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1.2 Harvesting of Surface Runoff Water

The technology of harvesting runoff water has long been known in a number of the countries in the West Asia region, including Syria, Lebanon, Jordan and Yemen. It is based on constructing structures that allow for collecting and storing of surface water runoff, to be used where needed throughout the dry season and in different locations. The harvested water can be used for irrigation and drinking purposes, rather than being lost via evaporation. Surveys carried out both in the Arab region and at the itnernational level have demonstrated the existence of different types and forms of these structures. Nevertheless, they all can be categorized under one of two groups of structures for the collection and storage of either surface or groundwater.

Technical Description

Surface structures for runoff water harvesting

• Small dams or hill reservoirs

These structures are small dams consisting of earthfill, aggregate or concrete. They are typically constructed across the entire width of the watercourse (rivers, valleys, flood relief channels) so that a small lake is formed behind them. The storage volume of the lake is proportional to the height of the dam, with the dam height not exceeding 15 m in most cases. The dams are built on the higher upstream regions of rivers or valleys, where the watercourse is narrow and deep. As a result, the storage volume is larger for the same lake surface area created upstream of the dam. The dams are usually equipped with a spillway to allow the release of floodwater, and to divert it into the river course or the river valley downstream of the dam after the lake water level reaches a certain height.  The dams are also equipped with low-level outlets (so-called Mofaregh). The low-level outlets are wells with large diameters, excavated just upstream of the dam structure, and connected to a canal extending underneath the dam structure to the downstream side of the dam. Their purpose is to facilitate emptying of the lake and provision of irrigation water for land downstream of the dam. Thus, it is necessary to carefully study the dam site from several perspectives, including its geologic, topographic and geomorphologic characteristics, as well as the soil erosion characteristics of the watershed. Hydrological studies also must be conducted to ensure dam safety throughout its lifetime.

The dam foundation is usually in the form of a ditch, where either a concrete wall is built, or where it is filled with impervious soil penetrating through the surface layer of the river bed or the valley. The foundation must extend to a depth sufficient to ensure the dam is safe from failure due to water seepage through the dam structure when the lake is filled.

• Water-spreading dikes

Water-spreading dikes are a major construction for utilizing the seasonal water flow of river valleys for agricultural development and for pasture improvement activities. The dikes consist of the following main units:

• The main dam, which obstructs the watercourse at a suitable site to allow for temporary water storage until the water is spread on the land;
• Proper outlets for disposal of the excess water;
• A diversion system for the stored water;
• A group of embankments or low rock walls for distribution of the conveyed water;
• Appropriate outlets through the embankments or around their edges, to ensure the water is automatically distributed.

Different methods for spreading the stored water are as follows:

• Spreading uncontrolled floodwater -- the water is directed through artificial or natural outlets to the areas where the water is to be used. These areas are often near the watercourse and have a gentle down slope away from it to allow for the spreading of the floodwater under gravity (Figure 14).water intended to be spread. Often, these areas are near the watercourse, having a gentle down slope away from the watercourse to allow for the spreading of the floodwater under gravity;
• Spreading of controlled floodwater – the water is collected by constructing a dam across the valley course to a suitable height (3-4 m). The water upstream of the dam is diverted through a diversion channel to the area where it is to be spread. The water spreading can be implemented through construction of barrages on the main branches of the river valley (Figure 15).  Controlled floodwater are sometimes collected in embankment ponds and used later for agricultural activities or animal grazing (Figure 16).
• Water spreading by systematic flooding – the floodwater is spread through a dike and a diversion channel to the area where it is to be spread. The land is divided into suitable basins, and the water fills the basins.  After the irrigation is completed, the excess water is released through outlets to the next basin (Figure 17).

Figure 14. Spreading of controlled floodwater

Figure 15. Spreading of controlled floodwater

Figure 16. Earth ponds for collecting controlled floodwater

Figure 17. Water spreading by ponding

• Spades and Streams

These are natural or artificial shallow depressions spread along the banks of wide desert valleys. The spades (also called Mahafir) have various geometric shapes, and are usually created by manual or mechanical excavation. They are open from two sides, or one side in the direction that the water flows into the spade. The capacity of spades does not exceed a few thousand cubic meters, and their depth is usually not more than about 2 meters. Spades (Mahafir) generally exist in natural depressions, where streams are formed in valley beds on bends, in limited areas and volume depressions. They fill with water when the valley is flooded, and hold the water for a period of time after the flood recedes. Spades and streams fill with water, either via rainwater from the high areas surrounding them, or from floodwaters. They generally do not require specific water inlet or outlet structures, and their stored water can be directly used.

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