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Erosion & Sedimentation Risk Assessment:
Planning for Environmentally Responsible Road Construction
Robin McKillop, M.Sc., P.Geo.January 21, 2015
SCOPe
OutlineOutline
• The Challenge
• The Strategy
• The Application
THE CHALLENGE
BackgroundBackground
• Road construction commonly necessitates vegetation removal and earthworks
• Exposed soil susceptible to erosion– Water– Wind– Gravity– Ice (freeze‐thaw)
• Erosion sedimentation
So What?So What?
• Erosion and sedimentation are natural processes
• BUT…unnaturally widespread, severe and/or frequent erosion problematic
• Risks– To environment (ecological)– To project (financial, delays)– To proponent (legal)
Ecological RiskEcological Risk
• Risk to aquatic species, if sediment enters watercourse or waterbody
Erosion & Sediment ControlErosion & Sediment Control
• ESC measures to minimize occurrence and severity of erosion and sedimentation, thereby mitigating risks
• ESC plan specifies types and configuration of ESC measures
The DisconnectThe Disconnect
• ESC plans are prepared to mitigate risks, yet rarely are the risks explicitly identified in advance or communicated to review agencies when submitted
• Need to understand risks in order to ensure appropriate environmental protection…
THE STRATEGY
What is E&S Risk Assessment?What is E&S Risk Assessment?
• The classification of a construction corridor into areas of similar erosion and sedimentation risk
Source
Receptor
ObjectivesObjectives
• Identification of site‐specific risks
• Risk‐commensurate ESC measures– Low risk ≠ Moderate risk ≠ High risk
• Streamlined approvals through improved agency communication and demonstrated risk awareness
• 3‐step, GIS‐based model using public and project‐specific data sets
• Raster‐based overlay analyses with weighted factors
• 3 maps:– Map 1 – Erosion Potential– Map 2 – Potential Ecological Consequence of Off‐site
Sedimentation– Map 3 – Overall E&S Risk
3 2 1 1 1
3 2 1 1 1
3 2 2 2 1
3 3 2 2 1
3 3 3 2 1
1 1 2 2 2
1 1 2 2 3
1 1 2 3 3
1 2 2 3 3
1 2 3 3 3
3 3 3 3 3
2 2 2 2 3
1 1 2 3 3
1 1 2 2 2
1 1 1 1 1
7 6 6 6 6
6 5 5 5 7
5 4 6 8 7
5 6 6 7 6
5 6 7 6 5
Basis for E&S Risk AssessmentBasis for E&S Risk Assessment
THE APPLICATION
Freegold Road Extension
• 120 km
• 2‐lane gravel
• Valleys with discontinuous permafrost
Casino Project
• The likelihood of soil particles being transported downslope from their origin by surface runoff
• Erosion Potential = f (Soil Erodibility, Slope Gradient, Slope Length)
Map 1: Erosion PotentialMap 1: Erosion Potential
• Soil erodibility– Terrain mapping polygons
– Permafrost probability interpretation
Map 1: Erosion PotentialMap 1: Erosion Potential
Permafrost Probability MappingPermafrost Probability Mapping
17
Moderate HighVery Low Very HighLow
Probability of encountering shallow (<1 m) permafrost
• Soil erodibility– Terrain mapping polygons
– Permafrost probability interpretation
• Slope steepness– L = <10%, M = 10‐30%, H = >30%
• Slope length– N/A in this case
Map 1: Erosion PotentialMap 1: Erosion Potential
19
Moderate
High
Low
Map 1: Erosion Potential
Map 2: Potential Ecological Consequence of Off-site
Sedimentation
Map 2: Potential Ecological Consequence of Off-site
Sedimentation
Aquatic• Habitats
– Watercourses– Ponds/lakes
• Sensitivities– Fish habitat High
(or connected to fish habitat)
– Non‐fish habitat Low
Potential for Sediment Delivery• Topographic position of receptor
– Downslope– Upslope
• Length of flow path– L = >100 m, M = 30‐100 m, H = <30 m
• Ground cover (e.g., vegetation)
DEM‐based flow paths
Map 2: Potential Ecological Consequence of Off-site
Sedimentation
Map 2: Potential Ecological Consequence of Off-site
Sedimentation
22
Moderate
High
Low
Map 2: Potential Ecological Consequence
Map 3: Overall Erosion and Sedimentation RiskMap 3: Overall Erosion
and Sedimentation Risk
• Combines results of Map 1 and Map 2
#1Erosion Potential
#2Potential Ecological
Consequence of Off‐site
Sedimentation
#3Overall
Erosion & Sedimentation
Risk
24
Map 3: Overall Erosion & Sedimentation Risk
Moderate
High
Low
