32 Maroto 17 - Procedia-ESEM · Maroto et al./Procedia Environmental Science, Engineering and...

Procedia Environmental Science,

Engineering and Management http://www.procedia-esem.eu

Procedia Environmental Science, Engineering and Management, 4 (2017) (4) 245-254

International Symposium on Soil and Water Bioengineering in a Changing Climate,

7th-8th September, 2017, Glasgow, Scotland, UK

ERESMA RIVER SLOPE: STABILIZATION AND RESTORATION PROJECT (COCA, SEGOVIA, SPAIN)*

Rodrigo Maroto1, José Carlos Robredo1, José Luis García1,

Martín Giménez1, Guillermo Tardío2

1Escuela Técnica Superior de Ingeniería de Montes, Forestal y del Medio Natural, Campus Ciudad Universitaria Avda. de la Moreras s/n, Madrid, Spain

2SCIA, Paseo de Lugaritz 25 bajo, 20018 Donostia-San Sebastián, Gipuzkoa, País Vasco, Spain

Abstract The aim of the project discussed in this paper is is to perform geological, climatic and hydrological studies, to design actions focused on solving the problems of instability of one of the slopes of Eresma River when crossing the town of Coca (Segovia, Spain). Studies revealed the necessity to act mainly at the base of the slope, protecting it from the river erosion and increasing its stability. It is also necessary to restore the middle and upper part of the slope, where continuous landslides occur. To size the actions, two software applications have been used: (i) IBER©, used to model possible floods in the river and get information about the height of the water sheet; (ii) GEO5©, used to calculate slope stability parameters. Based on the results, a gabion wall was designed at the base of the slope, as the main solution. In the rest, it is need to recuperate the vegetation to consolidate the soil and avoid landslides. Soil bioengineering techniques that can be used in combination with this objective are: live slope grids, organic mats, reinforced earth structures, live crib wall. Finally, with the proposals for the areas described, the stability of the slope will be increased considerably and the risk of landslides will be reduced. Key words: bioengineering, erosion, landslides, river flood, slope 1. Introduction

The Eresma River passes through the town of Coca (Segovia, Spain) and has

undergone progressive erosion in this slope for several hundred years (Ballesteros-Cánovas et al., 2015; Rodriguez-Morata et al., 2016). This process has been accentuated since 1970

Selection and peer-review under responsibility of the ECOMED project consortium Corresponding author: e-mail: [email protected]

Maroto et al./Procedia Environmental Science, Engineering and Management, 4, 2017, 4, 245-254

246

due to the planting of a cove on the opposite bank, which has caused an increase in the narrowing of the river against the foot of the problematic slope. Time after the plantation a first intervention was made consisting of the elevation of several sections of wall formed into a gabion. This action was able to effectively protect the dwellings located in the area near the beginning of the slope.

After the first landslides of the slope, Coca City Council expresses its concern to the Duero Hydrographic Confederation (CHD). Recognized and identified the problem, in 2003 the CHD drafted the project Improvement of the drainage capacity of the river Eresma and protection of banks in Tower of Saint Nicholas, municipality of Coca (Segovia). Its execution begins in 2007, ending in December 2008. The actions of this project are summarized in the clearing of the right bank - including the expropriation of a strip of contiguous land with the consequent cutting of poplars - and the construction of a rip-rap breakwater wall on the left bank in order to avoid erosion of the foot of the slope. This was done in two areas: on the upstream slope of the Roman access bridge to Coca and on the downstream slope (Fig. 1). The result was satisfactory in the first zone, stabilizing the slope, slowing the erosion and avoids significant collapses. This is not the case in the other area where landslides have occurred, highlighting the magnitude of the event in 2015.

Fig. 1. Tower of San Nicolás and the slope The Project has mainly a socio-cultural justification, which is none other than

preserving the Tower of St. Nicholas. This monument, built in the 12th century and declared of Cultural Interest in 1931 is one of the tourist attractions of the town of Coca together with its famous castle. The Tower is located just 40 m from the head of the slope seeing its future stability compromised in case of not restoring the area. In addition, about 80 m from the head of the slope are located the first houses of this town which despite being far away from the problematic slope, remain exposed to the consequences of the lack of stability.

The above reasons are more than enough to carry out this project, through which it is intended to guarantee the safety and stability of the Tower and the aforementioned houses through the stabilization and restoration of the slope.

Eresma river slope: stabilization and restoration project (Coca, Segovia, Spain)

247

The objective of this paper is to describe and analyze the actions of the project which entails the restoration of slope on the left bank of the river Eresma as it passes through the town of Coca (Segovia, Spain). The main aim is to stabilize and prevent landslides, proposing a series of actions along the slope as well as in the base. Before the establishment of the actions, a multiple study of the situation in the area and its problems are planned to be carried out. This study will be the basis of the project and will constitute the technical support for the justification of the solution adopted.

