FLOOD04017

• County: Jefferson
• County: Denver
• Community: Columbine Valley, Denver
• Stream Name: Dutch Creek, Lilley Gulch, Coon Creek,
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 5/1/1978
• Prepared For: Jefferson County / Arapahoe County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

I

I

I

HYDROLOGIC AND HYDRAULIC DETERMINATIONS

Design Rainstorms

I

Basin Parameters

The project lies predominantly in T5S and R69W and extends east and

west into Ranges 68 and 70. The rainfall data for T5S and R69W as

presented on pages 254 and 255 of the "Project Reuse Report" was used

for the entire project. These data are presented in Table B-1 of

Appendix B. The depths and durations were computed for the five

design rainfalls (2-, 5-, 10-, 50- and 100-year) for a 180 minute

duration at 10 minute intervals.

I

I

Each sub-basin was physically described in terms of tributary area to

each design point, sub-basin length, distances from sub-basin centroid

to des.ign point, degree of perviousness throughout the basin and the

selection of representative coefficients typical of the basin. Numer-

ous field visits were made to become familiar with the physical charac-

teristics of each sub-basin, to aid in making necessary judgments con-

cerning adjustments to coefficients relating peak flows and time to

peak. All available records of existing storm sewers were gathered from

the jurisdictions in the Basin and were studied to determine the extent,

capacity and effects on runoff of the existing storm drainage facilities.

Infi ltration Rate/Retention

I

I

I

Per cent of imperviousness was determined by measuring impervious surface

areas in each of the sub-basins located within the total basin. The

various types of existing and future land uses measured within each sub-

basin were assigned a degree of imperviousness from which a composite

percentage was computed for each sub-basin. Field visits verified the

development characteristics of the various sub-basins.

Soil types prevalent within the study area and their characteristics

were determined from publications of the Soil Conservation Service,

namely Soil Survey of Arapahoe County; General Soil Map for Jefferson

County, dated June 1972; and a Table for Soil Characteristics (First

Draft). East of the hogback all of the soils were reasonably similar

and an infiltration rate of 1/2 inch per hour was used. In Basin D-l,

west of the Hogback, soils with extremely high infiltration rates are

present. A composite infiltration rate of 2.1 inches per hour was

estimated for Basin D-l.

I

I

I

I

C (coefficient reflecting time to peak) was determined from Figure 4-2
t

"Relationship between C and Imperviousness", as revised 7/15/1977 from
t

the Urban Storm Drainage Criteria Manual, and modified where necessary to

reflect steep slopes and degree of sewering of the basin.

Pervious and impervious retention amounts were obtained from Table 2-2

of the Drainage Manual. A value of 0.3 inches was used for pervious

areas and 0.1 inch for impervious areas.

I

I

C (coefficient related to peak rate of funoff) was determined from
p
Figure 4-3 "Relationship between C and C " as revised 5/15/1975 and
p t.
modified to reflect steep slopes and degree of sewering.

Hydrographs

I

Table A-I in Appendix A lists the information developed for the various
parameters associated with the study Basin hydrologic investigation,
based on present and future development within the study area.

Hydrographs were run using a computer program developed by the Urban

Drainage and Flood Control District with the assistance of URS Company.

Six hydrographs were calculated for each sub-basin, resulting in a

total of 126 hydrographs covering the 2-, 5-, 10-, 50-, 100-year future

and 100-year existing conditions. Figure B-2 in Appendix B is an

I

I

I

7