Délimitation des zones de protection autour de la retenue du barrage Hachef (Maroc) par télédétection et SIG

Abstract

La présente étude concerne le barrage Hachef (Barrage nommé 9 Avril 1947), mis en service en 1995, situé dans le Rif Occidental Marocain à environ 30 km au sud de la ville de Tanger et 15 km à l’est de la ville d’Asilah. La retenue du barrage d’une capacité utile de 270 millions de m 3 reçoit un apport solide de 609 000 tonnes/an à cause de l’érosion des versants favorisée par le déboisement. Des foyers de pollution sont également identifiés dans les régions limitrophes du barrage. Dans le but de préserver et protéger cette ressource destinée à l’alimentation en eau potable de la population des régions voisines, il s’est avéré nécessaire de mettre en place des périmètres de protection. Ceci entraîne également une plus grande rentabilité des investissements et infrastructures réalisés. Dans ce travail, une solution informatisée pour la délimitation des périmètres de protection a été conçue et réalisée. Ainsi, un modèle d’analyse spatiale par le SIG basé sur l’évaluation de la vulnérabilité à la contamination de la ressource a été élaboré. Toutes les données disponibles, aussi bien celles issues de la télédétection que celles du terrain, ont été modélisées et intégrées au sein d’une base de données géographique gérée par un SIG. Dans cette expertise, les différents aspects de la problématique concernant la sauvegarde de la ressource en eau ont été cernés et les besoins des gestionnaires ont été recensés. L’utilisation de cette méthode a permis de délimiter les trois zones de protection autour de la retenue du barrage (zones de protection immédiate, rapprochée et éloignée). Due to the degradation of the quality of Moroccan water resources, it is necessary to set up mechanisms and actions to ensure their conservation and protection. The creation of protection zones is one of the more effective means for establishing this protection. The study region involved the Hachef dam, operating since 1995 and located in the Moroccan Occidental Rif, approximately 30 km south of Tangiers City and 15 km east of Asilah City. The watershed area is 220 km 2 with an annual average precipitation of 900 mm. The useful capacity of the dam is 270 million m 3 . By storing 90 million m 3 /year in normal years, this dam supplies a water supplement to the existing resources, covering drinking water requirements and irrigation for the region located between Tangiers and Asilah beyond the year 2015. In order to classify the importance of protection from pollution, the watershed was divided into three zones (zones I, II and III), as defined by their distance from the resource to be protected. In the immediate protection zone (zone I), it is necessary to assure protection of the dam against water pollution. The dimension of this zone should allow for appropriate intervention in the event of an accident and also allow for planting of a forest or a zone of protective vegetation. In the next protection zone (zone II), one should assure protection against pollution and other anthropogenic stressors that could threaten the harnessed waters. In zone III, located furthest away from the resource to be protected, the protection of waters should focus on major pollutant problems, in particular those cases difficult to eliminate such as chemical or radioactive contamination. The demarcation of these zones requires several disciplines and different expertises.In order to facilitate the management, storage and analysis of necessary data for the demarcation of protection zones, we used a Geographic Information System (GIS), in which remote sensing allowed the description of the natural environment (land use, crop types, infrastructure, hydrographic network, etc.). The following cartographic approach represents the analysis of the spectral signatures of the main types of land use by a supervised classification based on the minimum distance method. The study of data in the context of a dam allowed us to identify the information flows, as well as to analyse and collect data in the form of information layers including both cartographic and alphanumeric data (Table 1).In the demarcation of the protection zones around the reservoir, we conducted adjustments and multi-criteria analyses while taking into account a number of parameters (Figure 3). The manipulation of the system was assured by structured tools that offer to the user all the possibilities of GIS management, from data entry to the output of results (Figure 2). The examination of the study region by remote sensing data (Spot HRV image of 17-08-96 and Landsat TM image of 28-03-96), allowed the discrimination of the distribution of the different land use classes (Figure 4). The interpretation of aerial photos and observations allowed us to establish the ground truth against which the image processing results were compared. The adopted GIS contained software to plan the various tasks required for the demarcation of the protection zones demarcation.After analysis of all the potential factors to be considered for the demarcation of the protection zones (geologic, hydrogeologic, climatic, anthropogenic factors), we noted that geologic and hydrogeologic factors make a negligible contribution to the pollution of the reservoir. In contrast, morphology, degradation of vegetation, the hydrographic network and agriculture are important factors for determining the protection zones. Integrating the previously mentioned factors allowed the demarcation of three protection zones, as presented in Figure 5.This work has taken advantage of GIS implementation to define protection zones for a potable water supply (dam). This research has also defined different necessary stages for the conception of a prototype that integrates the remote sensing data into a GIS for the delimitation of these zones. The implementation of these zones represents a possibility for effectively protecting water resources and extending their potential lifetime. This action also generated a better profitability for the investments and the infrastructure.