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<br />The triangular-shaped areas (Map-I) betwl~en the 93
<br />main watershed outlets were not separated fcr tabulation and
<br />illustration of the sediment yield. Howev'~r, the database
<br />contains the sediment yield rates based 011 the landtype-soil
<br />units in those zones. These interfluve zom:s typically contain
<br />a large amount of trails and dirt acce,ss roads and are
<br />frequented by off-road vehicles. The Davis County Planning
<br />Commission report (1980) suggested that these trails and
<br />access roads increase the overland flow and concentrate these
<br />flows enough to cause erosion and depo:;it s"diment in
<br />ephemeral channels, Most of this sediment will not be
<br />transported to main.stem drainages but can impact properties
<br />directly downslope of these interfluve areas,
<br />
<br />POST-FIRE SEDIMENT YIELD MODELS
<br />
<br />A post-fIre sediment yield rate for :l burned area can
<br />help planners prioritize emergency mitigation by targeting
<br />those areas with the highest hazard to life or property, Post.
<br />fIre sediment yields were modeled assuming a Low-Intensity
<br />and High.lntensity burn of the entire watelsh,~d. A bum over
<br />the entire watershed is considered a won;t.case scenario
<br />appropriate given the proximity of wate~hed outlets and
<br />debris flow history.
<br />The post-fIre sediment yield hazard is l:ompounded by
<br />the additional hazard of debris (rock, sill:, sand, gravel,
<br />organic material) in channels along the Wasatch Range,
<br />Most of the debris flows observed in 1983.84 picked up a
<br />high percentage of their volume from the:hmmel banks and
<br />bottom (Wieczorek & others, I 989). Wieczorek and others
<br />contend that for the short-term, partly deta(:hed slopes and
<br />high ground-water levels, appear to increa;;e ':he potential for
<br />
<br />Table 2, Average PSIAC factor ratings fo)' soils in the study area,
<br />
<br />'''..
<br />
<br />further movement leading to debris flows ant
<br />hyperconcentrated floods. Hyperconcentrated lloods, 0:
<br />debris floods are less stratifIed, and have lower clay conten ."
<br />than debris flows. They also usually do not form levees ant
<br />the woody debris does not have a preferred orientation (Lips
<br />1983).
<br />Watersheds in a burned area typically develop rills anI
<br />gullies which increase the delivery ratio and volume 0'
<br />sediment yield off the watershed slopes to the tributaries an' ,
<br />main drainages. Studies of mountain streams after watersh.. '
<br />disturbance suggest that the sediment transport rate is II
<br />function of the supply of soil or sediment in the stream rathe'
<br />than increased runoff (Booker & others, 1993).
<br />
<br />BURN INTENSITY
<br />
<br />Various defmitions have been used to defIne bun
<br />intensity (fIre intensity). Vierick and Schandelmeir (1980
<br />defined it as the effect of the fIre on the ecosystem, whethe:
<br />it effects the forest floor, tree canopy, or some other part 0"
<br />the ecosyslem. Sediment yield data was developed for Lo"
<br />and High-Intensity bums, The PSIAC rating factors wer.
<br />adjusted accordingly to model the effects on sediment yield.
<br />The PSIAC rating factors that are affected in the even
<br />of a fIre are: Runoff, Ground Cover, Land Type &
<br />Management, Upland Erosion, and Channel Erosion an,
<br />Sediment Transport. The other factors, Geology, Soils
<br />Climate and Topography will not change with alter..
<br />watershed or management conditions, The average PSIAC
<br />factor ratings for the various terrain in the study area lIT1
<br />listed in Table 2.
<br />
<br />PSIAC ]~rese'nt Low Intensity Higb Intensity
<br />Factor Condition Burn Burn
<br />Geology 0.8 0.8 0.8
<br />Soils 3.0 3.0 3,0
<br />Climate 6.0 6,0 6.0
<br />Runoff 3.9 5,0 7.0
<br />Topography 19.0 19.0 19.0
<br />Ground Cover .4,0 -0.8 5,7
<br />Land Type & .6,0 1.6 8,0
<br />Management Quality
<br />Upland Erosion 3.5 8.5 17.0
<br />Channel Erosion & 4.5 8,0 14.0
<br />Sediment Transport
<br />Totals 30.7 5LI 80,6
<br />
<br />The Present Condition, Low. Intensity bum and High-
<br />Intensity burn factor ratings equate to ,m average annual
<br />sediment yield from sheet and rill erosion cf 0.75 tons per
<br />acre, 1.6 tons per acre and 4,6 tons per acm mspectively, An
<br />average sediment density of 90 pounds per cubic foot was
<br />used to calculate tons per acre,
<br />
<br />LOW-INTENSITY BlJRN
<br />SEDIMENT YIELD MODEL
<br />
<br />Typical low-intensity burn characteristics Were
<br />consistently used for each of the PSlAC ratings. Table 3 lists
<br />the criteria for a Low-Intensity burn. This ,:riteria includes
<br />some Forest Service characteristics for a Medium-intensity
<br />
<br />172
<br />
<br />bum, This was done to limit the number of PSIAC rating;
<br />needed to model the potential upland sediment yiel, I
<br />following a typical Low/Medium- Intensity burn.
<br />
<br />Table 3. Low-Intensity Burn Criteria used in PSIAC model
<br />
<br />a.
<br />b.
<br />c.
<br />d.
<br />e.
<br />f.
<br />g.
<br />
<br />Perennial roots and vegetation intact
<br />Crowns of trees burned
<br />Scorched trees
<br />Low plants and grasses still somewhat moist
<br />Leafy litter consumed
<br />1/2 of the decomposed organic layer consumed
<br />Loose grass consumed, sticks and stumps remain
<br />
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