Pebsham Landfill Stockpile Planning ... - East Sussex · Agency’s website indicates that the site lies in Flood Zone 1 (low probability of flood risk), see Drawing No. 1. The Environment

Pebsham Landfill Stockpile Planning Statement

APPENDIX 1

Surface Water Management Scheme and Flood Risk Assessment

Biffa Waste Services Limited

January 2008 SLR Ref: 407/0034/00262

Biffa Waste Services Limited 1 SLR Ref: 407.0034.00262 Appendix 1: SWMS & FRA January 2008

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CONTENTS

1.0 INTRODUCTION......................................................................................................... 1

1.2 Planning Policy Statement 25 (PPS25)................................................................ 1 2.0 SITE HYDROLOGY AND DRAINAGE CHARACTERISTICS.................................... 2

2.1 Potential Sources of Flooding – Level 1 Screening Study................................ 2 2.2 Application of PPS25 ............................................................................................ 2

3.0 SURFACE WATER MANAGEMENT.......................................................................... 3 3.1 General Arrangement............................................................................................ 3 3.2 Maintenance........................................................................................................... 3

4.0 SUMMARY AND CONCLUSIONS ............................................................................. 5

TABLES Table 1 Potential Risk Posed by Flooding Sources

DRAWINGS Drawing No. 1 Local Hydrology Drawing No. 2 Balancing Pond Design & Sections


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1.0 INTRODUCTION This Appendix presents a flood risk assessment and proposals for the management of incident rainfall and surface water runoff for the temporary development of soils stockpiles to the north of the existing Pebsham landfill. Although the topsoil and subsoil stockpiles will only be present for a period of up to four years it is recognised that without due regard to flood risk and surface water management they have the potential to have the following potential impacts on surface water resources: • locally increase the rate of surface water runoff. If the runoff is not controlled this could

increase the extent and magnitude of flooding on land adjacent to the landfill including Combe Haven Site of Special Scientific Interest (SSSI) to the north of the site; and

• impair the quality of surface water runoff from the site. Surface water runoff may contain

elevated concentrations of suspended solids, if not appropriately controlled. Planning Policy Statement 25 – Development and Flood Risk – identifies that any development constructed without due regard to flood risk can endanger life, damage property and require wasteful expenditure on remediation. The surface water management plan has therefore been developed to: • ensure that runoff is controlled at all stages of the site development and that the

drainage system has sufficient capacity to safely discharge surface runoff without impairment to the stability;

• ensure that there is no increase in flood risk either at the site or downstream of the site;

and • ensure that surface water discharged from the site is of a suitable quality. 1.2 Planning Policy Statement 25 (PPS25) PPS25 provides technical guidance in relation to development and flooding in England and sets out a precautionary framework to guide planning decisions. Development advice has been generated to determine when flood risk issues need to be taken into account in planning future development, based on the Environment Agency’s indicative floodplain maps.


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2.0 SITE HYDROLOGY AND DRAINAGE CHARACTERISTICS 2.1 Potential Sources of Flooding – Level 1 Screening Study The site is currently undeveloped. A summary of the potential sources of flooding and a review of the potential risk posed by each source at the application site is presented in Table 1.

Table 2 Potential Risk Posed by Flooding Sources

Potential Source Potential Flood Risk at Site? Justification

Fluvial flooding No Site shown by EA mapping to be in Flood Zone 1 (low probability flood risk)

Tidal flooding No Site shown by EA mapping to be in Flood Zone 1(low probability flood risk)

Flooding from rising / high groundwater No Proposed to develop stockpiles above

existing ground levels

Overland flow flooding No

Runoff from adjacent higher ground associated with Pebsham landfill is

collected by perimeter drains and routed to attenuation ponds

Flooding from artificial drainage systems No None identified

Flooding due to infrastructure failure No Site remote from infrastructure

The site lies within the catchment of the Combe Haven and review of the Environment Agency’s website indicates that the site lies in Flood Zone 1 (low probability of flood risk), see Drawing No. 1. The Environment Agency website confirms that the Combe Haven is defended along its length within the vicinity of the site. No other potential sources of flooding have been identified in the screening study. 2.2 Application of PPS25 PPS25 – Development and Flood Risk – records (Table D.1) that all uses of land are appropriate in Flood Zone 1. In addition, PPS25 states that for development proposals on sites comprising one hectare or above the vulnerability to flooding from other sources as well as from river and sea flooding, and the potential to increase flood risk elsewhere through the addition of hard surfaces and the effect of the new development on surface water runoff should be considered in the flood risk assessment. A consideration of sources of flooding is given above. Given the site setting and the proposed temporary duration of the stockpiles it is considered there is no need to assess flood risk to the proposed development area further. The following section details measures to manage surface water runoff from the stockpiles to ensure (a) the flood risk to third parties is not increased, and (b) the quality of runoff is managed as a result of development.


