Risk Assessment with continuous data

John Naylor

Unit 4

Risk Assessment 1 Risk Assessment?

2 Conceptual Models 3 Generic Assessment

4 1D Modelling 5 Continuous Data Interpretation

6 Selection of Protection

Risk Assessment?

Click to edit Master title style Risk Assessment What is risk?

• Tolerability of risk • Voluntary & Involuntary • Loss to Life, Property,

Environment... • Damage to Life, Property,

Environment... • Perception

RISK = HAZARD x CONSEQUENCE

Click to edit Master title style Current Risk Meaning (CIRIA C665) Very high risk There is a high probability that severe harm could arise to a designated

receptor from an identified hazard, or there is evidence that severe harm to a designated receptor is currently happening. This risk, if realised, is likely to result in a substantial liability. Urgent investigation (if not undertaken already) and remediation are likely to be required.

High risk Harm is likely to arise to a designated receptor from an identified hazard. Realisation of the risk is likely to present a substantial liability. Urgent investigation (if not undertaken already) is required and remedial works may be necessary in the short-term and are likely over the longer-term.

Moderate risk It is possible that harm could arise to a designated receptor from an identified hazard. However, it is either relatively unlikely that any such harm would be severe, or if any harm were to occur it is more likely that the harm would be relatively mild. Investigation (if not already undertaken) is normally required to clarify the risk and to determine the potential liability. Some remedial works may be required in the long-term.

Low risk It is possible that harm could arise to a designated receptor from an identified hazard, but it is likely that this harm, if realised, would at worst normally be mild.

Very low risk There is a low possibility that harm could arise to a receptor. In the event of such harm being realised it is not likely to be severe.

Assessor Walkover

Desk Study

Initial Conceptual

Model

Investigation Monitoring

Generic Assessment

Quantitative Assessment

Revised Conceptual

Model

Risk Assessment Tool Kit for Continuous Monitoring

Qualitative • Conceptual Model • Hazard Assessment • Pollutant Linkage

Discussion • Prediction

Quantitative • Gas Screening Value • Basic Calculation • Prediction

Detailed Qualitative • Pollutant Linkage

Detailed Analysis • Ground Gas Behaviour • Prediction

Detailed/Semi Quantitative • 1D Modelling • Sensitivity Analysis • Continuous data

assessment • Prediction • Setting Design Criteria

Click to edit Master title style Future Influences All gas risk assessment needs to include

prediction

The better the data, the better your understanding, the better the confidence...

The better to base prediction, the less conservative you need to be

Click to edit Master title style Risk Assessment • Use available information to establish or discount

pollution linkages – Source gas discussion and generation potential (zoning) – Physical pathway consideration (including migration) – Receptor investigation – Temporal changes (short and long term)

• Justification – All data requires interpretation to place it in context – Robust justification required

• Uncertainty – With many sites we may not be 100% sure on pollution linkages – Assessment should acknowledge such uncertainty and make

recommendations and/or allowances

Conceptual Models

Click to edit Master title style Preliminary Conceptual Model In order to determine where and how to monitor, a preliminary conceptual model should be prepared prior to the investigation. This requires identification of: - potential sources of hazardous gases based on a review of the current and

previous uses of the site and neighbouring land, and the underlying natural and man-made geology and hydrogeology;

- potential human and other receptors (e.g. buildings and structures, flora, the atmosphere, ground water and surface water) both on-site and off-site;

- credible pathways of possible exposure of the receptors taking into account what is known about the geology and hydrogeology, building construction and services layout, etc;

- forseeable events such as flooding, changes in groundwater level, global warming, extreme weather conditions, the closure of mines, and possible changes to the gas regime caused by future development.

