Flow survey for housing development

Detectronic were commissioned to undertake a flow survey within two neighbouring residential housing developments in Yorkshire, UK.

The monitoring catchment area features a housing development which is bordered by a main road to the east. There is a commercial development to the north and a recreation ground to the west. To the south of the catchment is a second housing development owned by another developer.

A flow survey was required to record foul water flows from the industrial development alongside flows from both housing developments to better understand the hydrology of the whole area with a view to preventing flash flooding and pollutions.

Objectives

To gather robust, highly accurate data for a level data comparison
To gather robust, highly accurate data for a flow data comparison
To gain a clear insight into suspected rainfall induced surcharging
To measure I&I across the catchment

Planning and implementation

Following several site visits, a bespoke flow survey plan was drawn up and agreed. Detectronic site engineers then installed a total of six flow monitors at strategic points across the development. Rainfall was recorded using a 0.2mm remote monitoring rain gauge positioned on the roof of an electricity substation in the vicinity.

Hydraulic flow data was calculated using the continuity equation for partially filled pipes. Depth data obtained from a hydrostatic pressure sensor was used with the pipe diameter to calculate the wetted cross-sectional area of the hydraulic flow. When multiplied by the velocity data (obtained from a wetted Doppler velocity sensor located at the invert of the channel) the data analysis team was able to calculate flow rate.

Results

Initially planned as a 4 week flow survey, but extended due to very dry weather, data was gathered from each of the six Detectronic flow monitors. The data illustrated clear evidence of surcharging across the catchment, and this was particularly evident in the northern half of the development which is at a lower elevation. Periods of surcharging are consistent with two distinct mechanisms:

Rainfall results in surface water flows entering the foul water drainage system. This was evident in the pattern of surcharging across the catchment in October and November which coincided with shorter periods of intense rainfall. Levels peak quickly in response to rainfall however, with a lack of prolonged rainfall levels begin to recover quickly.
Prolonged rainfall results in rising groundwaters which enter the foul water drainage system, through joints within pipework and inspection chambers. This was evident in longer periods of surcharge witnessed in December and January. Levels peak quickly in response to rainfall however recover much more slowly, or remain high for prolonged periods as rainfall continues to feed groundwaters.

Even though both housing developments are of a similar size, the average daily volumetric flow from one development was significantly higher than volumetric flow from the other. Data from the monitor located in the northern boundary of development one recorded surface and infiltration flows from the commercial development. These contributed to recorded flows passing into the pumping station.

The strong correlation in the hydraulic patterns of the locations serving the one of the housing development indicate a hydraulic issue at the pumping station downstream. From the data, we can identify the flow rates at the point they begin to surcharge (i.e. they exceed the pipe diameter). This occurs on several occasions during the survey period with flow volumes ranging from 5.044 l/s to 7.193 l/s. From this, we can conclude that the pumping station struggles to accommodate flows in excess of 5 l/s.
Simon Hardacre, Detectronic data analyst