SOUTH WALES COAL TIPS
Key Project Details
Location: South Wales
Client: Welsh Government & the Mining Remediation Authority
Sector: Mining
Services: GIS Analysis, Soil Moisture Mapping, Visualised Digital Terrain Model (DTM), Hydrological Analysis, Weighted Susceptibility Analysis
Resilience to climate change is a fundamental aspect of future asset management. Rainfall-induced shallow landslides are hazardous phenomena triggered by intense, concentrated rainfall. These landslides develop in unconsolidated tip deposits, generally in the form of translational or shallow rotational failures, that can lead to debris flow type failures. The topography of South Wales, combined with the location of many tips on natural slopes, increases the risk of such failures.
Central Alliance was commissioned by the Welsh Government and The Coal Authority (now known as the Mining Remediation Authority) to carry out a satellite-based soil moisture mapping study of 69 colliery spoil tips in South Wales.
All the tips were rated “Category D” (“has potential to impact public safety”) – the highest rating available. The analysis also covered surrounding areas around the tip, including part of any catchments/watersheds that may impact or influence the tip. The data will assist in the evaluation of landslide susceptibility of the tip and assist in prioritising the sites regarding sites for visits, proactive interventions and targeted monitoring. There were two rounds of images, the first captured in September 2021 and the 2nd image was captured in April 2022.
The area has a high-profile history of spoil tip related landslips, with particular attention following the Tylorstown and Wattstown failures in 2020. More recently, this was evidenced by the collapse and subsequent landslide in Cwmtillery in November 2024.
Using high resolution imagery from L-Band SAR data supplied by the Japanese Aerospace Exploration Agency ALOS-2 satellite, the topsoil layer (10-30 cm depth) was analysed using algorithms that utilise the backscatter signals to determine the soil moisture levels of the study area. Various GIS analyses were conducted incorporating the Soil Moisture data. The following techniques and analysis carried out as well as maps and overlays providing soil moisture mapping, visualised Digital Terrain Model (DTM), hydrological analysis, and weighted susceptibility. The Weighted susceptibility analysis combines soil moisture and slope angle to provide a susceptibility rating. The Hydrological analysis depicts flow accumulation paths and flow direction based on the elevation model.
To prioritise failure susceptibility and therefore guide colliery spoil tip maintenance inspections, based on the Soil Moisture data and associated analysis, the following susceptibility scoring matrix has been devised. This comparative measure scores spoil tips on seven different metrics, to form a quantitative prioritisation method. These metrics are:
- Tip Soil Moisture (TSM) – Soil moisture on the spoil tip is categorised into ten equal intervals, each of which is weighted to form the score for this metric.
- Catchment Soil Moisture (CSM) – Soil moisture in the spoil tip catchment area, i.e., the areas at greater elevation to the coal tip, from which rainfall run off would flow into the spoil tip. Some spoil tips share catchment areas due to their geographic proximity. This metric measures the catchment area to a maximum of 1km upslope from the tip.
- Slope Above Soil Moisture (SASM) – Soil moisture in the area immediately above the spoil tip, in which slope failure would flow into and impact the spoil tip. The area studied for this metric was created by consideration of lateral and topographical constraints of the land.
- Slope Below Soil Moisture (SBSM) – Soil moisture in the area immediately below the spoil tip, in which slope failure may undermine the stability of the spoil tip. The area studied for this metric was created by consideration of lateral and topographical constraints of the land, as well as rivers channels and valley bottoms.
- Weighted Susceptibility (WS) – By applying a weighted overlay combining the soil moisture and slope angle, susceptibility maps can be produced highlighting where higher soil moisture and steeper slopes combine.
- Drainage Path Analysis (DPA) – The maximum rate of change of flow through flow accumulation paths intersecting the spoil tip, combined with the overall volume of flow through the point. This is derived from the Unit Hydrograph, which is a measure of direct runoff from one unit of constant intensity rainfall over the accumulation path drainage basin. This metric is suited to detecting rapid washout-type failures.
- Land Cover Sensitivity (LCS) – A metric studying steep slopes (>20°) in the catchment areas which are tree covered. These areas are sensitive to changes in landcover, i.e., deforestation, due to the subsequent increased likelihood of soil loss if trees are removed. The scores were weighted depending on the type pf trees occupying the land, with a distinction between broadleaf and coniferous tree types.
In addition to the matrix, Tip Inspection sheets were also created to highlight various features discovered during the analysis. These sheets helped the Coal Authorities inspectors pinpoint areas to investigate, saving them time, rather than having to cover the whole tip.
