First Flush Event 28th August 2003

On 28th August 12.5mm of rain fell on the roads and other impermeable surfaces in the Bourne stream catchment. 

The past 12 days had been dry, hence rainfall on 28th August represented a 'first flush' event. 

Graphs below illustrate SUDS performance:

1. During a 'first flush' event, and

2. The effect of heavy rainfall on stream water quality

See enlarged plan to locate the three SUDS sampling points referred to

1.  SUDS performance during a 'first flush event'

The charts below illustrate how the two SUDS (Phase II the wetland, Phase I the lagoon system) acted to treat these water, reducing bacterial contamination, settling out the suspended solids that many pollutants will attach themselves to, and - in the case of the lagoons, reducing the Biological Oxygen Demand (BOD), all of which improve the quality of water going downstream.

2.  Illustrating the effect of heavy rainfall on stream water quality

The graphs below compare the vast difference in the levels of pollutants washed into the stream from surrounding roads and other impermeable surfaces (roofs, forecourts, etc).  Water samples were taken at the uppermost reaches of the stream close to Ringwood Road (U/S of SUDS Phase II).

20th August (a dry day) compared with 28th August (12.5mm rainfall)