FLOOD02223

• County: Arapahoe
• Community: Littleton
• Stream Name: Upper Slaughterhouse Gulch
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 3/1/1983
• Prepared For: Littleton, Arapahoe County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The evaluation of the Powers Park detention pond and spillway was made by
establishing a stage/discharge relationship for the spillway from the as-constructed
drawings. The area/capacity relationship was determined by integrating the areas
from the 2-foot contour interval maps and from construction drawings,

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CHAPTER IV - HYDRAULIC ANALYSIS

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A. General

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The purpose of the hydraulic analysis in a major drainageway planning study

is to define the floodplains for the various frequency storms and evaluate the

adequacy of the existing drainage facilities, such that the damage potential and

viable alternatives can be defined and evaluated, The methodology used and the

results of the analysis are presented in this chapter. The floodplains were defined

for the 2-, 5-, 10-, 50-, and lOa-year flood flows for the future basin develop-

ment condition, The evaluation of the existing drainage facilities is limited to

determining the capacity and general condition and does not include evaluating

the structural integrity of the facility.

C. Floodplain Determination

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B, Evaluation of ExistinR Drainage Facilities

The floodplain for the study area was determined using a computerized backwater

program, The program is titled "HEC-2 Water Surface Profiles", The required input

to the program is the channel cross-section coordinates, the hydraulic properties

of each sub-section within the cross-section, and the discharge value, The back-

water calculations are initiated by inputting a known water surface elevation and

discharge value. The longitudinal spacing of the cross-sections varies with channel

condition, but generally the spacing was less than 200-feet. The starting water

surface elevation was obtained from the results of the SWMM model output at Powers

Park, At each of the initializing sections, another section was considered immedi-

ately upstream to stabilize the backwater calculations, Discharge and corresponding

water surface elevations for the future development condition 2-, 5-, 10-, 50-, and

lOa-year flood and for.the existing development condition loa-year flood were input

and backwater profiles calculated. These data were then plotted on the 2-foot

contour interval project mapping and the corresponding floodplains was outlined,

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The evaluation of the existing drainage facilities was performed using the
I" = 100', 2-foot contour interval mapping prepared by Landmark Mapping Company
under a separate contract with Urban Drainage and Flood Control District (UD&FCD)
and field survey and documentation provided by the City of Littleton, The
facilities that were evaluated included channels, storm sewers, and detention
ponds and spillways,

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The capacity of the storm sewer systems was determined using the guidelines
presented in the Urban Storm Drainage Criteria Manual (Reference-5), The capacity
of the system was estimated by evaluating the pipe and the inlets to determine which
facility controls (limits) the capacity, The full flow capacity of the pipe was
calculated using the "minilllUm" pipe slope and approximate roughness coefficient.
The capacity of the inlets was calculated based upon the type (i,e" grate, curb
opening, or combination) and whether the inlet was lion-grade II or in a "sump" COn-
dition, then applying the suggested "allowable capacity" reduction factor (Reference-
5).

D, Evaluation of Proposed Drainage Facilities

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Capacities of all proposed storm sewer pipes were determined using the guid-

lines presented in the Urban Storm Drainage Criteria Manual (Reference-S), The

evaluation of the proposed Caley detention pond facility was made by establishing

a stage/discharge relationship for the spillway and outlet works, The area/capacity

relationship of the detention reservoir was determined by integrating the areas

from the 2-foot contour interval maps, The affects of the proposed detention

facility were evaluated by re-running the SWMM program with the proposed detention

pond in place. The results are presented on the peak flow diagram in Figure 111-6,

From these peak flows the capacities of the proposed storm sewers were subtracted

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