Resilience to climate change is a fundamental aspect of future asset management. Rainfall-induced shallow landslides are hazardous phenomena triggered by intense, concentrated rainfall. These landslides develop in unconsolidated tip deposits, generally in the form of translational or shallow rotational failures, that can lead to debris flow type failures. The topography of South Wales, combined with the location of many tips on natural slopes, increases the risk of such failures.
Central Alliance was commissioned by the Welsh Government and The Coal Authority (now known as the Mining Remediation Authority) to carry out a satellite-based soil moisture mapping study of 69 colliery spoil tips in South Wales.
All the tips were rated “Category D” (“has potential to impact public safety”) – the highest rating available. The analysis also covered surrounding areas around the tip, including part of any catchments/watersheds that may impact or influence the tip. The data will assist in the evaluation of landslide susceptibility of the tip and assist in prioritising the sites regarding sites for visits, proactive interventions and targeted monitoring. There were two rounds of images, the first captured in September 2021 and the 2nd image was captured in April 2022.
The area has a high-profile history of spoil tip related landslips, with particular attention following the Tylorstown and Wattstown failures in 2020. More recently, this was evidenced by the collapse and subsequent landslide in Cwmtillery in November 2024.
Using high resolution imagery from L-Band SAR data supplied by the Japanese Aerospace Exploration Agency ALOS-2 satellite, the topsoil layer (10-30 cm depth) was analysed using algorithms that utilise the backscatter signals to determine the soil moisture levels of the study area. Various GIS analyses were conducted incorporating the Soil Moisture data. The following techniques and analysis carried out as well as maps and overlays providing soil moisture mapping, visualised Digital Terrain Model (DTM), hydrological analysis, and weighted susceptibility. The Weighted susceptibility analysis combines soil moisture and slope angle to provide a susceptibility rating. The Hydrological analysis depicts flow accumulation paths and flow direction based on the elevation model.
To prioritise failure susceptibility and therefore guide colliery spoil tip maintenance inspections, based on the Soil Moisture data and associated analysis, the following susceptibility scoring matrix has been devised. This comparative measure scores spoil tips on seven different metrics, to form a quantitative prioritisation method. These metrics are:
- Tip Soil Moisture (TSM) – Soil moisture on the spoil tip is categorised into ten equal intervals, each of which is weighted to form the score for this metric.
- Catchment Soil Moisture (CSM) – Soil moisture in the spoil tip catchment area, i.e., the areas at greater elevation to the coal tip, from which rainfall run off would flow into the spoil tip. Some spoil tips share catchment areas due to their geographic proximity. This metric measures the catchment area to a maximum of 1km upslope from the tip.
- Slope Above Soil Moisture (SASM) – Soil moisture in the area immediately above the spoil tip, in which slope failure would flow into and impact the spoil tip. The area studied for this metric was created by consideration of lateral and topographical constraints of the land.
- Slope Below Soil Moisture (SBSM) – Soil moisture in the area immediately below the spoil tip, in which slope failure may undermine the stability of the spoil tip. The area studied for this metric was created by consideration of lateral and topographical constraints of the land, as well as rivers channels and valley bottoms.
- Weighted Susceptibility (WS) – By applying a weighted overlay combining the soil moisture and slope angle, susceptibility maps can be produced highlighting where higher soil moisture and steeper slopes combine.
- Drainage Path Analysis (DPA) – The maximum rate of change of flow through flow accumulation paths intersecting the spoil tip, combined with the overall volume of flow through the point. This is derived from the Unit Hydrograph, which is a measure of direct runoff from one unit of constant intensity rainfall over the accumulation path drainage basin. This metric is suited to detecting rapid washout-type failures.
- Land Cover Sensitivity (LCS) – A metric studying steep slopes (>20°) in the catchment areas which are tree covered. These areas are sensitive to changes in landcover, i.e., deforestation, due to the subsequent increased likelihood of soil loss if trees are removed. The scores were weighted depending on the type pf trees occupying the land, with a distinction between broadleaf and coniferous tree types.
In addition to the matrix, Tip Inspection sheets were also created to highlight various features discovered during the analysis. These sheets helped the Coal Authorities inspectors pinpoint areas to investigate, saving them time, rather than having to cover the whole tip.
Key Project Details
Location: South Wales
Client: Welsh Government & the Mining Remediation Authority
Sector: Mining
Services: GIS Analysis, Soil Moisture Mapping, Visualised Digital Terrain Model (DTM), Hydrological Analysis, Weighted Susceptibility Analysis
