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

Sanderson Gulch Major Drainageway Plan
Baseline Hydrology Report October 2012
3.0 HYDROLOGIC ANALYSES
3.1 Overview and Methodology
This study updates the major drainageway design flood flows based upon current criteria for the entirety of
the Sanderson Gulch watershed including the tributary North Sanderson Gulch.
The approach to development of the updated hydrologic modeling for the watershed was established in the
kickoff meeting. The hydrology should not be changed simply due to the use of different hydrologic models.
Therefore, it was important to re-create the 1972 model for Sanderson Gulch and Weir Gulch. The
Sanderson Gulch portion of the 1972 MDP was extracted as it is hydrologically not connected to Weir Gulch
and for the initial calibration exercise excluded the Mississippi Gulch watershed since no flows came to
Sanderson Gulch from that watershed until the completion of a minor storm transbasin outfall in 1981. This
re-created model, termed the “Duplicate Hydrologic Model,” was then edited using corrected rainfall and
impervious values, and was termed the “Corrected Hydrologic Model.” The following two-step process was
followed:
1) Duplicate Hydrologic Model – Since an electronic file was not available for the 1972 model, the previous
model was re-created. The watershed and sub-basins were delineated similar to the 1972 study;
however the sub-basins were further subdivided to be less than 130 acres to meet current UDFCD
criteria. Basin hydrologic parameters and rainfall from the 1972 MDP were adopted. This information
was entered into the most current versions of UDFCD’s hydrologic models, and the results calibrated to
match, as closely as possible, the 1972 MDP discharges at selected design points.
2) Corrected (Baseline) Hydrologic Model – Starting with the Duplicate Hydrologic Model, stage-storagedischarge
curves for each detention pond in the 1972 model for Sanderson Gulch were revised and
updated as necessary to match current conditions. Current basin imperviousness was applied to each
subbasin. The design rainfall values were updated to match current values according to the UDFCD
criteria manual.
For both the Duplicate and Corrected (Baseline) Hydrologic Models, the Colorado Urban Hydrograph
Procedure (CUHP 2005), version 1.3.3 was used to generate hydrographs for each subwatershed for the 2-,
5-, 10-, 25-, 50-, and 100-year storm return periods. Hydrographs for the subbasins were routed using the
Environmental Protection Agency Stormwater Management Model (EPA SWMM), version 5.0.022, to
determine peak discharge rates at design points.
Model development and results for each of the two major modeling steps is described in the following
sections.
3.2 Duplicate Hydrologic Model
In order to create a Duplicate Hydrologic Model, the 1972 MDP basin delineation was revised to meet
current UDFCD criteria while attempting to leave basin parameters unchanged. Subbasin delineation,
changes to infiltration and pond routing and subsequent calibration of the model results to arrive at the
Duplicate Hydrologic Model are described in the following sections.
3.2.1 Subwatershed Delineation
The 1972 MDP divided the Sanderson Gulch watershed into 20 subbasins. These original 20 subbasins
were subdivided into a total of 89 new subbasins to comply with current UDFCD modeling criteria.
Subdividing of the original basins was based on topographic mapping, road profiles, curb and gutter,
cross pans, previous drainage studies and site observations. Mapping used for the delineation consisted
of 2-foot contour interval topography based on a 2008 LIDAR survey provided by Denver, one-foot
contour interval mapping flown by Accurate EngiSurv in June, 2012, and 10-foot contour interval USGS
mapping.
In accordance with UDFCD guidelines, subbasins within the study area were delineated so that no single
subbasin exceeded 130-acres in area, and individual subbasins averaged no more than 100 acres.
Subbasin delineation parameters are summarized in the Table 3-1. The maximum subbasin area is 150
acres because Ward Reservoir Number 5 could not be further sub-divided. The second largest subbasin
area is 122 acres.
In addition to the Sanderson Gulch watershed, two-off site basins contribute runoff through storm
drainage systems. The off-site basins are the Mississippi Gulch basin and Federal Boulevard / Lowell
Avenue basin. These two off-site basins are not included in the Duplicate Hydrologic Model because they
were not part of the 1972 MDP. They are included in Table 3-1 because they are used later in the
Corrected Hydrologic Model.
Table 3-1 Subbasin Delineation Summary
Total Area
Average
Sub-basin
Area
Minimum
Sub-basin
Area
Maximum
Sub-basin
Area
(sq. mi.) (acres) (acres) (acres)
Total No.
of Sub-
Basins
Study Area
(Sanderson Gulch Watershed)*
Offsite Area
(Federal Blvd and Lowell Ave)
Offsite Area
(Mississippi Gulch)
7.6
0.48
0.92
54.6
61.4
84
1.16
38.1
37.4
150
93.1
122.7
89
5
7
* Duplicate Hydrologic Model does not include the off-site Mississippi Gulch and Federal Boulevard / Lowell
Avenue basins.
Subbasin flow paths and slopes were determined using the project topography previously described. For
the subbasins with drainageway slopes greater than 0.04 ft/ft, Figure RO-10, Slope Correction for Natural
and Grass-Lined Channels in the UDFCD Urban Storm Drainage Criteria Manual (USDCM) was used for
the adjustment. Figure B-1 in Appendix B shows the delineation of subbasins. The recommended
watershed shape factor, which is the drainage basin length-to-width ratio, should not exceed 4 to 1 per
UDFCD guidance. All subbasins were delineated for compliance with the shape factor criteria which
unavoidably produced some small subbasins.
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