Draft BS8576

Click to edit Master title style Cross Section Conceptual Model

Draft BS8576 requires cross sections for migration of permanent gases

Click to edit Master title style Revised Conceptual Model • Confirm or discount identified potential pollution

linkages • Site data with risk assessment to verify

conceptual model • Robust justification and identify assumptions or

ambiguity (evidence and sensitivity analysis) • Provide enough detail to enable Options

Appraisal and to allow for most design scenarios

Generic Assessment

Click to edit Master title style Generic Quantitative RA • Gas Screening Value (GSV) • Broad brush • Commonly used • Conservative Screening Assessment* • Several key assumptions in the calculations • Data used and assumptions differ slightly

between authors • New build on gassing ground

Gas Screening Value (GSV)

• GSV – designed for use on a gassing source

•Inherent uncertainty is covered by high FoS

Background: A generic risk screening test. GSV = borehole flow rate (l/h) x maximum concentration (%) A)GSV is compared against published figures

for Characteristic Situations (CS1 to CS6)

B)Characteristic Situation determines the level of protection required

Click to edit Master title style Gas Screen Value (GSV) (aka hazardous gas flow rate BS8485)

Chg – Concentration of a specific hazardous gas expressed as a percentage of total gas volume (%v/v)

q – Total gas flow from a borehole in litres per hour (l/hr) Qhg - Calculated flow rate of a specific hazardous gas from a

borehole reading

GSV = Concentration/100 x flow or

Click to edit Master title style GSV Input Values • Worst Credible Calculating the highest GSV from each monitoring round and using the

maximum values obtained

• Worst Possible

Calculate the GSV from the highest concentration and highest flow recorded during all monitoring rounds

• Most Realistic ?

Calculating the GSV using concentration data from continuous monitoring?

But, make sure you know what the dominant processes are!

Click to edit Master title style CIRIA Situation A

Modified Wilson & Card and BS8485 • Holistic approach • Gas source is on the site (does not consider lateral

migration) • Uses concentration and flow rate • Back analysis of measurements from underfloor voids • A black box – do not use the calculation for anything other

than intended • Includes trigger values • GSV determines Characteristic Situation and protection

requirements

Click to edit Master title style CIRIA Situation B

NHBC Traffic Lights System • Gas source is on the site (does not consider lateral migration) • GSV calculated separately for CH4 & CO2 (initial & steady flow) • Uses the Pecksen correction method • Minimum 150mm void and floor plan of 8m x 8m • Small room dimensions 1.5m x 1.5m x 2.5m • One complete volume change per day • 10% room volume contributed leak through sub-floor • Max 2.5% v/v CH4 equilibrium volume in void • Max 0.5% v/v CO2 equilibrium volume in small room • GSV determines Traffic Light and protection requirements • Includes trigger values

Click to edit Master title style Comparisons of Methods

Situation A Classification (inc BS8485)

Characteristic Situation (everything not covered

by Situation B)

GSV

CH4 or CO2

Situation B Classification

Traffic Light

(low rise residential with 150mm void)

GSV

CH4

GSV

CO2

1 <0.07 (<1 CH4, <5 CO2)

Green <0.16 (<1 %)

<0.78 (<5 %)

2 <0.7 (<70Ltr/hr)

Amber 1 <0.63 (<5 %)

<1.56 (<10 %)

3 <3.5 Amber 2 <1.56 (<20 %)

<3.13 (<30 %)

4 <15 Red >1.56 >3.13

5 <70

6 >70

Situations A and B are slightly different and for general comparison only!

Click to edit Master title style GSV Calculation from

Continuous Data Assuming that the continuous data has identified the main driving mechanisms and fits with the conceptual model:

• Maximum concentration data easily obtained from the data

• Flow rates can be monitored by traditional methods

• Purge & recovery tests not directly applicable

1D Modelling

Click to edit Master title style 1D Modelling • Using as many real site values available

– Monitoring data (concentrations, volume, differential pressure etc) (transect monitoring)

– Investigation data (permeability, depth, water table, soils etc) – Building data (footprint, void volume, small room size etc)

• Using appropriate 1D physical modelling – Diffusion Driven – Fick’s Law – Pressure Driven – Darcy’s Law

• Tools – Time Series Data / Concentration Duration – Flux Calculation – Modular 1D Approach

• Sensitivity Analysis

Click to edit Master title style Modular Approach • Put forward in a paper by Steve Wilson “Modular approach to analysing vapour migration

into buildings in the UK” • Detailed in the Ground Gas Handbook or CIEH • Breaks down the process into modelling parts • One dimensional model • Based on equations from Johnson & Ettinger and

VOLASOL • Considers UK typical construction

Click to edit Master title style Modular Approach

Source Concentration

Model flow from source to sub-floor void

Calculate dilution in sub-floor void

Model flow from sub-floor void to indoor air

Calculate dilution to give indoor air concentration

Measurements taken from the ground

Choose advective or diffusive driver (Darcy or Fick) OR Measure!

