# Monitoring of Ground Movement and Groundwater Changes in London Using InSAR and GRACE
## 5 Discussions
### Site-Specific Ground Movement and Its Possible Reasons  
The analysis of site-specific monthly subsidence velocity revealed a complex pattern of movements. Multiple sites show abrupt changes and indicate distinct land movement which might be induced by changes in groundwater, fault-controlled or dominated by various other controlling factors. The tributaries of the River Thames form a dense drainage network which directly affects the topography, geomorphology and geology of the study area. Additionally, the study area has flood plains containing alluvium, clay areas, open sandy layers, landslide gradients and deep-seated underground tectonic constructions, making the land surface prone to displacements. In Figure 8a-d, land movement patterns for different sites were identified and reasons for such movements are discussed below. These sites were selected based on distinct land movement pattern and available literature. In the Figure 8a, the area marked covers 1.63 km\({}^{2}\) and crosses multiple important anthropogenic structures like Westminster Bridge above the River Thames, the Commonwealth Secretariat, Supreme Court and Parliament Square, Waterloo train station, Nelson square, London Bridge station,  
Figure 8: PSInSAR site specific subsidence maps for London. (**a**) Area crossing Thames river; (**b**) area on the north of Thames river and passing through CTRL (Channel Tunnel Rail Link); (**c**) area on the south of Thames river showing distinct subsidence; (**d**) area on the north of Thames river overlaid on groundwater contour provided by EA. Buckingham Palace, London Eye, and sections of the British rail network. This area is a low-lying river flood plain with elevations between 5 to 10 m above Ordnance Datum (mOD), and the bedrock geology is dominated by the London clay formation. Superficial deposits in the area include alluvium, peat, Kempton park gravel formation, Langley silt member and Taplow gravel formation. This area also coincides with the location of the Jubilee Line extension, which was constructed between 1993 and 1999 [54]. The total number of PS points obtained within the area marked are 3131, and the average land movement for all the PS points is \(-\)0.87 mm/year, where the negative sign shows land movement away from the sensor, therefore subsiding. This subsidence in Figure 8a, can be attributed to the continuous effect of subsurface construction works, which were carried out for the Jubilee Line extension and compression of alluvium because of groundwater abstraction. The area marked in the Figure 8b spans over 0.29 km\({}^{2}\) and crosses multiple important anthropogenic structures like Highbury and Islington station, Canonbury station, Alwyne Castle and, St. Andrew church. It also lies along the path of Channel Tunnel Rail Link (CTRL), which is a high speed 1 (HS1) railway, joining the Channel Tunnel and London (operational since 2007). The total number of PS points obtained within the area marked are 323 and the average land movement for all the PS points is \(-\)0.83 mm/year. The part of CTRL track between Caledonian Road and Canonbury (area marked) was constructed during our study period. We can attribute the land movement of the area marked to the construction of CTRL and to the possible compaction of subsurface particles around the tunnel along its track. Our results are in close agreement with the results obtained by PanGeo Project [39], as the authors obtained subsidence rates of \(-\)1.09 mm/year around the same area, marked as PGGH_London_015. The area marked in Figure 8c is a heavily built-up area and contains buildings, roads and other man-made structures. It contains the train-line between Beckenham Hill, Bellingham and Catford. It contains other important structures like Kings church, schools, hospitals and large residential areas. Additionally, the area contains around 38 boreholes, from which groundwater is directly pumped out to meet the local requirements.