Bilan hydrologique et envasement du barrage Saboun (Maroc) Hydrological balance and siltation of the Saboum dam (Morocco)

Résumé

La présente étude s'inscrit dans le cadre d'un programme de recherche sur la gestion des lacs et barrages collinaires en zone méditerranéenne HYDROMED. Elle a pour objectif le suivi hydrologique et la sédimentation du barrage collinaire Saboun, situé dans la zone de Tanger au Nord-Ouest du Maroc. Un dispositif hydro-pluviométrique automatique a été installé à ce sujet depuis novembre1997. Il a permis le suivi du bilan en eau de la retenue et l'analyse de son fonctionnement hydrologique. Parallèlement, une étude de la bathymétrie par sondage ponctuel a montré que le taux de sédimentation est de l'ordre de 2% par an. Cette perte en eau de la capacité totale de la retenue est une conséquence directe d'une forte dégradation des sols du bassin versant estimée à 115 T/ha/an.

Mots-clés:

• Barrage collinaire,
• bilan en eau,
• hydrologie,
• envasement

Abstract

It is proposed that many Mediterranean countries could make better use of their water resources by constructing small hill reservoirs. Since 1980, Morocco has built more than 158 small dams in order to meet the requirements of rural population water use. Due to both environmental degradation and bad management, these small infrastructures are now in a critical situation. Furthermore, pollution and siltation, resulting in a deterioration of water quality and a reduction of water capacity, are problems at these sites.

The present study is part of the HYDROMED research program on the hydrological balance of small dams in the Mediterranean area. Its objective is the monitoring of hydrological balance and siltation of a small dam, Saboun, located in Tangier in Northwest Morocco. A remote hydrological station was installed in November 1997. This station allowed the examination of the dam water balance and analyses of its hydrological function.

The analyses of water balance demonstrated that the volume of water stored in the Saboun reservoir increased rapidly in November 1997, and was then followed by a progressive decrease. The load factor varied from 1.9 in 1997/98 to approximately 2 times the current capacity of reservoir, to only 0.3 in the following year 1998/99. These results show that even for a short hydrological period (2 years), the hydrological balance of the Saboun reservoir varied greatly as a result of the irregularity of its hydrological parameters. The essential reasons for this variability were the rainfall characteristics that control the runoff from the watershed and bad management practices.

In addition, a bathymetric survey was carried out by a punctual method to estimate the rate of dam siltation and sedimentation from the watershed. The results of this control demonstrated that the dam siltation rate was about 2 % per year. This loss of total water retention capacity of the dam is the consequence of high specific soil degradation of the watershed. The rate of sediment delivered from watershed and deposited in the reservoir was estimated at 37 T/ha/year. The total soil erosion for watershed was estimated at 115 T/ha/year by integrating both the sediment evacuated at the time of dam overflow and the sediment deposited in stream flow. This erosion, which comes essentially from cultivated soil (95 % of the watershed surface), may have serious effects on the hydrological balance of the dam, particularly by reducing the reservoir capacity of the dam, and on the reservoir water quality.

Keywords:

• Small dam,
• water balance,
• hydrology,
• siltation