Since phenomena like erosion, sediment and sedimentation are regarded as significant factors in the rivers’ water catchments and the settlement of sediment load in sedimentary basins and engineering-civil projects, specifically dams’ reservoirs, various studies have been done in accordance with the importance of project and different methods of estimating the amount of sediment. Few studies have been held to compare the methods applied to assess the amount and type of the parameters affectivity. Lali area is located in Khuzestan province, Iran. It is almost 200 km away from the north of Ahwaz, the province center; it covers approximately 300 km2 surface areas (Figure 1). From geological aspect, it is situated in the structural-sedimentary zone of the folded Zagros  . According to the fact that Lali area holds all the nine factors producing sediment and causing erosion, it is possible to investigate and measure them in this area using the empirical PSIAC method. Thus, all the effective parameters are easily available in this area so as to analyze them. That is why Lali area has been chosen.
The surface quality and quantity contribute to consider erosion in rivers; however, engineering projects focus on the quality factors. Quality factors hold
Figure 1. The geographical situation and the accessible ways to Lali area, from Google earth.
determining the properties of water catchment and river bed, the amount and type of transported sediments and varying the bed gradation  .
Morphological changes of the river rely on the amount of sediment entering into the river and the sediment removing from the water catchment basin. Moreover, it is essential to figure out how the rivers erosion is influenced by sedimentary load transportation of upstream, the plant coverage and tectonic and geological phenomena to analyze the eroding pattern in rivers’ sedimentation and its effects  and  . Hence, it is possible to assess the sedimentary yield produced by erosion and sedimentation using PSIAC (Pacific Southwest Inter Agency Committee) PSIAC Empirical method and other similar quality methods practiced in this research  .
The erosion amount of rock units and sediments transported into sedimentary basins is identified as parameters specifying the geomorphologic properties and the characteristics of the accumulation and/or replacement of sedimentary masses. The surface percentage of rock units in the intended area is applied to calculate the erosion index of the area’s geological units and determine the sediment frequency in Lali area  .
Both the quantity and quality made by sedimentation and erosion phenomena depend on the amount of its extension and the situation of water catchment. In the current research, the water catchment is Karun River, one of the largest water catchments in Iran (Figure 2).
Regarding the trend of global warming process and the extreme extension of dry and semi-arid areas all across the world like Iran and irregular rainfall distribution in these areas during a year, it is important to retain and store water resources in dams’ reservoirs. As a huge ratio of runoff in Iran flows into Khuzestan Plain through Karun River, it is significant to investigate and store this amount of runoff to be used in dry seasons of year. In consequence, preserving dams’ reservoirs and lengthening their life span should be measured more in
Figure 2. The situation of Taraz water catchment in Lali area, Khuzestan, Google earth.
order to store more water resources. Moreover, the amount of sedimentation in dams’ reservoirs and sedimentary basins should be inspected as well. Finally, factors influencing erosion and sedimentation in the intended areas should be analyzed and evaluated and the type and the amount of discharges with the ratio of water flowing into basins and reservoirs should be exactly specified.
This research aims at finding out the answers of the following questions:
- How much is the affectability of each one of the parameters in PSIAC method?
- Which of these parameters affect the rate of erodibility the most?
- How can the climatic and geographical conditions influence erosion?
- How can the geological conditions and the formations of the area’s soil and stone affect sedimentation and erodibility?
- How the effective parameters influence each other?
2. Review of Literature
Several base studies have been done in this regard, for example, various investigations have revealed the relationship between sediments produced by water catchment basins with the climatic hydrologic and geomorphologic factors.
Norlan (1986) evaluated the amount of erodible formations as the origin of floating yield in California  . Kasimir, et al. (1995) indicated that small basins with the quantity of looser formations influence the load of floating sedimentary  . Khojeini and Nezhadha Shami (1998) demonstrated the influence of erodible formations on sediment production in water catchment of Taleghan, Iran  . Ghadimi Arusmahalleh and Aminsobhani (1998) showed the effect of sedimentation on Shale and marl and erodible formations in Namak Lake of Iran  and  .
Alijani (2016) indicated that the type of lithology and formation of the area affect the amount of sedimentation and erosion. Stronger formations had less sediment than weaker ones like Bakhtiari conglomerate formation and Asmari lime formation  and  .
