Introduction

Las Glorias beach is located in the Gulf of California on the coast of the State of Sinaloa, between meridians 747 000 m and 753 000 m and the parallels 2 798 000 m and 2 802 000 m of the zone 12 north of the UTM system (Figure 1). It has an approximate extension of 5 km. It is limited at its ends by rock breakwaters. Its morphology as seen from above is practically straight, with parallel bathymetry. Two hydrological basins discharge towards the beach: the Sinaloa river basin and the Bahía Lechuguilla - Ohuira - Navachiste basin. The climate is arid and semi-arid with rains mainly in the summer (^{García, 2004}). On the beachfront, occur mixed astronomical tides with semi-diurnal predominance (^{SMN, 2018}), that is, two high tides and two low tides occur during each lunar day, and the levels of two consecutive high tides or low tides are generally similar. The amplitude of these tides is located in the micromareal range (less than 2 m) according to the ^{Davies classification (1964)}.

Figure 1 Location of "Las Glorias" beach, Sinaloa. 

According to ^{Alcántar (2007)}, in the 80's it was possible to observe the formation of sand arrows in front of the mouths of the Sinaloa river and the Navachiste estuary, due to the interaction between waves, currents and coastal transport. The arrow in front of the Sinaloa River caused the river to change its course towards the northwest and to end at approximately the middle of Las Glorias beach. In the same decade, the Lic. Gustavo Díaz Ordaz and Lic. Guillermo Blake Aguilar dams were completed on the Sinaloa river beds and on its main tributary, the Oconori stream, respectively, which caused a drastic reduction in the sediment supply to the beach, resulting in a sedimentary decompensation which began the erosion of the beach. Later, at the beginning of the 90's, a canal was dredged on the beach with the purpose of relocating the mouth of the Sinaloa River to the southeast. At the same time two rock-based breakwaters were built with the purpose of horizontally stabilizing said outlet. Consequently, on the one hand, in the back of the beach small lagoons were formed as a result of the old bend of the Sinaloa River, and on the other, a physical barrier was generated that interrupted the passage of coastal sediment transport to the Las Glorias beach side. This aggravated its sedimentary decompensation. Regarding changes in the beach coastline, ^{Alcántar (2007)} estimated that this fell back 149 linear meters between 1980 and 2004, with an average erosion rate of 6.2 m / year. Finding that the highest rate of erosion occurred between 1990 and 1994, this being 9.2 m / year, erosion that could be associated with the construction of breakwaters at the mouth of the Sinaloa River in 1992 and 1993. As for the profiles, reports that the erosion occurs in summer, while accretion occurs in the winter.

With the years some actions focused on the restoration and recovery of the Las Glorias beach have been carried out, the most important were carried out in 2006, which consisted of a beach nourishment in a 2 km extension, an average width of 71.50 m, with an elevation of +1.0 m above the Mean Low Water (MLW) and the construction of a breakwater at the mouth of the Navachiste estuary, although it was designed for a length of 311.90 m, only 146.90 m were built (^{Limón, 2010}). Regarding the response of the beach to these works, ^{Zayas (2010)} reported that between 2003 and 2008 there was both an advance and retreat of the beach coastline. In general, the coastline advanced northwest of the beach due to the accumulation of sediments in the breakwater, while in the rest of the beach the coastline receded mainly in the area of construction.

According to this, it is concluded that the beach is under considerable conditions of anthropogenic alteration, mainly by the construction of engineering works both on the beach and in the Sinaloa River, observing an advanced state of deterioration due to the erosion impacting said anthropic alteration.

Given the unstable situation of the beach, the need to establish measures for its restoration and recovery is evident, and therefore a detailed study to understand the degree of balance it maintains and the processes involved in it. In this regard, the present study is focused on providing and analyzing beach swell, sediment and morphology data, which on the one hand will allow us to understand the relationship between the temporal change of the shape of the beach profile and the wave dynamics that involve sediment transport and, on the other hand, provide a basis for further studies on its morph dynamics.

Methodology

Measurement of beach profiles

For the measurement of the beach profiles, geodesic control points were established by means of satellite measurements in static mode with dual frequency GPS receivers to link the topo - bathymetric points to the active national geodesic network (RGNA) of the National Institute of Statistics and Geography (^{INEGI, 2010}), particularly at the apex of Culiacán.

In this way, the geodesic points of horizontal and vertical support were established, with their geodesic and known UTM coordinates, as well as their elevation above the Mean Sea Level (MSL) that allowed, through the transverse sectioning of the coastline, to obtain the characterization of the main elements that, according to ^{Carranza & Caso (1994)}, make up the profile of the beach.

To achieve this characterization, the elevations of the natural terrain along the beach profile were measured with varying distances of between 5 and 20 m, depending on the changes in slope of the relief, which goes from the upper limit on the ground of the beach to the breakers area, usually during low tide, to cover a greater length of beach. Such measurements were obtained using an electronic tachymeter (total station) and stick prisms, that is, by tachymetry or trigonometric leveling, and do not include the part of the beach profile that goes beyond the swell zone in the direction of the sea.

