FLOOD03506

• County: Jefferson
• Community: Lakewood
• Stream Name: South Lakewood Gulch
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 8/1/1978
• Prepared For: Lakewood
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECT! ON I V
FLOODPLAINS

The data base required for this program consists of cross sectional channel

shapes, roughness, and survey information about culverts and bridges. The

majority of the information is provided by 2-foot contour I-inch to IOO-foot

scale aerial topographic mapping and surveyed culvert information. This was

supplemented by a field trip in which detailed notes were taken of various

soil, channel, and culvert conditions.

GENERAL

Floodplain delineation in a dependable manner is the cornerstone to

reasonable flood plain controls. Private property is being regulated for

the good of the private property owner, his neighbor, the general public,

and to reduce the liability of government. The flood discharge and the

floodplain boundaries must be established in accordance with reasonable

and accepted engineering methods.

Floodplain mapping was prepared In plan format from calculated flood profiles
and will be used for flood control and land use planning purposes.

FLOOD FREQUENCY

The frequency of occurrence of the regulatory flood for land use regula-

tion purposes is the I percent flood event (IOO-year flood) with future

development. This flood has a I percent chance of occurring, or being

exceeded, in any given year.

Field inspection of the floodplain was undertaken to check the map for
conformance to actual field conditions.

The floodplain mapping was submitted to the Colorado Water Conservation
Board in the form of a Flood Hazard Area Del ineation Report dated July, 1977
for Official designation and was so designated on March 15, 1978.

The I percent event criteria provides for a basic waterway which will

function during larger, as well as smaller floods and is a reasonable

basis for public safety and protection.

PRINCIPLES OF FLOODPLAIN CONTROLS

FLOOD MAGNITUDE

The overall purpose of floodplain controls is to:

. Protect the public health, safety, and general welfare.

. Minimize flood losses in areas subject to flood hazards.

. Promote the wise use of the floodplain.

. Provide for the preservation of floodways and floodplain storage

areas in order to protect those downstream from increased flooding.

The rate and volume of the I percent flood has been determined for the

existing and future development and is presented in Section III. The flood

is defined using procedures of the Runoff Section of the Urban Storm

Drainage Criteria Manual supplemented by additional modeling techniques.

FLOOD PROFILES

Floodplain controls are critical factors in drainage management. Successful

floodplain controls will preserve a reasonable path for floodwaters by con-

trolling development in flood hazard areas, prevent loss of floodplain

storage volume and prevent the introduction of structures, property or

land uses into the floodplain which would result in damages.

The floodwater surface profiles are computed using standard engineering

techniques. The basic procedure in determining the ft-oodplain was the

HEC-2 backwater program developed by the USACE. A backwater program is

necessary to determine the flow depth upstream of the multitude of bridges,

culverts, and various channel constrictions of South Lakewood Gulch.

(Ref. 5, 6).

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