Table 11: Proposed consequence scale for subsidence caused by drawdown in aquifers. Water resource Low consequence Moderate consequence High consequence (asset) group Groundwater users Impact of subsidence is Without modification, Aquifer, without modification of (aquifers) within annual variability in current pumping regime current pumping infrastructure function and operating costs cannot meet water demand such as deepening bores, of current and future users. due to subsidence. cannot meet demand requirements as a result of subsidence. Surface water users Subsidence results in either Subsidence results in a Subsidence results in a material (rivers) no change or a material material change in river change in river flow, with change in river flow, with no flow, with measurable significant impairment of users’ measurable impairment of impairment of ability ability to access entitlement. users’ ability to access to access entitlement. users’ entitlement. Surface water Subsidence results in no Subsidence results in Subsidence results in material ecosystems material change in ecosystem material change in change in ecosystem condition, condition, with no measurable ecosystem condition, with with significant impairment of impairment of ecosystem measurable impairment of ecosystem function. function. ecosystem function. Table 12. Proposed likelihood assessment criteria for subsidence as a result of onshore gas development. 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 years HIGH Normally consolidated Moderate likelihood High likelihood High likelihood sediments MODERATE Over-consolidated sediments Low likelihood Moderate likelihood Moderate likelihood with >20% clay LOW Over-consolidated sediments Low likelihood Low likelihood Low likelihood with little clay 6.5 Summary For the Gippsland region it is summarised that: There is a low risk of subsidence from onshore tight and shale gas development. There is a moderate risk of subsidence within 30 years from coal seam gas development.
Table 11: Proposed consequence scale for subsidence caused by drawdown in aquifers. 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 river 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 water ecosystems Subsidence results in no 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. Table 12. Proposed likelihood assessment criteria for subsidence as a result of onshore gas development. 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 years HIGH Normally consolidated sediments Moderate likelihood High likelihood High likelihood MODERATE Over-consolidated sediments with >20% clay Low likelihood Moderate likelihood Moderate likelihood LOW Over-consolidated sediments with little clay Low likelihood Low likelihood Low likelihood 6.5 Summary For the Gippsland region it is summarised that: There is a low risk of subsidence from onshore tight and shale gas development. There is a moderate risk of subsidence within 30 years from coal seam gas development.