Using building dimensions and air changes

Consider mass concrete, cracks, penetrations, membranes

Compare against hazard or flammable gas criteria

Click to edit Master title style Modular Approach Example • Detailed in the Ground Gas Handbook or CIEH • Breaks down the process into modelling parts • One dimensional model • Use NHBC assumptions

Click to edit Master title style Model Lateral Migration

Qv = flow of gas being considered, in m3/s through area A Ki = intrinsic permeability of material through which gas or vapour is flowing in

m2 γ = unit weight of gas in N/m3 µ = viscosity of gas being considered in Ns/m2 A = area of migration perpendicular to migration direction in m2 i = pressure gradient along migration route (as a fluid gradient for the gas

considered) (gas pressure/unit weight)/length. The units for gas pressure in this equation are Pascals(Pa)

Using 1960’s Landfill Example

Click to edit Master title style Calculate Dilution in SFV

Q = q[(100-Ce)/Ce)] Q = Fresh air flow required

q= surface emission rate of the gas to the under floor void

Ce = equilibrium gas concentration in the void we want to achieve

Q = (0.000037x64) x [(100-2.5)/2.5)] = 0.00237 x 39

= 0.09 ltr’s per hour

10 hour test = (0.09x10/1000)/(64x0.15) = 0.00005%

Standard building regulations provides 0.4 m3 per hour which is much greater than surface emissions and therefore will not build up to hazardous concentrations

Continuous Data Interpretation

Environmental Correlations Multi-parameter continuous data allows correlations to

be drawn between environmental parameters and ground-gas concentration, to identify ground-gas drivers

Ground-gas drivers include: • Atmospheric pressure • Borehole pressure • Temperature • Ground-water levels

Atmospheric variables showing no correlation with ground-gas concentration may be eliminated

Atmospheric Pressure & Water Level Describing the time series data set

Click to edit Master title style Differential Pressure

Differential Pressure

Boulder Clay (very low permeability)

Topsoil

High Atmospheric Pressure

Sandstone Bedrock

Turf

Response zones need to be designed too!

Low Atmospheric Pressure

Coal Gas Reservoir

Landfill Containment Failure

Fuel leak

• Residential housing adjacent to a petrol filing station

• UST leak, quantities unknown • Resident noticed petrol fumes in his

basement, but only first thing in the morning

• GGS installed a GasClam® with TVOC detection functionality in the basement

Variable Data TVOC

VOC intermittent in basement

970

980

990

1000

1010

1020

1030

0

20

40

60

80

100

120

26 Feb 28 Feb 02 Mar 04 Mar 06 Mar 08 Mar 10 Mar

Pres

sure

(mba

r)

Conc

entr

atio

n (p

pm)

Pressure in basement TVOC

Diurnal Variation Plots

Maximum concentrations at night

0.0

10.0

20.0

30.0

40.0

50.0

60.0

00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00

Conc

entr

atio

n (p

pmv)

Time of Day

Time Averaged Diurnal Variation

TVOC

Click to edit Master title style Using Concentration Duration

Graphs in Risk Assessment • Primarily a qualitative / semi quantitative tool • Use for selection of elevated or steady state

concentrations in detailed risk assessment and/or sensitivity analysis

0

5

10

15

20

25

27 Nov 29 Nov 01 Dec 03 Dec 05 Dec 07 Dec 09 Dec 11 Dec

Con

cent

ratio

n (%

)

