FLOOD06450

• Designation Number: 92
• County: Jefferson
• Community: Arvada
• Stream Name: Ralston, Leyden Creeks
• Basin: South Platte
• Title: Major Drainageway Planning - Ralston/Leyden Creek, Arvada, Jefferson County, CO
• Date: 8/1/1977
• Designation Date: 1/1/1978
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I

VIII-]

I

SECTION VIII

wi 11 be used in all grass-l ined channels. The low flow channel (depending On
the reach) wi 11 be sized to hand I e up to 200 cfs.

European-Type Channels. European-type channels used in this master plan
will be used in areas where increased channel capacity is required and where
right-of-way constraints do not allow the use of a grass-lined channel. This
type of channel uses textured concrete channel walls and a natural grass
bottom to provide a more hydraulically efficient channel. The channel will
aesthetically blend into the local environment of selective plantings adja-
cent to the channel and can be used for a local recreation belt along the
continuous length of channel. Unless otherwise noted, the new channel will
be located over the existing channel. Drop structures are to be incorpor-
ated into reaches of this type of channel in order to keep depths and veloci-
ties of flow within allowable criteria. Channel walls will be vertical
or sloped at 0.5 to 1. Trickle channels will be sized to handle the normal
row flow of the stream. Vehicular maintenance access points will be pro-
vided at regular intervals at selected street crossings,

PHASE B MASTER PLAN

I

GENERAL COMMENTS

I

The master plan presented in this Section was developed using criteria set
forth by the local entities and the Urban Drainage and Flood Control District,
The refinement of the master plan included the finalization of the hydrolo-
gical and hydraulic analyses of Ralston Creek and Leyden Creek for the 100-
year flood for future basin development. This information is presented on
mapping in this report.

~;~,

I'

I

The following section describes the general types of improvements used in
this master plan. Unless otherwise noted, all improvements utilize the de-
sign criteria contained in the Denver Regional Council of Governments' Urban
Storm Drainage Criteria Manual, Volumes 1 and 2. Specific variations of the
proposed improvements from the typical improvements are described later in
this Section,

I

It is anticipated that most dwellings that are acquired in order to build a
European-type channel will be purchased as they come on the market to mini-
mize the sociological impact of the improvements.

Nonstructural (Modified) Channels. The use of a wide range of nonstructural
and semi-structural tools for flood plain management is involved in this
type of channel. The inclusion of culvert and bridge replacements and minor
channel erosion control work introduces costs which are not normally part
of a nonstructural solution, thus this is a "modified" alternative. Non-
structural channel techniques include:

Flood plain management includes all measures for planning and action which
are needed to determine, implement, revise, and update comprehensive plans
for the wise use of flood plain lands and their related water resources.

I

Of special note to the master plan is the assumption as to Blunn Reservoir,
The overall approach was to assume that Blunn Reservoir, if built, would not
significantly affect the downstream flooding. This approach is valid because:

I

. Blunn Reservoir would not reduce the flood peak on Ralston
Creek for either future or existing conditions below its
confluence with Leyden Creek (See Section VI).

I

. Development is sparse in the basin on Ralston Creek upstream
of Leyden Creek.

I

. Due to the possibility of catastrophic losses for events larger
than the 100-year flood, the flood plain on Ralston Creek above
its confluence with Leyden Creek should be regulated without
considering Blunn Reservoir.

I

I

TYPICAL MASTER PLAN IMPROVEMENTS

I

Grass-l ined Channel. Grass-l ined channels used in this master plan wi 11 be
designed for natural waterway hydraulic and appearance characteristics while
increasing channel capacities to contain the 100-year flow with future basin
development. The channels will aesthetically blend into the local environ-
ment and will be usable for a local recreation belt along the continuous length
of channel. Unless otherwise noted, the new channel will be located over the
existing channel. Drop structures are to be incorporated into reaches of this
type of channel to keep depths and velocities of flow within allowable criteria,
Channel side slopes will generally be nO steeper than 4 to 1. The channel will
be seeded with a combination of natural grasses which reauire only occasional
mowing or at the option of the local entity, the channel may be seeded or
sodded with an irrigated type of grass requiring extensive mowing. Trickle channels

10.
lL

I

I-
I

I

L

Evaluation of existing flood plain
. Include monuments and sians. and show high water elevations
for 1 percent flood and selective flood proofing
Selective flood proofing
Flood plain regulations
Revise building codes and subdivision regulations
Develop plan for urban area renewal of flood plain over
period of time (phased acquisition)
Early warning system
Insurance and public education
Relief and rehabilitation
Minor structural modification
. Bridges and culverts
. Debris protection
Parks, recreation, and open space (phased acquisition)
Evaluation on regular basis

2.
3.
4.
5.

6.
7.
8.
9.