18.1 Purpose: To protect the area bordering the main river channel, promoting the forestry use of those flat, low-lying parts, which sometimes present minor or abandoned channels, prone to reactivation during floods slightly greater than the normal maximums during the rainy season; 1. Río Quebradas: from the Quebradas Zone to the "El Panameño" hill, 50 meters: on both sides of the river.
2. Río San Isidro from the "El Panameño" hill to its confluence with Río Jilguero, 15 meters on both sides.
3. Río Jilguero up to the SNAA treatment plants, 15 meters on both sides.
4. From the treatment plants passing through El Hoyón, to the extent of the Regulatory Plan boundary, 50 meters on both sides of the river.
5. Río Rivas and Buena Vista, General Viejo, Pejibaye, Platanares; 50 meters on both sides of the indicated rivers.
6. The alignments on the rivers and streams shall be those determined by INVU through the application of the Forestry Law (Ley Forestal) number 7575, articles 33 and 34, that is, immediately after the limit up to where the Regulatory Plan extends.
18.2 Rivers that cause problems in the Canton of Pérez Zeledón.
18.2.1 Río Quebradas: In the northern sector of this depression lies Río Quebradas, Río Jilguero, tributaries of Río Pedregoso and the latter of the Pacuar, which crosses the southeastern part of the city of San Isidro.
During periods of intense rain, the community of Quebradas, located 5 km to the north, is affected; and on July 28, 1996, it was no exception, where the most important infrastructure works such as bridges and roads, in addition to dwellings, were damaged.
18.2.2 Río General, Río Buena Vista, and Chirripó Pacífico: Their most important characteristic is that they present a channel with a steep slope, where many rivers and streams converge, contributing sufficient water and material due to the effects of landslide and undercutting of the channels downstream. The tributaries of Río Buena Vista are mostly streams with a high gradient and a large contribution of material, among which are: Azul, Ernesto, Pueblo Escondido, Boquete, Gemelas, Pueblo Nuevo, among others. The confluence of the rivers Buena Vista and Chirripó Pacífico forms the Río General, where great dynamism is evident in the surfaces traversed by the main river and tributaries, where microrelief features, such as recent alluvial terraces and abandoned channels, extend to the confluence with Río Pacuar.
Historically, the report of the largest floods caused by Río General, which is the main tributary of the Río Grande de Térraba, occurred between October 12 and 14, 1955, as a result of a strong storm that hit the country, where bridges and several dwellings were affected in the localities of Hermosa and Peñas Blancas.
This feature of the fluvial process highlights the dynamics of undercutting and outward migration of the outer bank of the river, this displacement coinciding in some sections with the right-of-way of main roads where erosion is very strong, causing the washing away of access roads to Rivas, Pueblo Nuevo, Palmital, and Buenavista, which in turn could not be easily rectified, as it is located between the river channel and mountainous terrain, where the hillsides have steep slopes.
Río General flows through an alluvial plain, characterized by meandering dynamics manifested by the sinuosity of its main channel and the presence of abandoned channels, which are easily reactivated by heavy rains or extraordinary hydrometeorological events, a typical case being Hurricane César.
18.3 Permitted Uses: The zones delimited as High Risk on the zoning map at a scale of 1:10,000 and hazard zones at a scale of 1:50,000, correspond to the area adjacent to the fluvial channel, prone to periodic flooding, which by provision of laws, shall be considered as "Protection Zones," consequently it is prohibited to carry out agricultural or livestock activities within them or to remove vegetation; only reforestation with plant species native to the zone shall be permitted.
18.4 Communities located on the banks of the rivers:
La Palma: The most vulnerable sector is located at coordinates (493 E) and (367,150 N), sheet San Isidro, in the vicinity of Río Pacuar, where due to the effects of undercutting and overflowing near the bridge known as La Palma, it was damaged, in addition to the destruction of several dwellings.
The potential effects of Río Pacuar overflowing, upstream of the La Palma bridge, are high; therefore, the adjacent zone (left bank) may at any time be very vulnerable, mainly in the surroundings of the restaurant.
