FLOOD02174

• County: Denver
• Community: Lakewood
• Stream Name: Dry Gulch
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 7/1/1979
• Prepared For: Denver
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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13

IV HYDRAULIC ANALYSIS

between the floodway and the flood fringe is made in Reference

l. The floodway is the general flood channel which could convey

the entire lOO-year flow with a water surface not exceeding that

of the normal lOO-year flood by more than 0.5 feet. Conversely,

the flood fringe is that portion of the flood plain which could

be blocked or filled with earth without causing an increase in

water surface or more than 0.5 feet. This distinction between

floodway and flood fringe essentially divides the flood plain

into an area which carries a major portion of the flow, and an

area which is relatively ineffective in terms of conveying the

flood. The flood fringe would be flooded by relatively slow

moving water.

Hydraulic computations were made along the channels of Dry and

North Dry Gulches to determine the extent of flooding which would

occur in the four predicted floods. The principal tool used in

the hydraulic analysis was the computer program developed by the

U.S. Army Corps of Engineers entitled "HEC 2, Water Surface Pro-

files" (Reference 4). Data relating to channel slope, roughness,

cross section shape, and existing drainage facilities were

developed from the topographic maps, facilities plans, and field

reconnaissance. These data were then utilized by the computer

program to calculate flow depths, widths and velocities at

selected cross sections along each channel.

The water surface elevations in each storm were determined at

each cross section, and the topographic maps were used to inter-

polate flooded outlines between cross sections. The development

of these flooded area maps made it possible to estimate which

properties and structures would be expected to be inundated in

the five-, ten-, fifty-, and one hundred-year floods.

After the proposed facilities for the Master Plan were selected,

flooded outlines were prepared for the lOa-year flooding which

would occur after construction of the facilities. This "resi-

dual" lOO-year flood plain limit is illustrated on each of the

Major Drainageway Planning Sheets of Section VII.

The anticipated extent of flooding in a lOO-year flood is illus-

trated in detail in "Flood Hazard Area Delineation, Dry Gulch

and Tributaries" (Reference l), and on the Major Drainageway

Planning Sheets of Section VII of this report. A delineation