**4.2 Lagoon behavior in the dry season and its relationship with the tides**

The water level behavior within the lagoon system is mainly determined by the tidal condition and its influence reaches the Mexcaltitán lagoon, at a distance of around 25 km from the Camichín mouth. With the operation of the PHLC during the dry season, there is an increase in the level of the lagoon system, in the Mexcaltitán lagoon, the average increment is around 0.06 m, while in the Toluca lagoon is around 0.02 m. Thus, when comparing and calculating the maximum amplitude of the water levels in the sea and in each of the lagoons, it was found that in the sea the amplitude is 1.85 m, while in the Toluca lagoon is 0.95 m without and with the dam, and in the Mexcaltitán lagoon is 0.3 m under current conditions with an increment of 0.33 m with the dam. As one can observe, the amplitude of the water levels in the Toluca lagoon does not have a significant impact due to the operation of the hydroelectric project. However, in the Mexcaltitán lagoon, the amplitude of the water levels increases by around 3 cm due to the contributions of the San Pedro river during the dry season. Thus, in the Toluca lagoon, the percentage of tidal damping is around 48% for both scenarios, while in the Mexcaltitán lagoon, it is around 84% under current conditions and around 82% under hydroelectric project conditions.

*Interconnection among River Flow Levels, Sediments Loads and Tides Conditions and Its Effect… DOI: http://dx.doi.org/10.5772/intechopen.109175*

In addition, velocities values do not present significant differences in the dry season considering the PHLC operation as increments of just 0.01 and 0.02 m s−1 were observed. Therefore, the sediment transport conditions are not substantially modified in the lagoon system. This was confirmed since the velocities are determined by the tide condition, when the maximums are reached, close to the high and low tides, the system has the capacity to put the sediment in suspension.

As the largest discharges in the entire system were at the mouth of Camichín, it is the site where the main exchange of fresh water with the sea occurred. The maximum flow entering the system under current conditions was calculated at around 427 m3 s−1, whereas the one leaving the system was 423 m3 s−1. With the operation of the hydroelectric project, a reduction of the maximum flow that enters the system through the sea of the order of 4 m3 s−1 was observed; however, the flow that leaves increased by the order of 2 m3 s−1. On the contrary, at the mouth of Palapares, under current conditions, the maximum flow that entered the system through the sea was 157 m3 s−1 and the one leaving was 115 m3 s−1. With the operation of the project, there was a decrease in the flow that enters of around 1 m3 s−1, while the flow that leaves increased in the order of 1 m3 s−1.

#### **4.3 PHLC operation during the rainy season**

Applying numerical modeling during the period of flash floods, the PHLC operation does not have significant modifications in the hydrodynamics and salinity of the lagoon system. Thus, the water level behavior in the Mexcaltitán and Toluca lagoons, in both conditions (without and with the project), is mainly determined by the contributions of the San Pedro river. Unlike the mouths of Camichín and Palapares, where their behavior is mainly determined by the influence of the tide, with an increase in its average level due to the contributions of the river. In general, the water level in the lagoon system for the flash flood season, is above the mean sea level, of the order of 18 cm at the Camichín mouth, 14 cm at the Palapares mouth, and 1 and 1.6 m at the Toluca and Mexcaltitán lagoons, respectively. Thus, if the project comes into operation, it would be expected an average increase in the water level of the order of 1 cm in the Camichín and Palapares mouths. Whereas, the maximum water levels in the Mexcaltitán and Toluca would reach around 2.8 and 2.5 m, respectively.

Regarding the flow velocity, this increase considerably throughout the lagoon system in the rainy season. In the dry season, the velocities analyzed never were above 1 m s−1, while in the flood season, the speeds were close to 2 m s−1 at the mouths of Camichín and Palapares. On the contrary, the simulation with the operation of the PHLC showed that the speed does not present significant differences with respect to the current conditions calculating increments between 0.001 and 0.002 m s−1.

Looking at the discharge values during the rainy season, and for most of the simulation time, the discharge moves out through the mouths toward the sea and exceeds 1000 m3 s−1, which corresponds to the peak of the flood. With the operation of the project, there is an average increment in the system discharges of 1 m3 s−1. In this way, it is considered that during flash floods, practically a constant gradient of the lagoon system toward the sea is established.

Undoubtedly, the greatest change observed in the rainy season is in the salinity concentration of the entire system. This is due to the contribution of freshwater from the San Pedro river at that time that reaches 0 PSU in a short period (2 days) and this behavior is not modified with the operation of the PHLC.

