**2.1 PHLC: reservoir characteristics**

Las Cruces Hydroelectric Project (PHLC) curtain is located in the state of Nayarit on the San Pedro river, approximately 80 km from the river's discharge to its floodplain to later drain into the MaNas lagoon area and, finally, to the Pacific Ocean. **Figure 2** shows a general scheme of the PHLC contribution basin, having considered a tributary area of 24,879 km2 , as well as the general plan of the reservoir, which has a maximum flood length of approximately 60 km on the San Pedro river considering the elevation of the OMWL (ordinary maximum water level) at 238 masl.

The most important data of the reservoir concerning this study are: (a) OMWL capacity of 2267 × 106 m3 and (b) extraordinary maximum water level (EMWL) capacity of 2485 × 106 m3 . **Figure 3** shows the runoff and sediment volumes at the inlet and outlet of the reservoir, as well as the hydrometric stations (EH) of Pajaritos

**Figure 2.** *General plan of the reservoir and the contribution basin to the PHLC. Source: Ref. [25].*

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

#### **Figure 3.**

*General plan of the reservoir: runoff and sediment load. Source: Ref. [25].*

and San Pedro. The bathymetry of the reservoir was obtained from the LIDAR information provided by [25], and the hydrometric information used to obtain the average monthly discharges was obtained from Ref. [26].

**Table 1** shows the monthly average discharge values and the monthly mean concentrations of transported fine solids obtained from the monthly sediment averages. The monthly mean concentrations of fine solids were obtained from the quotient of the monthly sediment volumes divided by the runoff volume.

Annual precipitation throughout the San Pedro river basin shows high variability. However, there is a clear trend, with precipitation being greater as one move toward the outlet of the basin. In fact, the highest rainfall occurs in the MaNas area, where it reaches more than 1.5 m per year. The average annual discharge calculated at the EH San Pedro station was 87.2 m3 s−1, the maximum 204.7 m3 s−1, and the annual minimum 24.6 m3 s−1. According to the records, the maximum annual average discharge was presented in 2008, whereas the minimum was in 2005. The San Pedro river basin recorded 53 floods from 1944 to 2004, causing significant socioeconomic damage [26]. It was determined that the flood season occurs mainly from July to November. These floods are associated with the fact that the San Pedro river basin is subject to extraordinary meteorological events. In particular, the rainfall in this area is due to the presence of the Mexican monsoon, which causes above-average rainfall, although not always with the same intensity during the months of June, July, and August. The monsoon is produced by a break in the atmospheric circulation in the northwest that generates the release of latent heat and a rise in temperature and wind and, consequently, an increase in humidity. The humidity flow begins in the so-called intertropical zone of convergence and is accentuated by the tropical waves caused by the trade winds, which come from the central Atlantic, generating a displacement of humidity toward the Pacific coastal zone. But, also, droughts occurrence was identified for two historical periods, from 1976 to 1980 and 1995 to 2001, with 5–7 successive years of null flow, respectively. Regarding the river regime, flows between 1000 and 1400 m3 s−1 were observed in the case of very dry years, whereas in average years,


#### **Table 1.**

*Average monthly discharges and concentration data.*

the accumulated flows are around 2200 m3 s−1, and in very wet years of a more than 4000 m3 s−1. If one considers a design flood with a return period (Tr) of 10,000 years, peak flows of 12,237.8 m3 s−1 and 12,867.4 m3 s−1 were obtained. When this design avenue passed through the spillway, a maximum discharge rate of 9048 m3 s−1 was obtained, reaching a maximum level of 2.3 m, which is less than the EMWL.