The method applied in this study is based on field work, local investigation and sampling in the intended area. In addition, this research has also focused on factors affecting sedimentation and erosion on geological formations, morphology, topography and sediments in Lali area specially the nine factors of (Pacific Southwest Inter Agency Committee) PSIAC method  . Initially, it concentrates on the geological formations of this area such as rock formations, alluvia sediments and probably Aeolians. Next, stronger formations are identified. After that, it scrutinizes the area’s soil profile and type, the existing trenches, the sedimentation pattern at rivers in Lali area, the area’s topography, unpaved areas, and the amount of plant coverage and the nine parameters of PSIAC method. For the next step, the data are analyzed and the scores are compared by empirical formulas and simulated. Holistically, each one of the nine factors is classified as low, moderate and high from the aspect of sedimentation and erodibility based on the scores  . In consequence, the average of the scores is determined using statistical methods. Thus, the statistical values are figured out such as minimum, maximum, median, mode, standard deviation and the amount of sedimentation and erodibility in the area.
Remarkably, this research has analyzed the effects of each influential parameter on sedimentation and erodibility, separately. For example; the effect of runoff phenomenon on the area’s plant coverage, the soil type depending on geological formations or the relationship between the area’s topography and the erodibility rate of the river and sediment transportation.
4. Setting of Empirical Estimation of Sediments
The minimum and maximum scores have been recommended for these nine important factors affecting the erosion. The amount of the erosion potential of the water catchment will be calculated by the sum of scores and determination of the whole score  . The statistical information mostly obtained through considering the nine factors of PSIAC method affecting erosion and sedimentation. Analyzing and investigating each one of these nine factors can figure out which one of them is the most effective factor and which one is the lowest one. Surface geology, runoff, type of plant coverage, topography and unpaved areas considered as the most influential ones  and  .
It should be pointed out that the amended PSIAC method suggests using the above mentioned Empirical formulas along with identifying and evaluating the quality factors of all the nine factors in order to exactly determine the score related to each factor. The main points affecting erosion are as the following: surface geology, soil, weather, runoff, unpaved areas, plant coverage, ground usage, high lands erosion, river erosion and sediment transportation  and  . Therefore, some tables have been presented to classify the erodibility of sediments as the following.
Table 1 presents the classification of the nine important factors affecting erosion and producing sediments erodibility based on high redundancy of sediments.
Table 2 indicates the classification of the nine important factors affecting erosion and producing sediments erodibility based on moderate redundancy of sediments.
Table 3 shows the classification of the nine important factors affecting erosion and producing sediments erodibility based on low redundancy of sediments.
Table 4 indicates the relationship and the amounts between the erosive factors of the soil of the under study area and the calculated numerical scores of these sediments.
Table 1. The classification of erodibility in case there are lots of sediments.
Table 2. The classification of erodibility in case there are moderate sediments.
Table 3. The classification of erodibility in case there are low sediments.
Table 4. Estimating the scores of erosion factors using PSIAC method in Harkesh river catchment to the central situation of the intended area (basin surface = 323.5 km2).
Score sum: 64.5. Regarding the amounts, the total sum of the basin scores has been calculated 64.5. The average amount of the annual erosion can be estimated in the basin. Qs = 38.77 exp (0.358 R). Where: R: the sum of the scores. Q = the erosion amount of the basin in m3/km2 per year.
PSIAC empirical method is identified as one of the most significant methods estimating the rate of sedimentation and erodibility in water catchments. This method has also been applied in this research to calculate the erodibility score of sedimentary basins in Lali area. This study has analyzed nine factors affecting erosion due to the area’s climatic conditions and has scored the maximum, minimum points. The total point achieved by adding up these scores altogether and the erodibility amount of the water catchment is estimated  and  . The limits of co-efficiencies and influential factors are presented in Tables 1-3. In addition, Table 4 presents the estimation of erosion factors using PSIAC method in Harkesh river catchment to the central situation of the intended area.
