GENERAL40564

• Permit No: M1989065
• IBM Index Class Name: General Documents
• Doc Name: PRE MINING CONDITIONS
• Media Type: D
• Archive: No

a~g's equation is • A culvert or bridge can act as a reservoir outlet if
there is significant storage behind it. The
1.49 r~ s~ (Eq ~~ procedures in TR-55 can be used to determine the
V = peak flow upstream of the culvert. Detailed
n storage routing procedures should be used to
determine the outflow through the culvert,
average velocity (fUs),
=hydraulic radius (ft,) and is equal to s/pw,
= cross sectional flow area (ft~,
pW =wetted perimeter (ft),
=slope of the hydraulic grade line (channel
slope, fUft), and
n = Manning's roughness croefficient for open
channel flow.
i~ng's n values for open channel flow ran be
otained from standard textbooks such as Chow
;~or Linsley et al. (19821. After average velocity
puted using equation 34, Tt for the channel
agment can be estimated using equation 3.1.
tervoirs or lakes
.:mes it is necessary to estimate the velocity of
I through a reservoir or lake at the outlet of a
watershed. This Lravel time is normally very small
can be assumed as zero.
~itations
. Manning's kinematic solution should not be used
r sheet flow longer than 300 feet. Equation 33
as developed for use with the four standard
rainfall intensity-duration relationships.
n watersheds with storm sewers, carefully
dentify the appropriate hydraulic flow path to
estimate T~. Storm sewers generally handle only a
mall portion of a lazge event. The nest of the
ak flow travels by streets, lawns, and so on, to
the outlet. Consult a standard hydraulics textbook
determine average velocity in pipes for either
~ressure or nonpressure flow.
The minimum T~ used in TR-55 is 0.1 hour.
e
Example 3-1
The sketch below shows a watershed in Dyer
County, northwestern Tennessee. The problem is to
compute T~ at the outlet of the watershed (point D).
The 2•year 24fiour rainfall depth is 3.6 inches. All
three types of flow occur from the hydraulically most
distant point (A) to the point of interest (D). To
compute Tc, fast determine Tc for each segment
from the following information:
Segment AB: Sheet flow; dense grass; slope (s) _
0.01 ft/ft; and length (L) = 100 ft.
Segment BC: Shallow concentrated flow; unpaved;
s = 0.01 ft~ft; and L = 1400 ft.
Segment CD; Channel flow; Manning's n = .05;
flow area (a) = 27 ftz; wetted
perimeter (pW) =282 ft; s = 0.005
ftlft; and L =7300 ft.
See figure 3.2 for the computations made on
worksheet 3.
(210-VI•TR-55, Second Ed., Jcme 19867