PCWA-L 467.pdf - PCWA Middle Fork American River Project ...

150 MEASUREMENT OF STAGE AND DISCHARGE
urements are made from the upstream side of the bridge, it is the
relative location of the upstream end (nose) of the pier bat is relevant;
for measurements made from the downstream side it is the
location of the downstream end (tail) of the pier that is relevant. If
any part of the pier extends into the measurement cross section, the
area of the pier is excluded. However, bridges quite commonly have
cantilevered walkways from which discharge measur€'ments arc
made. In that case the measurement cross section lies beyond the end
of the pier-upstream from the nose or downstream from the tail,
depending on which side of the bridge is used. In that situation it is
the position and direction ofthe streamlines that determines whether
or not the pier area is to be excluded. The hydrographer, ilhe had not
previously noted the stationing ofthe sides ofthe pier when projected
to the measurement cross section, does so now. Ifthere is negligible or
no downstream flow in that width interval (pier subsection)-that is,
if only stagnation and (or) eddying exists upstream from the nose or
downstream from the tail, whichever is relevant-the area ofthe pier
is excluded. Ifthere is significant downstream flow in the pier suhsection,
the area of the pier is included in the area of the measurement
cross section. The horizontal angles ofthe streamlines in and near the
pier subsection will usually be quite large in that circumstance,
Footbridges are sometimes used for measuring the d·ischarge of
canals, tailraces, and small streams. Often a rod suspension can be
used for the meter when measuring from a footbridge. In low velocities
the procedure for determining depth when using a rod suspension
is the same as that used for wading measurements. For higher velocities
depth is obtained from the difference in readings at an index
point on the bridge when the base plate of the rod is at the water
surface and when it is on the streambed. The measurement of depth
by that method eliminates errors in reading the depth caused by the
fast-moving water piling up on the upstream face of the rod. Handlines,
hridge cranes, and hridge boards are also used from footbridges.
When using a sounding reel, depths and velocities are measured by
the methods described in the preceding section of the manual on
cableway measurements (p. 146-148). \'\-'hen using a handline, depth
is determined by first lowering the sounding weight to the streambed
and then raising the weight until one of the tags is at the water
surface. The distance that the weight is raised is measured along the
rubber-covered cable (p. 106) with either a steel or metallic tape, a
folding foot-rule, or a graduatod rod. The total depth of weter is then
the summation of (1) the distance the particular tag is above the
meter cups, (2) the measured distance the meter and w"ight were
raised, and (3) the distance from the hottom ofthe weight to the meter
cups.