Post-Fire Sediment Transport and Erosion in the Water Canyon and Canon de Valle Watershed, Jamez Mountains, NM

Post-Fire Sediment Transport and Erosion in the Water Canyon and Cañon de Valle

Watershed, Jemez Mountains, NM DRAKOS, Paul, RENEAU, Steven, SCHULTZ-FELLENZ, Emily, RIESTERER, Jim, KELLEY, Rick, MILLER, Elizabeth, GOETZE, Phillip, and CHAMBERLAIN, Paul

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Pajarito Plateau watersheds

Water Canyon/Cañon de Valle investigation conducted 2010-2011 to assess contaminant

inventory in post-1942 sediments • 1:200 scale mapping in 25+ reaches

• Characterization of post-1942 sediments using hand-dug pot holes (identified based on presence of exotic clasts, buried soils, bedrock, etc.)

• Dendrochronology utilized to provided supplemental information to constrain the age of flood deposits

• Reworked ash from 1977 La Mesa fire and 2000 Cerro Grande fire preserved in situ within sediment deposits provides additional age control

• Measurement of cross sections following post-fire floods on 8/3/11 and 8/21/11

• 8/21/11 flood est. ~3000 cfs in CDV, larger than 8/3/11 flood

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Extent of 1977 La Mesa fire within Water Canyon/ Cañon de Valle Watershed

Extent of 2000 Cerro Grande fire within Water Canyon/Cañon de Valle Watershed and maximum post-fire discharge (6/2000)

840 cfs

274 cfs

740 cfs

Extent of 2011 Las Conchas fire within Water Canyon/Cañon de Valle Watershed, and estimated discharge from 8/21/2011 flood

~3000 cfs (estimated)

1600+ cfs (estimated)

3500 cfs (est.)

Sediment delivery after Cerro Grande wildfire (Reneau et al., 2007)

• Over 90% of ash delivered to LA Canyon reservoir in the first year after the fire

• Slurries dominated by ash flow (suspended sediment) follow first convective storm after the wildfire

• Sediment transport rates declined after first year, but remained above pre-fire levels five years after the fire

• Significant increase in erosion rates in the first year following the fire

• Rapid decline in fine-grained sediment transport, whereas coarse-grained sediment transport is prolonged (bedload transport during snowmelt runoff)

• Post-fire sediment is primarily deposited within first two years after the fire

Preservation of in situ ash in post-1942 sediment deposits

Cerro Grande muck deposit (June 2000)

Methods – 1:200 Scale Mapping

Methods – Characterization of post-1942 sediment deposits

Methods – Characterization of post-1942 sediment deposits

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Methods – Measure geomorphic cross sections Location selection based on contaminant concentrations and

dendrochronology, prior to Las Conchas fire

Water Canyon/Cañon de Valle watershed showing reaches

Reach WA-2 Geomorphology, cross section location

c1

f1

c3

Qbt

c2

Reach WA-3 Geomorphology, cross section location

c1

f1

c3

Qc

c2 Qt

c1

f2

Cross sections in Reaches WA-2 and WA-3 showing post-1942 sediment deposition and post-Las Conchas fire muck deposits

CG ashy si

CG ashy si at base

LM deposit at base

CG ashy si

LM(?) ashy vfs

17 of 127 cm post-fire sed (c3)

~58 of 123 cm post-fire sed (c2)

~58 of 123 cm post-fire sed (f1)

10 of 10 cm post-fire sed (f2)

CG ashy si-fs

23 of 58 cm post-fire sed (f1s)

Water Canyon/Cañon de Valle watershed showing reaches

Cross sections in Reach CDV-1E showing post-1942 sediment deposition and post-Las Conchas fire muck deposits and erosion

measured after floods on 8/3/11 (left) and 8/21/11 (right) > 50% (+/-75%) of post 1942 sediment deposits are post-fire seds

CG ashy si

CG muck at base

Cross sections in Reach CDV-2W showing post-

1942 sediment deposition and post-Las

Conchas fire muck deposits and erosion measured after flood

on 8/21/11

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CG muck

CG ashy si+vfs

Note in lower cross section Some channel incision but most post-1942 sediments remain in place.

Cross sections in Reach CDV-2W showing post-1942 sediment deposition and post-Las Conchas fire muck

deposits and erosion measured after flood on 8/21/11

Note erosion of c2 sediments next to channel, preservation of c3/f1 deposits on higher surface, deposition of muck from 8/3/11 and coarse sediment from 8/21/11 flood.

Sampling location CV-613603 in reach CDV-2W where c2 unit was completely eroded during 8/21/2011, flood.

Reach CDV-3 Geomorphology, cross

section locations

f1

c3

c2

Qbt

c1

Qc

Cross sections in Reach CDV-3 showing post-1942 sediment deposition and post-Las Conchas fire muck deposits measured after flood on

8/3/11; upstream (left) and downstream (right); post-fire sediment <20% of total post-1942 deposition

Water Canyon/Cañon de Valle watershed showing reaches

Cross sections in Reach WA-4 showing post-1942 sediment deposition and post-Las Conchas fire muck deposits; < 20% are post-fire deposits

CG dk vfs+si

Reach WA-5 Geomorphology, cross section location

c1

f1

Qc

c2

Tb

c1

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Cross sections in Reach WA-5 showing post-1942 sediment deposition and post-Las Conchas fire muck deposits and erosion

measured after floods on 8/3/11 (left) and 8/21/11 (right)

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8/23/11 Flood coarse sediment deposits and scouring

Coarse sediment deposits in Cañon de Valle from August 21, 2011, flood. Gravel bar adjacent to channel in reach CDV-1E.

Coarse sediment deposits in Cañon de Valle from 8/21/2011 flood. ~2 to 4 m wide gravel deposit buries 0.5m wide

channel in Reach CDV-2W

Scour hole in reach CDV-1E demonstrating channel erosion in Cañon de Valle from August 21, 2011, flood.

Water Canyon fan deposit at Rio Grande (left) and detail showing ~10 cm thick L.C. muck deposit (right)

Photos taken 11/30/11

Muck from 8/3/11 flood?

Coarse (ms-cs) sediment from 8/21/11 flood?

Water Canyon Fan

Post-Las Conchas sediment deposits along Rio Grande

Conclusions • First post-Las Conchas fire flood on 8/3/11 deposited muck layer

(ashy silt) with a maximum thickness of 22 cm at cross section locations

• Some erosion also occurred

• Second post-Las Conchas fire flood on 8/21/11 was larger discharge event. Coarser-grained sediment deposits up to 50 cm thick at cross section locations were observed in both overbank and channel settings.

• Considerable scour and bank widening also observed during second flood event.

• Total post-fire related sediment deposits from LM, CG, and LC fires at randomly selected cross sections ranges from up < 20% to 75%, shows a general down-stream attenuation trend

• Post-LC muck and coarser-grained sediment deposits observed at mouth of Water Canyon at Rio Grande.

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Pending Investigation (2012)

• Systematic assessment of post-Las Conchas erosion and deposition in reaches investigated in 2010/2011

• Reoccupy and resurvey cross sections to quantify post-Las Conchas fire erosion and deposition at specific locations

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Acknowledgements Steve Veenis

LANL Environmental Programs TerranearPMC.com

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