QUANTIFYING THE RISK OF MINE PIT CAPTURE

Andrew Nelson, Dave McLean, Peter Brooks, Karen Hodges

Yakima County Water Resources

water resource specialists What are Floodplain Mine Pits?

Collins (1995) Pit Capture Hazards • and Abandonment

1995 Channel

2000 Channel

Lewis , WA 2000 Aerial Photo (USGS) via Google Earth Pit Capture Hazards • Avulsion and Channel 4 m of Scour Abandonment • Upstream Knickpoint Migration and

Tujunga Wash, CA (Bull and Scott 1973)

Tujunga Wash, CA (Bull and Scott 1974) Pit Capture Hazards • Avulsion and 1962 2001 Channel Abandonment • Upstream Knickpoint Migration • Rio Paraiba do Sol, Brazil (NHC 2014) Starvation Downstream Restoration Opportunity

• Riparian Surface Area • Off-channel rearing habitat • Channel Complexity

Terrace Heights Pit, Yakima, WA 41 years after capture (Bing Maps Birds-Eye Image) Quantify Hazards to Manage Pit Connections Empirical Approach • 31 Pit Captures and Connections • February 1996 • Supplemented with 5 examples of cutoffs and fall.

(Bull and Scott, 1974; Collins, 1995; Cui et al., 2014; Czuba et al., 2011; Dunne et al., 1980; Hilldale and Godaire, 2010; Kelly, 2003; Kondolf, 1994, 1997; Major et al., 2012; NHC, 1995, 2005, 2012, 2014a, 2014b; Norman et al., 1998; Scott, 1973; Wampler et al., 2007; Weatherly and Jakob, 2014; Yakima River Floodplain Mining Impact Study Team, 2004) Avulsion vs Connection

Channel Shortened, Channel Expansion Steepened, & offset by Expanded Lengthening

Major Disturbance Less Disturbance Potential Potential Connection Connection

Primary Hazard Local Channel Migration Avulsion Avulsion

Primary Hazard Upstream Channel Incision Avulsion vs Connection

Channel Shortened, Channel Expansion Steepened, & offset by Expanded Lengthening

Major Disturbance Less Disturbance Potential Potential Predicting Avulsion: Theory • Slope Ratio: 퐴푣푢푙푠𝑖표푛 푃푎푡ℎ 푆푙표푝푒 > 3 to 5 퐶ℎ푎푛푛푒푙 푆푙표푝푒

• Superelevation: 퐿푒푣푒푒 퐻푒𝑖𝑔ℎ푡 퐴푏표푣푒 퐹푙표표푑푝푙푎𝑖푛 > 0.5 to 1.1 퐶ℎ푎푛푛푒푙 퐷푒푝푡ℎ Will a Connection Evolve into Avulsion: Empirical Data N=23

Slope Ratio >1.7 Channel-Pit favors Connection With: avulsions No Avulsion through Avulsion pits Logistic

Regression Avulsion Likelihood Avulsion

Slope Ratio Knickpoint Height: Theory • Knickpoint Height (z) governed by change in hydraulic base-level (NOT pond depth) • This can be predicted by the geometry of the avulsion path 풛 = 푺풊(푳풊 + 푳풑 − 푳풂) Knickpoint Height: Theory • Knickpoint Height (z) governed by change in hydraulic base-level (NOT pond depth) • This can be predicted by the geometry of the avulsion path Initial Slope Initial Length 푧 = 푆𝑖(퐿𝑖 + 퐿푝 − 퐿푎) Pit Length Avulsion Length Knickpoint Height:

Empirical Data Actual Knickpoint Height (ft)Knickpoint Actual

Predicted Knickpoint Height (ft) N=11 Knickpoint Migration: Theory • Knickpoints in unconsolidated sediment evolve through diffusion (Cedar R., NHC 2014) • Knickpoints trend indefinitely towards oblivion.

(From Brush and Wolman 1960) Knickpoint Migration: Theory • Knickpoints in unconsolidated sediment evolve through diffusion (Cedar R., NHC 2014) • Knickpoints trend indefinitely towards oblivion. • A threshold (t) 푘푛𝑖푐푘푝표𝑖푛푡 표푏푙𝑖푣𝑖표푛 푠푙표푝푒 must be applied to 푟푒푎푐ℎ 푠푙표푝푒 define functional oblivion Knickpoint Migration: Theory • Knickpoints in unconsolidated sediment evolve through diffusion (Cedar R., NHC 2014) • Knickpoints trend indefinitely towards oblivion. 풛 풙 = • A threshold (t) 풕 푺 must be applied to 풊 define functional oblivion Knickpoint Migration:

Empirical Data N=11 • t empirically ranges from 0.6 to 4

• best predictions achieved using

t=1.15 Actual Knickpoint Distance Knickpoint Actual Predicted Knickpoint Distance Downstream and Coarsening

Few quantitative observations. Impacts of a single pit capture probably modest, but cumulative impacts can be significant! Channel recovery timescale: Theory • Here “recovery” defined as a return to bedload conveyance through pit. 푽 • 푃𝑖푡 푃푒푟푠𝑖푠푡푎푛푐푒 = 풑풊풕 푸풃 Pit, Clackamas River OR, 16 years after capture (Bing Maps Birds-Eye Image) Channel recovery timescale: Empirical Data Faster than expected recovery

Qb + headcut vol. +

some Qs Actual Years to to Recover Years Actual Expected Years to Recover N=6 Quantify Hazards to Manage Pit Connections

Lewis River near La Center, WA 16 years after capture (Bing Maps Birds-Eye Image) Quantify Hazards to Manage Pit Connections

Evaluate Avulsion Probability Quantify Hazards to Manage Pit Connections

Evaluate Avulsion Probability Low

Primary hazard is short-term accelerated channel migration (Easy to Mitigate) Quantify Hazards to Manage Pit Connections Must Consider Upstream Knickpoint Impacts

to Infrastructure and Channel High

Evaluate Avulsion Probability Low

Primary hazard is short-term accelerated channel migration (Easy to Mitigate) Quantify Hazards to Manage Pit Connections

Consider Upstream Knickpoint Impacts to Infrastructure and Channel • Abandonment of Side Channels • Undercutting of Structures & • Possibility of Cascading Effects

Mitigate unacceptable hazards & let the river restore itself.

Lewis River near La Center, WA 16 years after capture (Bing Maps Birds-Eye Image) Questions?

Andrew Nelson [email protected]

Yakima County Water Resources

water resource specialists References & Data Sources

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