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

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IV-l

SECTION IV

FLOOD PLAINS AND FLOOD DAMAGES

TABLE IV-1
TYPICAL STRUCTURAL AND FURNISHING VALUES

FLOOD PLA INS

Once the flood magnitudes were determined for future conditions, the flood
plain could be determined. The basic procedure in determining the flood
plain was the HEC-2 backwater program developed by the U,S. Army Corps of
Engineers (USACE). A backwater program is necessary to determine the flow
depth upstream of the multitude of bridges, culverts, and various channel
constrictions of Ralston and Leyden Creeks.

Type

Total Value
Land and Structure

Structure

Furnishings

House

$30,000
35,000
45,000

$24,300
28,400
36,500

$10,000
12,000
15,000

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 I-inch to 100-foot scale aerial
topographic mapping and surveyed culvert information. This was supplemented
by a field trip in which detailed notes were taken to note various soil
,
channel, and culvert conditions. This report shows the lOO-year flood plain
for future basin development for the existing channel and for the Master Plan
Channel, This flood plain will be submitted to the Colorado Water Conserva-
tlon Board for official designation.

Apartment
Complex
Commercial
Trailer

25,000/unit
161 s . f.
7,000

4,000/unit
20/s.f.
5,000

FLOOD DAMAGES

TABLE IV-2

The flood plains for the 2-, 10-, and 100-year storms for developed basins and
existing channels plotted On the I-inch to 100-foot scale mapping revealed the
extent of flooding by incremental reaches of the creeks, The extent of flood-
ing was related to general first floor levels of structures. Damages were then
tabulated according to type of structure and similarity of levels of flooding
for each reach,

RALSTON CREEK
FLOOD DAMAGE PROBABILITY DISTRIBUTION
FUTURE BASIN DEVELOPMENT WITH EXISTING CHANNEL
BASED ON 1976 VALUES

Federal Insurance Administration damage curves (September 1970) were used to
calculate dollar damage values for structures and furnishings for the various
levels of flooding.

Phase A Reach Annual Private Annual Publ i c Annual Total
Reach Description Damages Damages Damages
R"l Clear Creek $ 389,600 $15,400 $ 405,000
to Pierce
Street
K-2 Pierce Street 979,100 22,900 1,002,000
to Garr i son
Street
R-3 Garrison Street 1,828,600 21,400 1,850,000
to Leyden Con-
fluence
R-4 Leyden Conflu- 536,700 5,800 542,500
ence to Xenon
Street

Table IV-l presents some of the typical values used for structures and furnish-
ings in Arvada and JefferSOn County. These values were the basis for estimating
flood damage costs for the 2-, 10-, and 100-year flood events.

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Tables IV-2 and IV-3 give the annual flood damages calculated durina Phase A
for Ralston and Leyden creeks in Arvada and Jefferson County for pr~sent channel
conditions and future basin development,