Chapter 2. Description of the study area and its flooding problems

Table 2.5: Morphometric coefficients for two sub-basins of the Basin (Garc´ıa, 1990)

Coefficient Formula Turrialba Basin Colorado Basin

P Compacity coefficient K = A 0.44 1.16

Aw Shape Factor index FF = L2 0.25 0.18

Average slope in % 19.66 33.99

Nu Bifurcation ratio Rb = Nu+1 3.75 3.30

Lu Length ratio Rl = Lu−1 2.06 2.05

Lt Drainage density Dd = A 2.84 3.53

C Drainage frequency Df = A 2.62 4.56

P = Perimeter of the basin, A = Area of the basin, Aw = Average width of the basin, L = length of the basin, Nu = Number of river segments of order u, Lu = Average length of the river segments of order u, Lt = Total length of river segments within the basin (km), C = Amount of river segments within the basin

2.2.1 Location of the study area

Turrialba City is located in the coordinates 9◦5402000 North and 83◦4100800 West, at 640 metres above the sea level. It forms part of the Turrialba District, which belongs to the canton of the same name. The population of Turrialba Canton, by 2000, was 68510 inhabitants, with 46.7% of them (32004 inhabitants, 50.1% women, 49.9% men) living in Turrialba District (INEC, 2001). Specifically, the study area has a square shape that is defined by the Lam- bert coordinates 208320–211950 N and 569680–573770 E of the topographical sheet, scale 1:50,000, of the National Geographical Institute of . It includes the centre of Turrialba City, as well as some surround- ing neighbourhoods, such as Campabadal, Las Americas, Nochebuena, San Cayetano, Poro,´ La Margot and La Guaria (see Figure 3.1). The rivers found within this area are: Turrialba, Aquiares, Azul and Col- orado, as well as the Gamboa Stream with its tributary Tunel´ Stream. In addi- tion, the streams Poro,´ Puente Vigas and Barahona can be cited as tributaries of the Colorado. As can be seen in Figure 2.4, there are several minor water- courses that do not have a name. Also, some sections of the Gamboa Stream, the Colorado River and one of its small tributaries run underground through culverts.

2.2.2 Geomorphology of the study area

Six major geomorphological units were identified in the study area (see Fig- ure 2.5) using photo-interpretation and fieldwork observations.

11 2.2. Definition and characterisation of the study area

Figure 2.4: Map of the rivers included within the study area. A differentiation is given between rivers, streams and underground sections

They are described as follows:

• Denudational hills Located in the southwest and northeast corners of the study area. They consist of very steep slopes (30–60%, according to Calvo (1993), on lavas (southwest hills) and pyroclastic rocks (northeast hills) strongly affected by river incision and mass movements. The top of these hills has been intensely weathered, originating a thick soil layer of fine texture.

• Debris avalanche slopes Present in the north-central and northwest parts of the area. This unit is characterised by an irregular topography with slopes ranging from 3% to 20% (Calvo, 1993) and numerous rounded hills separated by depres- sions. Huge lava blocks form these hills. Another important feature is the absence of a superficial drainage network, due to the large amount of

12