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Newsletter and Technical Publications 1.4 Groundflow Collection through Underground Structures The technologies outlined in this section are based on utilization of groundwater with the use of structures excavated in a horizontal direction, in contrast to the traditional approach of vertical excavation, through mechanisms capable of reaching depths of up to several hundred, and in some cases, several thousand meters. Technologies that depend primarily on horizontal tunnel excavation into aquifers include the Aflaj systems (also known by other names, such as Fajjara in Syria and Kahariz in Iraq) infiltration galleries, and hand-dug wells (sometimes referred to as radial wells) connected to tunnels. The main characteristic of these technologies is that most of the water flows through tunnels or galleries excavated horizontally, although vertical excavation is conducted initially to explore the aquifer or to reach the groundwater levels permitting utilization of the aquifer in a horizontal direction. These technologies use structures that can function for hundreds of years, in contrast to the tubular wells, which are susceptible to corrosion and whose working lifetime does not usually exceed 50 years. More important is the suitability of these technologies to the conditions of the dry areas. Renewable groundwater resources in arid and semi-arid zones are usually available in valleys, in scattered aquifers of limited extensions in horizontal and vertical directions. Thus, it is not feasible to utilize these aquifers in a sustainable manner using tubular wells, and water levels soon fall as the aquifer is partially or completed depleted. This situation was experienced in the Al-Qalamoun basin in Syria, where the Aflaj systems were replaced with systems with tube wells. After a period of time using the Aflaj system to facilitate development of the area, the groundwater aquifers were depleted with the use of tube wells. This caused the local relevant authorities to excavate wells to depths that sometimes exceeded 600-700 m. Studies indicate that deep soil layers in countries of the West Asia region are mostly fossiliferous in nature, in which water is utilized using a groundwater-mining technique. Water of alluvial layers is widespread in wadis, being hydraulically connected to the wadi’s surface runoff. Alluvial layers are usually shallow and characterized by their renewable water. Isotope studies on alluvial water indicated the surface runoff in wadis is one of the most important sources of continuous recharge for groundwater aquifers. Aflaj and shallow wells discharges may decrease in drought periods, but recover their natural equilibrium conditions when these periods end. The advantage of conserving the water balance and the sustainability of this limited water resource is that it complies with a sustainable development concept. On the other hand, using the aquifer water at high rates can result in water deficiency and quality problems. For very thick and extended aquifers which receive substantial recharge because of their large extension or high rainfall intake areas, these aquifers are utilized via borehole vertical wells penetrating through several shallow and deep aquifers. It is clear thatsustainable water development can be facilitated by effective management and continuous monitoring of the impacts of water use on the storage and quality. Technology Description Al-Aflaj Al-Aflaj are sub-horizontal tunnels with widths between 0.5-1 m, heights between 1-1.5 m and maximum depths about 50 m below the land surface. The length of the Aflaj systems varies from several hundreds meters to several lilometers (e.g., the Al-Gaddey Falaj in Sultanate of Oman has a length of 15.5 km). The length of the Falaj is inversely proportional to the slope, whereas the length ranges between 0.5-3.5 km in very steep areas. Al-Aflaj systems obtain their water from shallow groundwater characterized by high water levels relatively close to the ground surface. In general, they are excavated in alluvial and loose layers. Although Al-Aflaj systems are rarely excavated in rock layers, if it is necessary to do so they are excavated in weathered, broken and cracked parts. From a hydrological perspective, Al-falaj consists of two parts. The upper part containing the tunnel lies under the groundwater surface (Figure 28) and is known as the absorption part (in Syria) or Falaj utilities (in Sultanate of Oman). This part of the Falaj works as a horizontal well, acting as a tiled drain collecting water through fissure carbonate (or crystalline) layers or porous media (gravel, conglomerates, sand, sandstone, alluvium, flood deposits). This part of Al-Falaj can consist of several side tunnels in different directions opening into the main tunnel (Kahariz in Iraq). The lower part of Al-Falaj is mainly a channel above groundwater level, and may leak part of the water collected in the upper part of Al-Falaj unless measures are taken to control or prevent the leakage. The Falaj channel conveys the water to the ground surface. The water is then distributed to the ground surface through a special trap called “Al-Sharee’ah” or “Al-Kasryah”, which is the first point where users can obtain the water. In cases of relatively high water discharges from Falaj, the water can be distributed in two stages (Figure 29). In the first stage, a group of channels are branched at the Al-Kasryah, ending at a secondary distributor to discharge water in the designated channels. It is noted that the upper parts of the Falaj represent the well openings used to extract the tunnel excavation material. They also are used later for ventilation and maintenance. A tank is attached to the Falaj in some cases to collect some of the water flowing out of the Sharee’ah. Falaj projects depend on a good location of the orignal well, and the Falaj systems generally rely on local water flow systems. The absorption channels (Falaj utility) are excavated outward from the original well at an excavation depth about 1.5 m below the groundwater surface. The channels are excavated as much as possible parallel to the direction of the groundwater flow. This covered part of the Falaj is usually excavated via excavation of a group of vertical well (diameters between 1-5 m) about 5 m between the neighboring wells for steep areas, and about 30-60 m for relatively level areas. The tunnel is then excavated between these vertical wells to a width between 0.5-1 m and a height between 1-1.5 m. The tunnel bottom and sides are lined, whereas the vertical wells are protected with mortared rocks. The well top is usually above the ground level, with the excavation material piled around the wells to protect them.