El Hoyón: A sector adjacent to the school in the place known as El Hoyón was drastically affected by the undercutting (socavamiento) of the right bank of the Jilguero River, where several houses were completely destroyed, in addition to the erosion of a stretch of the road that disappeared entirely. A part of the soccer field of the El Hoyón educational center disappeared, leaving exposed and at high risk part of the sewage infrastructure, generating pollution problems.
The El Hoyón educational center must be relocated, as it is located in a High Risk zone:
Hospital Nuevo and Santa Cecilia: Severe erosion occurred on the right bank of the Jilguero River, where both homes and bridges were seriously affected. Due to this situation, it is necessary not to authorize any construction permit in the High Risk area of the Jilguero River.
Rivas: The community of Rivas, and particularly the Pueblo Nuevo neighborhood, was one of the most vulnerable localities in the canton of San Isidro, where part of the neighborhood disappeared almost completely. This was because many of the homes were built on old channel fill (rellenos de cauces), which were reactivated by the heavy rainfall caused by Hurricane César.
Unfortunately, the population of Rivas is settled on old alluvial terraces (terrazas aluviales) of the Buena Vista River, with little height, making them susceptible to flooding by the dragging of logs, rocks, mud, etc. For this reason, a High Risk zone is established where urban development is not permitted, along with regulatory policies for land use (uso del suelo), promoting the reforestation of denuded areas and agricultural practices that do not favor erosion.
Similarly, the road between Rivas and Pueblo Nuevo was completely altered, as the highway was entirely eroded and became part of the river. A part of the new route was built along a rather unstable hillside, where the cuts of the slopes (taludes) and terrain characteristics favor the occurrence of landslides, in addition to small streams that have a high gradient, which drag a large amount of material from the upper parts.
La Hermosa: Historically, the most severe damage from floods generated by the General River occurred on October 12 and 14, 1955. By July 1996, the community of La Hermosa, located in an area of flat topography on the left bank of the General River channel, benefited from a greater impact caused by the overflowing of the waters, with the dragging of a large amount of sediment, rocks, and logs that caused destruction to all homes located in this area, where the impact was greatest.
Because it is considered an area extremely susceptible to being affected, and given the evidence of old abandoned channels, rocks in adjacent lands that were deposited by ancient floods reflect being faithful witnesses that at some point in the river's history, the area was floodable by possible extraordinary events. From the photogrammetric and hydrological analysis, it is concluded that it corresponds to a HIGH RISK ZONE.
District of Pejibaye: The location of the neighborhood called "El Colegio" on the right bank of the Platanares River, in an area of clear fluvial influence, favored its overflow destroying the entire neighborhood completely.
The clear evidence of a floodable area is determined by the signs of ancient floods, where blocks of rocks and trunks of old trees are buried under a certain layer of sediment, being faithful witnesses that the area was not suitable for settling homes.
The location of the district of Pejibaye can reflect that, to a certain extent, the area comprising the central sector can be flooded if the conditions in the basin are favored by the contribution of much sediment due to landslides and heavy rainfall, as well as extraordinary climatic events. The High Risk zone was delimited on a map at a scale of 1:50,000 where urban development will not be permitted.
Locality of San Rafael de Platanares: The zone is characterized by presenting a very irregular topography where landslides in the upper basin of the Platanares River, caused by soil saturation, the steep slopes of the adjacent lands, and high runoff rates due to deforestation, caused the channeling (encañonamiento) of water into narrow channels that filled the few flat areas on their banks, flooding them and depositing a large amount of mud and logs. This was favored by the blockage of the small bridge over the Platanares River on the road leading to Bolivia. The entire town is located in a High Risk flood zone.
El Brujo de Savegre: The settlement of El Brujo was located in a flat topography area of clear fluvial influence, mainly near the División River, where the town was razed by the overflowing waters. The high instability observed along the road that provides access to this community favors and contributes to a future where, constantly, there is greater sediment transport towards the channel, contributing more volume and sediment dragging, overloading the river and streams, causing overflow mainly in the flatter parts and completely flooding these sectors.
Therefore, due to the direct impact of the División River, it is established that the flat area located between the access road and the channel is highly vulnerable to river overflow, and no type of construction should be authorized in that zone.