Carnell

IEMA, Ireland Regional Event Are SUDS the Answer for Drainage? Dublin, 12th Dec 2007 “Filter Drains as a SuDS System” presented by Andrew Todd, Atkins Ltd on behalf of Carnell

Filter Drains CIRIA C697 The SuDS Manual (2007) Trenches Shallow Rubble / stone filled Temporary sub-surface storage Infiltration system Conveyance to downstream SuDS Component Introduction

Excavated trench 1-2m stone aggregate fill Upstream pre-treatment to remove –Sediments –Fine silts Infiltration remote from vulnerable groundwaters Remote from pollution risks Key Criteria

Advantages Infiltration: –Reduces run off rates –Reduction in pollutant load: High removal of suspended solids Low / medium nutrient removal High removal of heavy metals Fit within landscaping and at road sides

High clogging potential Not suitable for sites with fine particulate Build up of pollution difficult to see Historic failures due to: –Poor maintenance –Wrong siting –High debris Limited to small catchments High cost of replacing filter medium Disadvantages

Depends on the criteria used to determine sustainability Source control Water Quality Pollutant removal Sustainable construction Natural materials Sustainable?

Infiltration system –Suitable ground conditions –Remote from vulnerable groundwaters –Linear infiltration trench –Part of a conveyance system Attenuation –Increased flow path reduces velocity –Reduces peak discharge rate –Storage within the filter medium Sediment removal Source Control

Water Quality Sediment removal –“Does what it says on the tin” –Basic function of the filter drain is to filter –Traps sediments –Sediments contain pollutants

Pollutant Removal Pollutants in particular heavy metals attach to finer sediments –Lower velocity means less energy to carry sediment –Sediments attach to the filter medium –Pollutants retained Pollutants removed –Heavy metals Copper Zinc Cadmium Etc –PAH –Suspended Solids

Filter drains require a filter medium –Stone aggregate Quarried rock Crushed Beach shingle All transported by road –Alternative materials Crushed concrete Rubber shreds Glass? Blast furnace slag All these have potential contamination issues Sustainable Construction?

Quarried stone is certainly a natural material but can its use as a filter medium be justified as sustainable? Maybe Natural Materials

Road Drainage System The use of filter drains is not new Used as a road drainage system for many years UK Highways Agency has 1000’s kms on Trunk Roads and Motorways Not specifically a SuDS system

Drain the road surface Drain the road foundation Lower the ground water table Filter Drains perform all three Safety is the primary concern Rapid water removal is a key requirement –Function of stone grading –Road drainage specifies Type B –Type B is at least 80% in the range mm & a max of 5% below 10 mm SuDS is secondary as a road drainage function Road Drainage Function

Advantages Drains both surface and sub-surface Removes sediment Removes pollutants Minimal land take Provides some flow attenuation Disadvantages Perceived poor performance affecting the road structure Blocks with sediment Stones scattered from the surface Requires maintenance Road Drainage Concerns

Stone Filter Medium The one aspect that has to be addressed How to eliminate or reduce the reliance on stone filter medium –Alternative materials have issues It will inevitably become contaminated with sediment and block –Needs to be replaced More quarrying More vehicles Sustainable? –Recycle

Recycling Filter Medium Quarried Stone contaminated with sediment –Replace with new - not desirable –Remove off site clean and replace – still large number of vehicle movements –Remove, clean and replace in one operation Reuse of existing material –Prolong the drain life –Minimal import of new material –Minimal impact on the environment Sustainable Before Refurbishment After Refurbishment

Filter aggregate removed, cleaned and replaced in one operation Aggregate reuse extends drain life Minimises vehicle movements Recycling

Clean filter medium returned to trench in one operation Reduced vehicle movements Minimal used of new quarried product Sustainable Recycling

Safety Issues Stone scatter Vehicles sinking in to drain medium Safety Aspects of Road Drainage

Safety Geo-grid in the top layer of the filter medium: –Improve vehicle stability –Minimises stone scatter –Reduces maintenance Addresses primary road drainage issue.

Sustainable? Do Filter Drains provide Attenuation? Do Filter Drains improve Water Quality? Can Filter Drains provide Infiltration? Do Filter Drains use Natural Aggregate? Can the aggregate be recycled? Can the drain surface be stabilised? Filter Drains are a SuDS system Summary Yes

Any Questions