Exercise A
35
c. Histogram
The hydrograph should properly be shown as in Figure A4 and Figure A5 by
plotting the results directly.
It may be found that the best-shaped storm hydrographs are obtained when
the "rain" is stopped just before the maximum run-off is obtained. That is to
say, the duration of the storm is slightly less than the
time of concentration
for
the catchment. If the rain persists after the water table reaches the surface
then
direct run-off
over the surface occurs. When the rain stops before this
occurs, the run-off is only in the form of
ground water
flow.
It is recommended that different slopes and surface profiles be tried until the
most suitable hydrograph is obtained.
Stream flow from multiple storms
Connect the flexible piping from the river inlet tank to the quick release connector on
the 5 l/min flow meter. Connect the flexible piping from the overhead spray nozzles to
the quick release connector on the 3 l/min flow meter.
The sand bed should be allowed to drain following any previous use of the
apparatus.
This experiment can be carried out by arranging a first storm of duration rather less
(say 50%) than the time of concentration, tc (as obtained in the previous experiment).
Follow it by a second storm of the same duration while the recession limb of the first
one is still quite high. The discharge values must be recorded continuously from the
start of the first storm, and the resulting double hydrograph when plotted will show
the much larger run-off values obtained for the second storm which falls on a
previously saturated catchment. The method for drawing the hydrograph, outlined in
"Stream Flow for a Single Storm", may be used.
Stream flow from an impermeable catchment (urbanisation)
After investigating the rainfall run-off relationships for a permeable catchment, it is of
interest to reduce the permeability of the catchment surface by covering part or all of
it with the impermeable Polythene sheet provided with the accessory items. If only
the upper part of the catchment (away from the discharge end) is sealed in this way,
then the run-off from the plastic sheet is lost in the sand in the lower part. If, however,
the lower part of the catchment only is covered, the run-off is more immediate and
the effect on the hydrograph more marked. The plastic sheet provided should be
trimmed with a knife or scissors to fit the required catchment area.
Stream flow from a highly vegetated catchment
The effect of a highly vegetated catchment may be simulated by covering part or all
of the catchment surface with the absorbent material provided.
Stream flow with reservoir storage
The effect of a flood detention reservoir on the run-off from a standard storm can be
demonstrated by using the accessories provided. The circular ended ring can be
used when partly buried in the sand to form a circular reservoir, and the closed ring
can, similarly, be used to retain the rain that falls on it and to release the water slowly
through the centre aperture. It may prove necessary to use all available vessels to
Summary of Contents for S12-MKII
Page 1: ...Advanced Environmental Hydrology System Instruction Manual S12 MKII ISSUE 21 February 2014...
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Page 8: ...4 Equipment Diagrams Figure 1 Front View of S12 MKII Hydrology System...
Page 9: ...Equipment Diagrams 5 Figure 2 Plan View of S12 MKII Hydrology System...
Page 10: ...Armfield Instruction Manual 6 Figure 3 End View of S12 MKII Hydrology System...
