Access

Access to the reservoir is via the C-415 main road towards Caravaca de la Cruz, a two-lane road which branches off the A-7 Motorway in the Municipality of Alcantarilla, near the capital of the Region.

From Caravaca de la Cruz (at about 75 km from Murcia) one continues north on the local secondary road C-415 for about 13 km towards Moratalla.

From this town one takes the local minor road MU-703 which goes round the southern flank of the Sierra de Los Álamos. After about 23 km one arrives at Campo de San Juan, from where one takes the access road.
 

Access to La Risca Dam

The access road to the dam goes round the right shore of the reservoir and reaches the dam after about 1,500 metres. After passing the dam now on the left shore the road continues with a branch of about 350 m which leads to the local La Risca road.

The road is 5 m wide with two hard shoulders of 0.50 m each. The road surface is made up of, from the bottom upwards, a 0.25 m sub-base of a natural sand-gravel mix, a 0.20 m base of artificial sand-gravel mix and a top layer of a hot type S-20 bitumen mixture.

Other possible access routes to the dam if the MU-703 road is cut off from Moratalla are as below:

• From Caravaca de la Cruz on the local road C-330 for about 13 km until the turning for Archivel. From this turning, in the direction of Archivel, on the local road MU-702 for about 34 km until reaching the junction with the MU-703. From this junction, taking the MU-703 towards Moratalla, the turning towards the dam is in about 5 km.
• From Caravaca de la Cruz towards Moratalla. From Moratalla on the local secondary road C-3211 for about 21 km until the junction with the local minor road B-30. From this road, to Benízar and then towards El Sabinar, for about 10 km to the junction with the local road which leads to the dam. From the junction to the dam the access road is slightly less than 5 km long.

The location of the reservoir can be seen in the following map:

Characteristics

Imágenes

Purpose

Phenomena such as torrential rain and floods are not unusual in the Segura river Basin. The damage caused by the water has historically always been a worry for local residents both because of economic losses and the losses of human lives.

As a result of the catastrophic floods in the Region of Murcia in the month of October 1973, the “Flood Control Plan of the Segura river Basin” was drawn up. In this document initially no work on the River Moratalla was contemplated.

It was later, once the canalization of the Segura river between Murcia and Guardamar had been analyzed, that the need was recognized for inclusion of defence works on the River Moratalla in the aforementioned Flood Control Plan.

In the Study of Solutions for flood prevention on this river, performed in 1989, the construction of the La Risca reservoir is proposed.

The purpose of the La Risca reservoir is that of flood abatement in this basin, characterized by high intensity rainfall and short concentration times.

By doing this, the peak flow rate is reduced, firstly of the River Moratalla and subsequently of the Segura river after the former flows into it.

The body of the mixed dam is distributed as follows: concrete gravity dam with a straight plan triangular profile on its left abutment and central body, and an earthfill dam on the right abutment. Landslides from the walls of this earthfill dam are prevented upriver by a buttressed wall made of reinforced concrete, and downriver by a wall of reinforced earth. The earthfill dam is completed with a rockfill covering.

The crest in the concrete area is at an altitude of 1,080 metres above sea level, whilst in the earthfill section it rises up to 1,081 metres above sea level. It is 291 metres long, and the slopes in the concrete area have a gradient of 0.75/1 in the upriver face and 0.05/1 in the downriver face; in the earthfill area they have a gradient of 2.5/1 (on both shoulders).

The total height of the concrete dam from foundations to crest is 26.88 metres.
 

Cross section of the concrete dam

The dam is divided into different sized blocks; thus, going from the right bank to the left, the following areas can be defined:

• A 35 metre long stretch which makes up the earthfill dam
• A block of 9.5 metres, denominated B-6, made of rolled concrete which serves as the joint between the earthfill and the gravity dam.
• Three 20.00 metre blocks denominated B-4-2, B-2-1 and B-1-3, which encompass the full length of the spillway.
• Twelve 15.00 metre blocks, blocks B-5 to B-27, odd numbers, with a typical concrete gravity dam section.
• One 6.50 metre block, block B-29, finishing off the left abutment.

The joints between the concrete blocks were sealed using 0.40 metre PVC strips placed at 0.50 metres from the upriver face.

Detail of La Risca Reservoir Elevation
 

The triangular profile of the dam ends at the crest with a slightly trapezoidal stretch, which makes the upriver vertical face whilst the downriver face has a gradient of 0.05/1. The total width of the crest is 7.50 metres, achieved by two cantilevers measuring 1.50 metres each.

Territorial and Administrative Context

Physical characteristics of the basin.

