IEMA -“Are Suds the Answer for Drainage?”

Risk Based Approach

to the Impact of

Road Drainage

on

Hydrogeology

Geological Survey of Ireland, 12th December 2007

• Principle• Suds and Road Drainage• GSI \ EPA \ DOE Guidance• Implementation of procedure

– Risk from accidental spillage

– Routine runoff Assessment Criteria

– Application of the Assessment Criteria

• Additional Criteria• Procedure Flow Chart• Practical Example

Introduction

• “Utilise concept of aquifer vulnerability and source protection to assist the initial assessment of all proposed developments upon the groundwater environment, in the context of roads”

– Source - Road

– Path - Substrate

– Receptor - Aquifer Vulnerability

Principle

• Suds – Components commonly used in road drainage– Components commonly used in

Road Drainage• Filter Drains• Swales / Open Channels• Attenuation ponds• Infiltration basins• Soakaways

– ‘Sustainable’

– Offers choice of open and closed systems

Suds and Road Drainage

1. ALL DI ME NS IONS A RE I N MI LLIM ET RE S UN LE SS I ND IC AT ED OTH ERWISE.

2. ALL DI ME NS IONS A RE T YP IC AL, RE FE R TO A PP EN DI X 1/ 5 FOR AC TU AL D IMENSIONS.

3. WH ER E POSS IB LE T HE LEN GT H TO B RE AT H RA TI O SH OU LD B E 1: 4 OR GRE ATER.

4. WH ER E RE QU IR ED P ON D TO B E LI NE D WI TH 300mm OF IM PE RM EA BLE CLAY T O 300m m AB OV E DE PT H OF I NT ER CE PT OR VOLUME

5. Ø RE FE RS T O DI AM ET ER OF OU TLET PIPE.

6. * ALL DI ME NS IONS I ND IC AT IV E. S EE T EC HN IC AL P AP ER F OR A CT UA L DI MENSIONS

7. WH ER E IT I S RE QU IR ED T O LI NE T HE P ON D AN I MP ER ME AB LE C LA Y LI NE R SH ALL BE LAI D 300m m THICK TO

A H EI GH T 500m m AB OV E TH E OU TLET LEVEL.

8. UN LE SS OTH ER WI SE I ND IC AT ED A LL P ON DS S HA LL B E SE CU RI TY F EN CE D TO R CD /300/1 A ND P ROVI DE D WI TH A LOC KE D GATE

T O RC D/ 300/ 8 TY PE 2.

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T itle:

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R PS SC D 572

DO NOT SCALE USE FIGURED DIMENSIONS ONLY

By App.

R PS C on su lt in g En gi neers, W est Pi er B usin ess Campus,

D un L ao gh ai re, Co . Du bl in , Ireland.

T : +353 1 288 4499 - F : +353 1 283 5676

E : i rel and@ rps group. c om W : w ww.rpsgroup.com/ireland

SPECIFIC CONSTRUCTIONDETAIL

A01 27/09 AF MF

TYPICAL ATTENUATIONPOND DETAIL. A02 23/10 AF MF

C01 26/11 AF MF

Typical Filter Drain

Typical Attenuation Pond

• Risk and Risk Management– Hazard

• Potential of contaminant loading

– Vulnerability• Likelihood of contamination if a contaminant event occurs

– Consequences• Depends on the ‘Value’ of the groundwater

GSI \ EPA \ DOE

DOE, EPA, GSI, (1999). Groundwater protection schemes

• Groundwater Protection Responses matrix– Available for each activity or group of activities

• Landfills

• Organic Land spreading

• Single Houses

GSI \ EPA \ DOE

• No Protection Response matrix for roads

DOE, EPA, GSI, (1999). Groundwater protection schemes

• To classify the risk arising from Road Runoff, a procedure for assessing the risk from runoff was implemented

– Risk of accidental spillage assessed

– Method of scoring Risk associated with road runoff• Risk from routine runoff• Risk scoring

– Additional Criteria assessed• Positive Hydrostatic pressure• Public water supply• Karst Features

– Flow chart

– Practical Example

Implementation of Procedure

• Assessment of the risk of an acute pollution impact - HA 216 / 06– Risk expressed as annual probability– Guideline probability > 1% acceptable

• May be less for SAC’s etc.

