Geoscience Reference
In-Depth Information
regular dredging. This is really only possible for
small streams as the power of large rivers at high
velocities would place enormous strains on the
gauging structure.
particle the shorter the wavelength. This type of
instrument works in small streams (less than 5 m
width) and requires some suspended matter.
Measuring hillslope runoff
The difference between flumes and weirs
The measurement of runoff may be required to
assess the relative contribution of different hillslope
runoff processes; i.e. throughflow, overland flow,
etc. There are no standard methods for the measure-
ment of runoff processes; different researchers use
different techniques according to the field conditions
expected and personal preference.
Although flumes and weirs perform the same func-
tion - measuring stream discharge in a continuous
fashion - they are not the same. In a weir the water
is forced to drop over a structure (the weir - Figure
5.11) in the fashion of a small waterfall. In a flume
(Figure 5.12) the water passes through the structure
without having a waterfall at the end.
Overland flow
Ultrasonic flow gauging
The amount of water flowing over the soil surface
can be measured using collection troughs at the
bottom of hillslopes or runoff plots. A runoff plot is
an area of hillslope with definite upslope and side
boundaries so that you can be sure all the overland
flow is generated from within each plot. The upslope
and side boundaries can be constructed by driving
metal plates into the soil and leaving them pro-
truding above the surface. It is normal to use several
runoff plots to characterise overland flow on a slope
as it varies considerably in time and space. This
spatial and temporal variation may be overcome
with the use of a rainfall simulator.
Recent technological developments have led to
the introduction of a method of measuring stream
discharge using the properties of sound wave propa-
gation in water. The method actually measures water
velocity, but where the stream bed cross-sectional
area is known (and constant) the instrumentation can
be left in place and combined with measurements
of stage to provide a continuous measurement of
river discharge. There are two types of
ultrasonic
flow gauges
that work in slightly different ways.
The first method measures the time taken for an
ultrasonic wave emitted from a transmitter to reach
a receiver on the other side of a river. The faster the
water speed, the greater the deflection of the wave
path and the longer it will take to cross the river.
Sound travels at approximately 1,500 m/s in water
(dependent on water purity and depth) so the instru-
mentation used in this type of flow gauging needs
to be extremely precise and be able to measure
in nanoseconds. This type of flow gauging can be
installed as a permanent device but needs a width
of river greater than 5 m and becomes unreliable
with a high level of suspended solids.
The second method utilises the Doppler effect to
measure the speed of particles being carried by the
stream. At an extremely simple level this is a
measurement of the wavelength of ultrasonic waves
that bounce off suspended particles - the faster the
Throughflow
Measurement of throughflow is fraught with diffi-
culty. The only way to measure it is with through-
flow troughs dug into the soil at the appropriate
height. The problem with this is that in digging,
the soil profile is disturbed and consequently
the flow characteristics change. It is usual to insert
troughs into a soil face that has been excavated and
then refill the hole. This may still overestimate
throughflow as the reconstituted soil in front of the
troughs may encourage flow towards it as an area
allowing rapid flow.