2. Material and methods

The project presented was done following the scheme showed in Fig. 2. It is divided in five phases, beginning from a descriptive study phase of the zone to an exhaustive hydrological analysis with different tools that can guarantee the results after the establishment of the actions.

Fig. 2. Project framework

2.1. Geology

The cartographic, geomorphological and hydrogeological information of the area was consulted in the Geological and Mining Institute of Spain (IGME). In addition, provincial maps of the National Soil Erosion Inventory (INES) have been used, identifying the geological risks present in the site.

Soil samples have been taken in two locations in the slope, one on the head and the other one on the base. Both paving stones have allowed knowing in detail the most relevant characteristics of the soil. Both points in the slope are characterized by a poorly developed and very sandy soil, circumstances that in no way promote stability.

The hillslope has a high inclination and very low vegetation cover. This last factor is due to the bad soil conditions and the continuous landslides that leave the surface totally


248

exposed to erosion. It is important to highlight the high pH in the area, conditioning the selection of plant species to be used for revegetation and restore this part of the slope.

The sands and scarce clays that form the base of the slope are continuously subject to erosion produced by the river. Also, at the middle part of the slope, soil erosion is generated due to waste water of unknown origin (Fig. 3) which promotes that the slope is loaded with water and the weight of the slope exceeds its limit of stability. 2.2. Climate

The characterization of the climate has been elaborated from the data registered for the weather station located in Nava de la Asuncion. The variables analysed were temperature, rainfall, humidity and wind, giving the first two, the greatest relevance. The study compiles the information recorded in the last 15 years and manage it in such a way that the most influential parameters are obtained in terms of the possible choice of species for the restoration of the slope and the more adequate temporal distribution of the work during the project execution.

After analyzing in detail the climatology of the zone, their influence in the river floods was discard. There is not an area where the torrential rainfall can have significant effects on the slope. This area is characterized by an average annual precipitation and temperature of 415 mm and 11ºC respectively. It is a semi-arid zone, with a mild temperate climate with cold winters and three months in the year of drought. Frost is strong and highlights the great seasonal thermal contrast.

Fig. 3. Seepages at the base of slope generated by waste water 2.3. Hydraulic and hydrologic study

A simulation model of floods has been created for observing the behaviour of the water in the area during hypothetical situations. After creating the model, we proceeded to validate it by comparing the results of the simulation against a real flood happened this same year with the measurements and observations taken in the field. The result was satisfactory, with an error of around 2%. Once validated, a simulation was carried out with the flow rate


249

whose return period was 500 yr. flood. The objective to accomplish was to obtain two parameters that would condition the sizing of the actions at the base of the slope: - maximum levels reached by the water sheet - maximum speeds reached by water

The first of these parameters allows to know the height affecting the base of the slope and therefore, justify the corresponding dimensioning of the performance. The velocity will reflect the points where the shear force of the water is greater, and therefore the zones that will need more protection. The results of the simulation indicate the following: - Along the slope, the water table is located at 746 m above sea level, which means that approximately 6m in vertical from the base of the slope are flooded. - The velocity of water is maximum in the initial area of the slope just at the exit of the pool that is situated upstream of it.

Based on the results, it was decided to carry out the stabilization action of the slope base, from the point where the maximum velocity is reached, to the area where the slope ends. The length of this performance is about 350 m. Once this data is entered, it is still necessary to establish the point of the section where we are interested in knowing the hydrograph. This point is the one corresponding to the slope of Coca which is located 22 km from the station 2053 (Fig. 4). Adjusting the above parameters in WinCaudal3 program to carry out this new calculation gives as an output of the program the hydrograph (Fig. 5) generated by the river crossing Eresma on the slope.

Fig. 4. Reach of simulation

2.3.1. Simulation with IBER 2D program The objective is to be able to observe the behaviour of river during the possible floods

in the area (Fig. 6), with the intention of obtaining two fundamental data in the area of the slope: - Height reached by the water sheet - Slope points where maximum speed conditions are generated


250

2.3.2. Validation of the model The hypothesis is simple, through the various marks left after the flood (Fig. 7) is

going to proceed to estimate. The level reached by water in some points of this area to later verify if this quota, in those same points, corresponds with the one that throws the model.

Fig. 5. Hydrograph of Eresma river in the base of slope

This information will justify the measures to propose to protect the slope as well as the dimensioning of the same.

Fig. 6. Scheme of Eresma´s reach by IBER2D


251

Fig. 7. Poplar in the left bank of Eresma River 3. Results

Once the model has been validated, the simulation will be carried out from which the velocity and height values of the water sheet will be obtained which will condition the sizing of the performances (Fig. 8). In order to carry out this simulation only the hydrographs of the IBER2D must be modified using now those calculated for a period of return of 500 years flood.

