FLOOD04566

• County: Boulder
• Community: Boulder
• Stream Name: Gregory, Bear Can, Bluebell Can Creeks, Viele Chan
• Basin: South Platte
• Title: Major Drainageway Planning Volume 1, Part C
• Date: 9/1/1970
• Prepared For: Boulder County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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23

TABLE IV-l

SOUTH BOULDER 100-YEAR DESIGN RAINFALL

Time from
Beginning of Total Incremental Design
Storm Rai nfall Rainfall Rai nfall
0 0.00 O.OU 0.00
10 1. 35 1. 35 0.10
20 1.80 0.45 0.14
30 2.15 0.35 0.45
40 2.29 o. i4 1.35
50 2.40 o. II 0.35
60 2.50 0.10 0.11
70 2.58 0.08 0.08
80 2.64 0.06 0.06
90 2.69 0.05 0.05
100 2.74 0.05 0.05
110 2.78 0.04 0.04
120 2.81 0.03 0.03
TOTALS 2.81 2.81
Infi ltration

Part of the rainfall that occurs infiltrates into the ground and does not
become part of the immediate storm runoff. As South Boulder 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 pasture
to lawn grass. In some cases, the infiltration will increase, and in other
cases it will decrease.

The infiltration rates used in thIs study are based on the probable future
land cover. To obtain these rates, numerous field infi ltration tests were
performed in the Boulder area. Some of these tests were performed on lawn
grass while other tests were conducted in the foothills area that is trib-
utary to the South Boulder drainage ways. Based on these tests and from
various soils maps of the area, South Boulder was divided Into two infil-
tration zones. The western zone, whIch includes the foothi lls and recent
alluvial deposits, was estimated to have an Infiltration rate of just over
one inch per hour. The eastern zone, which includes most of the existing
urban area, was estimated to have an infiltration rate of one-half inch per
hour.

These infiltration rates are for the 100-year design storm where it is
assumed that the ground surface is saturated before the storm occurs. For
storms that have no antecedent rainfall and where the ground is assumed not
to be saturated, the initial infiltration would be much higher.

Per Cent Impervious Land

Land that is totally impervious will produce nearly a 100 per cent runoff.
Pervious land wi 11 absorb and retain a significant amount of rainfall. In