IEMA -“Are Suds the Answer for Drainage?”

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

Introduction • 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

Principle • “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

Suds and Road Drainage • 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 Typical Filter Drain Typical Attenuation Pond

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

GSI \ EPA \ DOE • Groundwater Protection Responses matrix • Available for each activity or group of activities • Landfills • Organic Land spreading • Single Houses DOE, EPA, GSI, (1999). Groundwater protection schemes • No Protection Response matrix for roads

Implementation of Procedure • 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

Risk from Accidental Spillage • 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

Routine Runoff risk scoring • For routine runoff the factors attributed to the risk from road runoff are attributed a risk weighting – HA 216 / 06 Source Pathway Degree of Risk X Weighting = Risk Score

Criteria 1 – Traffic Density • 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 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

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

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

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

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

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

Application • 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 X Weighting = Risk Score

Additional Criteria • Positive Hydrostatic pressure and the path of least resistance • Public drinking water supply / Source Protection • Karst Features

Positive Hydrostatic Pressure • 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 Road Runoff Water table Positive Hydrostatic Pressure

Public Drinking water supply • 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)

Karst Features • Assessment of vicinity to works from GSI Groundwater Vulnerability mapping • Assessment of specific features from available Ground Investigation information

Procedure Flow chart REGIONALLY IMPORTANT AQUIFER Point of Discharge (Base) Drain / Ditch / Filter Drain Conventional Drainage GSI Assessment Low / Moderate Vulnerability < 3m Low k Subsoil From BH Logs / EW-MLA series Continuous Hydrostatic Pressure Competent Rock (RQD > 40%) and confirmed by on site inspection Yes Conventional Drainage Additional Criteria Conventional Drainage No Distance from Karst, Sinkhole, Fault HA216/06 Method Low Risk Score < 150 Conventional Drainage Sealed Drainage High Risk Score > 250 YES Conventional Drainage Distance from Public / Private Water Supply 2m of Low k subsoil ** below drainage level above aquifer Medium Risk Score 150 - 250 Condition Satisfied? Sealed Drainage Sealed Drainage NO From BH Logs / EW-MLA series Lined Filter Drain Lined Interceptor Drain * Sealed Drainage • May consist of compacted clay • base with bentonite mix ** 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)

Practical Example • Initial Assessment rock & gravels within 3m of drainage  Sealed Drainage required • Secondary Assessment groundwater level between 0.9m & 2.5m depth below drainage level therefore no continuous hydrostatic pressure • Risk Assessment Score = 190 Medium Risk, examine material over aquifer – insufficient buffer  SEALED

References • 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

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 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.

IEMA -“Are Suds the Answer for Drainage?” Thank You Geological Survey of Ireland, 12th December 2007

IEMA -“Are Suds the Answer for Drainage?”