FLOOD01405

• County: Jefferson
• County: Arapahoe
• Community: Jefferson County, Arapahoe County
• Stream Name: Massey Draw, North Tributary SJCD
• Title: Major Drainageway Planning
• Date: 7/1/1979
• Prepared For: Jefferson County / Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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15

V, PLAN FOR FLOOD CONTROL

IV. HYDRAULIC ANALYSIS

General

Drainage problems were identified in the Phase A Major Drainageway Planning

Report for Massey Draw et al (2), published in March, 1978. Alternative

flood control schemes were developed and evaluated which range in complexity

from maintaining the existing situation with adequate enforcement of an

effective flood plain ordinance to installation of major structural improve-

ments, The most promising of the alternative plans for the reaches studied

were assembled to form the recommended plan which was presented in conceptual

detail in the Phase A Report, Following review and acceptance of the plan

by the UDFCD and Jefferson and Arapahoe Counties, this Phase B Report was

prepared.

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 (7),

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.

The recommended Flood Control Plan is designed in sufficient detail to pro-

perly plan bridges and culverts for new vehicular thoroughfare, to permit

the acquisition of needed channel right-of-way and to permit the planning of

new subdivisions and buildings with full knowledge of future channel and flood

plain locations and characteristics. The recommended plan is shown schematic-

ally on Sheet No, 3 of the drawings in detail on Sheets Nos. 4-24, A drawing

by drawing description is also provided, Backup design information is on file

with UDFCD. The flood hazard area of Massey Draw, et al, was delineated in a

separate report published in May, 1978 (3),

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.

Recommended Plan

The first items of the recommended plan are adoption of the 100 year flood

plain limits, adequate regulation of the flood plain with effective zoning,

acquisition of flood insurance for all buildings within or adjacent to the

flood plain and annual notification of potential flood hazards for property

owners, Acquisition of a 12-foot access right-of-way along the entire length

of the drainageways is recommended, Ideally it should be located above the

10 year flood profile. Advantage should be taken of opportunities to acquire

such right-of-way. Improvements generally are designed to control overtop-

ping at road crossings, lessen where possible the damage potential for existing

structures, eliminate two areas of widespread flooding, provide flow separation

sttuctures for irrigation ditches and return SJCD (South)-Basin 6200 to the

South Platte River. Important traffic routes including Ken Caryl Road,

Chatfield Avenue, Platte Canyon Road, and So. Garrison Street at Tributary

No.1 of SJCD (South), have been designed to pass the 100 year flood without

Backwater computations were made for the 2, 5, 10, 50 and 100-year floods for the

future basin development condition and existing channel conditions, Sub critical

flow was assumed. The 100-year flood plain based on future development within

the basin and existing channel conditions is shown schematically on Sheet 2,

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

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

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

and Flood Control District. A separate report titled nFlood Hazard Area

Delineation, Massey Draw and North Tributary, SJCD (south) 6200 and Tributariesn

(3) was published in Y~y, 1978. It contains detailed flood plain and floodway

information, which is keyed to cross-sections.

~ Leonard Rice Consulting Water Engineers, Inc.