7.3.3: Storm and seasonal changes

In the foregoing section we treated the beach state, which is a result of everyday morphological response to the antecedent and concurrent forcing. Here, we look at the response of the upper shoreface profile due to storm events with low surge heights and – largely in line with this – to the summer-winter response of the upper shoreface.

The cross-shore profile responds differently to summer and winter conditions, leading to a seasonal behaviour and profile characterisation as shown in Fig. 1.3. In response to the milder summer wave conditions, the offshore bars of the ‘winter’ profile move onshore and finally attach to the shore and rebuild the wider berm associated with the ‘summer’ profile. Hence, in summer the beach is ‘rich’ in sediment (a high beach profile; slope high) and in winter it is ‘poor’ in sediment (a low beach profile; slope gentle). In the classification of Wright and Short (1994), typical summer profiles resemble reflective beaches and winter profiles dissipative beaches (Sect. 7.3.2).

The summer-winter seasonality is strongest for the Northern Hemisphere storm wave climate that exhibits a large seasonality (Sect. 4.3). Note that situations also exist in which the seasonal behaviour is reversed. For instance, the beaches in the Rhone delta on the Mediterranean coast show a reverse behaviour due to the summer mistral.

Figure 7.12: Left: profile during a calm period with normal water level and wave conditions (‘summer’ profile, see Fig. 1.13 for details). Right: profile as a result of storm wave height and surge conditions (‘winter profile’). The post-storm profile is computed according to Vellinga, see Fig. 7.18 for further info on the Vellinga profile and the considered storm conditions.

Figure 7.13: Upper figure: shoreline change between October 13 and 20 and between October 20 and 25 along a stretch of coast that is approximately 80 km long. FRF is the location of the Field Research Facility of the Army Corps of Engineers. Lower figures: the significant wave height at 8 m depth and tidal variation in time. The three vertical lines in the lower figures indicate October 13, 20 and 25 respectively. Note that surge is included in the tidal level variation. (Adapted from List et al., 2005; List & Farris, 1999).

The summer-winter behaviour can also occur when a relatively calm period is interrupted by a storm event (Fig. 7.12). This behaviour was recorded by List and Farris (1999) along a stretch of the eastern USA coast (see Fig. 7.13).

Figure 7.13 shows that after a calm period a ‘summer’ beach is present which turns into a ‘winter’ beach during a storm (note the relatively low storm surge heights). After the storm the beach nearly completely returns to its summer state: the beaches breathe so to speak. What is interesting is that there is a large variation in the degree of breathing. The average recession/accretion is \(10\ m\) but some locations show much less variation and others reach \(15\ m\) to \(20\ m\). These last locations are also known as ‘hotspots’, locations with more erosion than their environment. The reasons behind this are not well-known, but it is likely that a variable offshore bathymetry may cause wave energy focusing (see Sect. 5.2.3 and specifically Fig. 5.6) and that the position and height of dissipative bars play a role.