Saudis test RWH and storage techniques
A two-part project to test and implement rainwater harvesting (RWH) and storage techniques in Saudi Arabia was motivated by the need to maximise the retention of resources in this dry environment. Abdul Malek A Al Sheikh of the Prince Sultan Research Center for Environment, Water and Desert (PSRCEWD) at King Saud University in Saudi Arabia explains the project's design and execution.
The project plan is divided into two stages: implementation of the project in different parts of the kingdom follows the success of the first trial stage. The project is based on the use of two major methods of water harvesting and storage: artificial ponds and artificial groundwater recharge wells in the dam basin.
The trial project started in September 2004. Three suitable sites were selected to construct three artificial ponds in Dhruma, AlKharj and Asherat Sudair. Another four locations were selected for water storage through recharge wells behind dams in Al-Ulb and Ad-Diriyah and the Huraimla dam, Al-Hariq dam and AI-Muzairah dam in Al-Ammariyiah.
This technique of water harvesting is based on constructing large artificial storage catchment ponds. Rainwater and runoff is diverted towards these ponds and the location is selected carefully taking into consideration the climatic conditions, topography and environmental nature of the area.
Remote sensing satellite imagery and 3D digital elevation models were used to specify the best location for the construction of these ponds. The artificial pond is constructed on any water channel located near places where water is needed.
The main purpose of these ponds is domestic watering, not groundwater recharge, and this is achieved by preventing water infiltration to the ground by building insolating layers on the base of the pond. The dimensions of the pond are engineered to suit the need of domestic consumption and to reduce evaporation through increasing its depth, planting trees as wind breaks and by making the pond perpendicular to the common wind direction in that area.
Water can be used by pumping it through pipes constructed to the level of the pond's base and raised outside to supply tanks with water. The PSRCEWD team started working on the artificial pond in Dhroma at the end of December 2004.
The works included the excavation of a 320m-long, 120m-wide and 8m-deep pond. The soil dug from the site was used to construct embankments surrounding the pond to provide a protection for the pond against any collapse and sand movement.
These embankments add almost 2m height and are suitable for tree-planting. The base and sides of the pond were lined with a low-permeability mud to a thickness of about 0.2m and were well compacted.
The front of the pond and the entrance were lined with stones and a concrete plane (0.2m thick, 64m long and 32m wide) to avoid soil erosion caused by water flow. The base of the nearby valley adjacent to the entrance of the pond was cleaned and extended to act as a slack area, where water stands for a while and gets rid of its sediment before getting into the pond.
Embankments parallel to the sides of the valley were constructed to route the flow of water towards a dam of 20m length, 9m width and 3.8m height. This dam will hold water and channel part of it towards the pond during runoff.
Pipes of 260mm diameter were placed to convey water from the pond to a safe and feasible place for tanker transportation of water for domestic use. The site was fenced for protection.
Water conservation through storage is a vital part of any integrated management strategy. The aquifers and existing geological layers at the valleys are economically and environmentally suitable areas to store the recharged water and preserve it against evaporation and surface pollution.
In Saudi Arabia, hot and dry weather prevails, evaporation is high and rainwater sporadic. Therefore, the government was keen to construct many dams to preserve and utilise water during the non-rainy seasons.
However, water stored in surface lakes is exposed to extensive evaporation, especially in the summer when temperatures rise and the evaporation ratio exceeds the natural water recharge. A logical solution is to increase the water recharge rate by digging artificial water recharging wells in the dam's lake bed.
Artificial water recharge wells are used to store rainwater and runoff in the strata. Geophysical and surveying studies identified the optimal dams and best locations to dig the wells.
The depth of the well is determined by the depth of the layer holding groundwater, and its nature. In addition, control observation wells were drilled in the vicinity of the dam to check the level of groundwater and the efficiency of the artificial water recharge process.
Artificial water recharge is used for three main purposes:
- Hydro-geological control - it is possible to raise the level of groundwater using artificial water recharge and in turn control the rate and direction of groundwater movement inside the layer and thus controlling the movement of low quality water (like sea water) in the desired direction.
- Water storage - artificial recharge is used to store water inside unconfined and non-saturated underground reservoirs through injection of the water into the internal cavities of the rocks. These cavities provide huge spaces to store huge quantities of water. Such water might be used when needed through pumping from the reservoir to the supply regions, which in turn helps the slow movement of the underground water in the reservoir and reduces the losses. Water can be stored using this method for long or short periods of time depending on the right choice of selecting the structure of layers, location and the geological status.
- Water quality control - artificial water recharge means mixing two different kinds of water, so if the quality of water already present in the ground reservoir is lower than the water injected using artificial water recharge wells, then the quality of the reservoir's water will improved. In addition, ground reservoirs work as fIlters to enhance the quality of water being injected because it filters all materials suspended in that water.
In the recharge and pumping process, the underground water level increases and decreases accordingly and, in turn, creates a drainage system inside the underground reservoir. The wells can be distributed in accordance with local requirements and preserve underground water levels to avoid ground collapse or any environmental impacts, which might occur due to the decreased underground water level.
The dams of Al-Ulb in Ad-Diriyah, Huarimla, Al-Hariq and Al-Muzairah in Al-Amareyah were chosen to drill artificial water recharge wells for the ground layers. All necessary geophysical studies were carried out to specify the best location of those wells based on the underground layers nature of each location.
For Al-Ulb dam in Ad-Diriyah, five artificial water recharge wells were designed and drilled behind the dam at a depths ranging between 33-40m. A 457mm-diameter recharge pipe was then installed into each well, with multiple holes to a distance of 18m below surface level. A length of 6m remained above surface, fitted with three 203mm diameter valves at different levels.
After the fieldwork was completed, it rained heavily during February 2005, and the water runoff started reaching Al-Ulb dam. The flow continued continuously for four days.
A large part of the dam lake was filled and water covered the three valves for all wells. The recharge valves were opened and all necessary measurements were taken.
The preliminary results showed that artificial water recharge is very effective in recharging the layers holding groundwater. It was observed that after two weeks of opening the 15 valves of the pipes installed on the five wells, groundwater was recharged with more than 65% of the water volume collected in the dam's lake.
Moreover, the underground water level measurements indicated a remarkable increase of the water after having opened the feeding pipes. Farmers reported that the quality of underground water was improved. It is hoped that this project will be extended into different regions of the kingdom.