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3.0 SURFACE WATER MANAGEMENT 3.1 General Arrangement It is proposed to collect and manage runoff shed from the proposed stockpiles. It is proposed that a shallow lagoon is excavated to the immediate north of the stockpiles which will be used to collect runoff shed from the stockpiles. Water quality in the temporary lagoon would be subject to routine quality sampling and inspection. With agreement of the Environment Agency it is proposed that the water in the lagoon is discharged to either adjacent watercourses (as currently occurs) or is discharged by pump to the landfill perimeter drainage network. The lagoon would attenuate runoff and provide opportunity for the settlement of suspended solids prior to discharge. Discharge from the landfill perimeter drain network is made with existing Consent from the Environment Agency. The layout of the proposed surface water management scheme is shown on Drawing No. 2. Hydrological calculations have been undertaken using the WinDES suite of programs to size the proposed stockpile lagoon. Given the following design parameters: • catchment area: 3.6Ha; • standard percentage runoff: 70%; and • pump rating: 40l/sec. It is calculated that the stockpile lagoon should have a volumetric storage capacity of no less than 500m3 to contain all flows generated from a storm up to and including a 10% annual probability of occurrence (1 in 10 probability of occurrence in any year). Additional analysis, to assess potential discharge rates associated with lower probability events has shown the following peak discharge rates from the development area in addition to the 40l/sec removed by the pump: • 4% (25-yr) annual probability of occurrence: 48l/sec; • 2% (50-yr) annual probability of occurrence: 233l/sec; and • 1% (100-yr) annual probability of occurrence: 552l/sec It is proposed that an emergency spillway is constructed in the lagoon to ensure flows associated with these lower probability events do result in erosion. 3.2 Maintenance Appropriate routine maintenance of the proposed surface water management scheme will extend the effective life and overall efficiency of the scheme and reduce residual risk as required by PPS25. In the absence of good site practice and appropriate maintenance, the gradual accumulation of solids might reduce the capacity and effectiveness of the system. It is proposed therefore to incorporate a number of operational practices to ensure that the surface water system performs efficiently. These would include: • site operatives will routinely monitor the efficiency of the surface water scheme. The

accumulation of sediment within the perimeter ditches and lagoon will be checked and any obstructions (debris etc.) within the system will be removed. When necessary, sediment will be removed from the ditches and lagoon and disposed of appropriately; and


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• the construction and overall design of the scheme will be co-ordinated to prevent

interference with the site works. All aspects of the surface water management system would be constructed in accordance with appropriate Environment Agency consents (e.g. land drainage).


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4.0 SUMMARY AND CONCLUSIONS The Environment Agency’s Flood Risk Standing Advice for England (PPS25) Ver. 01, March 2007 details the requirements for developments in Flood Zone 1; the table below details the information required and where it is presented in this flood risk assessment. A: PLANS

Location Plan See Drawing No. 1 Existing Site Plan Figure 00262-1 of Planning Application

Development Proposal Plan Figure 00262-2 of Planning Application Identification of any Structures which may Influence Local

Hydraulics Not applicable – site is shown to be outside the floodplain

B: SURVEYS

Site Levels Related to Ordnance Datum (Existing & Proposed) See Drawing No. 1 and Drawing No. 2

C: ASSESSMENTS

Information detailing Current Surface Water Disposal Measures on Site and the State of Maintenance of these

Services None identified

An Assessment of the Volume of Surface Water Runoff Likely to be generated by the Proposed Development See Section 3.0

Proposals for SuDS with the Aim of not Increasing, and Where Practicable Reducing the Rate of Runoff from Site as

a Result of Development See Section 3.0

Estimate of how Climate Change Could Affect the Probability and Intensity of Flood Events Not applicable given short duration of development

Information About the Potential Sources of Flooding (e.g. rivers, sea, surface water runoff, sewers, groundwater,

artificial sources etc.) Surface water runoff is identified as most significant potential

source of flooding. See Section 2.0

Information on how Potential Sources of Flooding will be Managed Safely within the Proposed Development See Section 3.0

Consideration of the Proposal Relative to any Existing SFRA Not applicable – SFRA has not been published Confirmation of whether EA Consent is required for any

Aspect of the Work and if this has been Applied for See Section 4.0

Consideration of the ‘Dry Island’ Effect and how this may Affect Access/Exit to the site Not applicable

It is concluded that the proposed development with respect to duration, site setting and surface water management is appropriate to reduce flood risk to acceptable levels.

Pebsham Landfill Stockpile Planning ... - East Sussex · Agency’s website indicates that the site lies in Flood Zone 1 (low probability of flood risk), see Drawing No. 1. The Environment

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