– Inputs• RL = Length of road• AADT = Annual Average Traffic Daily Flows• % HGV = percentage of Heavy Goods Vehicles• SS = Spillage rates – UK data

• Ppol = Probability for a given accident of serious pollution occurring – UK data

– Output• Pacc= Probability of a spillage

• Pacc = RL x SS x ( AADT x 365 x 10-9)x(%HGV / 100)

• Pinc = The probability of a spillage accident with an associated risk of a serious pollution incident occurring

• Pinc = Pacc x Pol

Risk from Accidental Spillage

• For routine runoff the factors attributed to the risk from road runoff are attributed a risk weighting – HA 216 / 06

Routine Runoff risk scoring

Component Weighting

Traffic 15

Rainfall 15

Soakaway Geometry

15

Unsaturated Zone

20

Flow type 20

Effective Grain Size

7.5

Lithology 7.5

Degree of Risk X Weighting = Risk Score

Source

Pathway

• It is believed that heavy travelled roads such as motorways and multilane highways (AADT >30,000) produce higher concentration of pollutants than roads located in rural areas (Barrett et al, 1998)

• Therefore for this assessment the risk associated with runoff has been categorised into three levels:

• Low Risk AADT < 15,000

• Medium Risk AADT 15,000 – 50,000

• High Risk AADT > 50,000

Criteria 1 – Traffic Density

Component Weighting

Traffic 15

Rainfall 15

Soakaway Geometry

15

Unsaturated Zone

20

Flow type 20

Effective Grain Size

7.5

Lithology 7.5

Criteria 2 – Rainfall

• The larger the rainfall, the larger the runoff however the longer the antecedent period, the more pollutant, therefore:

• Two rainfall components:– Volume

• Low Risk < 740mm

• Medium Risk 740-1060mm

• High Risk > 1060mm

– Intensity in one hour for 1 in 100 year• Low Risk < 35mm

• Medium Risk 35-47mm

• High Risk > 47mm

Component Weighting

Traffic 15

Rainfall 15

Soakaway Geometry

15

Unsaturated Zone

20

Flow type 20

Effective Grain Size

7.5

Lithology 7.5

Criteria 3 – Soakaway Geometry

• Applies to soakaways and linear draina gefeatures such as filter drains and open ditches

• Risk depends on the directness of path and distribution of pollutant

• Therefore soakaway geometry categories:

• Low Risk Linear feature• Medium Risk Shallow Soakaway• High Risk Single point serving

high road area

Component Weighting

Traffic 15

Rainfall 15

Soakaway Geometry

15

Unsaturated Zone

20

Flow type 20

Effective Grain Size

7.5

Lithology 7.5

Criteria 4 – Unsaturated Zone

• Considerable depths to water table can allow for attenuation of the pollutant

• Therefore the risk associated with depth of unsaturated zone has been assessed as depth to water table of:

• Low Risk >15m

• Medium Risk 15 – 5m

• High Risk < 5m

Component Weighting

Traffic 15

Rainfall 15

Soakaway Geometry

15

Unsaturated Zone

20

Flow type 20

Effective Grain Size

7.5

Lithology 7.5

Criteria 5 – Flow type

• Intergranular flow offers maximum opportunity for beneficial interaction between migrating fluids where as fissures by there definition offer direct paths to the water table

• Therefore for this assessments the risk associated with Flow type has been classified as

• Low Risk - Unconsolidated or Non fractured consolidated flow

• Medium Risk - Consolidated deposits

• High Risk - Heavily consolidated deposits, igneous and metamorphic rocks

Component Weighting

Traffic 15

Rainfall 15

Soakaway Geometry

15

Unsaturated Zone

20

Flow type 20

Effective Grain Size

7.5

Lithology 7.5

Criteria 6 – Effective Grain Size

• Finer materials provide the greatest moisture storage and the longest delay in migration from the surface to the water table.

• Therefore the risk has been classified by the effective grain size encountered:

• Low Risk Fine sand & below• Medium Risk Coarse sand• High Risk Very Coarse sand

& above

Component Weighting

Traffic 15

Rainfall 15

Soakaway Geometry

15

Unsaturated Zone

20

Flow type 20

Effective Grain Size

7.5

Lithology 7.5

Criteria 7 – Lithology

• Significant clay minerals and organic content offer increased potential for beneficial attenuation

• Therefore the risk associated with runoff has been categorised into three levels

• Low Risk > 15% Clay minerals

• Medium Risk 15%<Clay minerals>1%

• High Risk < 1% Clay minerals

Component Weighting

Traffic 15

Rainfall 15

Soakaway Geometry

15

Unsaturated Zone

20

Flow type 20

Effective Grain Size

7.5

Lithology 7.5

• Using the above stated criteria and testing regime, a risk score is establish based on weighting and score– Low 1 – Medium 2– High 3

• Taking the final Risk Score– Low Risk of Impact < 150 – Medium Risk of Impact 150 – 250 – High Risk of Impact > 250

• Using this rational: – Low risk = minimal mitigation required– Medium risk = further consideration of particular situation is required– High risk = consider sealed system

Application

X Weighting = Risk Score

• Positive Hydrostatic pressure and the path of least resistance

• Public drinking water supply / Source Protection

• Karst Features

Additional Criteria

• Occurs where ground water is naturally above level of pathway

• Where the direction of flow is into the receiving pathway, the risk of groundwater pollution is naturally mitigated as pollutant follows path of least resistance

Positive Hydrostatic Pressure

Water table

Positive Hydrostatic Pressure

Road Runoff

• Source Protection Zones

– Each locations requires case by case examination of the location of road run off in relation to ground water extraction point.