CH4 Continuous Data

0

5

10

15

20

25

0 20 40 60 80 100

Con

cent

ratio

n (%

v/v

)

% Time

CH4 Concentration Duration Curve

Concentration Duration Curves Convert to a ‘Concentration Duration Curve’

IRP-IGM Characterising gas regimes Data Sets Conc. Dur. Curves Data Sets Conc. Dur. Curves

Families of curves describing different behaviour

Calculating ground-gas flux • Using the results of the Purge & Recovery Test

and gas filled borehole dimensions you can calculate gas flux:

Where Q = Gas Flux V = Volume of the internal vadose zone of the borehole c = Change in gas concentration expressed as a percentage t = change in time over which the change in concentration was

measured

Example: - A 50mm diameter installation with a standing water level of 10m provides a vadose volume of 19.63 litres

- Following P&R, methane concentrations increase from 0 to 10 % v/v over a period of 1 hour

- Therefore this equates to a flux of 1.963 ltr/hr

Click to edit Master title style Post 1960’s landfill waste

Click to edit Master title style Post 1960’s Landfill Waste –

Purge & Recovery

Flux (Q) SWL = 10.5 m V = 20.6 ltr t2-t1 = 30 min C2-C1 = 33 %v/v Q = 13.6 ltr/hr

C1

t1 t2 C2

Click to edit Master title style Old Ashy Landfill Waste

Click to edit Master title style Old Landfill Waste –

Purge & Recovery

Flux (Q) SWL = 11m V = 21.6 ltr t2-t1 = 65 min C2-C1 = 3 %v/v Q = 0.6 ltr/hr

t1 t2

C1

C2

Borehole over layer of peat

2 weeks

Stabilised Made Ground

Organic material

Click to edit Master title style Ratio Analysis

• Ratio’s and concentrations of gases can provide additional lines of evidence for gas source, modification and migration

• Trends can provide information on the gas regime variability or stability

• Can even be used to check borehole integrity

Click to edit Master title style CH4 / CO2 Ratio Analysis • The CH4/CO2 ratio can provide a good

indicator of processes and confirm source • As this trend change over time can provide

information on the gas regime variability or stability

• Can be used to demonstrate gas stream modification – enrichment or depletion

• Use site specific information to assist with interpretation

Click to edit Master title style CH4 / CO2 Ratio Analysis

CH4 (%v/v)

CO2 (%v/v)

O2 (%v/v)

CH4/CO2 Ratio

Possible Interpretation

60 40 0 1.5

90 5 0 18

75 20 5 3.75 5 20 10 0.25 0 0 20.9 0

0 0 0 0

Click to edit Master title style Balance Gases • A sample volume of gas is composed of

100%v/v of gaseous components • Commonly the components being measured

do not add up to 100%. The remaining percentage is termed the balance

• In most cases the balance is composed of nitrogen plus a minor amount of trace gas Balance = 100 – (CH4 + CO2 + O2)

Click to edit Master title style Free Nitrogen • Free nitrogen is nitrogen no longer

associated with oxygen

N2(free) = Bal(N2) – (3.77 x O2)

Click to edit Master title style Borehole Integrity

Direct Air Leak!

Click to edit Master title style Verification by Measurement (Sub-floor Monitoring)

It’s Best to Test!

Summary

Lines of Evidence

- Increase confidence

- Verify gas regimes

- Identify migration

-Confirm Pollution Linkages

Click to edit Master title style Summary • Risk Assessment is very complex, will have

different meaning to different stakeholders • Lines of Evidence Approach • In terms of ground-gas contamination, we need

to think long term – Risk Prediction • Many modelling tools available, so choose the

right model(s) for the site specific circumstances • Use site data to verify your conceptual model • Continuous monitoring can add several lines of

evidence to improve risk assessment

Thank you

Discussion / Questions?

Contact: john.naylor@ground-gassolutions.co.uk 07856 244 224 or 0161 232 7465

Specialist Ground-Gas Consultancy • Investigation and Monitoring • Ground-Gas Risk Assessment • Protection Measure Design • 3rd Party Verification Services • Awareness & Technical Training