FLOOD08228

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Urban Drainage and Flood Control District
• Date: 3/1/1990
• Prepared For: Boulder Creek
• Prepared By: Goerge V. Sabol
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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Hydrologic Design Manual
for Maricopa County

Rainfall Losses

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r:~,6~ Surface Retention Loss
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~ll~!~1 Surface retention loss, as used herein, is the summation of all rainfall losses other
than infiltration. The major component of surface retention loss is depression
storage; relatively minor components of surface retention loss are due to intercep-
tion and evaporation, as previously discussed. Depression storage is considered to
occur in two forms, First, in-place depression storage occurs at, and in the near
vicinity of, the raindrop impact. The mechanism for this depression storage is the
microrelief of the soil and soil cover. The second form of depression storage is the
retention of surface runoff that occurs away from the point of raindrop impact in
surface depressions such as puddles, roadway gutters and swales, roofs,lrrigation
bordered fields and lawns, and so forth.

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A relatively minor contribution by interception is also considered as a part of the
total surface retention loss. Estimates of surface retention loss are difficult to obtain
and are a function of the physiography and land-use of the area.

The surface retention loss on impervious surfaces has been estimated to be in the
range 0.0625 inch to 0.125 inch by Tholin and Keefer (1960), 0.11 inch for 1 percent
slope to 0.06 inch for 2.5 percent slopes by Viessman (1967), and 0.04 inch based on
rainfall-runoff data for an urban watershed in Albuquerque by Sabol (1983). Hicks
(1944) provides estimates of surface retention losses during intense storms as 0.20
inch for sand, 0.15 inch for loam, and 0.10 inch for clay. Tholin and Keefer (1960)
estimated the surface retention loss for turf to be between 0.25 to 0.50 inch. Based
on rainfall simulator studies on undeveloped alluvial plains in the Albuquerque
area, the surface retention loss was estimated as 0.1 to 0.2 inch (Sabol and others,
1982a). Rainfall simulator studies in New Mexico result in estimates of 0.39 inch for
eastern plains rangelands and 0.09 inch for pinon-juniper hillslopes (Sabol and
others, 1982b). Surface retention losses for various land-uses and surface cover
conditions in Maricopa County have been extrapolated from these reported es-
timates and these are shown in Table 4.1.

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Ifi~:~1 Infiltration is the movement of water from the land surface into the soil. Gravity
and capillary forces drawing water into and through the pore spaces of the soil
matrix are the two forces that drive infiltration. Infiltration is controlled by soil
properties, by vegetation influences on the soil structure, by surface cover of rock
and vegetation, and by tillage practices. The distinction between infiltration and
percolation is that percolation is the movement of water through the soil subsequent
to infiltration.

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