26 
BULLETIN 1408, U. S. DEPARTMENT OP AGRICULTURE 
to line the canal for a short distance beyond the structures. For 
size No. 24 and smaller in good soils a straight wall may do, but 
the end of the flume should extend back farther than for other types. 
Figure 17 shows connection between metal flume and end struc- 
tures. The end of the metal sheet should not be turned into the 
concrete. The flume, when filled with water, sags at bottom and the 
opening in the head wall must be shaped approximately to cor- 
respond to this form. The bars across the top may be omitted from 
small sizes of some types. This is desirable where large weeds may 
No. of flume = perimeter 
of semi- circle in inches 
Substructure Dimensions 16 ' Span 
No. of 
Flume 
Stringer 
Bent 
Posts 
Knee 
Braces 
Sway 
Braces 
Knee-Brace 
Spreader 
12- 21 
2"y 6" 
4"x4" 
2"x 4" 
2"* 4" 
j'x 2" 
24-42 
2"* 8" 
4'* 4" 
2\6" 
2'" 6" 
2'x3" 
48-72 
3**8" 
4"* 6" 
3"*6" 
2"* 6' 
2"x3" 
ELEVATION OF BENT 
SECTION A-A ^° SECTION C-C 
Fig. 17. — End connections and substructure for metal flumes 
cause overflow. In all cases a liberal free board should be pro- 
vided. 
Creosoted wood-stave flumes are coming into favor and it is 
believed that their cost of maintenance will be low. The substruc- 
ture required is in some respects similar to that for metal flumes. 
Where the bottom of the drain is very soft or the structure may 
be endangered by floods, trusses similar to those in Plate 1, J, are 
recommended for small flumes. Corrugated pipe, requiring no sup- 
port, is a convenient means of carrying small laterals across narrow 
ditches. Reinforced concrete is desirable for large flumes. (See 
pi. 2, K, and pi. 2, L.) 