Figure 28. Ideal cross-section for a Falaj
Figure 29. Distribution of Falaj water After allocating sufficient water volumes for the Falaj, surveys are conducted to determine the longitudinal section of the water withdrawal channel. The channel is usually excavated in an inverse manner, with the excavation starting from the outlet point on the feeding tunnel’s side. The tunnel excavation technology may be replaced in the lower parts of the Falaj, where the bottom approaches the ground surface. This part is excavated like the trench of an open channel. After lining it with a cement or clayey mortar, the excavation material is replaced. At the Falaj outlet, the excavation process starts with an open channel for users to withdraw the water. This part can be covered to control evaporation and leakage losses, and to protect the Falaj water from contamination. In regard to the water basin, the lengths of the open and covered parts of the Aflaj vary according to the region and location of the Falaj. The feeding part of the Falaj or the water collection area extends to short distances in mountainous regions, whereas the part that conveys Falaj water to users extends for long distances and is usually meandering to follow the land topography. In contrast, the Aflaj of plains rely on tunnel excavation technology, with its covered parts reaching several kilometers. The covered length of the Barrimy oasis, for example, extends about 10 km. Since the main characteristic of the Aflaj is a gravity-based water flow from the supply to the consumption areas, a mild slope (typically ranging between 1/500-1/2,500) is required for the channel bed. This optimal slope minimizes silting and deposition in the Falaj. The proper design of the Aflaj has several economic benefits, including reducing water leakage losses in the covered parts, and evaporation losses in the open parts. Proper site selection also can facilitate the drainage process for agricultural lands irrigated by the Falaj. On the basis of experience acquired from study and management of Aflaj systems in the southeastern region of the Arabian Peninsula (Oman and United Arab Emirates), Aflaj are classified into three main types:
The first type of Aflaj systems is common in valleys’ beds and comprise either perennial systems, or seasonal systems in which there are water flows of short duration. The Ghayli Aflaj are usually open channels, with the surface runoff as the main source of recharge. They may also be partly covered to reduce evaporation and protect them from sediment deposition. The depth of the Ghayli Falaj does not normally exceed 4 m, and its length can range from tens of meters to several kilometers. Because this class of Aflaj depends on the subsurface flood flow, its flow can sometimes be improved by constructing small embankments. Examples of this type of Aflaj include Eastern Masfout Falaj, Sahm Falaj and Bokaa Falaj in United Arab Emirate, and Simaa Falaj, Al-Sald and Al-Janibi in Sultanate of Oman. The Dawoudi Aflaj obtain their water from unconfined aquifers, especially the upper flow systems. They are characterized by long tunnels up to several kilometers in length excavated under the groundwater surface, with depths of up to fifty meters (Al-Ohee Falaj in Sahar state). The length of the Dawoudi Aflaj systems in the Buraimi area of Oman ranges between 1-15 km, with depths ranging between 25-50 m. This class of Aflaj systems is characterized by a continuous flow and high discharge. Their flow pattern depends on their location, characteristics of the supplying aquifer, and the management and maintenance procedures. Examples of Dawoudi Aflaj systems include Naslah and Hueid in the United Arab Emirates, and Wad Batha, Mudeihi and Buraimi in Sultanate of Oman. The Aflaj Al-Ayniah in the Sultanate of Oman are characterized by high temperature resulting from the relatively-deep groundwater movement. The water of these Aflaj systems are used for beneficial medical purposes. Examples of Al-Aflaj Al-Ayniah are Ain-El Maghoor and Sukhna in the United Arab Emirates, Al-Kasfe in Nakhel in Oman, and Arak (Foggaras) in the Palmyrian region of Syria The following factors should be taken into account in selecting the Falaj site: Topographic specifications The existence of a gentle ground slope towards the areas where the water is to be utilized is preferred, considering the village location, residential concentrations, and the fertile lands. The supply area is subject to regular and long durations of good rainfall rates. Hydrogeologic specifications The presence of a relatively shallow aquifer with a high water level is desirable. A reliable source of aquifer recharge also is desirable, especially where the aquifer has a limited thickness and extent. Hydrodynamic specifications There should be no large seasonal or annual variations in water levels. Also not desired is a significant or continuous fall in the groundwater table as a result of pumping in neighboring wells or group of wells utilizing the same aquifer as the Falaj.
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