A CHARACTERIZATION METHODOLOGY FOR POST-WILDFIRE FLOOD HAZARD ASSESSMENTS Page: 4 of 7
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HEC-HMS can utilize five different unit
hydrographs (UH) to simulate runoff. The SCS UH
and SCS rainfall abstraction loss rate [SCS, 1993]
were selected in this study to characterize the
relationship between rainfall-runoff and peak
discharge. The Muskingum method was selected to
route computed flood flows through downstream
subbasins because channel losses and flood-wave
attenuation in individual watersheds have not been
fully characterized. Hence these losses were
assumed to be zero even though they are known to
be relatively high in certain pre-fire stream channel
reaches (e.g., those channel reaches with relatively
thick alluvial deposits). Muskingum routing
parameters were computed from average channel
flow velocities using Manning's equation. In
addition, level-pool reservoir routing was selected
to move water through road culverts with high
embankments and for flood detention structures.
Pre-fire SCS curve numbers (CN) were determined
for all watersheds [McLin, 1992] and formed a
starting point for post-fire simulations. These pre-
fire CN values typically ranged from the mid-50s
and 60s for wooded alpine forests, to 70s and 80s
for mountain brush and pinon-juniper woodlands.
These values were originally obtained using a
quasi-model calibration procedure for ungaged
watersheds [McLin et al., 2001].
The post-fire CN values were initially modified
from original values using weighting factors based
on the percent of subbasin areas that were burned.
These burned areas were subdivided into low (57%
of total burn area), medium (8% of total), and high
(34% of total) severity burned areas as defined by
the US Forest Service's Burned Area Emergency
Rehabilitation team [BAER, 2000]. This
classification is qualitatively linked to changes in
soil texture and infiltration capacity. High burn
severity areas are located in those areas where the
surficial soil structure has been altered. These soils
typically have a hydrophobic layer that was formed
during the fire. This layer is located approximately
6.4 mm (0.25 in) below the surface and is between
6.4 to 76 mm (0.25 to 3.0 in) thick. These
hydrophobic soils develop when high temperature
fires produce heavy volatile organics that migrate
into soils and condense [Imeson et al., 1992;
Dekker and Ritsema, 1994]. For the Cerro Grande
wildfire, these hydrophobic soils are preferentially
located on north-facing canyon slopes with heavy
ponderosa pine forests. They occur on
approximately 22% of the total burn area. Medium
severity burn areas show little or no
hydrophobicity and are concentrated on south-
facing canyon slopes with sparser vegetation, on
mesa tops, and in canyon bottoms. Low severityburn areas are generally located along the
perimeter of more severely burned areas. This
hydrophobic soil distribution is related to the
distribution of fuels, temperature, and heavy winds
during the fire.
Post-fire CN values of 65, 85, and 90 were
assigned to the low, medium, and high severity
burn areas, respectively. Unburned areas retained
their original pre-fire CN values. A composite CN
value was then computed for each subbasin using
four burn severity weight factors and four
estimated CN values. These weight factors were
computed according to the fraction of burned area
within each subbasin area (i.e., unburned, low,
medium, or high severity). Each respective weight
factor was multiplied by each respective CN value
and the results were summed to obtain the
composite CN value. Details of the HEC-HMS
simulations are described in McLin et al. [2001].
3. HEC-RAS FLOODPLAIN MAPPING
For the modeling efforts described here, stream
channel cross-sections at varying locations were
obtained from the Laboratory's computer-based
graphical information system (ArcView GIS). For
this study, cross-sections are located approximately
every 61 m (200 ft) along each reach. Topographic
data are automatically extracted from a
triangulated irregular network (TIN) that is created
from the DEM database. This procedure minimizes
channel-surveying tasks. The data extraction
process is performed for each cross-section
following the pre-selected channel reach pathway.
Each point along the cross-section forms an (x, y,
z) topographic point that is geo-referenced to the
New Mexico State Plane coordinate system. A
typical 30 m (100 ft) long cross section contains
between 15 and 50 data points. These cross-
sectional features are exported to the HEC-RAS
model using HEC-geoRAS, an ArcView extension
capability developed by the USACE-HEC.
In order to verify this data extraction process,
approximately 1 % of all channel sections were
surveyed using a network of precision benchmarks.
Differences between 51 surveyed and DEM low-
point elevations from channel sections are shown
in Figure 2. These elevation differences (i.e.,
surveyed minus DEM elevations for identical
points) are normally distributed and appear
random. They have a mean difference of 0.34 m
(1.11 ft) and a standard deviation of 0.64 m
(2.11 ft). These differences range from +1.81 m
(5.92 ft) to -1.19 m (-3.89 ft). The affect of these
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LIN, S. G. MC; EECKHOUT, M. E. VAN & AL, ET. A CHARACTERIZATION METHODOLOGY FOR POST-WILDFIRE FLOOD HAZARD ASSESSMENTS, article, July 1, 2001; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc723861/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.