FLOOD07371

• County: Jefferson
• Community: Arvada
• Basin: South Platte
• Title: Ralston/ Leyden Creeks Hydrology Report
• Date: 8/1/2002
• Prepared For: UDFDC
• Prepared By: Boyle Engineering Corporation
• Floodplain - Doc Type: Floodplain Report/Masterplan

SECTION 3.0 - HYDROLOGIC ANALYSIS

3.1 Background

This study and report were conceived in order to convert the hydrologic model for the Ralston Creek
watershed from MITCA T to CUHP-SWMM and to account for reductions to the 100-year floodplain as a
result of the construction of and/or enlargement of various reservoirs, Because results from previous studies
were used in development of the published Flood Insurance Study (PIS) and associated floodplain mapping,
it was necessary to perform a calibration procedure to insure that results from this study compared well with
those previously published,

The Arvada/Blunn Reservoir was not included in previous hydrologic models, Leyden Lake was omitted
from the Phase A report, but was considered as a separate model for the Phase B report, However, Leyden
Lake was improved in 2001, and no longer operates as represented in the Phase B modeling, For these
reasons, in order to determine consistency between the models, it was first necessary to model the Ralston
Creek basin without considering effects from Arvada/Blunn Reservoir and Leyden Lake, Ralston
Reservoir, which was in place before the Phase A report was published and has not been modified since.
remained in the model.

Once the new CUHP-SWMM model was sufficiently calibrated to previously published results, effects of
the new reservoirs were added to the model in order to determine an updated hydrologic response for the
watershed, These results will be used to determine the extent of the 100- year floodplain, as well as the
magnitude of flooding for other events, for the future developed condition,

3.2 Methodology

Although this study is essentially an update of previous studies, it utilized a new modeling application, In
addition, mapping and computer-generated data files were unavailable from the previous studies, For this
reason, other than the calibration procedure to insure consistency, this study essentially resulted in the
development of a completely new model.

Because there is no comprehensive, single-datum mapping for the entire Ralston Creek watershed, USGS
7,5' quadrangle maps were used, Arvada, Golden, Ralston Buttes and Black Hawk were the maps used for
this study, Available electronically, these maps could be combined with computer aided drafting to develop
accurate watershed runoff characteristics,

Design rainfall was then prepared for various points in the watershed, Design events considered include the
10-,50-, 100-, and 500-year events, Rainfall patterns for a number of various areal corrections were
developed and used in the analysis, To more accurately represent rainfall, the entire basin was developed
into upper, central, and lower regions, with separate rainfall patterns developed for each,

Using this information, CUHP models were prepared and run for the various design frequencies and
correction scenarios, Output hydrographs from these models were then run through the stream network
using UDSWMM in order to determine peak flow rates at critical design points,

3.3 Subwatershed Delineation

The first step in the procedure was to delineate the entire Ralston Creek basin and separate the Ralston,
Leyden, and Van Bibber subbasins, These basins were then delineated into subwatersheds, For areas
upstream of Arvada/Blunn Reservoir and Leyden Lake, subwatersheds were delineated with a maximum
basin area of five square miles, For areas below these reservoirs, a detailed delineation was performed with
maximum basin areas of 130 acres, Because the Van Bibber basin was only included in this study to
determine its effects on the Ralston Creek basin (an analysis of the Van Bibber floodplain was not included
in this study), it was not subject to the same areal criteria as the Ralston and Leyden basins, The Van
Bibber Creek basin was delineated based on previous studies where the sub watershed maximum area limits
did not apply,

Once delineated, a number of geographic variables for each subwatershed were calculated for input into the
CUHP model. These variables include the following:

. Contributing Drainage Area (sq, mi,)
. Catchment Length (mi,)
. Centroid Distance (mi,)
. Catchment Slope (ftlft)

These variables were determined graphically using computer aided drafting, and this electronic map is
included on the attached CD-ROM, The watershed characteristics are shown in Tables 1,2, and 3 of
Appendix A for the Ralston, Leyden, and Van Bibber basins, respectively,

The subwatershed delineations are shown in Figures 1 through 5 of Appendix A, Figures 6 through 9 of
Appendix A portray a routing schematic for the basin,

3.4 Watershed Imperviousness

A critical variable for development of subwatershed hydrographs is the percentage of impervious surfaces in
each subbasin, Impervious surfaces increase the hydrologic response of a subbasin, and the hydro graphs
generated for each subbasin are particularly sensitive to this value,

It was understood that any changes to planned development relative to the Master Plan were local rather
than regional in nature, For this reason, aggregate impervious percentages developed in previous studies
(which utilized much larger subbasin areas) were kept the same, However, a zoning map provide by the
City of Arvada was utilized to determine actual impervious percentages for each individual subwatershed,
For each large subwatershed in the previous study that was subdivided into smaller watersheds for the
present study, a weighted average was computed to maintain consistency over larger regions,

The values determined for watershed imperviousness are included in Tables 1,2, and 3 of Appendix B,

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