Priority Outfall for Catchment Management of Water Quality
Case reference EIR2026/00290
Received 24 February 2026
Published 27 May 2026
Request
I am a member of the Pillhill Brook Association who monitor water quality at 9 sites along the Pillhill Brook which is a rare and precious chalk stream. The Pillhill Brook is a head water of the River Test and we work closely with the Test & Itchen Catchment Partnership a collaborative, community-led group working to improve the water environment of the Rivers Test and Itchen in Hampshire. The PBA recently leant that there is a National Highways priority outfall for catchment management of water quality just south of the A303 flyover by Mullens Pond (Easting 429,644.19, Northing 145,672.27). Interrogating the https://theriverstrust.maps.arcgis.com I have been able to ascertain that the outfall has 3 FID numbers associated with it: FID 18967, 9064 & 6911. Not being familiar with your outfalls I am writing to you to request the following:
1. the purpose of this outfall and the area that it services?
2. why does it have 3 FID numbers?
3. construction drawings that show how the road drainage is connected to this outfall
4. how the outfall is designed to work i.e. what sediment traps/treatment is performed on run-off before it enters the brook?
Response
1. The purpose of this outfall and the area that it services?
The National Highways drainage assets do not discharge to the Pillhill Brook, they drain to soakaways. There is a ditch between Westfield and the A303, which appears to show an outfall on our Geotechnical and Drainage Management System (GDMS) database but it is not confirmed on the as-built drawing, which shows a soakaway trench.
2. Why does it have 3 FID numbers?
The three flood event identification numbers (FID) are not on the A303 at Pillhill Brook. FID 18967 is on the M5, FID 9064 is on the A1 and FID 6911 is on the M62. FID 6039 (2008) is historic and not assessed. We have checked our records and found the following FID’s at this location, FID 6316 (2010) is on the eastbound carriageway for blocked gullies drain to soakaway. FID 29620 (2019) and FID 29621 (2020) are on the westbound off slip road were sent in by members of the public using Fix-My-Street (FMS) but were not confirmed because on inspection nothing was found.
3. construction drawings that show how the road drainage is connected to this outfall
Please see as-built drawing attached.
4. how the outfall is designed to work i.e. what sediment traps/treatment is performed on run-off before it enters the brook?
Soakaways rely on ground infiltration, not direct discharge to the brook. The ditch ends at a soakage trench and has no outfall to the brook. There is no road run-off into this ditch.
Documents
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