FLOOD00230

• Designation Number: 121
• County: Adams
• County: Arapahoe
• County: Douglas
• Community: Denver Metro Region
• Stream Name: Massey Draw and North Tributary
• Title: FHAD - Massey Draw and North Tributary, SJCD South 6200 and Tributaries, Jefferson and Arapahoe Counties, CO
• Date: 5/1/1978
• Designation Date: 11/1/1978
• Floodplain - Doc Type: Floodplain Report/Masterplan

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15

IV. HYDRAULIC ANALYSIS

Information from the hydrologic analysis was used to define flood profiles for

the various flood frequencies and development conditions. Computations were

made with the aid of the U. S. Army Corps of Engineers' backwater program HEC-2.

Input data for the program consists of flood discharges, cross sections des-

cribing the ground geometry at appropriately spaced intervals, estimates of

roughness coefficients, information describing culverts and road profiles at

road crossings, and measurement of channel and overbank distances between cross

sections. Input data was obtained from detailed topographic mapping, field

inspection and engineering judgment. Cross section locations were identified

on stereo paired aerial photographs and were coded up in HEC-2 format by Delta

Aerial Surveys who also provided the base mapping. The cross sections are also

located on the plan and profile drawings. Culvert information was taken from

field notes of surveys made by Delta Aerial Surveys, supplemented by field in-

spection. The potential for blockage at each culvert was determined by con-

sidering size and type of culvert, the magnitude of flood flow, and the potential

for debris generation upsteam.

Vegetation within the flood plain ranges from nothing in areas of new construc-

tion to irrigated lawn and landscaping in parks and urban areas. Generally

native grasses prevail. Roughness coefficients used in the backwater calcula-

tions were n ; .04 for channels and n ; .045 for overbank areas.

Backwater computations were made for the 10 and lOa-year floods for the future

basin development condition and existing channel conditions. Subcritical

flow was assumed. The lOa-year flood plain based on future development within

the basin and existing channel conditions is shown schematically on Plate II,

and in detail on the plan and profile drawings. The 10 year flood profile is

also shown where it can be distinguished from the lOa-year profile. All hy-

draulic calculations and backup information are on file with the Urban Drainage

and Flood Control District. Tables IV-l through IV-5, which summarize the

flood hazard area information, are keyed to the cross sections shown on the

plan and profile drawings.

~ Leonard Rice Consulting Water Engineers, Inc.