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<br />which coats soil panicles and reduces water
<br />penetration. This condition reduces water infil-
<br />tration rates and moisture storage apaciry rc:p
<br />suIring in increased runoff and erosion rates.
<br />. After fire. soils are no longer protected by
<br />vegetative cover from turbulem air. Wind is an
<br />erosive force in these conditions, blowing
<br />slopes dean of loose soil particles. The wind-
<br />blown soils arc usually deposited down slope
<br />and in sUearn channels for latcr movement
<br />during storms.
<br />The development of rill networks and gully
<br />erosion increases pen-fire loss during the rainy
<br />season when soils are wet or saturated. Infiltra-
<br />tion rates arc decreased on bare slopes. and
<br />therefore, runoff, or overland flow increases and
<br />the: sediment carrying capacity increases. The
<br />result of this type of erosion is the movement of
<br />sediment and debris into stream channels, caus-
<br />ing clogged drainage ways, mud flows and de-
<br />bris flows. Since the race of runoff is higher and
<br />che sediment and debris load is higher, the po_
<br />tential for flooding is also increased.
<br />Soil slippage can occur during heavy rains
<br />when the amount of water emering the soil
<br />layer exceeds the capacity of the parent rock to
<br />transpon water. This leads to supersaturated
<br />soils, and soon the stress on the soil exceeds its
<br />strength, resulting in sloughs and slumps. After
<br />fires, even moderately heavy rainfall can super-
<br />saturate soils denuded of vegeration.
<br />There is generally a higher flooding risk as a
<br />~ult of a fire. This increased risk may arise
<br />~om increased watershed runoff due to changes
<br />in the surficial soil and vegetation characteris-
<br />tics as previously described; diversion and/or
<br />overflow of conveyance facilities due to in-
<br />creased sediment loads from the barren water-
<br />sheds; and [he possibili')' of additional flooding
<br />from ineffective sediment basins (Figure 4).
<br />Post~fire conditions can also result in re-
<br />duced-stability landslides and other geologic
<br />hazards. such as
<br />erosion of supporting rocks or at the tOe of a
<br />pre-existing slump or landslide;
<br />damage to a landslide stabilization measure
<br />(such as a drainage or dewacering system);
<br />and
<br />. damage to e<il:rth retaining Structures or other
<br />slope stabilization measures.
<br />
<br />Issues and concerns
<br />
<br />Erosion, flooding. mud and debris flows fol-
<br />lowing fires ace considered by some geologists
<br />and geomorphologists as naturally occurring
<br />phenomena that do not require human inter-
<br />vention. Government officials and the people
<br />in their jurisdictions who are directly impacted
<br />from post-fire hazards tend to think of them as
<br />~ything but natural and demand effcctive and
<br />.mediate mitigation measures. Three of the
<br />questions that arise after a fire are, "Should we
<br />do anyrhing at all?" "What should we do?"
<br />
<br />and. "How much is enough?" Alrhough rhe
<br />technical answers associated with appropri3[e
<br />response following fires may be years away from
<br />resolution, i.e. whether to mitigate, revegetate
<br />or evacuate, the realities of the hazards and im~
<br />pacts on human populations require some type
<br />of action.
<br />It might be instructional to consider the agri-
<br />cultural concept of" T," or tolerable soil loss.
<br />Tolerable soil loss is considered to be that
<br />amount of soil which can be lost on an annual
<br />basis without affecting a she's productivity. or
<br />irs ability to support a multitude of uses. A cer~
<br />rain amount of erosion might be permissible in
<br />an agricultural setting. with losses offset by ad-
<br />justing management inpucs; erosion of outlying
<br />wild land areas is offset by nacural soil forma-
<br />rion over time. But, when the effects of acceler~
<br />ated erosion from fires affect people's lives,
<br />property and community infrastructure, there is
<br />no tolerable soil loss: if 99 percent reduction in
<br />erosion still results in 1 percent of the sediment
<br />filling up someone's living room to the ceiling,
<br />then there is no soil loss which is colerable in
<br />the urban environment.
<br />In the case of Southern California, logic dic~
<br />tated that remediation could not be pursued on
<br />all of [he areas affected by [he fires. Not only
<br />were the costs prohibitive, but natural regenera~
<br />tion in the extensive area of affected wild lands
<br />occurs at a rate much more rapid than human's
<br />effort to augment it. limhed economic and
<br />human resources were then directed. as they
<br />should have been, towards the affected commu-
<br />nities of Laguna Beach. Malibu, Alradena,
<br />Thousand Oaks. and pares of Orange Coun')'.
<br />
<br />Hazard assessment
<br />
<br />Figure 3. Re-establlshment
<br />of vegetation following
<br />fire; Laguna Beach, March
<br />1994
<br />
<br />The planning and implcmemarion of POSt-
<br />fire hazard mitigation measures require a docu-
<br />
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