Armfield S12-MKII, Инструкция по эксплуатации

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Exercise A - Rainfall-runoff relationships (storm
hydrographs)
Theory
Rain falling on a catchment area will make its way to the point of concentration where
it will leave the catchment. In a gravity flow situation, this is bound to be the lowest
point in the catchment. If the discharge is by means of ground water movement, the
situation is more complex and the flow can be distributed over a wide front but as the
flow is constrained to leave this model catchment at a single point, we shall not
consider this case here.
In practice, a catchment area is defined only once the point of concentration has
been fixed and, as steam flow data is needed here, the site of a new pre-existing flow
measurement structure is usually chosen. When rain falls on the catchment, the time
taken for the water to reach the point of concentration will depend on the horizontal
distance it has to travel and also on the velocity.
Figure A1 shows lines of equal flow time for a catchment of similar proportions to the
model in which the flow velocity is everywhere the same. Figure A2 illustrates a
valley catchment in which the flow velocity is assumed to increase once the water
has entered the stream channel. Flow outside the stream could be either by surface
or ground water flow, or both.
Figure A1
Figure A2
The greatest time taken for rain falling on the catchment (the far corners) is called
time of concentration.
A graphical record of flow and time is called a hydrograph and Figure A3 shows a
typical hydrograph resulting from a single rainstorm. The timing and intensity of the
rainfall is shown by the block in the upper part of this figure and if the rainfall persists
for longer than the time of concentration of the catchment, the run-off hydrograph will
level off at the peak value on the catchment. Under these circumstances, the
recession curve part of the hydrograph is delayed until the rain stops.