FLOOD07727

• County: Jefferson
• Community: Arvada
• Stream Name: Ralston/Leyden Creek
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 8/1/1977
• Prepared For: Arvada
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION I I

LOWER RALSTON CREEK BASIN (RURAL)

BASIN DESCRIPTION

The basin between
main components:
Creek, The lower
15, and 16,

Ralston Reservoir and the City of Arvada consists of three
the Ralston Creek mainstem, Leyden Creek, and Van Bibber
basin is comprised of subbasins 7, 8, 10, 11, 12, 13, 14,

The following section summarizes the
Creek, and Van Bibber Creek basins.
reach will be identified by the same
through (i.e" reaches 10, 11 and 16
descriptions follow.

descriptions of Ralston Creek, Leyden
For the purpose of simplification, a
number as the subbasin it is flowing
form Leyden Creek). Deta i 1 ed bas i n

Ralston Creek Mainstem

UPPER RALSTON CREEK BASIN

The tributary area between Ralston Reservoir and the proposed site for Blunn
Reservoir is characterized by a transition from foothills to high plains,
Elevations range from 6,000 feet to 5,400 feet with an average ground slope
of between 16 and 18 percent. Soi 1 depths vary from deep in flat areas such
as the flood plains and plateaus, to shallow in the steeper areas. Develop-
ment is mainly rural, with urban residential sites beginning to spread into
the area from the lower end,

The mountainous, forested basin above Ralston Reservoir is divided into
subbasins 1,2,3,4,5,6, and 9, as depicted on the Basin Map, These subbasins
have their headwaters near State Highway 119. Elevations range from 10,000
to 6,000 feet, and the terrain ranges from rolling grassy meadows surrounded
by forest to extremely steep, rugged mountainous terrain,

LOWER RALSTON CREEK BASIN (URBANIZED)

The average ground slope varies from approximately 20 to 50 percent. The
creek itself begins as a low point in the valley in the upper-most reaches,
and finally shows an identifiable channel near the lower end of subbasin 1.

The lower urbanized portion of the Ralston Creek Basin is primarily in
the City of Arvada. This area is divided into two subbasins, differing
primarily by present levels of development, Subbasins 17 and 18 make
up this region.

Table 11-1 summarizes various physical parameters for each of the subbasins
in the upper Ralston Creek basin, Only present conditions were analyzed with
respect to runoff parameters here, since no significant development is
antipicated for the upper basin.

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TABLE 11-1
UPPER BASIN PARAMETERS
Over 1 and Overl and Streambed
Sub- Area Pe rcen t Flow Length Slope Slope
bas in (Sq, Mi,) Impervious (Feet) Percent Percent
1 6.4 10 1,200" 20 7
2 4,0 10 900 45 4
3 17,0 10 700 40 5
4 2,9 15 1 ,200" 25 10
5 1.5 15 1 ,200* 25 10
6 8,6 15 1 ,000 50 7
9 5.6 15 900 30 5

As Ralston Creek flows through the upper portion of Arvada, it joins with
Leyden and Van Bibber Creeks, Elevations range from 5,600 to 5,200 feet and
the average ground slope varies between 2 and 3 percent, The stream channel
is well defined, often passing through residential developments. Flood
plain encroachment is most severe in this basin, and many sharp turns in
the channel point out obvious potential problems in times of flooding, There
are several bridges in these subbasins.

Individual subbasin characteristics for the lower (urbanized) Ralston Creek
basin are tabulated in Table 11-2 below,

TABLE 11-2

LOWER (URBANIZED) BASIN PARAMETERS

*Maximum overland flow length used is 1,200 feet; defined channel generally
occurs at greater lengths.

Overl and Overland Streambed
Sub- Area Percent Flow Length Slope Slope
basin (Sq, Mi .) Impervious (Feet) Percen t Percent
17 4.1 20 120 to 1,200* 3 0,6
18 3.2 30 120 4 0.6
"-Varies, depending upon degree of urban i zat ion