Fig. 8. Results of IBER 2D simulations 4. Discussions

The construction of a support heel for the slope by building a wall of gabions (Fig. 9) was proposed. The wall will have a height of 8 m and will extend along the 350 m of the slope. Its base, 3 m wide shall be placed 1 m below the lowest point of the riverbed. In


252

addition, despite invading part of the channel, it will be located some distance from the base of the slope due to the steep slope and poor condition of the terrain. The space of separation between wall and slope is filled with organic material (Fig. 10), which will hinder the exit of natural terrain through the gaps between gabions, allowing e.g. the plantation of poplar cuttings. The last two rows of the wall will be waterproofed on the exterior face by a geocomposite of clayey core in order to avoid that the water can penetrate in the back of the wall.

In addition to this action, an extension of the channel is carried out to avoid that the water sheet rises as consequence of reducing the cross section. Due to the sandy texture of bed, large movements of this during the floods occur, which could imply a risk of undermining of the wall. To solve this problem, we chose to locate a series of background thresholds formed by gabions along the length of the action (Fig. 10). At the same time, rock from the rock will be spread on the surface of the bed, which will make even more difficult the movement of the sands.

Fig. 9. Plan of the proposed solution

Fig. 10. Cross section of the proposed solution


253

The action proposed in the base, constitutes the point of support necessary for the consolidation of the slope, avoiding its undermining. The current situation on the hillside presents some instability that will tend to readjust over time. In order to ensure the stabilization of the slope surface, four solutions based on bioengineering were proposed: organic blankets, living geocells, reinforced earth walls and living walls. From those, the most appropriate technique in this case, is the use of geocells (Fig. 11).

Fig. 11. General arrangement of the organic geocells solution (after: https://www.controlerosion.es/productos/geoceldas)

5. Conclusions The study reveals the case study of Eresma River when crossing the town of Coca (Segovia, Spain) which showed instability of one of the slopes, and the outcomes of the project Improvement of the drainage capacity of the river Eresma and protection of banks in Tower of Saint Nicholas, municipality of Coca (Segovia), drafted to solve the problem. The project was structured in five phases, starting with a descriptive study phase of the zone to an complete hydrological analysis by applying different tools so as to guarantee the results after the establishment of the actions. A simulation model of floods has been developed for detecting the behaviour of the water in the area during hypothetical situations. The model was validated by comparing the results of the simulation against a real flood. Based on these results, some solutions were proposed as support necessary for the consolidation of the slope References Ballesteros-Cánovas J.A., Rodríguez-Morata C., Garófano-Gómez V., Rubiales J.M., Sánchez-

Salguero R., Stoffel M., (2015), Unravelling past flash flood activity in a forested mountain catchment of the Spanish Central System, Journal of Hydrology, 529, 468-479.

Bladé E., Cea L., Corestein G., Escolano E., Puertas J., Vázquez-Cendón M.E., Dolz J., Coll A., (2014), Iber: numerical flow simulation tool in rivers, On line at: ftp://www.gidhome.com/pub/GiD_Convention/2010/papers/p148.pdf.

Bonterra Iberica S.L., (2017), Bonterra geocells for soil stabilization, On line at: https://www.controlerosion.es/wp-content/uploads/geoceldas-para-la-estabilizacion-de-suelos-2.pdf.


254

CEDEX, (1994), Practical aspects of the definition of the maximum ordinary flood, Center of Studies and Experimentation of Public Works (MOPTMA), On line at: https://www.rijksoverheid.nl/.../04/.../flood.../flood-risk.pdf

CEDEX, (2011), Mapa de Caudales Máximos, Centro de Estudios y Experimentación de Obras Públicas (MOPTMA), On line at: http://www.mapama.gob.es/es/agua/publicaciones/Mapa_caudales_maximos_Espana_tcm7-28761.pdf.

IGN, (2007), Manual of the SIOSE Database version 1.12. National Technical Team SIOSE Project. S.G. of Cartographic Production, On line at: http://www.siose/documentacion.jsp.

MAPAMA, (2016), National System of Cartography of Floodplains (SNCZI) Ministry of Agriculture and Fisheries, Food and Environment, On line at: http://www.mapama.gob.es/es/agua/temas/gestion-de-los-riesgos-de-inundacion/snczi/.

Rodriguez-Morata C., Ballesteros-Cánovas J.A., Trappmann D., Beniston M., Stoffel, (2016), Regional reconstruction of flash flood history in the Guadarrama range (Central System, Spain), Science of the Total Environment, 550, 406-417.

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile () /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/CreateJDFFile false /Description > /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ > /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles false /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ]>> setdistillerparams> setpagedevice

32 Maroto 17 - Procedia-ESEM · Maroto et al./Procedia Environmental Science, Engineering and...

Documents

Transcript of 32 Maroto 17 - Procedia-ESEM · Maroto et al./Procedia Environmental Science, Engineering and...