Prefeasibility Access Road Design-Revised
23
VI AutoTURN ANALYSIS
AutoTURN 8.0 was used to simulate the travel path for the design semi tractor-
trailer vehicle. This software makes use of the design vehicle dimensions and axle
spacing to simulate the travel path at various design speeds for the proposed
horizontal alignment of a roadway. Designers may use the results of the travel path
analysis to determine if horizontal curves are adequate for the design vehicle and
speed. Care has been taken to evaluate all travel paths, regardless of AutoTURN 8.0
results, for vertical alignment, cross-slope, and pavement type.
Several applications for AutoTURN 8.0 results are available, but this project focused
on analyzing the travel path of the design vehicle on the proposed horizontal curves.
The results of this AutoTURN 8.0 analysis provided potential locations for reduced
speed curves and locations that were reassessed for satisfying horizontal alignment
design standards based on speed, vertical alignment, and other criteria.
VII HYDROLOGY
The design storm used was the 25 year, 24 hour event. From NOAA Atlas 2, the
precipitation depth for the area is 3.4 inches.
For the hydrological analysis, HEC-HMS 3.3 was used. The SCS Curve Number was
used for the loss method and the SCS Unit Hydrograph was used for the transform
method. The lag time (the time that accounts for the delay between the start of
rainfall and the start of runoff) was calculated using equation 15-4a from NRCS’s
National Engineering Handbook
, Part 630, Chapter 15.
For the loss method, the curve number was chosen over the rational method
because of the large size of the basins. The curve number method is an empirical
equation that uses three variables to estimate runoff: a rainfall amount, a curve
number (which accounts for soil type and land use/cover), and an initial abstraction
(which accounts for the soil’s infiltration and storage capacity). The curve numbers
were taken from figure 2.5 of
Soil Conservation Service Curve Number Methodology
by S.K. Mishra and V.P. Singh 2003. The land types used were a combination of
“juniper-grass” and “sage-grass”. These curve numbers (using hydrologic soil group
C) are, 66 and 54, respectively. The final curve number is calculated by assuming the
curve number of the forested part of the watershed is 66 and the curve number of
the rest of the watershed area is 54. Zero was used for the initial abstraction.
For the meteorologic model, the SCS hypothetical storm (a synthetic distribution of
rainfall) was used for precipitation. The storm type used was Type II and the storm
depth, from NOAA Atlas 2, was 3.4 inches. A timestep of two minutes was used.
