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 III

2. Field inspection, testing, and an~lysis of data pertaining to impervious
cover, slopes, infiltration rates and other physical phenomenon concern-
ing runoff flow, This information was partially presented in Tables
11-1,11-2, and 11-3.

HYDROLOGICAL SUMMARY

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GENERAL

3, Determination of runoff flows based on the data established in I and 2
above. Many methods exist for this, three of which were listed in the
Manual when it was publ ished in 1969, Since that time, however, many
complex computer models have been developed to perform these runoff
determinations more accurately and economically, resulting in a highly
useful set of tools for application by the engineer. One of these models
was used to compute runoff flows for Ralston Creek and its tributaries,
and is described briefly below.

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The extent of present flooding and the sizing and character of major drainage
works and other urban storm drainage facilities are determined by the magni-
tude of runoff, Furthermore, the design of the facilities must be based
upon expec~ed sediment problems, including degradation, aggradation, and
water qua 11 ty,

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~ master pla~ must be based on adequate and thorough hydrological studies
If the plan IS to have significant value and establish a firm factual
base. Hydrological studies must take into consideration volume of runoff
as well as peak rates of flow, It is when the volume characteristics are
known that one realizes the value of detention storage, both pond type and
channel storage, to reduce the peak rates of flow,

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The magnitude of a flood is usually judged by its maximum rate of flow, even
though the maximum rate may last for only ten minutes, The frequency of
a flood is judged by its return period, The return period is defined as
the average interval of time within which a given event will be equalled
or exceeded once, Thus, a flood having a return period of 100 years has a
one percent probability of being equalled or exceeded in any given year,

The Massachusetts Institute of Technology Catchment Model (MITCAT) was
developed to establish a better understanding of the effects of basin
development, channelization, onstream reservoirs, and other sensitivities
not considered by most runoff computational methods, The concept is to
use basic fluid mechanics to analyze separately the overland flow and
stream flow portions of surface runoff. The design rainfall is the
most basic input to the overland flow area, called a catchment, and the
catchment then inputs to the stream, One can then vary individual
parameters to understand the sensitivity of stream discharges to develop-
ment patterns. For instance, the amount of impervious area and the over-
land flow distances to collecting streams are easily controlled variables,

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Runoff flows were determined at many locations for return periods of two,
ten, and 100 years so that a determination could be made concerning the
storm frequency for which drainage facilities should be designed, The de-
tailed information of the computational methods was presented in the Phase A
Report, with the present section highlighting basic information,

RAINFAL~ PATTERNS

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The approach taken by the Denver Regional Counci 1. of Governments Urban Storm
Drainage Criteria Manual in the determination of runoff flows is:

The hydrology of a large drainage basin is complicated by the necessity for
areal rainfall corrections to account for the distribution of point rainfall
intensities over the entire basin,

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1. Determination of rainfall that would most likely occur during an event
with a given return period. This is done at present by use of isohyetal
(line of equal depth rainfall) Depth-Duration-Frequency maps that were
synthesized during the formation of the Manual, This provides a basic
input for the runoff computation. This information was analyzed for
the Ralston Creek watershed in several locations, resulting in three
rainfall zones: two in the upper basin above Ralston Reservoir and
one in the lower basin, '

In this drainage study, 21 separate rainfall patterns were generated to account
for the various hydrological events which could result to produce design flows
on Ralston Creek and Leyden Creek. Since the peak flows at various points in
the basin may not be caused by the same event, then it must be kept in mind
that peak design flows on tributaries may not correspond to the peak flows on
the mainstem, Representative examples of various rainfall patterns used are
included in the Appendix to this Section.

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INFILTRATION

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Part of the rainfall that occurs infiltrates into the ground and does not
become part of the immediate storm runoff, As the study area develops, the
overall infiltration rate will change because large areas will become im-
pervious, due to building and paving, and other areas will change from pas-
ture to lawn grass. In some cases, the infiltration will increase, and in
other cases, it will decrease,

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