Carnell

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Carnell
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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
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Introduction

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

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Key Criteria

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

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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

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Disadvantages

• 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

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Sustainable? 

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

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Source Control

• 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

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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

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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

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Sustainable Construction?

• 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

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Natural Materials

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

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Road Drainage System

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

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Road Drainage Function

• 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 20-40 mm a
  max of 5 below 10 mm
• SuDS is secondary as a road drainage function

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Road Drainage Concerns

• 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

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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

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Recycling Filter Medium
Before Refurbishment

• 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

After Refurbishment
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Recycling

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

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Recycling

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

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Safety Aspects of Road Drainage

• Safety Issues
• Stone scatter
• Vehicles sinking in to drain medium

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Safety

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

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Summary

• 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

Yes Yes Yes Yes Yes Yes
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Any Questions