FLOOD06950

• County: Statewide
• Basin: Statewide
• Title: Techniques of Water-Resources Investigations of the US Geological Survey General Procedure for Gaging Streams
• Date: 1/1/1968
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

6

TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

choice of systems thus depends ou the charac-
teristics of the site.

Water-stage recorders

Both strip-chart and digital-tape water-level
re.corclers are in general use. Either recorder
lUay be uctuated by the float or bubble-gage
system. Figure :, shows a bubble-gage digital-
punch arrang:ement.

"',..

I~ ~,~!, :

Figure 5.-Bubble-gage digital-recorder arrangement. Gas
tank-on right; digital-punch recorder on left.

A strip-chart recorder produces " graphic
record of the rise and fan of the water surface
with resped to time. .\ ~ag-e-height scale of
1: f) and a time scale,of one (b.y being- eqnal to
2.+ inches ,He C'ommonly used. ContimlOll::-; re-
('orders ~llch as tlw Sten~llS .\-;1;') will operate
""attp",]pd for ppri",], of 611-011 dnys nnd pro-
,"irk a ypry satisf:.wt01"Y record of stage.
,\ digital stage reeordel' punchps coded values
of stage on paper tape nt preselected time inter-
vals. .\ timc intetTal of 15 minutes is normally
used. The Fisphel'-Porter recorder is battery
operated and will l'Un unattended for periods
of nO-DO <bys. The eode ,'onsists of fOllr g-l'OUpS
of four punches each: the four punches repre- ,
sent 1. :2, -t. and ~ in each gronp. The punching
of a 5tag-{' l'eql1in.'s only a n.l-inch advance of
paper tape. The recorder is aetnated hy it, cam
on a battel'y-driwnmechanical clock.
Digita] recorders are gradually replacing
strip-chart l'C'cOl'dC'I'S at p:aging' stations in the
rnited States. Tht' f'\\"o 1'('cordel's are about
"qual in accnmcy. reliability. and l'ost, but the

digital recorder is compatible with the use of
electronic computers in computing discharge
records. This automated system as developed by
the Geological Survey offers greater economy
and flexibility in the computation-publication
process than do manual methods associated with
graphical recording. However, the use of
graphical recorders will be continued at those
sites where a graphical record is necessary to
detect ice effects, backwater, or frequent mal-
functions of the recording system.

Reference gages

Because of the possibility of plugged intakes
or other malfunctions, a nonrecording gage is
installed So that the water level in the stream
can be directly measured. Comparative readings
on the inside and outside gages are taken dur-
ing each visit to the station by engineering per-
sonnel. Datum of all gages is checked at peri-
odic intervals-usually every 2 or 3 years. In
figure 2, the outside gage is on the bridge. Out-
side staff gages are visible in fignres 3 and 4
in the pools near the gage structures.

Discharge Measurements

Discharge measurements are made at each
gaging stat.ion to define the discharge rating for
the site. The discharge rating may consist of a
simple relation between stage and discharge or
" more complex relation in which discharge is
a function of stage, slope, rate of change of
stage, or other factors.
Discharge measurements are normally made
by the current. meter method, which consists of
determinations of velocity and area in the parts
of a. stream cross section. Ho\vever, indirect
methods are frequently used in determining
flood peak discharges. These methods utilize
hydmulic equations in conjunction with the in-
formation on channel characteristics and flood-
marks obtained in a field survey after the flood
event.
Discharge measurements may also be made by
the dilution method. This method depends on
determination of the degree of dilution of an
added tracer solution by the flowing water.