4.1. DSAS Results
Our study analyses the phenomenon of shoreline change along the coastal from Sidi Madjedoub to Oureah Beach using automatic calculation methods. The outcomes obtained in the current study of historical shoreline analysis, utilizing an automatic thresholding technique, are presented in (Fig. 4, 5 and, 6). In addition, the long-term shoreline evolution rates along the study area are listed in (Table 4) for each zone.
DSAS generated 720 transects perpendicular to the baseline, spaced at 20-metre intervals along a 15-kilometer stretch of coastline. For a complete diachronic analysis, the study area is subdivided into three segments (East, Central and, West). The first sector, between Sidi Madjedoub beach and Matarba beach, comprises transects from 1 to 181. The second sector, between Corniche cliffs and the Crique beach, covers transects from 248 to 413. Finally, the last sector, between the Sablette cliffs and Oureah beach, covers transects from 414 to 718.
Diachronic analysis of the shoreline evolution in our study area is illustrated in (Figs. 4, 5 and 6). They show the variation of the shoreline between Oureah Beach in the west and Sidi Madjedoub in the east and for 03 periods: 2003–2014, 2014–2023, and 2003–2023.
a- Period 2003–2014:
During the period between 2003 and 2014 (11 years), the coastal zone between Sidi Madjedoub Beach to Oureah Beach experienced important erosion over 85% of the study area. (Fig. 4), the global evolution of shoreline showed that the totality of the transects in the region studied (West, Central, and East) were exposed to high erosion with a mean rate of -0.8 meters/year. During this period, the most important regression was registered on the sandy beaches of the region (Sidi Madjedoub, Matarba, Salamander, Crique, and Sablette west), which reache − 20 m/year. The progression of the shoreline is more important in the central part, with a progression rate that varies between − 30 and 60 m.
b- Period 2014–2023:
During the period between 2014 and 2023 (09 years), the coastal zone between Sidi Madjedoub Beach to Oureah Beach indicated a situation of progression-regression-progression compared to the period from 2003 to 2014 (Fig. 5). The progression of the shoreline is more important in the eastern segment of the study area (East Sid Madjedoub), with a tendency rate that varies between − 23 and + 24 meters and an annual rate of -2.65 to 2.73 m/year. A maximum accretion rate is marked on the Matarba East beach.
c- Period 2003–2023:
The results obtained from the analysis of the global evolution of shoreline evolutions over the last 20 years (2003–2023) show that almost all the transects in the region studied (west, centre and east) have been exposed to relatively rapid erosion (Fig. 6).
In the eastern part, the average rates of erosion calculated using the EPR method vary between − 12.01 m/year and + 2.8 meters/year, with a mean rate of erosion of -0.28 meters/year and a high erosion rate registered in the Sidi Madjedoub beach with an EPR rate of -12 m/year.
In the central part between the corniche cliffs and the beach of the crique, shoreline change is more or less important, with a net rate of change of -0.29 m/year. With a maximum rate of erosion observed along the corniche cliffs, with a mean erosion rate is -2.68 meters /years.
In the western part between the beach of the crique and the beach of Stidia, we observed a more or less stable situation compared with the eastern and central part, with a mean rate of shoreline evolution at -0.13 meters/year.
Table 4
Long-term shoreline change rates.
Sector ID
|
Number of transects
|
coastal zone
|
Mean shoreline change (m/year)
|
Minimum shoreline change (m/year)
|
Maximum shoreline change (m/year)
|
1
|
1 to 65
|
Sidi madjedoub beach
|
-1.6
|
-11.98
|
+ 0.98
|
66 to 94
|
Matarba Beach East
|
0.59
|
-0.06
|
+ 1.32
|
95 to 181
|
Matarba Beach (Moulin bigour)
|
-0.07
|
-2.08
|
+ 1.34
|
2
|
|
Harbor of Mostaganem
|
Unmeasured Zone
|
248 to 335
|
Corniche the Cliff
|
-0.49
|
-2.68
|
+ 0.7
|
|
Harbor of Salamander
|
Unmeasured Zone
|
351 to 405
|
Salamander beach
|
-0.39
|
-1.25
|
+ 0.67
|
406 to 413
|
crique beach
|
+ 0.01
|
-0.52
|
+ 0.4
|
3
|
414 to 503
|
The cliffs of sablette
|
-0.05
|
-0.73
|
+ 0.87
|
504 to 531
|
East sablette beach
|
+ 0.08
|
-1.12
|
+ 0.9
|
532 to 603
|
West sablette Beach
|
-0.34
|
-0.99
|
+ 0.61
|
604 to 653
|
Chaabet nacira Beach
|
+ 0.02
|
-0.94
|
+ 0.78
|
654 to 690
|
Oureah Beach
|
-0.25
|
-0.81
|
+ 0.29
|
691 to 717
|
East stidia Beach
|
-0.25
|
-0.98
|
+ 0.87
|
Figure 4 Analysis of the shoreline change between 2003 and 2014 in the study area.
Figure 5 Analysis of the shoreline change between 2014 and 2023 in the study area.
Figure 6 Analysis of the shoreline change between 2003 and 2023 in the study area.
4.2. Numerical modelling of waves
Analysis of the results shows that north-northeast (22.5°) waves (Fig. 7) are more frequent in the study area. Arrive with an obliquity of 20 to 40° with regard to the coastline. The wave heights are lower when waves propagate in shallow water. The orthogonal lines are little refracted in the western part of the Oureah. In the central part, the swell does not conserve its energy offshore, as the area is naturally protected by the Harbour of mostaganem. Significant wave heights decrease to low values between 0.42 meters and 1.2 meters in the eastern sector of the bay and 0.1 m to 2.2 meters in the western part, with refraction coefficients varying between 0.03 and 0.8, so that these waves can generate rip currents. Very low values of significant wave heights less than 0.5m are observed in the sandy coasts like Sablette and Oureah.
Figure 8 shows the propagation of waves from the Northwest sector. Is frontal angle. Significant wave heights decrease considerably, with values between 0.4 meters and 1.6 meters and shoaling coefficients varying between 0.11 and 0.37 along the region studied. With the exception of the Corniche cliffs area, significant wave heights reach 3.2 m just near the principal jetty. This is generally due to the importance of the waves breaking as well as the diffraction wave on the jetties of the harbour of mostagnem and salamander.
The waves from sector west (270°) (Fig. 9) are generally winter waves. Significant wave heights decrease considerably, with grades varying between 0.2 m and 1.5 m and shoaling coefficients varying between 0.08 and 0.25 along the study area. The very low values of significant wave heights, which are less than 0.4 m are observed on the sandy coasts like Sablette and Oureah.
Figure 7 Propagation of north-northeast waves offshore (22.5°) simulated by Mike 21_SW model.
Figure 8 Propagation of Northwest waves offshore (315°) simulated by Mike 21_SW model.
Figure 9 Propagation of West waves offshore (270°) simulated by Mike 21_SW model.