The standard of the sediments levels in this study has been founded on identifying the levels of the sedimentary levels mostly based on determining the type of sediment, the rate of sediment stiffness or looseness. In fact, harder sediments with more condensed grading are found on higher sedimentary levels like the sediments made up of lime stone. In contrast, looser sediments with weaker grading are observed in areas with low topography like the sediments produced by marl and silt. Moreover, it is essential to identify the type of soil based on pediology standards and the standards specifying the sediments gradation like ASTM (American Standard Test Material) and BSTM (British Standard Test Material).
As observed in the aforementioned tables, the scores related to each one of the factors affecting erosion are represented as formulas and relationships so as to determine them more properly in PSIAC method  . These empirical formulas and other specific qualities related to the nine factors include: surface geology, soil, weather, run-offs, unpaved areas, plant coverage, land usage, high land erosion, river erosion and sediment transportation  and  .
Scores remarked for each of the erosive factors and the method or the reason of its quality amount is illustrated in Table 2.
1) Surface geology
Based on geological studies in the area of the case study, the coverage, rock units and geological formations are as the following:
- The lime stone of Asmari formation and somehow Sarvak with very low erodibility cover almost 42% of the under-study area.
- Shale and marl of Pabdeh Gurpi formations cover 10% of the area and have moderate to high erodibility.
- The quaternary alluvia deposits cover 7% of the area with very high erodibility.
Consequently, the surface below Lali area water catchment covers about 25% of the area with potential of high erodibility, 10% of the basin reflects high erosion, and 23% holds low erodibility and almost 42% of the water catchment gas low potential of erodibility.
As observed in Table 2, very high sediments scored 10, high sediment got 8, the moderate one got 3 and the low sediments marked 1, then the score 4.4 out of 10 was estimated for surface geology.
Based on field work and local investigations on the soil type of water catchment area, it was evaluated that the soil texture of this basin is rated moderate from producing erosion point of view and it was scored 5. The dominant soil contains dispersed pebbles with lime layers. Therefore, soil holds a moderate texture.
The following empirical formula was used in PSIAC method:
P2 indicates a six-hour-rainfall of water catchment with a two-year-returning period. The thunder rate of this water catchment is varied due to time intervals and different turning periods. It is estimated that the maximum thunder intensity is 7 mm within 2 years. This number is actually equal to 42 mm within 6 hours in a two-year-frequency. Holistically, the score related to the weather will be 8.4  and  .
PSIAC method is also practiced for run-offs. Hence, the following relation is applied to determine the scores:
R = the height of the annual run-off
Qp = the special discharge of the basin flood within a-two-year-turning period.
As there aren’t any measuring stations on the river of this area, the average of the flood runoff and the high discharge climax of the area’s hydrology specify the amounts of R and Q  and  . In fact, it is estimated at 3.2.
5) Unpaved Areas
As shown in Table 2, PESIC method and the given formula are practiced for this factor.
The relationship is as below:
S = the average slope of the basin in percentage
X = the score related to unpaved areas.
Based on the morphology of this area, the average slope of the area is 41% and the score given to unpaved factor is 3.5.
6) limited plant coverage
Due to local observations, it is estimated that the lands are stiff and have no plant coverage  .
The relation below is applied:
PB = the percentage of stiff land with no coverage
X = the score given to the condition of the land coverage.
It should be pointed out that plant coverage; stone and mush protect the soil. Most part of the soil is ready for stiffness and can’t have such coverage  and  . Therefore, plant coverage is scored 7.
7) Land usage. The following relationship is used in PSIAC method:
where b shows the plant coverage of high lands and X is the score. So, the score 6 is given to land usage.
8) high land erosion
Investigations specified that the amount of wind erosion is very low. It is concluded that most of the erosion is made by water in this area. Laminated layers are caused by this type of erosion. In fact, intense and short rainfall, deep steep and lack of plant coverage are the main reasons. The sol is almost made up of soft geological formations and the soil is erodible like marl and silt  and  .
Based on the amounts estimated for each of the nine factors, the total annual volume is 1.3 m/ton. The amount erosion is 507 ton/y/m2.
