FLOOD10334 ‎(2)‎

• Designation Number: 588
• County: Arapahoe
• County: Jefferson
• Community: Littleton
• Stream Name: Massey Draw and SJCD (South)
• Title: FHAD - Massey Draw and SJCD ‎(South)‎
• Date: 12/1/2005
• Prepared For: Urban Drainage and Flood Control District, Jefferson County, Littleton
• Prepared By: Olsson Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Pervious depression loss of 0.40 inch and impervious depression loss of 0.10 inch was used for the
area west of the hogback. These values match those used in the Final Stormwater Detention Study
''The Valley" (Muller Report, Reference 10). For the remainder of the study area, recommended
pervious depression loss of 0.3S inch and the impervious depression loss of O,OS inch from Table RO-
6 in the UDFCD Criteria Manual (Reference 4) were used.

UDSWM model elements, including subwatersheds, design points, conveyance elements, and
detention basins, are shown in Figures A-1 and A-2 in Appendix A. A schematic of the Massey Draw
model network is shown in Figures A-7 and A-8, and a schematic of the SJCD (South) model network
is shown in Figure A-9. Conveyance element parameters in the form of the UDSWM input files are
shown in Tables A-3 and A-4 for Massey Draw and SJCD (South), respectively.

The parameters for the UDSWM model conveyance elements east of the hogback were measured
from the 2-foot contour interval mapping of the drainageways that was prepared for the project. West
of the hogback, the USGS digital elevation models were used to measure the length and slope of the
routing elements.

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The study area is essentially fully developed, with vacant in-fill parcels scattered throughout the study
area, The subwatershed imperviousness was determined using GIS land use maps provided by
Jefferson County and aerial photographs. For those areas that are not yet developed, the South
Jefferson County Community Plan was used to determine the future anticipated land uses and related
imperviousness. The percentages of imperviousness within the study area are shown in Figures A-3
and A-4.

Depression Losses

3.7

UDSWM Routing Element

~:y~~---",
_!?g,:y~_a..~_,~~___
J"Q9:Y_~~E,
SOO-Year

Parameters

1.68
2.32
2,66
....._.__._....m....._._.
3,30

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Watershed

mperviousness

One-Hour Rainfa
Zone I
Below 6000'

ths
Zone IIA
Above 6000'
1,S7
..................m..........,..........
2,17
...-----....----.--
2.48
-~----
3,08

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Subwatersheds in the study area were initially delineated using USGS digital elevation models
(DEM's), Adjustments to the subwatershed boundaries were made, based on field observations, to
reflect the flow pattern changes that were the result of development. The areas of the subwatersheds
delineated for this study were compared to those delineated for the Original Plan. The comparison
was discussed in the Hydrology Report for Massey Draw and SJCD (South) (Reference 9). The
resulting delineated subwatersheds are shown in Figures A-1 and A-2 in Appendix A.

Table

The rainfall depths shown in Table 1 were entered into CUHP2000 to generate hydrographs for the
study area subwatersheds. Tables A-1 and A-2 in Appendix A show the subwatershed characteristics
in the form of the input parameters for the CUHP2000 model for Massey Draw and SJCD (South),
respectively. Each storm event was run separately, and Tables A-1 and A-2 show the 100-year storm
model input.

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Subwatershed Delineation

The following 1-hour rainfall depths from Table S01 of the Jefferson County Storm Drainage Criteria
(Reference 3) were used, The SOO-year 1-hour rainfall depth was determined by extrapolation of a
logarithmic plot of the 1-hour rainfall depths,

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A calibration model and a baseline model were prepared, The calibration model included two existing
detention basins on Massey Draw that are located west of the hogback. This model was used to
approximate the 1 OO-year flows from the Flood Insurance Study (FIS), Jefferson County and
Incorporated Areas (Reference 7) and the 1985 UDFCD memorandum (Reference 8), which updated
flows along Massey Draw. In the baseline model, the existing detention on Massey Draw, upstream of
Carr Street, and on SJCD (South), upstream of Kipling Parkway, was added Flows from the baseline
model were used in this study,

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Peak flows and hydrographs were developed for each of the 81 subwatersheds for the 10-, SO-, 100-
and SOO-year return period storms using the Colorado Urban Hydrograph Procedure (CUHP2000),
Version 1.1.0 (Reference S). Hydrographs for the subwatersheds were routed using the Urban
Drainage Stormwater Management Model (UDSWM2000), Version 1.4,6 (Reference 6) to determine
peak flow rates at select design points,

West of the hogback the soil infiltration parameters from the Muller Report were used, The initial
infiltration rate range was 3.9-4,S inches per hour, the final infiltration rate range was 0,6-0,7 inches
per hour, and the Horton's exponential decay rate coefficient range was 0.0016 to 0.0018. For the
remainder of the study area the parameters from Table RO-7 in the UDFCD Criteria Manual for type C
and D soils were used, For type C and D soils the initial infiltration rate is 3,0 inches per hour, the final
infiltration rate is O,S inch per hour and the decay coefficient is 0.0018. The Horton's exponential
decay rate coefficient was 0,0018

Ralnfa

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Soils in the watersheds are predominantly hydrologic soils group B, C, and D, as defined by the
NRCS, These soils are generally characterized by low permeability and high runoff, Figures A-6 and
A-7 show the hydrologic soil groups for the study area

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Methodology

Hydrologic Analysis

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City 0/' Little ton

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nfiltration

Flood Hazard Area Delineation
Massey Draw and SJCD (South)
December 2005