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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17

The second reach extends from South. Carr Street through. South Garrison Street.

In the stretch downstream of West Chatfield Avenue, the drainageway is poorly

defined and extends diagonally across numerous lots and the platted but unbuilt

South Cody Street and South Carr Court in the Meadowbrook Heights subdivision.

Upstream of West Chatfield Avenue the drainageway crosses a portion of the Fair-

view Heights Subdivision, and the upstream portion of the reach is unplatted.

No dedicated drainage right-of-way exists for the entire reach. Road overtopping

and probable failure will occur at South Garrison Street and West Chatfield

Avenue for floods greater than or equal to the 2 year event. Upstream of West

Chatfield Avenue the 100-year flood plain is fairly well defined; downstream it

is not well defined. The 100-year flood plain width averages 320 feet and

channel velocities average 4 feet per second. Overbank velocities average 3

feet per second where they exist.

however, the extent of flooding is difficult to determine because the topography

is constantly changing. Low flows of the drainageway are intercepted by the

Nevada Ditch while larger flows overtop the ditch bank and flow into the gravel

mining area. The North Tributary although presently not tributary is believed

to have been historically tributary to the main basin at the upstream end of

this reach. After drainage ~provernents are constructed in accordance with the

North Tributary's published drainage master plan, it will again flow into SJCD

(South). Definition of the flood hazard area accounts for this future

flow. Above the Nevada Ditch, the 100-year flood hazard area averages 340

feet in width and the channel velocities average five feet per second.

Overbank velocities average 4 feet per second. The reach is presently unplatted

and within Arapahoe County. It has no dedicated right-of-way for drainage. The

area downstream of the Nevada Ditch may be included in the City of Littleton's

flood plain park along the South Platte River.

The third reach of the North Tributary of Massey Draw extends from South Garrison

Street to the line dividing Sections 33 and 34. The area is presently unplatted,

although plans exist for a development called Galleria. It is possible that

proposed buildings will be within the flood plain. The 100-year flood plain

width averages 150 feet and channel velocities 5 feet per second. No existing

buildings would be damaged by the 100-year flood in this reach. Overbank velo-

cities, where they exist, average 3 feet per second.

The second reach extends from the confluence with the North Tributary to the

Arapahoe/Jefferson County line. Extensive areas north and south of the main

channel are subject to inundation because of overtopping at Platte Canyon Road

(located in reach 3). Flooding in these two areas does not correspond to eleva-

tions of floods in the main channel because the flows are physically separated.

North of the main drainageway, four residences may receive structure and content

damage. South of the drainageway an extensive area is being gravel mined. The

extent of the flood hazard area is difficult to determine because of the chang-

ing nature of the topography. A large stock pile of overburden exists across

the drainageway at the upper end of this reach. A channel was excavated through

it which is inadequate to convey the major flows.

The fourth reach of Massey Draw's North Tributary extends from the east line of

Section 33 to the upstream limit of the study. A portion of the reach is platted

as the Ken Caryl Ranch Plains development. Near the upstream limit, the 100-year

flood will overtop Continental Divide Road without causing significant damage.

A small onstream reservoir has been breached within the reach. The 100-year

flood hazard area averages 110 feet in width and the channel velocities average

5 feet per second. The reach is without a dedicated drainage right-of-way, al-

though land within the flood plain is being held by a homeowners association for

open space. OVerbank velocities where they exist average 2 feet per second.

The third reach of SJCD (South) extends from the Arapahoe County line through

Depew Street. Inadequate culvert and downstream channel capacity at Platte Canyon

Road causes overtopping of Platte Canyon Road and Canyon Avenue for floods

greater than or equal to the 5 year event. An extensive area in reaches 2 and 3

is included in the flood hazard area because of these road overtoppings. Below

Platte Canyon Road, a newly established car wash and an outdoor recreation

facility may be damaged by flood flows. Depew Street will be overtopped by the

2 year and larger floods. The roadways within this reach are overtopped without

significant flood damage. Above Platte Canyon Road the 100-year flood plain

The first reach of SJCD (South) - Basin 6200 extends from the South Platte River

to the confluence with its North Tributary. There is no defined outlet for the

basin to the South Platte River because the lower reach is being gravel mined.

An extensive area below the Nevada Ditch is included in the flood hazard area,

~ Leonard Rice Consulting Water Engineers, Inc.