The River Moratalla rises at 1,450 metres above sea level with the name of Alhárabe, at the foot of the Sierra de Arcaboche. After a course of 50 km it flows into the Segura river, at La Esperanza, close to the Sanctuary of the Virgin of the same name, the Patron Virgin of Calasparra. At the confluence, the Segura river has run for 170.5 km from its source and is at an altitude of 273 metres above sea level, the average gradient of the River Moratalla over its complete course being 2.4%.

La Risca Reservoir Inflow Basin

At its mouth, the River Moratalla basin is 345 km², whilst at the point of the La Risca reservoir natural downstream boundary, this is reduced to 76.8 km².

Environmental information.

The environmental factor played a decisive role in the final decision adopted in the Study of Alternatives performed in 1989, given the considerable ecological advantages which this reservoir site offered in comparison to the other options which were considered in the preliminary stages before drafting the Project.

In addition, as a result of the environmental research undertaken for the construction of La Risca reservoir, an Environmental Impact Assessment was performed which was later the base for the corresponding Environmental Impact Statement (EIS).

The results obtained showed that there was no need whatsoever for recovery measures to be taken, though the impacts caused were not so minimum so as not to perform certain corrective measures.

The corrective or impact minimizing measures adopted to alleviate the disturbances caused on the environmental surroundings by the construction and use of the La Risca Reservoir are briefly listed below:
 

• Replace tracks and accesses, as well as the creation of some new tracks.
• Protective and corrective work on the slopes aimed at preserving and consolidating the landscape.
• Revegetation tasks required to achieve adequate reforestation and restocking of both flora and fauna, in line with the intrinsic characteristics of the affected area, both in the works area and its surroundings, in order to hide and try to integrate them into the area’s landscape.
• Take measures aimed at preserving or even improving the existing faunistic habitats. For this, bioecological studies were performed and pools were created at the reservoir tail, including their corresponding shore vegetation, to offer animals shelter and nesting material.
• Establishment of compensation for the population affected, either by the creation of new jobs, or by direct economic compensation.

The EIS, published in State Bulletin (BOE) No. 281 of 24 November 1994, via the Resolution of 26 September 1994 of the Directorate General of Environmental Policies, established a series of conditions. Most of these cover the construction process of the dam, in order to guarantee environmental conservation of the area affected by the dam.

Thus, during construction, a series of actions were taken which are outlined below:

• Surplus products from the excavations were deposited in places where it was intended that they should contribute to an improvement, and not an impoverishment, of the place where they were deposited, recovering the topsoil which was not affected by the works and checking the residues to ensure that they would cause no further deterioration.
• Tipping and protection areas were reforested and restocked and irrigation installed over a total surface area of 2 hectares. The surface of the reservoir to be permanently flooded is that below the height of the undersluice, as the rest of the reservoir will only be flooded for a few hours during the rise of the water level caused by the floods.
• To get the aggregates for the concrete an existing quarry was used, to avoid opening new ones.

With regard to the use of the reservoir, in the EIS text certain conditions are stipulated to ensure that the works are environmentally viable. Amongst these conditions there is one which affects the reservoir’s use:

First. Protection of the fauna... “As it is a dam aimed exclusively at flood abatement, the undersluices shall habitually be left open, and as far as its capacity permits, water will not be collected nor will the flows be regulated, with the exception of the retarded flood evacuation, during which this discharge shall be handled in the most adequate way.”

In addition, La Risca Reservoir is within the “SCI” area (Site of Community Importance) called Sierra de la Muela with code number ES6200018, proposed by Government Council Agreement on 28 July 2000.
 

Geology and seismology.

Regional geology.

The La Risca Reservoir is located in the north-eastern area of the Baetic Mountains, which in turn are in the north-western segment of the Alpine Perimediterranean Orogenic Belt (alignment of mountain chains with sliding stratum structure originating from the Cretacean and Tertiary epochs which are arranged centrifugally with respect to the sea; Martín Algarra, 1987).

Within the Baetic Mountains, the area in question is considered as being within the external areas, just on the limit between the internal prebaetic and the external subbaetic.

General geological map of the Baetics Mountains

Geology and geotechniques of the reservoir.

The reservoir storage area is made up of Miocene sandstone, loams, limestone, although the same clayey loams which surface in the right abutment of the dam are clearly predominant.

Moreover, the materials dip, gently, towards the reservoir, which would seem to guarantee that the reservoir storage area is impermeable and watertight.

The impermeability of the storage area is, according to Amendment I to the reservoir Project, favoured by the almost continuous flow of water in the natural downstream boundary, where the River Alhárabe acts as the main drain. Similarly, the calcareous levels should be taken into account, as they are neither thick enough nor sufficiently long for the faults in the area to establish links and thus leak into the nearby valleys.

