2005-03-23_REVISION - M1977424

• Permit No: M1977424
• IBM Index Class Name: Revision
• Doc Date: 3/23/2005
• Doc Name: Submittal
• From: Western Water & Land Inc
• To: DMG
• Type & Sequence: TR5
• Media Type: D
• Archive: No

urban stormwater guidance manuals are concerned with flooding of residential structures
and therefore the conservative approach ofselecting aloes-frequency storm event is
justified. Garfield County also typically requires the use of 100-year storm frequency for
stormwater calculations associated with urban development. The SWMM requires that
IDF data be used to assess storm duration when using the rational method, and the IDF
data are presented in table format. The SWMM shows that for 100-year storm events
outside of the Grand Valley (Table A-1 b in the SWMM), of durations of 30 minutes, 45
minutes, 1 hour and 24 hours have corresponding intensities of 2.3, 1.76, 1.42
inches/hour, respectively.
2.3.2 Preparation of IDF Data
To better assess rainfall intensity at the Logan Wash Mine, data presented in NOAA
' Atlas 2 was used to prepare site specific IDF curves for return (storm frequency) periods
of 2, 5, 10, 25, 50, and 100 years. NOAA uses a statistical or probable rainfall intensity
for each frequency or magnitude of storm which is derived on the basis of rainfall data
' collected over the period of record. The data may be site specific or interpolated from
NOAA data bases, in this case, NOAA Atlas 2.
As outlined in Atlas 2, precipitation depths from isopluvial maps of Colorado were used
to interpolate values for the Logan Wash Mine site for the 6 and 24-hour durations for
each return period. Precipitation depths for other storm durations were calculated using
equations presented in Atlas 2. The precipitation depths were converted to average
intensity values and plotted on log-log scale using the electronic software Excel®. The
resulting IDF curve plot for the Logan Wash Mine site is shown in Figure 4.0. Tables
3.0 and 3.1 show the IDF calculations and the final tabulated data.
Figure 4.0 shows that the precipitation intensities associated with any given storm
duration increase with increasing return period. A 20-minute storm duration for return
periods of 2, 5, 10, 25, 50, and 100-years have storm intensities of approximately 1.3, 1.8,
2.25, 2.7, 2.95, and 3.3 inches/hour, respectively. For comparison purposes, the 24-hour
duration storms for the Logan Wash Mine area correspond to rainfall intensities of 0.05,
0.06, 0.08, 0.09, 0.10, and 0.11 inches/hour for the same return periods.
2.3.3 Selection of Design Storm
The basis For selecting a reasonable rainfall intensity for stonnwater design purposes is
partially bound by the method of solution and also by need to reduce construction costs in
stormwater control measures. The rational method is applied to storm durations at least
equal to the time of concentration. Storm durations significantly exceeding the time of
concentration may not realistically apply to the rational method as rainfall intensity tends
to vary within the catchment basin as storm durations increase. The use of a 24-hour
duration may be convenient and especially applicable to some urban runoff calculations,
Western Water & Land Inc. 9