Also, the natural conditions of flooding analysis were assessed (without ribs), and the results showed that there is no significant change because the disturbance in the average annual maximum discharge is not representative. In fact, it is expected a possible attenuation of floods during the rainy season, which is more related to the existence of the reservoir than to its operating policy. This is because, unlike the minimum annual discharges, which are very low in the EH San Pedro, any change in the operation of the reservoir would alter the flooding conditions. This means that only by reducing the operating volume to the desired minimum, the minimum average annual discharge frequency curves (ecological expenditure) could be preserved. Thus, the most important aspect is to establish operating policies that can meet the generation needs and that these policies are viable for the San Pedro river ecosystem conservation. Another determining factor observed was that flood volumes and times are related to sediment dynamics. Thus, larger floods stir up sediment deposition, while shorter flood times alter the rate of sediment deposition.

## **4.4 Coastal wetlands reduction**

The coastal marshes of the San Pedro river own a complex hydrological connectivity due to the existing interaction between freshwater and marine water, which affects largely the spatial distribution of salinity and vegetation in this region. The greatest changes observed in the Río San Pedro-MaNas ecosystem occurred in the alteration of the land cover and use, generating modifications of the hydrology and the local landscape, as well as altering the flood patterns responsible for the functional dynamics of the system, the maintenance, productivity and services that these ecosystems naturally provide to societies. Currently, the construction of any infrastructure can be seen as disastrous as it would profoundly modify the physiography of the entire basin, but at the same time dams, reservoirs and canals are an aid that would help to distribute hydrological resources in a constant manner and control possible flood events. Although the presence of a dam in the middle basin of the San Pedro-Mezquital river would generate initial changes, after the adjustment period, the system would be expected to work supportive of the MaNas conservation by reinforcing the marine influence in the area. This, in turn, would lead to a re-elaboration of the sediments of the coast, which may favor an effective wave that transports sand to the most deprived areas.

The fine sediment constitutes one of the most important aspects in the continuity of the sediment as it plays a determining role in the zone of the lagoons and mangroves for its sustainability. So far, what can be obtained from the work carried out is that the fine sediment will not be significantly retained in the PHLC reservoir. Rather it will re-distribute over time, that is, it will continue passing the same fine sediment but distributed differently over time. In fact, the first results indicate that the amount of fine sediment that would pass through the PHLC reservoir is not significant with respect to the total that reaches the mangrove area since the contribution of fine sediment from the San Pedro river is 0.28 × 106 m3 , while the amount contributed downstream of the EH San Pedro is 1.62 × 106 m3 and this reaches 2.19 × 106 m3 at the outlet to the sea. Therefore, it can be stated that in the hypothetical case, if all the fine material is retained in the PHLC reservoir, only 10% of the material that reaches the sea would be modified, but the reservoir, as mentioned, cannot retain this type of sediment, even if the dam acts as a blocking of the transported sediments to avoid that a lack of sediments downstream could become critical, maintaining the

#### *Interconnection among River Flow Levels, Sediments Loads and Tides Conditions and Its Effect… DOI: http://dx.doi.org/10.5772/intechopen.109175*

floodplain ecosystem. Auel et al. [28] indicated that it is possible to achieve sediment management to maintain the sediments downstream. Some techniques used for this purpose are flushing, sluicing, and bypassing [29, 30]. However, in the simulations carried out by [27], the main deposit of the thick material occurs in the tail of the vessel, for which its extraction must be done through a mechanical mechanism. In theory, the sediment can be moved to the curtain by emptying the reservoir several times, since it is a cannoned channel, that is, it is narrow and the water can reach high speeds for sediment transport. Therefore, if the coarse sediment is really needed downstream of the curtain, an option would be to use some mechanical methods such as dredging and transporting the material to the sites where the coarse sediment is needed.

The operation of the PHLC in the lagoon and MaNas zones shows that flash floods would be minimal, without any important consequence for the downstream areas. Even looking at the velocity results, the operation of the PHLC would not significantly modify the exchange conditions and water contributions to the lagoon system during a flash flood event. However, it is important to mention that with the operation of the PHLC, discharges on the San Pedro river would increase the volume of water that leaves the system toward the sea. Another important result of the operation of the PHLC is that the concentration of salinity would decrease due to the increase in the contributions of freshwater (constant flow) from the San Pedro river during the dry season.

It is during the dry season, when the water levels in the lagoon system increase as a result of the PHLC operation, the difference in levels between the lagoon system and the sea is modified according to:


Twilley and Brinson [31] observed that rising sea level is the main force affecting coastal wetlands, basically because there is no possible migration inshore of them. Migration takes place when the reduction of freshwater flow upstream accelerates salinity intrusion inland but also if the landscape has the possibility to allow mangroves to colonize inland marsh habitats. However, this kind of mangrove migration is almost impractical because of urbanization, tourism, agriculture, and aquaculture development in the zone. Thus, there is a significant reduction of land for this kind of refuge area in many coastal zones, such as the MaNas case.