The stability of the hillsides or embankments is good, guaranteed by the type of materials, their structural arrangement, and especially by the gentle topography of the reservoir.

Geotechniques of the Natural Downstream Boundary.

This area is characterized by the existence of three lithologies from wall to ceiling:

1. Reddish clayey loams: In the dam area there is a loamish or clayey loam mixture, with very sandy areas, formed by compact loams, some very sandy and others sandstoney, with other areas which include gravels (locally, clayey conglomerates). There is a predominantly reddish or brick red colour. Greenish or greyish green layers are interspersed, in general they are more sandy, classifiable as clayey sandstones, which also occasionally contain gravels.
2. Sandy limestone (calcarenites): They possess conglomerate and breccioid zones, covered towards the W and N by the previous level of calcareous sandstone, set in a flagstone type pattern, with a maximum thickness of 19 m.
   
   It is formed by sandy fossiliferous limestones, very sandy limestones or calcarenites and local conglomerates or breccioid areas, especially when in contact with the clayey-loam level.
   
   They are contaminated at their base by reddish clayey loams when it is breccioid or conglomerate.
   
   They are compact, with subhorizontal dips when they have been identified in samples.
3. Chalky limestone: It has stretches which are lithic and resistant, and others which are poorly cemented and are almost slightly clayey sand. On the left abutment there are slightly less resistant areas of sandstone in the form of less cemented sandstones (they are almost slightly clayey sand). In general, the unit dips about 10º towards the North and locally concordances appear with the lower calcareous level.
   
   The calcareous sandstones surface in a higher stretch of the left abutment. The dam’s foundations are keyed into sandy limestone (Calcarenite) on the left abutment and the river bed, whereas the right bank of the dam has its foundations on clayey loams.
   
   The separation between the clayey loams of the right bank and the sandy limestone is produced by a fault which has been recognized and mapped next to the riverbed, on its right bank.
   
   There are also many other faults and fractures which cross the reservoir and which have been discovered on excavating for the foundations. At the natural downstream boundary, these faults and fractures increase in importance from the left abutment towards the right, where there is greater vertical displacement between blocks. The reservoir site is a tectonically complex area of the internal prebaetic.
   
   The foundations for the concrete buttressed wall upriver from the dam have been keyed into limestone, in an area next to the fault, whilst the reinforced embankment downriver has its foundations keyed into the reddish clayey loams.
    

Seismology.

The seismic hazard differs depending on the place tested and it is standard practice to classify the land depending on this hazard. Following the criteria set out in the “Reservoir Safety Technical Guide. Geological and Prospecting Studies” published by the Spanish National Committee on Large Dams (SNACOLD) in section 3.3 these are classified in three main groups:

a) Areas of low seismicity ba < 0.04 g
b) Areas of medium seismicity 0.04 g < ba < 0.13 g
c) Areas of high seismicity 0.13 g < ba where ba refers to the area’s basic acceleration.

Following this classification, the La Risca reservoir is located in an area which we could classify as being of medium seismicity, as the basic acceleration for the Municipality of Moratalla is of 0.07 g.

According to the Reservoir Safety Technical Guide “Geological-Geotechnical and Prospecting Studies” and taking into account the land factor (using the directive on earthquake resistant construction NCSE-2002 and by analogy with the previous directive), the acceleration used in the calculations for a Project Earthquake of 0.12 g and for an Extreme Earthquake of 0.15 g or 0.18 g are considered, depending on each case.
 

Planos

• Geographical (3,5 MB)
• Cross-section (525 KB)
• Base of the reservoir (3 MB)

Ortofotos

• Photo of the dam (5,4 MB)
• Photo of the reservoir (5,4 MB)

Situation

The reservoir associated with the La Risca dam and its surroundings are within the Segura river Basin in the Region of Murcia, to the North West of the Autonomous Community. They are located on Map 889, MORATALLA, of the IGN (Spanish National Geographical Institute) at a scale of 1: 50.000. The land is in the province of Murcia, in the municipality of Moratalla.

The River Moratalla is a tributary of the Segura river, on its right bank, and on the Official Maps of the National Geographical Institute has two names, where it is indistinctly called the River Moratalla or Benamor. According to local residents, and backed up by the Autonomous Community of Murcia’s Regional Map in its 1982 edition, the River Moratalla is called the River Alhárabe from its source to its confluence with its main tributary on the right bank, which is the River Benamor. This confluence takes place 5 km downriver from the town of Moratalla, close to El Cobo.

The intersection of the La Risca dam axis with the River Alhárabe occurs at the following geographical coordinates:

Reference system ETRS89. Time zone= 30
X=581.670
Y=4.227.487

Geographical coordinates (based on the Greenwich meridian):

X 2º 03' 55'' West
Y 38º 11' 34'' North