The separation between successive beach profiles was approximately 250 m, giving a total of 18 profiles (Figure 2).

Figure 2 Location of beach profiles. 

The processing of the information obtained from both GPS and tachymetric measurements was processed by rigorous geodesic and topographic methods, in specialized software with equipment - computer interface to obtain control point and cross section information in the ITRF2008 Global Reference System and in the Universal Transverse Mercator (UTM) cartographic projection. Once the digital file of points was obtained, the cartographic drawing was carried out in Autocad®, Civilcad® and Matlab® software to obtain the coastline and the beach profiles.

Results

From the body of data from waves in deep waters it was observed that in all the seasons of the year the highest frequencies of occurrence correspond to waves with wave heights within the intervals of 0.5 to 1.0 m and 1.0 to 1.5 m with periods between 6 and 12 s, mainly from the south and southwest 22.5° (Figure 3). Likewise, the highest values for spring and summer of H- and H50 were estimated, which are quite similar, while the lowest values were estimated for winter (Table 2). However, the highest value for H100 was estimated for the autumn, followed by summer, spring and winter. Regarding the estimated values of T-, they were found to be low and showed little seasonal variability.

Figure 3 Wave roses for wave height and period by season. 

On the other hand, during the year there was not a significant difference in the waves in shallow water at the beach front in terms of their direction. This predominantly hit the beach practically perpendicular (Figure 4 and Table 3) . As for their height, it was also quite regular in a range of 1.0 to 1.5 m, except in winter where the heights are lesser, in the order of 0.5 to 1.0 m.

Figure 4 Wave roses illustrating waves affected by refraction and substraction. 

Regarding the profile of the beach, it was found that in all the control points the two extreme forms of profile are summer and winter (Figure 5). In general, the shape of the slope of the summer profile is more pronounced and with abrupt changes, in contrast, the winter profile shows a more or less continuous gentle slope, as well as the formation of a berm.

Figure 5 Beach profiles of the 18 control points measured quarterly from August (summer) 2015 to May (spring) of 2016. 

About the textural parameters of the sediments, these did not vary significantly along the beach or between seasons (Table 4), on average they showed a size of 2.477 φ, a standard deviation of 0.134 φ, asymmetry of -0.145 φ and kurtosis of 1,364 φ (in Phi (φ) units).

Discussion

The seasonality of the wave regime is reflected in the changes in the beach profile (Figure 5), since it is not the same throughout the year. The estimated values of the statistical parameters of the swell (table 4) indicate that in autumn wave height begins to decrease, in comparison with that observed in summer, until reaching winter swell, while, in spring, wave height increases and remains similar until summer. This behavior causes the profile of the beach to increase from autumn to winter, when the waves decrease in height until they reach low average values, and is eroded from spring to summer, when the waves are higher and more consistent. These changes in the beach profile occurred in virtually all control points along the beach, bearing great resemblance to those reported by ^{Alcántar (2007)}. However, the inter-annual cycle of change in the Las Glorias beach profile, which consists of phases of erosion and accretion occurring in spring-summer and autumn-winter, respectively, is not reversed as indicated by Alcántar (2007). , because the changes in the morphology of the beach profile respond to changes in wave conditions (^{Masselink & Hughes, 2003}), that is, high-energy waves produce erosion and low-energy produce accretion, as verified in this work.

The swell due to the effect of refraction and substraction tend to regularize in the beach front in terms of its direction and height (Figure 4). Nevertheless, higher waves dominate in spring and summer, while those of lower height dominate in autumn and winter (table 5). As the waves tend to impact, predominantly, practically perpendicularly on the beach in every season of the year, this indicates that little sediment transport occurs longitudinally to the beach, rather the transport of sediment is in transverse direction, that is, the higher waves erode the berm and tend to deposit the sediment in front of the beach in the form of a bar, which is then removed towards the beach by the lower height waves, forming a new berm.

In relation to the textural parameters of the sediments, since these did not vary significantly along the beach or between seasons, their distribution is spatially and temporally very homogeneous. Taking into account the limit values of the textural parameters described in the Coastal Engineering Manual of the Army Corps of Engineers of the United States of America (^{King & Galvin, 2002}) the sediments consist of very well classified fine sand, which is asymmetric towards thick and leptocurtic sizes. The homogeneity of the type of sediment suggests that it has a lower relative influence than the swell on the changes to the beach profile.

Conclusions

Higher waves dominate in spring and summer, while those of lower height dominate in autumn and winter. However, in autumn, the highest waves of the year occur; a condition where the beach can change its profile in a very short time.

Beach profiles show morphological changes with a marked seasonality, it can be observed that the maximum accretion is in winter and the maximum erosion in summer, this verifies that the inter-annual cycle of change of the beach profile responds to the seasonal regime of the swell.

The presence of a berm is typical of winter and the slope of the beach as a whole is lower than during the other seasons; On the contrary, the absence of the berm and the greater slope of the beach are typical of summer.

The distribution of the sediment that comprises the beach is spatially and temporally very homogeneous, fine sands predominate.