240
DOCK ENGINEERING.
-v2
Now the term - (w = 64 Ibs. for sait water) deviates by so little from
2
— that we can replace it by the latter, without sensible error. Whence,
or, ________
v^3*'..........................(39>
This value for 7q determines the amount of head absorbed in overcoming
friction. Its ratio to that given above (37) for simple discharge is expressed
by the coefficient: F The factor, / varies with the nature of the
surface of the conduit, and it is also found to depend, to a certain
extent, on the relative diameter of the conduit and the rate of flow, being
greater in small pipes than in large culverts, and at low velocities than
at high speeds. Its value is found, however, to lie between -005 and -01,
and -0075 may be taken as a serviceable mean for general use under normal
conditions.
The symbol, R, standing for the area of fluid section divided by the
perimeter, is referred to as the hydraulic mean radius, or the hydraulic
mean depth. For circular and square culverts running full, and for
circular culverts running half full, it is obviously equal to one-fourth
of the diameter.
There are other sources of friction than that investigated above, and
these cannot be overlooked in estimating the efficiency of the current
issuing from a sluicing culvert:—
I. There is the friction due to the form of inlet at the reservoir. If
an orifice in a thin plate, it has been found by experiment that
F2 = -055.
If the inlet has a square-edged entrance,
F2 = -505.
II. There is the friction at sudden enlargements or contractions of
culvert area. Let the ratio in which the effective area is suddenly
enlarged or contracted be designated r. Then, for abrupt enlargements,
F3 = (r - 1)2,
and for abrupt contractions the same formula may be used, although the
actual ratio of contraction is somewhat uncertain, being greater than the
apparent ratio. The loss of head is due to the enlargement succeeding
contraction.
III. For bends in circular culverts,
F. = - [ 0-131 + 1-847 \,