Iraqi Kurdistan Regional Government,
MINISTRY OF AGRICULTURAL
Pre-feasibility Study of Small Dam Sites of Kfree (Kfry or Bawa Shaswar) dam
By: Dr. Kamal Haji Karim, Assistant Professor,
Bakhtiar Qadir Aziz, professional in geophysics
Under supervising of:
Salahaddin Ali saed, Hydrogeologist
Sulaimaniya, Kurdistan Region – Iraq
1-Location and geomorphology of the site
This site is located in the northeastern Iraq, about (2 km.), to the north of Kfree town (Fig.1). The site is situated at the intersection of latitude (N: 340 43– 12=) and longitude (E: 440 58– 27=). The proposed dam site and reservoir are located in a valley stream called Kfree stream. The valley has north-south trend which begins at 12km north of the town and descend southward passing by Kfree town directly at its eastern border.
The dam site is located in front of small ridge which transected (cut traversally) by seasonal stream. Thus small gorge is formed which is about 70m and 200m wide at its base and top respectively (Photo 1). The height of the gorge is 20m. The stream drains a catchments areas has more than 70 square kilometers which may supply enough runoff, in winter and spring to fill a small dam of 20 height and 8milloin cubic meters of capacity. The catchments area is mainly hilly terrain and characterized by dendritic drainage pattern.
2-Geology of the site:
The proposed site is geologically located at Low Folded Zone, which is characterized by low amplitude anticlines and synclines. These structures are extending NW-SE. The simple geological setting of the area is complicated by two reverse faults. One of them located at 3km to the south of the dam (directly to the north of Kfree dam) while the other one is located at 25km to the northeast of the dam (Fig.3). These faults are reverse one. By these faults Lower Fars Formation is exposed and rest against Upper and Lower Bakhtiary Formation. The dam site is rested on the northeastern limb of an anticline we called it Kfree anticline. The other limb (southwestern one) is located under the town and disappeared by a fault possibly reverse one.
3-Stratigraphy of the site
Most (nearly 80%) of the lands of the watershed areas are covered with pebbly sandstone and claystone of Lower Bakhtiary and conglomerate of Upper Bakhtiary Formation which include the northern part of the catchments area. The southern part is covered by the sandstone and red claystone of Upper Fars Formation (Fig.3). The dam site is located on the latter formation which is composed of impervious thick beds (layers) of sandstone and red claystones. These layers dip toward northeast at angle of 40 degrees.
The result of weathering of Upper Bakhtiary Formation can be seen clearly in the area, especially in the channel of the stream. Huge quantity of gravel and boulder with sand is now rested on the stream bed (Photo 2) which transported from the catchments area. The lithology of the formation can be identified which include limestone, chert, igneous and metamorphic rock clasts (pebbles, cobbles and boulders).
The thin section study under polarized microscope revealed that the sandstone is composed of different type of terrigenous clastic grains such as, fragment of limestone (40%), chert rock fragments (30%), quartz grain (15%), igneous and metamorphic rocks (10 %) and other (5%). These grains are plotted on compositional triangle to show the percentage of each type of grains (Fig. 6). All theses grain is cemented by calcite cements and clay materials. On the surface the cement material (calcite) is partially weathered by water but beneath the ground they actually firm and coherent. No soluble grains such as gypsum minerals are found.
5-Condition of dam site and reservoir ground
As concerned to water tightness and stability, the site is good for dam construction. This is because both the dam site and reservoir floor is consisting mainly of alternation of thick layers of impervious sandstone and claystone. These two rocks are appearing in outcrop friable and loose but they are firm and sound below surface. Therefore the site has enough stability and watertight to be used for dam building. The layers are stacked perpendicular to the direction of water flow (to the stream flood).
There are no any fault near the dam site, the two existed fault in the area are too far to have any effects on the dam stability. As concerned to hydrogeological view, there is no any leakage or discharge of water. This is attributed to the imperviousness of the rock layers below the dam side and reservoir. We leave hydrological condition of the valley stream to the hydrological team to evaluate the rate of sedimentation and flood hazard analysis of the watershed in addition to water budget of the valley. .
on compositional triangle
Electrical Resistivity Method and
Vertical Electrical Sounding
The electrical resistivity method for subsurface study was used for practical purposes in France in 1912, since that time this method has proved it self to be among the most effective means of shallow subsurface investigation especially in sites investigation for engineering problem such as dam, high way, road, …ect. The main advantages of the resistivity surveying are the simplicity and portability of the equipment (resistivity meter) and its accessories. Vertical electrical sounding is designed to provide information on the variation of the subsurface condition with depth,
The resistivity method involves measuring the electrical resistivity of earth materials, by introducing an electrical current in to the ground and monitoring the potential field developed by the current. In most earth materials, electricity is conducted electrolytically by the interstitial fluid. The resistivity is controlled more by porosity, water content and water quality than by the resistivities of the matrix. In making a VES measurement for a resistivity survey a direct current is introduced into the ground through two electrodes and the potential difference is measured between a second pair of electrodes. Current and potential measurements are then used to calculate the apparent resistivity of the subsurface materials. Progress increase in current electrodes spacing increases the depth of investigation.
