Project Description: Surface Water Management Plans for Landfill Sites

Project client: Augean

Introduction:

The site is licensed under PPC Regulations for co-disposal of waste in Cells 1 and 2 and hazardous waste in Cells 3, 4 and 5. The site is to be progressively capped and restored and a surface water management system has been designed to attenuate and improve the quality of runoff from the site.

Project Brief

To determine whether the site restoration proposals, incorporating surface water management design, pose an unacceptable risk and to show that mitigation measures and site procedures reduce that risk to an acceptable level.

Egniol were commissioned to design the surface water system and prepare a risk-based assessment for the issue of runoff into the environment. This was to involve the development of a Surface Water Management Plan monitoring protocol with associated reactive procedures to ensure compliance with the Permit.

Project Delivery

A survey of the sensitivity of the site was made and an assessment prepared on the source - pathway- receptor principle.

The site is underlain by limestone in places and the protection of groundwater from potentially contaminated runoff, as required under the Water Framework Directive, was an essential requirement. To provide this, attenuation lagoons were built and lined with clay placed under CQA procedures. Control measures were incorporated on the outlet using a penstock for isolating the lagoons and the outflow rate regulated using an orifice. Additional freeboard was built into lagoon sizing to diminish the risk of overtopping and overland routing.

Only one local watercourse was available to receive site flows; an agricultural ditch for surrounding arable land that also received road runoff from the public highway. To avoid flooding in times of severe storm, a catchment model of the local hydrology was prepared in agreement with the Environment Agency and the site runoff incorporated into it for a series of return periods and durations up to a 1% event ( 1 in 100 years). The site controls were configured to comply with these criteria.

A hydraulic model was produced for the landfill site using Micro Drainage software based on the Wallingford Procedure. The model incorporated the final settlement profile, as consented for planning, and was tested also against post restoration contours. A worst case summer and winter rainfall duration was identified for the 1% probability event and the surface water drainage system designed for it.

It was originally intended to have two outfalls, one to the north and one to the south but the risk to groundwater was deemed too great and only the southern outfall was utilised. For the short term, a pumping station was designed, with storage requirements and pumping rate ascertained in the hydraulic model.

The surface water management plan was prepared to illustrate the site characteristics, the design of the management system, the baseline and continuing monitoring and the reactive procedures in the unlikely event of non-compliance with the permit.