**4. Description of the middle section of the hydrogeomorphological chart of the Lluta river**

The origins of the upper section of the Lluta river basin have been explained in detail, considering its foundations in terms of hydrographic and hydrological factors, which continue its journey downstream. Knowing the origins of its behavior and characteristics, the result of this is synthesized in the middle and low sections.

Each number in this section described in **Figure 15**, corresponds to the following:


6.Very Low Infiltration Capacity (Sedimentary and Mixed Rocks)

7.Temporary watercourse

8.Vegetation less than 25%

In this section, it is joined, always on the left, by the Socoroma ravine, the last contribution of surface water that the Lluta receives. It is born in the snowless cordon that divides the Lluta basin from the Seco river basin to the south, a tributary of the San José river. In Coca, the last place of cultivation, the excess expense of spills and filters totals 25 liters/sec (Hydrographic Holes of Chile I Region of Tarapacá DGA).

Starting from Dos Hermanos, 77 km from the sea, the valley decreases its slope and widens. It begins in Chironte, 73 km from Chacalluta, the middle and lower course of the river with Agriculture Valley as shown in **Figure 17**.

Between Chironta and Boca Negra, several streams that are usually dry fall into the Lluta, but tend to grow with the summer highland rains. The main ones on the right are those of Chironte, in the sector of that name, and Palmani in the Vila Collo sector. Further down, on the south side, the Lluta receives the Chaquire ravines in Chaspisca and even further below Molinos, the Boca Negra ravine. Both are born in the Sierra de Huaylillas. From this point downwards, the Lluta increases with spring waters. From Boca Negra, the valley widens and the slope decreases to a value close to 2%.

In order to estimate the existing risk of flooding in the event of a major hydrometeorological event, the maximum flows have been obtained for return periods of 5, 10, and 20 years. For this, data from the fluviometric stations corresponding to Lluta in Alcérreca and Colpitas in Alcérreca have been used.

In the middle section of the Lluta river, as has already been mentioned, there is the consequence of the active processes in the upper section of the basin, large material that has been dragged, eroded, and undermined until reaching the middle section. It mainly constitutes an area for depositing sediments, which have been increasingly

#### **Figure 17.**

*Representation of the hydro geomorphological chart of the Lluta river (middle section) (source, from the author).*

#### *Risk Reduction DOI: http://dx.doi.org/10.5772/intechopen.109404*

physically, chemically, and biologically weathered upon reaching the middle section, so the material arranged in this section is mainly in boulders.

This is reflected in the hydro geomorphological chart with an extremely low infiltration ability (sedimentary and mixed rocks) in the riverbed as such. After the bed riverbed, we find medium infiltration ability (fractured volcanic rock). And a high infiltration ability the further away from the bed riverbed.

Vegetation of less than 25% is found, which decreases with respect to the high section. As there is less vegetation in this area, it prevents the normal flow of surface runoff from being lost, which is why the normal processes of slopes are triggered, such as linear erosion, undermining, slip niches, producing dragging of material toward the lower section of the river and, due to the force of scour and drag with which the surface runoff comes from the upper section, temporary watercourses are generated.

Already within the analytical description, in the area of influence, it was determined that the western sector of the course of the Lluta near Putre is the one with the greatest slopes. In general, the horizontal, gentle, and moderate slopes are associated with the bed of the Lluta river, the colluvial glacis, and the volcanic terrace, see the Map of slopes. In these last two, the sediment movements are conditioned to the dynamics coming from precipitation as it drains on the surface as sporadic laminar flows.

The area of acute danger is identified as all those forms that show current activity: the active valley slopes and the excavation slope, whose slopes are complex (great gradient variability, i.e., between 50 and 100%) and that are potentially exposed to events of colluvial origin, such as rock falls, debris flows, translational and rotational landslides, and complex movements, with great granulometric variability, from fine to coarse.

The area of intermediate danger, are all those that can border with areas of acute danger, being forms of ancient or recent origin and that can present slopes of 5 to 20°. Its risk is medium due to the potential danger that its immediate contiguity offers to slopes with acute risk, which through mass movements, such as debris flows, rock falls, rotational or translational landslides, and complex movements, feed the slopes making the escarpment It is considered that these phenomena will be less recurrent than the areas of acute danger.
