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<br />A detennination of whether or not there is enough debris
<br />and sediment available in the channel(s) to produce debris
<br />'.' flows should be made. The potential damage will depend on
<br />(I) the sediment volume stored in a channel, (2) the sediment
<br />volume available for transport on hill slopes, and (3) the
<br />probability of a flood with enough flow to suspend and
<br />transport these materials through a channel (Foresl Service
<br />Handbook, 1978).
<br />
<br />GRAIN-SIZE DISTRIBUTION OF SOILS
<br />
<br />Soil samples were taken at 30 of the 46 PSIAC rating
<br />points to detennine the dominant grain size of the upper 2
<br />feet of material. The samples were tested for grain size,
<br />liquid and plastic limits, and organic matter content at the
<br />Natural Resources Conservation Service Soil Mechanics
<br />Laboratory in Lincoln, Nebraska The grain size distribution
<br />helps detennine the nature of the sediments available for
<br />transport from the watershed slopes to a stream or tributary.
<br />The mean grain-size distribution of the 30 samples collected
<br />was 36 percent fines (0.002 mm to 0,074 mm), 51 percent
<br />sand (0.075 mm to 4.76 mm) and roughly 13 percent gravel
<br />(4.77 mm to 76.2 mm). The gravel fraction, in reality, may
<br />be approaching over 20 percent based on visual observations
<br />throughout the study area.
<br />
<br />POST FIRE MITIGATION I EMERGENCY
<br />TREATMENT ALTERNATIVES
<br />
<br />Emergency mitigation alternatives have to be evaluated
<br />quickly and efficiently for cost effectiveness and the ability
<br />to immediately stabilize soil, control water, sediment and
<br />debris movement. The environmental sensitivity and public
<br />acceptability of the mitigation measures should also be
<br />considered, The GIS software facilitates decision-makers in
<br />selecting and prioritizing emergency mitigation zones and
<br />treatment by providing a ready database of sediment yield,
<br />watershed area, percent slope, vegetation, and soils.
<br />Emergency treatments include: (1) revegetation (2) wire
<br />fence grade stabilization structures to catch sediment and
<br />slow runoff (3) water spreaders to diffuse runoff (4) debris
<br />basins and (5) temporary measures such as straw bales to
<br />diffuse runoff and catch small volumes of sediment on the
<br />hillslope. Straw bales degrade rapidly and will eventually
<br />fail given enough time, so, careful planning and design is
<br />needed to place bales in zones where failure is not a hazard 10
<br />life or property, The Natural Resources Conservation
<br />Service in California restricts the use of straw bales to
<br />drainage basins less than one acre (oral comm. I.Grimm,
<br />NRCS).
<br />An interdisciplinary approach should be used to
<br />consider the variety of treatment alternatives available to
<br />produce a coordinated, technically viable, cost.effective and
<br />environmentally acceptable plan. Any emergency watershed
<br />protection plan along the Wasatch Range area will usually
<br />involve city, county, state and federal entities. With so many
<br />involved in this process, a speedy emergency plan will
<br />depend on good communication and coordination between
<br />the parties involved,
<br />The GIS database has numerous possible uses beyond
<br />those developed for this study. The database can be
<br />manipulated further for more detailed analysis of mitigation
<br />alternatives following a fire. Infonnation such as the
<br />location of stonn drains, parks, dams, debris basins, and
<br />many other cultural features can be added to the database in
<br />the future for a more comprehensive evaluation of any
<br />mitigation alternative proposed.
<br />
<br />SEEDING FOR EROSION CONTROL
<br />
<br />Seeding recommendations in a database of plant species
<br />are used in reseeding following a fire. The seeding
<br />recommendations include the site area or elevalion zone
<br />where a particular species or group of species should be used.
<br />Also included with these recommendations is the rate of
<br />application in pounds per acre if the seed is broadcast.
<br />(Appendix - B).
<br />Studies have shown that a watershed in good condilion
<br />(65% or more vegetation cover) prevents substantial runoff
<br />(Williams, 1991). It is very important to establish vegetation
<br />as sOOn as possible following a fire to help filter sediment
<br />and debris from runoff and control stonn flow runoff.
<br />Reseeding to reduce the immediate erosion hazard
<br />following wildfires in California has been debated. Some
<br />researchers argue that (1) seeding is not effective in the short
<br />tenn because most of the hazard occurs in the weeks and
<br />months following a fire, when seed would not have
<br />genninated; (2) seeding actually increases the erosion hazard
<br />because it attracts burrowing animals, and (3) seeding may
<br />suppress the natural reestablishment of native vegetation,
<br />thereby increasing the erosion hazard in the long tenn. In an
<br />effort to address these arguments, the NRCS is monitoring
<br />the perfonnance of aerial and hydro-seeding on reducing the
<br />sediment hazard created by the southern California wildfires
<br />of 1993.
<br />The NRCS has been involved with several reseeding
<br />efforts following wildfires in Utah, Where there was
<br />abundant existing native vegetation, the reseeded mixture
<br />yielded to the native vegetation, whereas open, bare areas
<br />usually produced a good stand of the reseeded mixture.
<br />Burrowing animals may be attracted to the freshly scattered
<br />seed mixture, however, this can be beneficial in that the
<br />borrowing can increase the infiltration of runoff during a
<br />stonn event.
<br />
<br />SUMMARY
<br />
<br />The watersheds with the highest sediment yield potential
<br />after a fire along the Wasatch Range in Davis and Weber
<br />Counties are: #32, #48, #59, #69 (Rudd Creek), #80, #82 and
<br />#24. These watershed boundaries can be located on the
<br />attached maps.
<br />The modeled Low-intensity and High-intensity bum
<br />sediment yield rates presented in this study can provide
<br />ready answers about the potential sediment load
<br />contributions from range slopes within a bum area The
<br />modeled sediment yield data can be accessed through the use
<br />of GIS software. The GIS software can calculate the total
<br />upland sediment yield potential from the burn area and
<br />provide decision-makers with infonnation needed to
<br />fonnulate emergency mitigation alternatives,
<br />Emergency mitigation involves identifying the primary
<br />goal whether it be protecting homes and other property from
<br />the impacts of accelerated erosion and sedimentation,
<br />protection of the existing stonn drain network or controlling
<br />sediment transport on upland slopes. Past emergency
<br />seeding of burned slopes in Utah has been considered
<br />successful even though there is no quantitative data at this
<br />time to document this. The University of Utah is presently
<br />conducting a study of the effects of seeding the burned
<br />slopes after the 1992 Midway Fire near Heber City, Utah.
<br />Recent trends show that urban development in steep,
<br />rugged, and fire-prone terrain is occuring not only along the
<br />Wasatch Range but also throughout Utab and the rest of the
<br />West. Development will likely continue and even accelerate
<br />
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