POST WILDFIRE SLOPE AND CHANNEL PROCESSES EXPECTED FOR THE MARTIN BURN FIREADAPTED GEOSYSTEM

1
POST WILDFIRE SLOPE AND CHANNEL PROCESSES
EXPECTED FOR THE MARTIN BURN FIRE-ADAPTED
GEOSYSTEM
B. Vasiliki Vassil, PG Associate Director, RCD of
Santa Cruz County Senior Project Scientist,
OBrien Gere Engineers, Inc. vassilbv_at_obg.com
2
WHAT IS A GEOSYSTEM?

• A geosystem encompasses the physical structures,
  characteristics and processes around which
  ecosystems develop and adapt.
• Rocks and Soil
• Geomorphic Features
• Geologic Processes
• Climate - Hydrology - Fire
• A geosystem is dynamic.

3
MARTIN BURN AREA GEOSYSTEM

• ROCKS
• SANTA MARGARITA FORMATION
• Upper Miocene, thick bedded to massive, medium to
  fine grained, white to light yellow-gray, friable
  sandstone
• METASEDIMENTARY ROCKS
• Mesozoic aged schist and quartzite - may be
  deeply weathered, differentially fractured
• SOIL
• DEVELOPS FROM PARENT MATERIAL
• Sandy on Santa Margarita - usually rapid
  drainage
• More clayey on metasedimentary rocks - slower
  drainage

4
GEOLOGIC MAP OF SANTA CRUZ COUNTY, CA SHOWING
MARTIN BURN AREA
Santa Margarita sandstone Metasedimentary schist
and quartzite Granite and adamallite Marble
Tsm
BURN AREA
ga
(Geologic map adapted from E.E. BRABB, 1997 Fire
boundaries from Chris Berry, city water
department)
5
MARTIN BURN AREA GEOSYSTEM

• GEOMORPHIC FEATURES

Deeply incised stream canyons Steep to
moderately sloping and semi-level upland terrain
6
MARTIN BURN AREA GEOSYSTEM
ACTIVE GEOLOGIC PROCESSES Periodic strong
seismic shaking High rates of uplift Landsliding
and erosion - these help offset high uplift rates
7
MARTIN BURN AREA GEOSYSTEM

• CLIMATE
• Mediterranean - dry summers, wet winters
• Average annual ppt, 1981-2001 48.3 inches
• 70-80 can occur in just a few storm events
• Ppt range 27.3 inches (1987) to 93.2 inches
  (1982)
• (Bonny Doon area rainfall
  data from http//www.cantrall.net/Rain/Rain.sht
  ml)
• HYDROLOGY
• Low flow to no flow during summer
• Intense flows, channel scour, bank failure,
  floods can accompany high intensity winter rain
  storms
• Sediment storage during low flows, rapid
  transport (sediment pulses) during high flows

8
MARTIN BURN AREA GEOSYSTEM

• WILDLAND FIRE
• Periodic burns normal in CA
• Hydrophobic soils in areas of high temperature
  fire lead to decreased infiltration, high runoff
  rates
• Loss of vegetation leads to increased runoff -
  less evapotranspiration, less detention by
  leaves, branches etc.
• Fire increases risk of floods, debris flows,
  erosion

9
ECOSYSTEM ADAPTED TO GEOSYSTEM

• Episodic influxes of large amounts of runoff and
  sediment are natural processes throughout CA and
  western North America - and are part of the long
  term dynamic-equilibrium of our channels.
• Rapid delivery of sediment from fires, intense
  storms and earthquakes prevents build up of
  unstable earth materials in the upper watersheds
  - clearing clogged channels and reducing risk
  from mega-debris flows.
• Channels transport sediment to the sea - this
  replenishes beaches and protects shorelines.

10
The ecosystem adapts manzanita and fern shoots
in Martin Burn Area
11
Small deer herd with young - browsing on new
growth 2 months after the Martin Burn
and the beat goes on..
12
NATURAL CAN STILL BE DANGEROUSWHAT TO LOOK FOR
AS WINTER APPROACHES

• when good slopes begin to fail

13
GREATEST DANGER AFTER FIRE

• DEBRIS FLOWS
• Sudden, fast moving streams of water and sediment
  that surge down slopes, usually in narrow
  drainages
• Armored front of rocks, trees and debris
• Deadliest type of landslide in CA - cant out run
  them
• Usually triggered by high intensity or long
  duration rain on saturated ground
• FLOODS
• Can occur downstream from burn areas
• Heavy February storms following July 1985
  Lexington Fire produced significant flooding and
  bank failures as upland runoff reached channels
  nearly instantaneously

14
Ive seen fire, Ive seen rain
(James Taylor)
LOW SEVERITY FIRE GROUNDCOVER REMAINS, OR
MULCH-TREATED MODERATE TO HIGH SEVERITY FIRE
MODERATE FIRE OR UNTREATED SEVERE
FIRE GROUNDCOVER CONSUMED
HIGH INTENSITY RAIN STORM
EXPOSED HYDROPHOBIC SOIL
PROTECTIVE SOIL COVER
MINIMAL OVERLAND FLOW
OVERLAND FLOW
INFILTRATION
SOIL EROSION
LOSS OF TOP SOIL, NUTRIENTS AND SEEDS
SOIL AND MOISTURE RETAINED
RAPID VEGETATION RECOVERY
MINIMAL SOIL LOSS
DEBRIS FLOWS
FLOODING
QUICK FIRE RECOVERY AND RESTORATION OF
PRE-FIRE HYDROLOGIC CYCLE
STREAM SEDIMENTATION, DEGRADED WATER QUALITY AND
HABITAT, RISK TO LIFE, PROPERTY AND
INFRASTRUCTURE
Adapted from Jordan et al., Post-wildfire soil
erosion, flood and landslide risk Wildland Urban
Interface Symposium, 2008, Penticton, B.C.,
Canada - available online at
http//www.for.gov.bc.ca/hfd/library/FIA/2008/FSP_
Y081004b.pdf
15
DANGEROUS CONDITIONS

• WATER SHEETING DOWN SLOPES, OOZING, FLOWING OR
  SPURTING FROM ROOT HOLES OR VOIDS ON SLOPE FACES
  (PIPING)
• MUDDY CAFÉ LATTE WATER FLOWING THROUGH RILLS,
  DRAINAGES OR ALONG ROADS
• DIMINISHED STREAM FLOW DURING STORM - POSSIBLE
  DEBRIS DAM UPSTREAM
• INCREASING FLOW WITH UNUSUALLY LARGE ROCKS MOVING
  THROUGH SMALL CHANNELS
• COLLAPSING STREAM BANKS, TILTING TREES, CRACKING
  OR BUCKLING PAVEMENT

16
POST-FIRE RISK MITIGATION

• MULTI-AGENCY, INTERDISCIPLINARY TEAMS
• ASSESSED VULNERABLE CHANNELS AND SLOPES AFFECTED
  BY RECENT FIRES
• PRESCRIBED TREATMENTS WHERE NEEDED
• NEXT SPEAKER, RICH CASALE OF THE NRCS
• INVOLVED IN POST-FIRE ASSESSMENTS AND TREATMENT
  PRESCRIPTIONS
• RCD OF SANTA CRUZ COUNTY
• A GREAT RESOURCE FOR MORE INFORMATION!