THE SHARING OF FLOOD WATERS IN THE KSOURS OF GHARDAIA AND BERRIANE (ALGERIA) HYDRAULIC STUDY

The present article deals with two systems of sharing flood waters used in the oases of Ghardaïa and Berriane. Based on bibliographic work, and data collection and investigations performed during the period between 2006 and 2018 near the Ksourian population as the floods are the only source of water for both oases. Ancestral hydraulic systems of sharing and regularizing floods were implemented in both oases. Each hydraulic system is composed of several hydraulic structures (e.g. dams, spillways, galleries, wells) that have been designed taking into considerations the geomorphology and topography of the region. This study compares the two systems and proves that although built 7 centuries ago, both the systems were built with intelligence. They have become parts of the national heritage and must be preserved for future generations.


INTRODUCTION
In dry regions like the Sahara, the rains are rare, but sudden and destructive floods appear in the seasons of autumn and spring. Twice or three times a year the rains can produce a high quantity of water. An important percentage of those drained waters evaporate in the atmosphere while a little quantity of water is drained in the ground to feed the groundwater. According to geological and hydrogeological conditions of a region, the population adapts while implementing necessary devices to catch groundwater. Local people usually build hydraulic systems for several years. Once completed and commissioned, each participant receives their share of water according to their contribution. For example, the ancestral dam designed in the valley of Tiout to collect water was destined to irrigate the gardens of Tiout oasis. For this purpose, the farmers take turns in sharing the water and each has their share of water converted into a number of hours [1]. In the Saoura Valley, the oases are equipped with foggaras that capture the waters of the Grand Erg Occidental (Taghit oases), spring water or groundwater. This water is used and shared among farmers based on an hourly mode [2] [3]. For the foggaras used in the oases of Touat and Gourara to capture groundwater, oasis people use a volumetric sharing system. For this purpose, a whole social and technical organization has been implemented for centuries to deal with groundwater sharing operations [4] [5] [6] [7]. The present study is interested in another ancient ancestral flood control system and the sharing of surface waters that has existed for over 7 centuries in the oases of Ghardaïa and Berriane. How did they manage to achieve this system, and what is the desired purpose of accomplishing it? Has the water in the two systems been divided randomly, or in a measured way? Are the two systems similar or do they differ?

Region of Study and Methodology of Work
Our research was conducted in the oases of Ghardaïa and Berriane, two oases located about 600 km southwest of Algiers (Fig. 1). To date, these two oases have evolved into cities, especially Ghardaïa, the capital of the M'zab Valley which is classified as a state according to the administrative division from 1987. The state of Ghardaïa has an area of 86,560 km 2 , of which the communes Ghardaïa and Berriane are the subject of our study. They are characterized by a dry to hyper dry climate. The maximum temperature can reach 50°C during the summer season. However, thanks to the existence of large palm areas, the temperature in the middle of a huge palm forest can go down to 15°C. During the winter period, the temperature drops to 3°C. The average annual rainfall is in the order of 100 mm. However, sudden and devastating floods are irregularly drained by all valleys of Chebka (basket) of M'zab, especially M'zab valleys (Ghardaia) ( Figure 2) and Bellouh (Berriane). People still well remember the last flood of October 1st, 2008 which was drained by the M'zab's valley with an estimated flow of 1200 m 3 /s. It killed 33 people and caused material and other damage. For more than 7 centuries, the aggressiveness of the valleys in the region has pushed the Mozabites to think about building hydraulic systems able to protect oases against floods and take advantage of these nutrient-laden waters for irrigation and groundwater recharge. The region of M'zab impressed us by the architecture of its 'ksours' and its floodwater sharing systems. Since 2006, we have carried out two to three missions a year to understand how these engineering systems work. Investigations and discussions with the Ksurian population and Oumana El Ma (water monitoring committee) were conducted at each mission. Investigations were carried out at the hydraulic works. Measurements were made at each hole or spillway. Documents and data were consulted at the libraries, the OPVM, the direction of hydraulics.

Principle of Watering System
Before the discovery of the deep aquifer, the Mozabites exploited the groundwater for more than 7 centuries. This is the only water reservoir for them. It is filled irregularly since it depends on the infiltrations from two or three floods that occur in the valleys: M'zab, Soudan or Bellouh. Thanks to the well system, the Mozabites irrigate their gardens and feed their homes with water. However, it is the raw flood water containing silts and nutrients that constitutes an indispensable element for the Mozabites to grow palms. In addition to minimizing damage in case of floods, Mozabite-designed hydraulic systems play an important role in sharing flood waters among farmers.

Ghardaïa Water Sharing System
To date, this system of sharing flood waters has worked perfectly for 7 centuries. It has been renewed several times. The last repairs were carried out after the great flood of 2008 at Tissembath level, and also the underground tunnels. The ancestral Ghardaïa water sharing system operates as follows. In case of floods in Aadhira's valley, the water goes to the valley of Bouchemdjane, then arrives directly to Tissembath ( Figure 3) [8].
In case the water level rises faster, the surplus water will flow towards the Bouchen dam [9]. Once the reservoir is filled with water, it feeds the groundwater of the western region of the oasis of Ghardaia. By penetrating the Tissembath work, the water takes the route in the underground canals to reach the alleys between the gardens ( Figure 4) [10]. In the times of floods, it plays the role of a channel. In the times of drought, it serves as an alley. Each canal feeds a specified part of gardens. These are equipped with a rectangular opening called Koua ( Figure  5). Its width is different from one opening to another according to several parameters: the distance of the garden from the arrival of the underground canal, the surface of the garden, the number of palm trees. When the garden is flooded, the surplus water flows into the M'zab valley.   In this study we use hydraulic laws to calculate and evaluate the hydraulic parameters of floodwater sharing system. We chose 4 gardens that are fed by one channel ( Figure 6). We compare the different quantities of water penetrated in each garden, and make a comparison between them according to the characteristics of the gardens. Then we determine the performance of the ancestral system using hydraulic laws.

