**4. Results and discussions**

To understand the effect that the PHLC would have on the flow downstream, a simulation was carried out considering two scenarios: a historical condition and the PHLC for dry and rainy seasons.

#### **4.1 PHLC functioning during the dry season**

**Table 2** presents the discharges observed in the lagoon system without and with dam during the dry season in the sections identified along the river. The negative signs in the average discharge indicate that the direction of the flow is toward the sea.

When the average flow rate is 0 m3 s−1, it indicates that during the analyzed period, the volume of water that entered the system is the same as the one that left. If this is different from 0 with a positive sign, it indicates that, during the studied period, the volume of water that enters is greater than the one that leaves and, on the contrary, if the sign is negative, it indicates that more water leaves than the one that enters the system. In this way, it is observed that with the operation of the PHLC, when the discharge in the San Pedro river increases, the volume of water that leaves the system toward the sea increases (**Table 3**).

Also, it was observed that when the levels in the lagoon system increase due to the operation of the PHLC during the dry season, the difference in levels between the system and the sea is modified by: (a) in high tide conditions the gradient is lower; therefore, the flow of water from the sea to the system decreases, a situation that is reflected in the reduction of the discharges that enter through the sea toward the lagoons; and (b) low tide condition, the gradient is greater; therefore, the flow of water toward the sea increases as well as in the flows that leave the system. In particular, in the Camichín mouth (SEC25), there is a major exchange of water between the sea and the lagoon system, leaving the Palapares mouth with a reduced exchange (SEC11). This water exchange defines the maximum, average, and minimum water levels at each site with respect to the two established conditions (without and with the PHLC) as shown in **Table 4**.

As highlighted in **Table 4**, in the four sites, the water level impact due to the operation of the dam is not significant. This is confirmed in **Table 5**, which shows the maximum, average, and minimum variations reached for each site.

The water level variation associated with the operation of the PHLC during the dry season showed that at the Camichín mouth there are no differences, but in the Palapares mouth and in both lagoons (Mexcaltitán and Toluca), there is an increase in the average level of the order of 1 cm. Thus, the average water level for the four sites is 0.18 m, 0.14 m, 1.0 m, and 1.6 m, respectively. In addition to the discharge and water


#### **Table 2.**

*Maximum (toward the system), reflux (toward the sea), and average discharges in the lagoon system, without and with dam during the dry season.*

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


**Table 3.**

*Discharges variation for the operation of the PHLC in the dry season.*


#### **Table 4.**

*Maximum, average, and minimum water levels without and with dam during the dry season.*


#### **Table 5.**

*Variation of levels due to the operation of the P. H. Las Cruces (PHLC) during the flash flood season.*

levels data, other aspects to be considered in the operation of the lagoon system are velocity and salinity.

Reviewing velocities for the four sites, one can find that the main changes were observed in both mouths of Camichín and Palapares. In the first, the average flow velocity for both scenarios (without and with a dam) is around 0.933 m s−1. However, the maximum velocity was 2.097 m s−1 for current conditions and 2.099 m s−1 for the project conditions, so there would be an increase of the order of 0.002 m s−1, which is not significant. At the Palapares mouth, an average velocity of 0.840 m s−1 for the current condition, and 0.842 m s−1 for the project condition were observed, which shows an increase of the order of 0.002 m s−1, which is also not significant. The maximum velocity registered was 1.807 m s−1 for the current conditions and 1.808 m s−1 with a dam; thus, there is an increment due to the operation of the project of 0.001 m s−1.

In 2016, the Institute of Marine Sciences and Limnology (ICMyL) of the UNAM established an initial salinity of the system and concluded that there is an important change from almost 0 in the river to close to 30 PSU in the lagoons, and then a gradual increase from the lagoon to the sea, as shown in **Figure 6**. Salinity is measured in practical salinity units (PSU) or ppt (parts per thousand).

To estimate the salinity of the system under PHLC operation, simulations were carried out considering that the San Pedro river has fresh water, while the lagoon

**Figure 6.** *Salinity measured in the system in May 2014.*

system presents high salinity concentrations (29–32 ppt), and close to the sea the salinity is 33 ppt. Thus, for the dry season, the average salinity of 32.9 ppt was obtained in the mouths of Camichín and Palapares, as well as in the lagoons of Toluca and Mexcaltitán in current conditions, and with the dam, the salinity decreased by only 0.1 ppt. The latter is due to the PHLC operation, where it is expected a greater contribution of freshwater coming from the San Pedro river; thus, the salinity concentration decreases mainly in the Mexcaltitán and Toluca Lagoons, with average values of 11 and 3 ppt, respectively. Similarly, in the mouths of Camichín and Palapares, the salinity decreases on average by 0.1 and 0.8 ppt, respectively.
