*3.2.1 Very high spatial resolution and short-term shoreline analysis at Zuydcoote*

Four photogrammetric survey campaigns were carried out in November 2017, May 2018, September 2018, and February 2019. Very high spatial resolution DSMs were then calculated (5 cm/pixel; **Figure 10a**). The DSMs were compared in pairs (e.g., **Figure 10b**). In order to synthesize the four DSMs, a statistical analysis was also conducted. For example, a map of the annual rate of change was produced by calculating a linear regression with the pixel heights at each date.

For example, between November 2017 and May 2018, the sediment budget was negative (−5041 m3 , i.e., −0.15 m3 /m2 ), 1368 m3 (0.22 m3 /m2 ) of sediment accumulated, and 6410 m3 was eroded (−0.25 m3 /m<sup>2</sup> ). Erosion was very mainly detected on the beach and dune front. The comparison map of the DSMs (**Figure 10b**) shows a flattening of the upper intertidal bar and the upper foreshore, and the disappearance of the aeolian sand accumulation features identified on the DSM of November 2017 (**Figure 10a**). The DSM comparison also shows that the dune front has been eroding, although blowouts were filling up.

The map of the annual rate of change calculated with the four DSMs (**Figure 10c**) confirms the previous observations: erosion of the sand bar, upper beach and dune front, and accumulation in the trough landward of the upper intertidal bar and in the blowouts. The topographic and statistical cross-shore profiles (**Figure 10d**) show that the highest negative evolution rates are corresponding to the foredune front. As indicated in Section 2.2.3 of this chapter, such map must be interpreted with caution. In bare areas, the geomorphological analysis can be validated, but in the vegetated zones, the changes detected may be due to vegetation growth and not to geomorphological changes. The interpretation of these DSMs must therefore always be combined with observation of aerial photographs.

#### **Figure 10.**

*Examples of (a) DSM computed from photogrammetry data (November 2017), (b) evolution map (November 2017 to May 2018), (c) map of annual rate of evolution, and (d) cross-shore topographical and evolution rate profiles.*

**105**

**Figure 11.**

*Recent Advances in Coastal Survey Techniques: From GNSS to LiDAR and Digital…*

Four topographic survey campaigns were carried out with an airborne topographic LiDAR in May 2008, March 2011, November 2012, and January 2014 on a 8 km long coastal stretch east of Dunkirk. For each measurement campaign, a DTM with a spatial resolution of 1 m was calculated. In order to understand sediment transfer between beach and dunes, these DTMs were used to calculate sediment volume variations in the mid foreshore, upper beach, and dune (**Figure 11**). The lower foreshore limit corresponds to the minimum elevation at the time of LiDAR measurements and therefore depends on the tidal level. The upper limit of the mid foreshore is the Mean High Water level (MHW, 2.83 m French elevation datum at Dunkirk). The upper beach is the area between the MHW level and the shoreline determined using the gradient method (see Section 3.1). The seaward limit of the dunes is the shoreline, whereas the inner limit is determined by photo-interpretation according to the type of dune vegetation identified in order to exclude areas with high vegetation (e.g., sea buckthorn) where the differences between the DTM

**Figure 11** shows that the shoreline east of Dunkirk experienced a sediment

eastern part, next to the Belgian border. The average vertical accretion is 1 m. In the whole study zone where the shoreline has been stable from 2008 to 2014, the sediment budget of the dunes is positive even if dune front erosion occurred in places. Estimates of changes in sediment volume indicate that accumulation in coastal dunes occurred primarily prior to erosive events in the fall and winter of 2013, particularly between 2008 and 2011 when a gain of more than 122 × 103

*(A) Map of shore evolution between Dunkirk and the Belgian border from 2008 to 2014 and (B) volumetric* 

*evolution of the foreshore, upper beach, and dunes (adapted from Ref. [38]).*

m3

from 2008 to 2014. Almost half of

m3

), mainly in the

m3

*3.2.2 Large-scale calculation of the sediment budget east of Dunkirk*

and the actual ground topography may exceed 50 cm.

this accumulation is observed in the coastal dunes (154 × 103

accumulation of approximately 326 × 103

*DOI: http://dx.doi.org/10.5772/intechopen.91964*

*Recent Advances in Coastal Survey Techniques: From GNSS to LiDAR and Digital… DOI: http://dx.doi.org/10.5772/intechopen.91964*
