Environmental drivers of earthen heritage deterioration in dryland regions

Earthen sites form a key part of our heritage: providing us with evidence for past civilisations, acting as a focal point for community identity and forming aesthetically pleasing buildings and structures. These sites are found on every continent but are concentrated in dryland regions, such as across northwest China and central Asia. Due to the low durability of earthen materials, many earthen heritage sites exposed to harsh dryland environments are experiencing extensive deterioration. If we are to pass on these sites to future generations, we need to understand how earthen heritage sites are impacted by environmental drivers of deterioration and how the risk of deterioration could be minimised. To address this challenge, this thesis determines the likely impacts of the main environmental drivers of earthen heritage deterioration under current and future conditions in dryland regions. The research presented in this thesis examines the impact of wind and rain as drivers of deterioration at Suoyang Ancient City. Suoyang is an archaeological city built out of rammed earth in the Han (206 BC–220 AD) and Tang (618 – 907 AD) dynasties and is located in Gansu Province, northwest China, on the ancient Silk Road. It exhibits a range of deterioration features from the detachment of surface material to wall slumps and collapses. In 2015, an experimental area was established at the site, which includes several test walls. This thesis uses a combined fieldwork-modelling approach. A controlled field experiment was used to investigate the deteriorative impact of key individual environmental processes on earthen structures. Geomorphological signatures were used to assess the relative importance of wind and rain as drivers of deterioration. These field results were used to develop and parameterise the Vegetation and Sediment TrAnsport model for Heritage Deterioration (ViSTA-HD), a cellular automaton model that spatially resolves the risk of environmental driven deterioration across earthen heritage wall faces. ViSTAHD was used to test the impact of future climatic and conservation scenarios on the risk of earthen heritage deterioration. Findings showed that even though individual wind events are of low magnitude, their high frequency occurrence results in a significant cumulative impact of wind-driven deterioration under both current and potential future conditions. Increasing natural vegetation cover was found to minimise the risk of deterioration under a range of climatic scenarios. Given the similarities in material properties, history and location of Suoyang to other earthen heritage sites in Northwest China and central Asia, findings in this thesis can be applied to many other earthen sites in these regions.