the purposes of this assessment the DELWP Resource Share Guidance approach is recommended as the basis of assessing likelihood, as shown in Table 11. Table 11: Proposed likelihood scale for subsidence caused by drawdown in aquifers. Consolidation response / LOW MODERATE HIGH Drawdown Range Less than 1 m drawdown Between 1 m and 10 m Greater than 10 m over 30 years drawdown over 30 years drawdown over 30 HIGH Moderate High High Normally sediments MODERATE Over-consolidated Low Moderate Moderate sediments with >20% clay LOW Low Low Low Over-consolidated sediments with little clay Table 12: Proposed likelihood assessment criteria for subsidence as a result of onshore gas development. Water Resource (Asset) Low Consequence Moderate Consequence High Consequence Group Groundwater users Impact of subsidence is Without modification current Aquifer, without modification (aquifers) within annual variability in pumping regime cannot of current pumping function and operating costs meet water demand due to infrastructure such as of current and future users. subsidence deepening bores, cannot meet demand requirements as a result of subsidence. Surface water users Subsidence results in either Subsidence results in a Subsidence results in a (rivers) no change or a material material change in river material change in river change in flow, with no flow, with measurable flow, with significant measurable impairment of impairment of users’ ability impairment of users’ ability users’ ability to access to access entitlement. to access entitlement. entitlement. Surface ecosystems Subsidence results in a Subsidence results in Subsidence results in material change in material change in material change in ecosystem condition, with ecosystem condition, with ecosystem condition, with no measurable impairment measurable impairment of significant impairment of of ecosystem function. ecosystem function. ecosystem function. Considering the range of values for clay content and the potential range of drawdown that may result from gas exploration or development, the expected range of subsidence as a result is likely to be in keeping with that experienced from current licensed groundwater extraction. That is, land subsidence would be expected to be in the range of millimetres to tens of millimetres. Give the indicative consequence table and the sediment types in the Otway region, there is a low risk of subsidence. Primarily this assessment results from the estimation of low drawdown as a result of gas exploration or development. 6.5 Summary
the purposes of this assessment the DELWP Resource Share Guidance approach is recommended as the basis of assessing likelihood, as shown in Table 11. Table 11: Proposed likelihood scale for subsidence caused by drawdown in aquifers. Consolidation response / Drawdown Range LOW Less than 1 m drawdown over 30 years MODERATE Between 1 m and 10 m drawdown over 30 years HIGH Greater than 10 m drawdown over 30 HIGH Normally sediments Moderate High High MODERATE Over-consolidated sediments with >20% clay Low Moderate Moderate LOW Over-consolidated sediments with little clay Low Low Low Table 12: Proposed likelihood assessment criteria for subsidence as a result of onshore gas development. Water Resource (Asset) Group Low Consequence Moderate Consequence High Consequence Groundwater users (aquifers) Impact of subsidence is within annual variability in function and operating costs of current and future users. Without modification current pumping regime cannot meet water demand due to subsidence Aquifer, without modification of current pumping infrastructure such as deepening bores, cannot meet demand requirements as a result of subsidence. Surface water users (rivers) Subsidence results in either no change or a material change in flow, with no measurable impairment of users’ ability to access entitlement. Subsidence results in a material change in river flow, with measurable impairment of users’ ability to access entitlement. Subsidence results in a material change in river flow, with significant impairment of users’ ability to access entitlement. Surface ecosystems Subsidence results in a material change in ecosystem condition, with no measurable impairment of ecosystem function. Subsidence results in material change in ecosystem condition, with measurable impairment of ecosystem function. Subsidence results in material change in ecosystem condition, with significant impairment of ecosystem function. Considering the range of values for clay content and the potential range of drawdown that may result from gas exploration or development, the expected range of subsidence as a result is likely to be in keeping with that experienced from current licensed groundwater extraction. That is, land subsidence would be expected to be in the range of millimetres to tens of millimetres. Give the indicative consequence table and the sediment types in the Otway region, there is a low risk of subsidence. Primarily this assessment results from the estimation of low drawdown as a result of gas exploration or development. 6.5 Summary
O2-Otway-impact-assessment-report-June2015.docx