FLOOD03351

• County: Statewide
• Basin: Statewide
• Title: Engineering and Design Hydraulic Design of Flood Control Channels
• Date: 7/1/1991
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

EM 1110-2-1601
1 Jut 91

Woolhiser and Lenz (1965). The principles of design and
operation of large debris basins as practiced by USAED,
Los Angeles. have been presented by Dodge (1948).
Ferrell and Barr (1963) discuss the design, operation. and
effects of concrete crib check dams used in the
Los Angeles County Flood ConlrOl District on small
Stre:lms.

(2) Debris storage. Debris basins. usually located
ne:lf canyon mouths at the upper end of alluvial fans. are
designed to settle out and provide storage space for debris
produced from a single major storm. In the Los Angeles
area. the debris basin design capacity has been based on
100.000 cubic yards (cu yd) per square mile of drainage
area, or 62 acre. feet per square mile. This quantity was
obtained as an envelope curve of observed debris pro-
duction during the storm of 1938 (Dodge 1948). Later
estimates by Tatum (1963), laking into account factors
affecting debris production such as fire history of the
area. indicated a value of about twice this amount. Debris
storage in the basin is usually maintained by reexcavation
after a major storm period. The debris stored in the basin
after anyone flood should not be allowed to exceed
25 percent of the basin capacity. When permanent debris
storage is more economical than periodic excavation. the
average annual rate of debris accumulation multiplied by
the project life should be used for storage capacity. Data
from the Los Angeles County Flood Conlrol District
(Moore, Wood. and Renfro 1960) on 49 debris dams and
basins give a mean annual debris production of 5.s00 cu
yd per square mile of drainage basin. This figure applies
in the Los An geles and similar are:lS. and can be used to
determine the economic feasibility of long-term storage
versus periodic debris removal.

(3) Debris basin elements. A debris basin consists
of five essential basic parts:

(a) A bowl-shaped pit excavated in the swface of the
debris cone.

(b) An embankment. usually U,shaped in plan. con-
slrUcted from pit material. located along the two sides and
the downslre:lm end of the pit. and joining the hillside at
each end where possible.
(c) One or more inlet chutes at the upsa-eam end of
the pit. when necessary to prevent excessive Stre:lmbed
degradation upsa-eam of the debris basin.

(d) A broad-crested spillway at the downSlream end
of the basin leading to a flood control channel.

4-2

(e) An outlet tower and conduit through the embank-
ment at the spillway for basin draining.

Plate 49 shows general design plans for a debris basin.
The basin shape. the inlets. and the outlet should be
located so that the debris completely fills the basin before
debris discharge occurs over the spillway.

(4) Design criteria. The slope of the upper surface
of the debris deposit must be estimated to determine the
proper basin shape and to estimate the tolal debris ca-
pacity of the basin. A value of 0.5 times the slope of the
natural debris cone at the basin site has been used for
design. The basin side embankments should be of suffi.
cient height and extend far enough upsa-eam to confine
the maximum debris line slope projected upslre:lm from
the spillway crest. The spillway should be designed to
pass the design flood discharge with the basin filled with
debris. The tops of the basin embankments should pr0-
vide 5 ft of freeboard with the foregoing conditions. The
design criteria for debris basins in the Los Angeles area
should be ased only for general guidance because of large
differences in geology, precipitation patterns, land use.
and economic justification in different parts of the coun-
a-y. The following conditions are peculiar to the
Los Angeles area:

(a) Phenomenal urban growth in the desirable land
area of the lower alluvial fans.

(b) Large fire potential.

(c) Hot. dry climate over a l:lfge pon/on of the year
which inhibits vegetative growth.

(d) Sudden torrential rainfall on precipitous mountain
slopes during a short rainy season.

(e) Unstable soil conditions subject to voluminous
slides when SatUrated. Debris and sediment production
rates vary throughout the couna-y depending on many
factors, some of which are conlrOllable by man. Exten.
sive conslrUction, strip mining operations. intensive agri.
cultural use. and timber cutting operations are only a few
eX:llRples of land uses that can have a profound local
effect on sediment production and thus determine the type
of sediment conlrOl necessary, Formulation of a sediment
conlrOl plan and the design of associated engineering
works depend to a large extent on local conditions.