Introduction
The ground model project is a 3D representation of Barton-on-sea cliff face, in order to study the cliff instability. The data collected will help to produce a 2D and 3D ground model images using systematic methods. The case study for this report will apply the ground model in the cliff of Barton-on-sea to show the changes that was caused by a landslide and study the soil types of the area.
Study Area
The Barton-on-sea is located in the south coast of England, Hampshire coastline’s.
The coast comprises of sedimentary strata, which consist of clays, sands and gravels of quaternary and Palaeogene geological ages. (Barton-on-Sea Cliff Instability Preliminary Study, 2011). The strata of this coastline are vulnerable to erosion, because the coastline is weak and has been historically prone to erosion and landslides. The cliff in Barton has experienced a recession rate of about 1 m per year in the 1960s (Moore, et,al, 2003). Due to the insufficient amount of soft rocks and beach material in Barton-on-sea, the frequency of landslides of the cliff line can be of concern.
Site investigation
In order to find the causes of Barton-on-sea’s coastal instability and the hazards related to it, site investigation took place in the last six decades. They used 77 boreholes, four shallow trial pits, 3 dynamic probes, 30 cone penetration test (CPT) probes 4 inclinometers, 60 groundwater monitoring installations and 12 slip tubes. Unfortunately most of the installed engineering equipment has been lost because of landslides (see the image below) (Barton-on-Sea Cliff Instability Preliminary Study, 2011). The highlighted area with red is where the lost equipments and its results are
Data collection and Methodology
The software used for this project was ArcGIS, which allowed the creation of a 3D ground model map.The data was collected for Barton-on-sea area and its sources included borehole, Digital Elevation Model(DEM), aerial photography and geomorphlogical maps. The DEM data was taken in 2009 and the aerial photo data was taken in 2008.
The systematic processes of making the 3D modelling image consists of these steps:
Data collected for Barton-on-sea cliff using Differential GbS (Dgbs) device and borehole samples.
Reviewing the borehole log to identify the strata bases, creating and cleaning the borehole data
Transferring the borehole data into a stratigraphic table and creating excel plans.
Converting the XYZ data collected by the Dgps into points to create Triangulated Irregular Networks (TIN), then creating a 2D transect lines to show the different type of soils. See the map below.
Exporting the data into graphs for further interpretations, see the graph below.
Calculated and used the modelled points to create a new surface, then subtracted an extrapolated the new Hillshade surface using DEM and the digitised value line to show eroded areas and its deposit within the landslides. See the 3D figure below, green represent the erosion and orange represent the deposit of the eroded area.
Creating a 3D strata model to the check the accuracy of the data collected. The extrapolated strata image below represents the contact layers between each soil type forming the boundaries within the landslide. The stratas show irregularity and some collide on each other, which indicate a possible error, for example the Chama sand strata collide with Barton clay F2.
Conclusion
3D modelling can provide insightful analytical view of the data especially when combined with 2D models, for example projects that require modelling the ground might need 3D modelling. This is because the ground most likely will hold information that can be displayed on the top of each other, so sometimes 2D modeling might not provide the full picture of what is happening within the ground. Hence using both practices can provide a better interpretation of the data and its results. For this project 3D ground model and 2D transects graphs was created for Barton-on-sea cliffs using ArcGIS and Excel, which allowed us to have a most accurate representation of the soil types and features within the landslide.
This then allowed for the creation of a geomorphological map that include all the landslide features needed to interpret the area, see the image below. This visualisation of the data allow for a more suited interpretation of the site. For example, looking at the circled areas, we can tell that the retrogressive mudslide headscarp and the mudslide lobe was caused by the erosion that was specified in previous sections. These mudslides are embedded within the Becton Sand and Chama sand formation as shown in the graph earlier. By understanding and interpreting different representation of the data we can have a better understanding of the incident, which will help us reach a better decision. I hope you found this project to be insightful.
References
Brown, S. and Barton, M. (2007). Downdrift erosion and the frequency of coastal landsliding. 1st ed. Southampton: University if Southampton.
Barton, M.E. & Thomson, R.I. 1984. Studies of the water Balance in a rapidly degrading soil cliff. In: Proceedings of the 4th International Symposium on Landslides, Toronto,
Canada, Vol. 1, 355–361).
Barton M.E. & Garvey P.M. 2011. Reactivation of landsliding following partial cliff stabilisation at Barton-on-Sea, Hampshire, UK. Quarterly Journal of Engineering Geology and Hydrogeology, 44, 233-248.
Hosseyni S., Torii N & Bromhead E.N. 2011. Discussion of the paper by M.E. Barton & P. Garvey, Quarterly Journal of Engineering Geology and Hydrogeology, 44, 233-248.
Moore R, Fish P, Glennerster M & Bradbury A 2003. Cliff behaviour assessment: a quantitative approach using digital photogrammetry and GIS. Proceedings of the 38th DEFRA Conference of River and Coastal Engineers, 08.3.1-08.3.13.
Rendel. 1993b. Soil Investigation at Barton-on-Sea, Hampshire (Phase III). Report by
Rendel Geotechnics Ltd for new Forest District Council (Ref: R/H240/01), September
1993.
Rendel. 2002. The Coastal Landslides at Barton-on-Sea, Hampshire, UK: Christchurch
Bay Strategy Study Input. Report by High-Point Rendel (Ref: 1589/R/01/02) for New
Forest District Council and Halcrow Group Ltd, October 2002.
Barton-on-Sea Cliff Instability Preliminary Study. (2011). 1st ed. Birmingham: Halcrow Group Limited.
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