– Guidance:• Inner Protection Area

– 300m fixed radius (GSI)– 50 day travel time, minimum 50m radius (HA216/06)

• Outer Protection Area– Outer protection Zone 1000m (GSI)– 400 day travel time (HA216/06)

Public Drinking water supply

• Assessment of vicinity to works from GSI Groundwater Vulnerability mapping

• Assessment of specific features from available Ground Investigation information

Karst Features

Procedure Flow chart

REGIONALLY IMPORTANT

AQUIFER

Point of Discharge (Base)

Drain / Ditch / Filter Drain

GSI Assessment

Low / Moderate Vulnerability

< 3m Low k Subsoil

ContinuousHydrostatic

Pressure

Conventional Drainage

Conventional Drainage

HA216/06 Method

Low Risk Score <

150

Conventional Drainage

High Risk Score >

250

Sealed Drainage

Lined Interceptor

Drain *

Lined Filter Drain

• May consist of compacted clay •base with bentonite mix

Medium Risk Score 150 -

250

2m of Low k subsoil ** below drainage level above aquifer

Sealed Drainage

YES

NO

Condition Satisfied?

Distance from

Karst,Sinkhole,

FaultSealed

Drainage

Conventional Drainage

Distance from

Public / Private Water

Supply

Sealed Drainage

Conventional Drainage

Additional CriteriaYes

No

** to be confirmed on site by BRE Digest 365 or similar approved

every 250m or as agreed with the DSR (Low k < 10-5m/s Lambe & Whitman,

1979)

Competent Rock (RQD > 40%) and confirmed by on site inspection

From BH Logs / EW-MLA series

From BH Logs / EW-MLA series

• Initial Assessment rock & gravels within 3m of

drainage

Sealed Drainage required

Practical Example

• Secondary Assessment groundwater level between 0.9m &

2.5m depthbelow drainage level therefore no

continuous hydrostatic pressure

• Risk Assessment Score = 190 Medium Risk, examine material over aquifer – insufficient buffer SEALED

Component Element Rating Weighting Factor

Score

1 Traffic Density

1 15 15

2 Rainfall 2 15 303 Soakaway

Geometry1 15 15

4 Unsaturated Zone

3 20 60

5 Flow Type 2 20 406 Effective

Grain Size2 7.5 15

7 Lithology 2 7.5 15190Overall Risk Score

• DMRB-UK,9 (2006). Design Manual for roads and bridges: Enviromental Assesment Vol11. Sec. 3 Environmental Assessment Techniques Part 10 (HA 216/06)

• DMRB-NRA (1996). Design Manual for roads and bridges: Geotechnics and drainage Vol4. Sec. 3. Part 3 HD 33/96 (NRA Erratum June 2001)

• DOE, EPA, GSI, (1999). Groundwater protection schemes

• M. Breun, P.Johnston, M.K.Quinn et al.(2006), Impact of Assessment of highway Drainage on surface water quality

• Ciria (2007). The Suds Manual, CIRIA Report C697

• TW Lambe, RV Whitman (1979), Soil mechanics, John Wiley & Sons

References

This paper is presented as a concept paper and due diligence should be exercised when addressing any issues contained within this presentation. Expert opinion should always be sought. Although every effort has been made to ensure that the accuracy of the material contained in this presentation, complete accuracy cannot be guaranteed. RPS Consulting Engineers accept no responsibility whatsoever for loss or damage occasioned or claimed to have been occasioned, in part or full, as a consequence of an person acting or refraining from action, as a result of a matter contained in this presentation.

Summary

• Risk Based Approach to the Impact of Road Drainage on Hydrogeology

– Suds and Road Drainage

– GSI \ EPA \ DOE Procedure

– Implementation of procedure• Risk from accidental spillage

• Routine runoff Assessment Criteria

• Application of the Assessment Criteria

– Additional Criteria

IEMA -“Are Suds the Answer for Drainage?”

Thank You

Geological Survey of Ireland, 12th December 2007