Describe and explain the hydrographs for the River Severn and the River Wye after
the storm of 6 August 1973.
From Reference Diagram Q2B, it is clear that the storm began at 11pm on the night of the 5 th August. It rained for
several hours, up until about 7am on the 6th, reaching a peak of approximately 25mm at 2am.
The River Severn was affected by this storm quite quickly. It is evident from the hydrograph that the discharge of
the River Severn started to increase at approximately 12am (0 hrs). This created the rising limb, which increased
steadily up until about 4am, where it reached a peak of approximately 25 cubic meters per second (cumecs),
before decreasing steadily to approximately 5 cumecs by 10am.
The River Wye however responded to this storm much more immediately. As soon as the rain began at 11pm the
discharge of the River Wye increased as well. Increasing rapidly, shown by the rising limb, to approximately 35
cumecs at 2am, mirroring the rainfall pattern. After 2am the River Wye decreased just as rapidly, falling to
approximately 5 cumecs by 9am.
The lag time of the River Severn is only approximately 1 hour, from the time of peak rainfall at 2am to peak
discharge at 3am. This may be due to a number of different factors. Firstly, the catchment area of the River
Severn is relatively steep, the land reaches a height of 670m, therefore the rainwater would find it’s way into the
River Severn’s channel, or one of its tributaries, very quickly, this would explain the short lag time. Unlike the
River Wye, the River Severn’s rising limb increases steadily between 11pm and 3pm. This might be because the
catchment of the River Severn is very well vegetated, and therefore a lot of the rainfall would be intercepted by
plants or affected by stemflow. Both of which are reasonably slow processes and would increase the time taken
for the rainfall to reach the river channel.
The River Wye appears to mirror the rainfall pattern; as soon as it began raining the discharge of the River Wye
began to increase rapidly, both the rainfall and discharge peaking at 2am. This may be due to a number of
factors. Firstly, the relief of land is very steep in the River Wye’s catchment area, similar to the River Severn, the
land reaches a height of 670m, and the lowest point is approximately 390m. Therefore, the rainwater would reach
the channel very quickly. If the rainwater was infiltrated into the soil, the steepness of the land would ensure that it
still reached the channel very quickly. Unlike the River Severn, the River Wye is not vegetated, at least there are
no trees within this catchment area, and therefore none of the rainwater will be intercepted or affected by
stemflow. This would explain why the River Wye responds so quickly to the rainfall.
Therefore, it is clear from the diagram the although the two rivers responded very quickly to the storm, the River
Wye’s discharge levels seemed to mirror that of the pattern of rainfall, rising rapidly and peaking at the same time,
whereas the River Severn’s discharge level, rose more steadily.