River Bank Erosion
Case Study: The Trans Canada Highway Bridge at Beaver River, Glacier National Park
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
Background: River Bank Erosion • What causes river bank erosion? • 2 main mechanisms: • Bank scour • Mass failure
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
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
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)
Affects of erosion at a bridge
Case Study: River bank erosion of Beaver River at the Trans Canada Highway Bridge
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
Our purpose • Why is the river eroding the bank? • How fast is the bank being eroded? • What are the implications of this bank erosion?
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
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)
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 km2 Max discharge in 1985 (429m3/s on May 20th) • Major flood in July 1983
Drainage area of the Beaver River
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
Major Field Observations
Site Diagram
Major Field Observations
Assessment
Field Methods: Flow measurements • “Pooh sticks” • Large error associated with method • More accurate methods: – Weir – Flow meter – Dye testing
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
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
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
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
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
Results: Assessment Rapid Assessment of Channel Stability Stability Indicator 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 )
Rating
Weight
Weighted Value
6
0.6
3.6
11
0.6
6.6
8 9
0.8 0.4
6.4 3.6
9 6 11
0.8 0.6 0.2
7.2 3.6 2.2
9
0.2
1.8
3 8
0.8 1
2.4 8
2
0.8
1.6
10
0.8
8
2 -
0.8 -
1.6 56.6 Fair
Ratings Excellent Good Fair Poor
Values Overall R (1-3) R < 32 (4-6) 32 3 < 6.4 (large pebble)
> 6.4 < 26 (cobble)
> 26 (boulder)
Grain Size (cm)
Pebble Count for Bar Closest to Bridge
Number of Pebbles
• Show slight difference downstream • Likely due to change in flow • Need more locations for this data to truly be useful
Number of Pebbles
60
80 70 60 50 40 30 20 10 0
75 58
6 < 3 (medium pebble)
1 > 3 < 6.4 (large pebble)
> 6.4 < 26 (cobble)
Grain Size (cm)
> 26 (boulder)
Results: Aerial photo analysis
1994 1986
2004
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
Results: Historical climate/drainage data Beaver River (at mouth) - Flow and Precipitation 450
700
400 600
350 500
400
250
200
300
150 200 100
100 50
0
0 1988
1991
1994 Year
1997
2000
2004
Total Precipitation (mm)
Discharge Rate (m 3/s)
300
WSC Discharge Rate Average relationship Annual Precipitation* Golden
• Represents glacial input to discharge • Evidence of other factors influencing discharge other than precipitation
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
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)
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 01-002 Woods, J.G., Glacier Country, (2004), Friends of Mount Revelstoke and Glacier, BC, ISBN 0-921806-16-7 http://www.wsc.ec.gc.ca/hydat/H2O/index_e.cfm?cname=WEBfrmPeakReport_e.cfm http://www12.statcan.ca/english/census06/data/trends/Table_1.cfm?T=CSD&PRCODE=59&GeoCo de=39019&GEOLVL=CSD http://www.th.gov.bc.ca/trafficData/tradas/inset3.asp http://www.transcanadahighway.com/britishcolumbia/TCH-BC-E5.htm http://atlas.nrcan.gc.ca/site/english/maps/archives/national_park/mcr_0219?maxwidth=800&maxheig ht=800&mode=navigator&upperleftx=4160&upperlefty=464&lowerrightx=7360&lowerrighty=3664 &mag=0.125 Google Earth http://images.google.com/imgres?imgurl=http://www.glossary.oilfield.slb.com/files/OGL98036.jpg& imgrefurl=http://www.glossary.oilfield.slb.com/DisplayImage.cfm%3FID%3D202&usg=__KiKSL2f QG-t5i2scmDiz4iWGsxI=&h=400&w=393&sz=69&hl=en&start=1&um=1&tbnid=cGZW6haL7ve7M:&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 http://www.pc.gc.ca/docs/v-g/bc/glacier/pd-mp/sec8/page1_E.asp www.arcc.osmre.gov/HydroToys.asp http://www.usbr.gov/pmts/hydraulics_lab/workshops/flowmeasurementworkshop_files/swoff er.jpg