Regarding to the outcomes of scoring nine effective factors of PSIAC method and analyzing the statistical results, the maximum, minimum, mode, medi 5 and standard deviation are as below:
Min = 3.2
Max = 13.50
Average = 7.167
Standard deviation = 3.22
Median = 7.9
Mode = 5
These scores indicate moderate erosion is dominant in this area. Moreover, it is not possible to depict a meaningful relationship between the erosive factors and the score based on the exponential relation of the equation, the high regression and low amount of regression number (R2). However, all the nine factors affecting erosion and sedimentation are evaluated as moderate  and  .
Therefore, the three following graphs have been depicted in order to specify the factors affecting erosion and sediment production of the soil of the under study area and also determine the equation as well as the regression co-efficiency of the above mentioned parameters.
The Graph 1 indicates the role of the most effective factors on erodibility and sediment production.
Graph 2 Presents the curve changes of sediment erodibility-the scoring based on PSIAC method in Lali area Water catchment.
Graph 3 illustrates the relationship of the erodibility of the nine factors based on PSIAC method.
PSIAC empirical method is recognized as one of the most significant methods estimating the rate of sedimentation and erodibility in water catchment basins. This method has also been applied in this research to calculate the adorability number of sedimentary basins in Lali area.
Regarding to the outcomes of scoring nine effective factors of PSIAC method and analyzing the statistical results, it is concluded that moderate erosion is observed across Lali area. Moreover, it is not possible to depict a meaningful relationship between the erosive factors and the score based on the exponential relation of the equation and the high regression and low amount of regression number (R2). However, all the nine factors affecting erosion and sedimentation are evaluated as moderate.
Graph 1. The curve changes of sediment and erodibility based on the most effective factors including weather, runoff, unpaved areas and high land erosion-the scoring based on PSIAC method in Lali area Water catchment.
Graph 2. The curve changes of sediment erodibility-the scoring based on PSIAC method in Lali area Water catchment. 1-surface geology, 2-solid, 3-weather, 4-runoff, 5-unpaved area, 6-limited plant coverage, 7-land usage, 8-high lands erosion, 9-river erosion and sediment transportation.
The identification of the main factors affecting the amount of sedimentation and erodibility in Lali area paves the way to apply more practical alternatives in civil projects like water transmitting canals, dams’ reservoirs and more appropriate structures. For example, in areas with looser soil and materials like marl, it is recommended to initially strengthen the foundation. In case the rivers’ banks are easily eroded, it is essential to build up embankments or provide berm or gabion.
Graph 3. The relationship of the erodibility of the nine factors based on PSIAC method. 1-surface geology, 2-solid, 3-weather, 4-runoff, 5-unpaved area, 6-limited plant coverage, 7-land usage, 8-high lands erosion, 9-river erosion and sediment transportation.
In areas where floods frequently occur, it is better to provide plant coverage or plant trees especially in watershed management projects.
The analyses of the results imply that this area has been situated in a moderate classification from the view point of adorability and sedimentation because there are both hard and strong formations along with the weak and loose ones. Conclusively, weak and loose formations are more probable to be exposed to erosion and the balance of these two types of formations ranges this area as a moderate one from the view point of erodibility and sedimentation.
The following items are suggested regarding the studies and observations in erosion phenomenon, sedimentation and sediment accumulation in dams’ reservoirs and sedimentary basins.
・ Fundamental studies on the amount of sediment discharge into dams’ reservoirs are led into identifying factors influencing the increase of sedimentary yield with water entering into reservoirs. Suitable alternatives are taken into account to diminish its harmful effects by analyzing the data of these studies.
・ These basins are exactly assessed to scrutinize the amount of sediment production in the geological formations located in water catchments leading into identifying formations ready to be eroded.
・ Detecting factors affecting the erosion process is led into selecting the most appropriate methods to diminish the effects of these processes and control its hazardous effects in the intended area before starting any civil project, specifically dam construction studies.
The author would like to highly appreciate all the ones, companies, researchers and scientific experts who contributed to prepare this research, Islamic Azad University Science and Research of Tehran and the dean of geology department, Dr. Ali Solgi. The author also thanks the water and electricity organization of Khuzestan for the sake of their contribution. The author respectfully appreciates the head manager of Ab-Niru consultant engineering, Dr. Hussein Jalali and the technical exclusive manager of Ab-Niru consultant engineering, Engineer Hasan Nabavi.