unit 4 - southampton 4 - risk...current risk meaning (ciria c665) click to edit master title style...
TRANSCRIPT
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: [email protected] 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