Two different types of resistivity meters were used for the fieldwork; the first is ABEM Terrameter, SAS 4000, which is very applicable for engineering site investigations. It measures voltage response created by the transmitter current while rejecting DC voltage and noise, the ratio V/I is automatically calculated and displayed digitally in ohm. The instrument has resolution of 0.02 milli-ohms for a single reading.
The second type of the resistivity meter is IRIS Syscal Jr switch-72; it is a new modern computerized type of resistivity meter was used in this study. The SYSCAL Switch type resistivity-meters of IRIS instruments are fully automatic instruments through the control of a microprocessor and they have been designed for intensive exploration of the ground.
Syscal Jr switch-72 is a new all-in-one multimode resistivity system; it is designed to perform automatically pre-defined sets of resistivity measurements with roll-along capability.
The first step before carrying out the electrical resistivity survey is preparing a suitable topography map of the area for plotting the exact location of the measurement stations, so by the aid of GPS about 350 measurements of geographic positions, bearings, horizontal distances and elevations were recorded. These data exported to the Surfer software program for the sake of construction of 2D and 3D topography map as illustrated in the figures (1) and (2).
For minimising the ratio of errors and increasing the percentages of Signal to Noise ratio three profiles running parallel to the strike of the out crops were laid out as shown on the 3-D view topography map of the are Fig (2), two of them were laid in the down stream and the last was in the up stream of the proposed dam. Each profile holds six vertical electrical soundings separate by a distance ranges between 40-50 m, so as a result 18 soundings using Schlumberger array were performed.
4-Interpretation and discussion
The field data represent a set of apparent resistivities obtained from different depths, these measurements were interpreted manually depend on Ebert method and by the aid of computer using special software. The interpretation of the field curves are shown in Fig (3), (4) and (5).
This profile is located close to the sandstone ridge that has thickness of about 36 m, and occurs at a distance of 50 m from the main sandstone ridge. All these soundings refer to the existence of four different layers below the ground surface, for illustrating the sub surface situation more clearly a geo-electrical section was drawn as shown in Fig (6), the quantitative description of this section is as follows:
1-The first layer is representing the upper thin layer of soil covers; it shows variable resistivity due to rapid changes in its physical and chemical properties. This layer always represent unconsolidated materials of highly weathered, it has thickness ranges between 1.1 – 1.8 m, as shown in Fig (6).
2-The second layer is clay; it shows very low resistivity ranges between 8.5-12.2 ohm.m, and it has small thickness ranges between 2.3-3.4 m.
3-The third layer is relatively thick, (31.2-39.4) m, it shows high resistivity, which denote to the existence of beds of sandstone and siltstone with some clay materials.
4-The fourth layer also represents a repeated cycle of clay with low resistivity ranges between 10.2-11.9 ohm.m.
It is plotted directly beneath the main sandstone ridge, which will be use as the embankments of the proposed dam. This ridge has thickness approaches 80 m and dipping toward the NE. The distance between profile-1 and 2 is equal to 30 m that is the maximum distance that the area is limited.
The geo-electrical section of this profile was constructed and displayed in Fig (7), the soundings denote to the existence of four layers as follow:
1-The topsoil layer has thickness range between 0.9-1.55 m was detected, composed of unconsolidated clay, sand and little gravel.
2-The second layer is clay, relatively shows higher thickness than the previous traverse, its thickness ranges between 6.73- 11.5 m.
3-The third layer is also clay according to its low resistivity, but it is characterize by existence of alternates thin beds of sandstone and siltstone.
4-The last layer also represents a clay bed of low resistivity ranges between 6.8-9.2 ohm.m.
This profile was laid out to investigate the upstream portion of the proposed dam. It holds six soundings as shown in Fig (2), the field curves and the geo-electrical section are shown in Fig (5) and (8) respectively.
Below is the quantitative interpretation of these soundings:
1-The topsoil layer has thickness ranges between 0.8-1.5 m.
2-A thin layer of high resistivity is appear at depths range between 0.8 –1.5 m represent a dense bed of sandstone, it has thickness equal to 1.56-3.55 m.
3-The third layer is also of relatively high resistivity, it is clay with existence of thin beds of sandstone and siltstone. The thickness of this layer is about 33.2 – 38.3 m.
4-The last layer of low resistivity (9.9-12) ohm.m, represent a clay layer.
1-The site is water tight and has enough stability for dam construction as the subsurface ground under consideration composed of thick layers of three rocks : Claystone, sandstone and siltstone, although the clay is a dominants component of the subsurface.
2- No faults were detected near the dam site, the two existed fault in the area are too far to have any effects on the dam stability.
3- The result of geophysical survey proved that there is no any subsurface cavity or zone of weakness (fracture zone).
4- There is no any leakage or discharge of water around the site. This is attributed to the imperviousness of the rock layers below the dam side and reservoir
5-We recommend extensive hydrological study of the valley stream by the hydrological team to evaluate the rate of sedimentation , flood hazard analysis of the watershed, capacity and possible dam height in addition to water budget of the valley.
6-The high resistivity of the sandstone layer denotes to coarse grain of this materials and existence of some porosity but not affect the dam water tightness.
7- Both geological and geophysical survey showed that the thickness of alluvium materials (stream deposits), in the stream, ranges between (1-6m) near the dam. It is mostly composed of gravel and boulder with sand.