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

Sanderson Gulch Major Drainageway Plan
Baseline Hydrology Report October 2012
The following reservoirs within the Mississippi Gulch hydrologic model were not included in the baseline
hydrology model because there is no formal storage volume or agreement for detention:
1. Garfield Lake
2. Huston Lake
Stage-storage-discharge curves were established using topographic mapping, design information and
previous studies. No design information for Smith Reservoir could be found and stage-storage-discharge
data was developed using 2012 aerial mapping and discharges from 1972 MDP. Kendrick Reservoir’s
stage-storage-discharge information from the 1972 MDP was found to be consistent with existing
conditions and was adopted into the Corrected (Baseline) Hydrology Model without change.
Reservoir routing information for Jewell Park Reservoir, Green Gables Entrance Road and Bit-O-Sea
Reservoir from the 1972 MDP were determined to be obsolete. The stage-storage-discharge curves were
developed using the structure survey data provided by UDFCD, topographic mapping and as-built
drawings of the outlet structures. Clairefield Reservoir information was derived from LOMR Case Number
03-08-0090P and modified to extend the stage-storage-discharge curve above the top of the control weir.
The Green Gables Park Pond is an irrigation pond that has no outlet. This pond can contain 100-year
runoff from the tributary basin because of the small tributary area. The reservoir routing information for this
pond was derived from topographic mapping and field measurement.
UDFCD's Detention Volume Estimating Workbook Version 2.03a was used to develop stage-storagedischarge
information for input into the SWMM model. Overflow elements were added within SWMM
where they were needed to convey the full 100-year storm event and ensure no inadvertent detention was
being modeled at any of the detention ponds. A summary of the detention pond stage-storage-discharge
information can be found in Table B-6 in Appendix B.
3.3.6 Hydrograph Routing
Parameters for the SWMM model conveyance elements were determined using the mapping described in
Section 1.4. Manning’s roughness values were determined based on the 2009 aerial image supplemented
by field observation. Table 3-4 provides the Manning’s n values selected for use in the Corrected
(Baseline) Hydrology Model and also provides the 1972 MDP study values for comparison. The
Manning’s n values were increased by 25 percent in accordance with the UDFCD-USDCM.
The underground storm sewer system was not modeled, except at locations where trans-basin inflows
occur. This is the result of storm sewers contravening topography where flows in the pipes are routed
down streets and do not follow surface flows based upon topography. There are two trans-basin inflows in
the updated hydrologic model and occur from Mississippi Gulch and Federal Boulevard/Jewell Avenue.
These storm pipes have less than 100-year capacity. Therefore, in the 100-year model, only the capacity
of the pipe is used to determine the trans-basin inflow.
Additionally, split flow design points were setup to analyze trans-basin outflows as shown in Appendix B,
Figure B-7. These occur where streets or pipe are conveying stormwater runoff into or away from the
Sanderson Gulch watershed. Rather than following surface topography, stormwater runoff can be
contained in the curb, gutter and cross pans of the roadway in a major event which divert water from the
natural watershed.
Table 3-4 Manning's Roughness Coefficients & Comparison
2012 Sanderson Gulch MDP and FHAD
Routing Element Description Original 'n' Increase 'n' by 25%*
Reinforced Concrete Pipe (old) 0.015 0.019
Asphalt Pavement 0.016 0.020
Engineered Channel, Ditch 0.035 0.044
Natural Drainageway with No Brush 0.035 0.044
Natural Drainageway with Brush 0.044 - 0.052 0.055 - 0.065
Natural Drainageway with Trees 0.060 0.075
1972 MDP
Initial Run with 0.035, final Run with "n" values determined from formula in SWMM manual
using invert slope to approximate friction slope.
*UDFCD-USDCM Chapter 4 EPA SWMM and Hydrograph Routing Section 4.2.2.
The SWMM input parameters and 100-year future conditions output are included in Appendix B as Table
B-7. SWMM model elements, including subbasins, design points and conveyance elements are illustrated
within the interactive Hydrology Map of Figure B-1 in Appendix B. Figure B-5 of the Appendix B provides
a SWMM routing schematic.
3.4 Mississippi Gulch Major Storm Hydrology
The Mississippi Gulch drainage basin is adjacent to the Sanderson Gulch basin, but does not have a good
outfall connection to the South Platte River. The watershed includes the existing Garfield and Huston Lakes
owned by Denver Parks and Recreation. Although these lakes provide attenuation and inadvertent
detention, there is no formal agreement to maintain any stormwater controls within these water bodies.
Therefore, their impact to stormwater has not been included in the baseline hydrologic model.
In the minor storm event on Mississippi Gulch, a 72-inch pipe transitioning to a 7-foot high by 4-foot wide
box culvert constructed in 1981 will convey stormwater from Mississippi Gulch into Sanderson Gulch. In a
major storm event, some water will flow easterly in Mississippi Avenue, some will flow down Mississippi
Gulch and some will be imported into Sanderson Gulch. UDFCD’s UD-Inlet Workbook for Streets was used
to determine the capacity of Mississippi Avenue during a major storm event where the flow in the gutter is
12-inches deep and the street slope is 2%. The amount of surface flow leaving the Mississippi Gulch
watershed and flowing to the Sanderson Gulch watershed currently is limited by the capacity of the storm
drain. Figure B-7 depicts major event storm flows from Mississippi where an estimated 90 cfs will be in
Mississippi Avenue, 987 cfs in Mississippi Gulch and 307 cfs will be imported by pipe into Sanderson Gulch.
The “duplicate effective” hydrologic model used to calibrate flows to the 1972 study did not include this
transbasin inflow since the pipe did not exist at the time. However, the “corrected effective” model used in
this study includes the 307 cfs imported Mississippi Gulch storm inflows.
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