FLOOD01619

• Designation Number: 320
• County: Jefferson
• Community: Arvada
• Stream Name: Lower Ralston, Van Bibber, Leyden Creeks
• Title: Major Drainageway Planning
• Date: 2/1/1986
• Prepared For: Arvada
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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INF IL TRATION
A portion of the design storm precipitation does not become storm runoff
because it infiltrates into the ground, it evaporates, or it is held as de-
tention in depressions. Depending on basin development conditions in the
future, appropriate iMiltration and losses were taken into account.

A diagram of flood discharges for various frequencies by location is pre-
sented in Figure III-l entitled "Peak Discharge Profiles - Future Condi-
tions".

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Data presented in Section II of the Phase A and Phase B reports were uti-
lized. The SCS curve numbers given were determined to be reasonable.

The flood discharge hydrograph for Van Bi bber Creek at the mouth for the
100-year event is presented in Figure 111-2. Hydrographs for the mouths of
Leyden Creek and Ralston Creek are shown on Figure 111-3 and 111-4, respec-
tively.

DETERMINATION OF DESIGN FLOODS
Flood flows were computed for numerous locations on the subject creeks for
the 2-, 5-, 10-, 50-, and 100-year frequencies. All results were checked
for reasonableness and cons i stency with adj acent area flood flows. When
questions arose, the runs were repeated as deemed necessary.

Present Conditions
The flood flows for present basin conditions are for 1984 levels of devel-
opment. This was based on the 1976 study updated to 1984.

FLOOD HYDROLOGY
The basel ine hydrology for the Phase A master pl anning study was for the
condition without Leyden Lake. Leyden Lake, at the time of the Phase A and
Phase B reports, is considered to represent risk of failure due to spillway
inadequacy. The spillway inadequacy is parti ally caused by dam embankment
settlement which would result in dam overtopping before the spillway capa-
city is reached.

The flood flows are given in Table III-3. The flood peak at the mouth of
Van Bibber Creek is shown to be lower for future development than for pre-
sent basin conditions by 3DO cfs. This result is due to the peculiar char-
acteristics of the basin whereby development in the Arvada area of Van
Bibber Creek causes storm runoff to occur earlier, thus reducing its con-
tribution to the main flood peak from upstream. This is illustrated by
Figure 111-2 where two distinct peaks are shown on the hydrograph.

The design floods were computed using the MITCAT model. The Phase A report
presents details on the model so that it can be reconstructed in the
future. A separate hydrological report was submitted to the District.

This master planning evaluation has indicated that a rehabilitation of Ley-
den Lake would provide flood damage reductions adequate to warrant such re-
habilitation. Therefore, design floods and related master planning for the
Phase B report are based upon Leyden Lake being in place. All flood values
are presented in Table 111-3 for comparison and ready reference.

Future Conditions
The flood flow values for future basin conditions (with and without Leyden
Lake) are presented in Table III-3 for various frequencies of occurrence.
These future condition flood peaks represent development in the basin in
accordance with reasonable projections of land use by Jefferson County and
the City of Arvada.

The flood peaks for future conditions are based on existing channel config-
urations. They include routing of floods though Leyden Lake and Blunn

Reservoir.

For planning purposes it can be assumed that the development projected for
the study area basins will be in place by about the year 20DD, however, no
attempt was made in the master planning to identify a specific year.