Berriane Water Sharing System
The Berriane water-sharing system is richer in elements (dams, weirs, dykes …) comparing to that in Ghardaia. The inflow of water occurs in two parts. The northwestern part is fed by several valleys, the most important of which is Zargui Valley. We assume it feeds and recharges all the western part of the palm grove. The northeastern part is the largest and the most important. It is fed by two main valleys: Lemkin which feeds a series of dams, e.g. Ballouh (Figure 7), to break the force of the water flow, and also feeds the palm grove. The Sudan Valley feeds the Sudan dam, the most important structure of the whole Berriane sharing system ( Figure 8) renewed during the colonial period in 1958; this dam is intended to recharge more than 60 % of the entire groundwater of the Berriane Valley. The water of these two parts meets in the Mlaga dam ( Figure 9). The water sharing system in the northeastern part runs from the Sudan dam, and the western part runs from the Ballouh Dam (Figure 7). Both dams regulate floods. For our study, we chose two gardens located downstream of the Sudan dam. Like in Ghardaia's case, the water sharing starts from Tissenbath which is on an alley that runs parallel to the palm grove. Each group of gardens is fed by a single opening hole (Koua). In this case, the slope of the natural terrain and the intensity of the flood affect the irrigated area. The excess water from the overflow is discharged to other gardens or valleys that are at a lower elevation.

Ghardaïa Water Sharing System
The velocity of free-surface flow in canals between gardens can be determined by the universal Chezy formula [11]: = √ and therefore the flow is determined: = × Q -the flow m 3 /s V -the velocity m/s S -the submerged surface m 2 C -coefficient that depends on the nature of the walls R -hydraulic radius m I -the average slope of the channel equals 0.015 With C coefficient of Bazin [11]: δ -the coefficient value is a function of the nature of the channel. We took δ = 1.30 (Table 1) R -hydraulic radius (m) The calculation is done for 3 water levels in this channel (Figures 11 and 12). The same calculation is made for the other openings. Figure 14 shows the different flow calculation lines in each point.  Table 2.

Berriane Water Sharing System
At the level of each opening hole of our case study that feeds a group of gardens, there are sub-orifices (openings in walls) that feed each garden. It should be noted that each garden has a surplus water evacuation system (overflow) as in the case of the oasis of Ghardaia.

Group 1 (Koua 1).
This hole feeds 3 gardens with a total area of 2820.5 m 2 . With a water level N = 0.4 m, the calculations with the Chezy formula give the following results:

Sharing waters between group 1 gardens
The sub-orifices are made from cement openings with almost the same width ( Figure 16). The garden 1 has a surface of 1313 m 2 which is fed by a 0.20 m wide opening hole. The nature of the feed channel has an irregular shape with an average width equal to 1 m. At the orifice, we will have q = V × S × C with V = 0.065 m/s. We have q1 = 0.0016 m 3 /s =1.6 l/s. which is fed by an orifice 0.18 m wide. The nature of the feed channel has an irregular shape with an average width equal to 1 m. Before arriving at the orifice, the remaining flow is:

Traditional calculation method.
On the site, we found that there is a lack of information on the designing of the various structures of this system. According to the local population, several parameters are included in the calculation of water shares, the most used of which are: the surface and position of the garden to the source of the water. It should be noticed that in the oasis of Berriane, the surface of the garden is directly related to the number of palm trees because there are no walls of separation between the gardens. So, we use the surface as the main parameter in the calculation. The results obtained are in Table 3.

DISCUSSION
The water catching and sharing system was designed to protect the Ghardaia oasis against floods. However, it is also intended to recharge the groundwater and submerge gardens with sediment-laden waters and nutrients. Our study shows that water shares attributed to a farmer takes in consideration the number of palm trees.
It should be noted that the sharing of waters in the second group is more precise because the nature of the sub-orifices is modified by gabion orifices. According to our calculations, the percentage of each part of water delivered is homogeneous between the neighbours according to the surface of each garden. In many cases, the quantity of floodwater entering this system is significant and the water reaches high levels (1 m). In this case, the sharing is done precisely. On the site we found some herbs, branches, stones and waste in the main canal and some opening holes and sub-orifices. This directly affects the flow and quantity of water entering each garden. The difference that exists between the two systems: Ghardaïa and Berriane is the presence of the separation walls between the gardens in the oasis of Ghardaïa. In the oasis of Berriane, the surface of the garden is connected directly to the number of palm trees; the surface of the garden is defined as follows: The distance between the palms is 3m. The distance between the last palm and the wall is 1.5 m (Figure 18). The surface of the garden is related directly to the number of palms, there are no dividing walls between the gardens.

CONCLUSION
As we mentioned at the beginning of this article, the two hydraulic systems realized at the level of the oases of Ghardaïa and Berriane consist of several structures: dams, canals, galleries and weirs, and are the work of a genius. Built 7 centuries ago with ancestral rules and rudimentary technical means, these structures today attract the admiration of engineers and technicians. Current hydraulic laws applied on both ancestral systems have given virtually the same results as those applied by the Mozabites. This is only a proof of the genius and knowledge acquired over centuries. Although, these hydraulic systems came into existence through painful long-term work, they have degraded a great deal recently. We argue that it is unacceptable to leave this hydraulic heritage abandoned. It is time for the government to take measures.