river bank erosion - university of · pdf filebackground: river bank erosion • what...
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River Bank Erosion
Case Study: The Trans Canada Highway Bridge at Beaver River,
Glacier National Park
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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
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Background: River Bank Erosion
• What causes river bank erosion?
• 2 main mechanisms:• Bank scour• Mass failure
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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
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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
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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)
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Affects of erosion at a bridge
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Case Study:
River bank erosion of Beaver River at the Trans Canada Highway Bridge
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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
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Our purpose
• Why is the river eroding the bank?• How fast is the bank being eroded?• What are the implications of this bank
erosion?
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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
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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)
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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
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Drainage area of the Beaver
River
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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
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Major Field Observations
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Site Diagram
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Major Field Observations
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Assessment
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Field Methods: Flow measurements
• “Pooh sticks”• Large error associated with
method• More accurate methods:
– Weir– Flow meter– Dye testing
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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
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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
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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
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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
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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
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Results: AssessmentStability Indicator Rating Weight
Weighted Value
Bank soil texture and coherence 6 0.6 3.6Average bank slope angle 11 0.6 6.6Vegetative bank protection 8 0.8 6.4Bank cutting 9 0.4 3.6Mass wasting or bank failure 9 0.8 7.2Bar development 6 0.6 3.6Debris jam potential 11 0.2 2.2Obstructions, flow deflectors and sediment traps 9 0.2 1.8Channel bed material consolidation and armouring 3 0.8 2.4Shear stress ratio 8 1 8High flow angle of approach to bridge 2 0.8 1.6Bridge distance from meander impact point 10 0.8 8Percentage of channel constriction 2 0.8 1.6Total - - 56.6Overall Rating (R ) - - Fair
Rapid Assessment of Channel Stability
Ratings Values Overall RExcellent (1-3) R < 32
Good (4-6) 32 <= R < 55Fair (7-9) 55 <= R < 78Poor (10-12) R >= 78
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Results: Flow measurements
• 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
Flow Rate Estimations
1.95
1.301.14
0
0.5
1
1.5
2
2.5
Upstream of bridge Just before bridge Downstream of bridge
Location
Flow
Rat
e (m
/s)
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Results: Discharge estimates • From estimated cross-section and estimated
velocity:– Discharge = 19.99 m3/s
• Compare with WSC hydrometric data for Sept 10 to 11th
• Ratio of average discharge over 2 days to the drainage area = 33.38 m3/s : 1150 km2
• and ratio of discharge over 2 days to OUR drainage area = x : 437 km2
• X = 12.68 m3/s• We were a little off…
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Results: Sediment
sieve analysis
•1.7m from surface
•Highest amt of very fine sand
•1.2m from surface, in organic layer
•Mainly muds, some very fine sand
•0.8m from surface
•Highest amt muds, some very coarse sand
Sample C
8.74.6 4.6 4.8 4.7 4.6 4.5 4.7
8.212.0
38.6
0.05.0
10.015.020.025.030.035.040.045.0
2.000 1.700 1.400 1.000 0.710 0.595 0.500 0.355 0.125 0.075 <0.075
Sieve Size (mm)Pe
rcen
t (%
)Sample B
5.4 5.2 5.3 6.2 6.0 5.6 5.5 6.1
13.4 12.6
28.6
0.05.0
10.015.020.025.030.035.0
2.000 1.700 1.400 1.000 0.710 0.595 0.500 0.355 0.125 0.075 <0.075
Sieve Size (mm)
Perc
ent (
%)
Sample A
4.7 4.7 4.7 4.8 4.8 4.7 4.8 5.2
31.3
16.1 14.3
0.05.0
10.015.020.025.030.035.0
2.000 1.700 1.400 1.000 0.710 0.595 0.500 0.355 0.125 0.075 <0.075
Sieve Size (mm)
Perc
ent (
%)
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Results: Pebble counts
• Show slight difference downstream
• Likely due to change in flow
• Need more locations for this data to truly be useful
Pebble Count for Upstream Bar
5
38
57
00
10
20
30
40
50
60
< 3 (medium pebble) > 3 < 6.4 (largepebble)
> 6.4 < 26 (cobble) > 26 (boulder)
Grain Size (cm)Nu
mbe
r of P
ebbl
es
Pebble Count for Bar Closest to Bridge
6
75
58
10
1020304050607080
< 3 (medium pebble) > 3 < 6.4 (largepebble)
> 6.4 < 26 (cobble) > 26 (boulder)
Grain Size (cm)
Num
ber o
f Peb
bles
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Results: Aerial photo analysis
1986
1994
2004
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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
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Results: Historical climate/drainage data
• Represents glacial input to discharge
• Evidence of other factors influencing discharge other than precipitation
Beaver River (at mouth) - Flow and Precipitation
0
50
100
150
200
250
300
350
400
450
1988 1991 1994 1997 2000 2004Year
Disc
harg
e Ra
te (m
3 /s)
0
100
200
300
400
500
600
700
Tota
l Pre
cipi
tatio
n (m
m)
WSCDischargeRate
Averagerelationship
AnnualPrecipitation*Golden
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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
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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)
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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 0-921-806-16-7
• 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=__KiKSL2fQG-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%3D1%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