Planning Applications

Flooding Report - Effects of draining the surface water from the fields into the river

Effects of draining the surface water from the fields into the river

Note the Environment Agency records its flow values in m3/sec so previous m3/hr values have been divided by 3600 for uniformity.

The River Blackwater has a number of monitoring points along its route designed to give early warning of flooding to residents and businesses. The two local monitoring points are at Tongham and at Farnborough.

As stated earlier the developer’s FRA claims that the existing greenfield runoff figures (desk based calculation) are between 44 and 48% (claimed in section 5.11 table 2 of the developer’s FRA). This represents at the 100 year return period rate a total of 8,851m3/hr (2.46m3/sec) being runoff (at 48% runoff) from the total 46.1ha site in 1 hour.

To illustrate the effect of disposing of this additional water, telemetry values from both stations have kindly been provided by the Environment Agency for 15 minute intervals for the July 2007 period. These are available to view on a separate spreadsheet.

Tongham gauging station has a normal level for the river of 0.56 – 1.45 metres above which flooding is likely. (Highest was 1.79 on 20th July 2007).

Farnborough gauging station has a normal level for the river of 0.36 – 1.30 metres above which flooding is likely. (Highest was 1.94 on 14th August 2006).

From the telemetry data for 20th July 2007 it shows that the flow rate at Farnborough at the start of flooding was 1.25m3/sec at 09.45 am with a peak of 2.26m3/sec being reached.

The telemetry shows that an increase of flow during the initial flooding stage from 1.3m3/sec to 2.23m3/sec caused an increase in level of 0.32metres at Farnborough and 0.34 metres at Tongham.

Thus an increase in flow of only 0.93m3/sec caused an increase of the flood depth of over 300mm.

The developers claimed runoff of 2.46m3/sec can now clearly be seen to be false as this rate by itself is virtually double the River Blackwater critical flowrate, without taking into account all the other flows into the river from the Thames Water surface water drainage system.