Soil Movement

Overview

The lack of soil moisture within reactive clay soils is the trigger for major soil movement and cracking. Lower-level suction can lead to clay soils drawing moisture away from the surface soils during drought. Some vegetation can also play a role in removing soil moisture. Soil shrinking and swelling can lead to extensive damage of assets if the foundations also move. Buildings on light strip footings or unstiffened slabs are vulnerable to soil movement, causing walls and ceilings to crack as the foundations move. The Climate Risk Engines account for soil movement due to drought using annual precipitation records. Soil movement due to drought can be widespread however it is rarely catastrophic and can be remedied if addressed promptly – typical repaid costs are less than 5% of the property replacement value. Note that this hazard is not the same as vegetation driven subsidence in seasonal drought, which is associated with tree roots beneath residential builds extracting water during a hot and dry summer.

Local Context Data Use

Soil movement (shrinking and swelling) for reactive soil types is a function of the amount of moisture within the soil relative to a normal condition for which the property was designed. This hazard is strongly correlated with drought conditions because the driver is a function of the soil moisture content. The Climate Risk Engines use soil clay content datasets to determine the clay exposure.

Baseline Hazard Data

Drought can be defined as ‘areas considered to be suffering from a serious or severe rainfall deficiency’. Serious rainfall deficiency occurs when rainfall lies above the lowest 10% of recorded rainfall but below the lowest 10% for the period in question. Severe rainfall deficiency is defined as rainfall among the lowest 5% for the period in question (Bureau of Meteorology 2021). The probability of droughts occurring at an asset’s location is based on data from the nearest historical weather stations and rain gauges.

Climate Change Projections

Downscaled data for ‘drought’ is drawn from the GCMs and RCMs. Generally, selections of GCM/RCMs for drought projection are made based on (a) local specialisation, (b) highest spatial resolution, and (c) projections that enable the prediction of drier futures. However, climate change projections within one model may increase the probability of drought in some locations and reduce it in others.