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

Sanderson Gulch Major Drainageway Plan
Baseline Hydrology Report October 2012
County of Denver Storm Drainage Design & Technical Criteria (January 2006). The City of Lakewood
Storm Drainage Criteria provides only 2-hour depths and distributions for the 2-, 5-, 10-, 50- and 100-year
events and these are slightly higher than both Denver and UDFCD criteria as shown in the appendix table.
After discussions with the project sponsors, it was decided to use the UDFCD-USDCM rainfall values,
given that the study area includes areas in both Denver and Lakewood, the Denver values yield lower
flows, and the UDFCD-USDCM values are believed to be more precise for the project area. UDFCD time
distributions of rainfall were also selected for use.
3.3.2 Watershed Imperviousness
A GIS-based approach was used to calculate the percent impervious area within the watershed. Zoning
data provided by the City and County of Denver and the City of Lakewood was used in conjunction with
Table RO-3 and Figures RO-3 through RO-5, from the UDFCD-USDCM, to generate percent impervious
values for each zoning type. Table 3-3 lists each zoning type, the corresponding UDFCD land use
description, and the associated percent impervious values used in the hydrologic model. GIS data
representing the zoning-based imperviousness was intersected with the subbasin polygons to calculate
area-weighted, or composite, percent impervious values for each subbasin. As previously described, the
Sanderson Gulch watershed is essentially fully developed and the project sponsors elected to create only
a future conditions model. The only significant future development anticipated is the redevelopment of the
152-acre Green Gables Country Club. No existing conditions modeling was completed because of the
insignificant change to the drainage basin.
Table 3-3 Zoning-Based Impervious Summary
Land Use Description
% Imp.
% of Watershed
Area
Business - Commercial Area 95% 2.9%
Business - Neighborhood Area 85% 2.7%
Industrial - Light Areas 80% 7.9%
Open Space - Conservation 2% 0.0%
Open Space - Park 5% 8.4%
Residential - Apartments 80% 1.0%
Residential - Multi-Unit (Attached) 75% 1.4%
Residential - Multi-Unit (High Density) 66% 0.1%
Residential - Multi-Unit (Low Density) 48% 0.5%
Residential - Multi-Unit (Medium Density) 61% 0.2%
Residential - Single-Family (Low Density) 28% 0.0%
Residential - Single-Family ~3.6 DU/AC 42% 3.9%
Residential - Single-Family ~4.2 DU/AC 43% 28.8%
Residential - Single-Family ~4.8 DU/AC 45% 0.2%
Residential - Single-Family ~5.1 DU/AC 45% 0.3%
Residential - Single-Family ~7.3 DU/AC 48% 37.7%
Residential - Single-Family ~9.7 DU/AC 55% 0.2%
Water 100% 3.7%
Basin input parameters including percent impervious for the Corrected (Baseline) Hydrology Model are
provided in Appendix B Table B-5. Future conditions percent impervious values are illustrated for the
watershed within the interactive Hydrology Map of Figure B-1 in Appendix B.
3.3.3 Depression Losses
Depression losses used are the same as the 1972 MDP. A pervious area value of 0.5 inches and an
impervious depression loss of 0.1 inches were used for the entire watershed.
3.3.4 Infiltration
Modeling of infiltration utilized Horton’s Infiltration Equation. 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.
3.3.5 Detention Ponds
Typically UDFCD’s policy is to model only regional, publicly-owned facilities. However, the decision was
made to include privately owned and maintained ponds since they were included in the previous study.
Inadvertent detention areas were not modeled since their use and function cannot be predicted in the
future.
The following seven detention ponds were included in the 1972 and current updated baseline hydrology:
1. Smith Reservoir, (private)
2. Kendrick Reservoir,
3. Jewell Park Reservoir,
4. Green Gables Entrance Road,(private and has been enlarged to detain the excess runoff from
the Green Gables Redevelopment)
5. Clairefield Reservoir,
6. Bit-O-Sea Reservoir, (private)
7. Green Gable Park Pond
In addition to these, four additional reservoirs have been included in the updated modeling:
1. Kendrick Reservoir Number 1,
2. Ward Reservoir Number 1,
3. Ward Reservoir Number 5, and
4. Harvey Park Lake
These reservoirs and their tributary basins were excluded from the 1972 MDP because there is no outflow
from these ponds for events up to at least the 100-year. Therefore, the results are the same whether they
are in the model or excluded because they fully contain a 100-year event runoff without a discharge
downstream. These four reservoirs have been included in the 2012 Corrected (Baseline) Hydrology Model
to make all subbasins connected. These four reservoirs are filled by irrigation ditches and receive very
little runoff from relatively small tributary areas and, consequently, have 100-year capacity. Agreements
should be developed with the reservoir owners to ensure their long-term use for 100-year capacity storage
of runoff is maintained.
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