Final Baseline Hydrology Report - October 2012 - Urban Drainage ...

Sanderson Gulch Major Drainageway Plan
Baseline Hydrology Report October 2012
3.2.2 Basin Parameters
All basin parameters were left unchanged from the 1972 MDP with the exception of soil infiltration loss
which was changed to Horton’s Equation because an unknown method was used in the 1972 MDP. The
infiltration coefficients used in the 1972 MDP were 0.75 inch/hour and an extra loss of 5 percent. These
are not applicable to Horton’s Equation and cannot be used in the current model. Soils in the portion of
the watershed within the City of Lakewood consist primarily of Hydrologic Soil Group (HSG) C, as defined
by the NRCS. There is no Hydrologic Soil Group information available for the City and County of Denver.
The City and County of Denver Storm Drainage Master Plan (Matrix, June 2009) assumed Type C soils
and this assumption was adopted for the 2012 Sanderson Gulch MDP. Figure B-1 in Appendix B shows
the distribution of soil types within the watershed. Initial and final infiltration rates and Horton’s decay rate
were determined using Table RO-7 in the UDFCD-USDCM. For HSG C soils, the initial infiltration rate
was 3.0 inches per hour, the final infiltration rate was 0.5 inches per hour. The decay coefficient was
calibrated to 0.00111.
3.2.3 Detention Pond Routing
Stage-storage-discharge curves for detention ponds within the watershed from the 1972 MDP were found
to not match the results reported in the 1972 MDP. For calibration purposes, the curves were adjusted so
that they produced the same pond discharges as the 1972 MDP for given inflows. Calibrated detention
pond inflow and outflow hydrographs are presented in Appendix B, Figure B-2.
3.2.4 Results Before Calibration
Revised subbasin delineations, updated infiltration and detention pond stage-storage curves were
incorporated into the modeling using the most current versions of CUHP and EPA SWMM. Modeling
results before calibration of results are shown in Column B of Table B-1 in Appendix B.
3.2.5 Calibration of Duplicate Hydrologic Model
The final step in creating the Duplicate Hydrologic Model was to calibrate runoff results to match as
closely as possible the 1972 MDP results. Calibration was done by adjusting the basin parameter C p and
the decay coefficient in Horton’s infiltration equation. The calibrated Duplicate Hydrologic Model results
are shown in the Column C of Table B-1 in Appendix B.
3.3 Corrected (Baseline) Hydrologic Model
Starting with the Duplicate Hydrologic Model, the following additional steps were carried out to develop a
final Corrected (Baseline) Hydrologic Model.
• Rainfall depths and distributions were updated to be in accordance with current criteria.
• Basin imperviousness was updated to correspond to current conditions and criteria.
• Stage-storage discharge information for each detention pond in the watershed was updated.
The Sanderson Gulch watershed is fully developed with the exception of the 152-acre Green Gables
Country Club. Green Gables is currently being redeveloped from an open space golf course to a mixed-use
commercial and residential site. Because Green Gables constitutes only a small fraction of the Sanderson
Gulch watershed, developing separate existing and future conditions models as is typical for most UDFCD
watershed master plans was not deemed necessary. Only a future conditions model was developed for this
study which includes the immanent redevelopment of Green Gables.
All analysis for the Corrected (Baseline) Hydrologic Model has been carried out in accordance with the April
2008 revision of the UDFCD-USDCM and is documented in the following sections.
3.3.1 Design Rainfall
Rainfall depth and distribution has been updated to current UDFCD-USDCM criteria for use in the
Corrected (Baseline) Hydrologic Model. Rainfall duration and depth-area reduction factors (DARF’s)
applied to point rainfall depths used within CUHP vary with the size of the watershed being analyzed. The
total contributing drainage area at the outfall of Sanderson Gulch to the South Platte River is 7.6 square
miles. For watersheds less than 10 square miles, the UDFCD-USDCM stipulates that the 1-hour point
precipitation depth be used and that no reduction in that depth for area be applied.
The one-hour point rainfall depths for the 2-, 5-, 10-, 25-, 50- and 100-year events were read from Figures
RA-1 through RA-6 in the UDFCD-USDCM. Figure B-3 in Appendix B shows the watershed boundary
on the isopluvial map for the 100-year, 1-hour event. Selected point rainfall depths for this and the other
design events are given in Table 3-2 below. Point rainfall data from the previous 1972 MDP study is
also shown in Table 3-2 for comparison. Table B-2 in Appendix B presents 1972 MDP point rainfall
values and associated basins.
Table 3-2 One-Hour Point Rainfall Depths (Inches)
2012 Sanderson Gulch MDP Update (Matrix)
2-Year 5-Year 10-Year 25-Year 50-Year 100-Year
0.98 1.4 1.65 2.01 2.3 2.57
1. Point rainfall depths are read from Figure RA-1 through RA-6 of USDCM.
1972 Sanderson Gulch MDP (Frasier & Gingery, Inc.)
Rainfall varies by sub-basin within the watershed
2-Year 5-Year 10-Year 25-Year 50-Year 100-Year
0.67-0.79 - 1.35 - 1.74 - - 2.00 - 2.26
The Sanderson Gulch watershed is 7.6 square miles and the UDFCD-USDCM requires the use of a 2-
hour storm for watersheds less than 10 square miles. Table RA-2 in the UDFCD-USDCM was used to
convert 1-hour point rainfall depths to the required 2-hour storm depth and distribution. Table B-3 in
Appendix B shows a comparison of the 2-hour rainfall distribution from the 1972 MDP and that derived
from the most current UDFCD-USDCM.
The Sanderson Gulch watershed is partially located within the City and County of Denver and partially
within the City of Lakewood and Jefferson County. Rainfall depth criteria differ somewhat in the
respective criteria manuals for Denver, Lakewood and UDFCD. A comparison of 1-hour and 2-hour storm
rainfall between the three different criteria is shown on the Table B-4 in Appendix B. The UDFCD-
USDCM one-hour point rainfall depths are slightly higher than those recommended in The City and
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