridge Sampled approx every 5 metres along transect, measuring 3 axes of 10 random pebbles Should have conducted at more locations, and one downstream

23 Field/Lab Methods: Sediment collection & sieving Collection of 3 samples at eroding bank Near water level, in organic layer, above organic layer Subject samples to standard set of sieves Weigh each sub-sample Should have used hydrometer for silts and clays

24 Lab Method: Aerial photos Acquired aerial photographs from 1986, 1994 and 2004 Attempted to measure movement of channel meanders, point bars and banks Unfortunately, most photos were at too small of a scale

25 Lab Method: Historical climate and discharge trends Examined maximum instantaneous discharge records for the WSC site Beaver River at Mouth Compared discharge events to precipitation levels over the same time period Goal: to determine the impact of non-precipitation sources on discharge Too many possible causes of discharge variation

26 Results: Assessment Rapid Assessment of Channel Stability Stability Indicator Rating Weight Weighted Value Bank soil texture and coherence Average bank slope angle Vegetative bank protection Bank cutting Mass wasting or bank failure Bar development Debris jam potential Obstructions, flow deflectors and sediment traps Channel bed material consolidation and armouring Shear stress ratio High flow angle of approach to bridge Bridge distance from meander impact point Percentage of channel constriction Total Overall Rating (R ) - - Fair Ratings Values Overall R Excellent (1-3) R < 32 Good (4-6) 32 <= R < 55 Fair (7-9) 55 <= R < 78 Poor (10-12) R >= 78

27 Results: Flow measurements Flow Rate Estimations 2.5 Flow Rate (m/s) Upstream of bridge Just before bridge Downstream of bridge Location Notice a decrease in flow rate from upstream of the bridge to downstream Possibly due to channel deepening or widening or subsurface flow Likely due to crude methodology

28 Results: Discharge estimates From estimated cross-section and estimated velocity: Discharge = m 3 /s Compare with WSC hydrometric data for Sept 10 to 11 th Ratio of average discharge over 2 days to the drainage area = m 3 /s : 1150 km 2 and ratio of discharge over 2 days to OUR drainage area = x : 437 km 2 X = m 3 /s We were a little off

29 Results: Sediment sieve analysis Percent (%) Sample C < Sieve Size (mm) m from surface Highest amt muds, some very coarse sand Percent (%) Sample B < m from surface, in organic layer Mainly muds, some very fine sand Sieve Size (mm) Percent (%) Sample A < m from surface Highest amt of very fine sand Sieve Size (mm)

30 Results: Pebble counts Pebble Count for Upstream Bar Show slight difference downstream Likely due to change in flow Need more locations for this data to truly be useful Number of Pebbles Number of Pebbles < 3 (medium pebble) > 3 < 6.4 (large pebble) 6 Grain Size (cm) Pebble Count for Bar Closest to Bridge 75 < 3 (medium pebble) > 3 < 6.4 (large pebble) > 6.4 < 26 (cobble) > 26 (boulder) 58 > 6.4 < 26 (cobble) > 26 (boulder) 0 1 Grain Size (cm)

31 Results: Aerial photo analysis

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33 Results: Aerial photo analysis Evidence of bar migration and change in river morphology A gross estimate of rate of erosion based on aerial photos We couldn t calculate one

34 Results: Historical climate/drainage data Discharge Rate (m 3 /s) Beaver River (at mouth) - Flow and Precipitation Total Precipitation (mm) WSC Discharge Rate Average relationship Annual Precipitation* Golden Represents glacial input to discharge Evidence of other factors influencing discharge other than precipitation Year 0

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36 Conclusions Why is the river eroding the bank? Due to river meander aggravated by high flow events in summer months, less-cohesive bank material, debris obstructions, poor riprap construction How fast is the bank eroding? Changes noted in the aerial photos but nothing directly related to the current erosion

37 Conclusions What are the implications? Undermining of bridge construction Wash out of TCH Closure of TCH would have huge impact on tourism (especially in summer months during high flow periods) economy (main route from BC to the east)

38 References Fahnestock, R.K., Morphology and Hydrology of a Glacial Stream White River, Mount Rainer Washington (1963), Geological Survey Professional Paper 422-A Lagasse, P.F., Schall, J.D., Richardson, E.V., Stream Stability at Highway Structures Third Edition, (2001), National Highway Institute, US Department of Transportation, Publication No. FHWA NHI Woods, J.G., Glacier Country, (2004), Friends of Mount Revelstoke and Glacier, BC, ISBN de=39019&geolvl=csd ht=800&mode=navigator&upperleftx=4160&upperlefty=464&lowerrightx=7360&lowerrighty=3664 &mag=0.125 Google Earth imgrefurl= KiKSL2f QG-t5i2scmDiz4iWGsxI=&h=400&w=393&sz=69&hl=en&start=1&um=1&tbnid=cGZW6haL7- ve7m:&tbnh=124&tbnw=122&prev=/images%3fq%3dudden%2bwentworth%2bscale%26um%3d 1%26hl%3Den%26rls%3Dcom.microsoft:en-ca:IE-SearchBox%26rlz%3D1I7GGLR%26sa%3DN er.jpg