Irlam, Boysnope - HWRC drainage 27th February 2008
• A new household waste recycling site (HWRC) is to be constructed on
a brownfield site located east of Cadishead Way and south of the River
Irwell (Old Course).
Existing Drainage Arrangement
• There is no existing drainage infrastructure within the site boundary.
The site generally falls towards the Manchester Ship Canal. It is
assumed that the site drains naturally through infiltration and surface
water run-off discharging to the canal.
• The proposed drainage is shown on Drawing LE10125/012
(GWMDA_DGP_EE_1323_P1). It is proposed to construct new
drainage for the whole site to comply with current standards. A welfare
building is to be constructed and a foul drainage connection will be
provided as indicated on the drawing.
• Surface and foul water is to be pumped from the proposed site,
beneath Cadishead Way and Liverpool Road, to the UU foul water
manhole located adjacent to the residential area to the west of the site.
• Drainage has been designed as follows:
o Initial design for 1 in 5 year storm event
o Checked to ensure no flooding occurs within the site for the 1 in
30 year storm event in accordance with Sewers for Adoption 6th
Edition (please note that internal sewers will not be offered for
adoption). Drainage amended as necessary to achieve this.
o Checked to ensure no floodwater spills beyond the site
boundary for a 1 in 100 year storm event (+10% allowance for
climate change based on 30 year design life) in accordance with
PPS25. Drainage amended as necessary to achieve this.
Microdrainage calculations are enclosed to demonstrate the above.
• As the proposed site topography falls towards the public highway all
flood water is to be attenuated underground. Underground tanks
located within the highway will be used to store the storm water.
• The proposed surface water drainage system also includes
surcharging of the upstream network to optimise the downstream
Revision P1 1
• Consideration was given to methods of disposal of runoff. Discharge to
existing ground via soakaway is not practical due to ground conditions.
The made ground does not lend itself to infiltration techniques.
• Discharge to the River Irwell or Liverpool ship canal is not practical as
only approximately 20% of the site can be discharged without trade
effluent consent. Therefore attenuation was provided to store excess
water and discharge to sewer at a reduced rate.
• Other methods of storm water storage were considered but were not
feasible due to the volume of storage required. In accordance with the
design principles detailed above 450 cubic metres of downstream
storage is required.
• Due to the fact there is no existing combined or foul sewer in the
immediate vicinity of the site it is necessary to pump off site. A
package pumping station and a rising main are proposed to be
constructed to pump foul and surface water flow west approximately
100 metres to the UU adoptable foul manhole located adjacent to the
residential area to the west. The proposed discharge rate is 6l/s.
• A Class 2 by-pass separator has been provided for the surface water
system in accordance with PPG3.
• The foul infrastructure as been designed to accommodate flows
generated from the welfare building which are less than 1.0l/s. It is
proposed to discharge the foul water direct to the pre-packaged
pumping station located downstream of the surface water attenuation
tanks and hydro-brake manhole.
• A hydro-brake manhole with flap valve located downstream of the of
the attenuation tanks controls the discharge rate to the pre-package
pumping station to 6l/s. The flap valve will prevent foul water that may
surcharge the surface water pipe from the pumping station, from
entering the hydro-brake manhole and attenuation tanks.
Proposed Discharge Rates
• The discharge to the 150mm diameter foul sewer has been proposed
at 6 litres/second. The trade effluent consent discharge rate has been
submitted to UU.
Revision P1 2