Erosion Rates in Dam Catchments in Jordan—Effects of Topography, Geology, and Urbanizations ()
1. Introduction
Knowledge of erosion rates in the different climatic zones of the Earth is of scientific and economic interest, and it has practical implication for agriculture, and water structures such as dams, weirs, terraces, drainage canals, roads, bridges and building constructions. In addition, knowledge of actual erosion rates is of importance to calculate historic erosion rates of rocks and terrains in past geologic times.
Direct measurement of erosion rates is a tedious task, complicated by the daily and yearly variations in the weather conditions expressed in changing rainfall rates and intensities, relative humidity, wind speed, temperature, water composition, rock and soil types and composition, topographic constellations, among others. This renders the calculation of erosion rates based on so many variables extremely complicated if not impossible [1] [2] [3] [4] .
This study tries to calculate average erosion rates in the different parts of Jordan based on the amounts of accumulated sediments in dam lakes during the last few decades and on the chemical loads transported with the floodwaters. The study assumes that accumulation of erosion products for a few tens of years reflects in a satisfactory way the weathering amounts in a catchment area.
This study considers that dry atmospheric precipitation (dust) and wind erosion compensate each other or their amounts are very small and negligible [5] .
2. Previous Studies
Worldwide, many studies dealt with erosion rates by direct measurements using markers on rocks and soils and measuring the changes taking place in series of years (e.g.: [2] [6] - [10] ), Other studies concentrated on the erosion of ancient historic marks on rocks, especially on human artifacts of all types (Petra in Jordan). These and other studies calculated erosion rates ranging from 1 - 16 m/Myr for different rock formations ranging from granite and schist to limestone and consolidated marls. The different studies concerned with erosion rates used pin markers in rocks and terrains, erosion of historic sites (changes in their morphology), sediment loads of river courses [11] [12] , cosmogenic nuclides data [2] , and others. This study concentrates on erosion rates in catchment areas of dams based on the accumulated silt in the reservoirs of dams constructed before many decades and distributed all over the country (Figure 1).
3. Methodology
Modern dam construction in Jordan started some five decades ago with the Shueib, Kafrain and Ziglab dams each with a capacity of a few MCM that was followed by the construction of bigger and smaller dams of up to 110 MCM. The quantities of sediments accumulated in these dams during the last few tens of years are used to calculate erosion rates considering the catchment areas’ extensions. In addition, the study tries to relate erosion rates to rainfall amounts, geology, topography and land use, especially urbanization and its effects on natural erosion. Comparison with erosion rates in other areas will also be attempted.
4. Analyses and Findings
Data on the quantity of sediments accumulated in dams and other information
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Figure 1. Location map of the dam catchment areas in Jordan (Background ASTER DEM 30 m resolution [13] ).
have been obtained from the Jordan Valley Authority [14] , the responsible institution for these dams.
Eroded rock materials from catchment areas are transported along watercourses in the form of solid particles and as chemically dissolved compounds of mainly evaporates carbonates, sulfates, and silicates. Generally, the dissolved materials do not precipitate in dams because their concentrations are generally very low and they do not reach the saturation state, and if they do, then in very small negligible amounts.
4.1. Precipitation- and Flood-Water Chemical Contents (Dissolution from Rocks)
A study on precipitation water quality in Jordan [15] concludes that precipitation water contains a weighted average of around 120 µS/cm of dissolved solids differing from one area to another according to precipitation front direction, area in Jordan and quantity of precipitation (Table 1). Khashman [16] in the course of his Ph.D. study on the southern part of Jordan reached at the same result obtained by Salameh [15] . Floodwater along wadi downstream areas (without base flow) contains 150 - 250 µS/cm (Table 2 and Table 3). This means that the floodwater dissolves from the respective catchment area an equivalent of around 100 µS/cm of solids mainly composed of carbonates and evaporates [15] that equals around 70 mg/liter, or 70 g/m3 of floodwater. If King Talal Dam is taken as an example with an average annual flood flow of 30 MCM [17] , then the amount of dissolved solids reaching the dam will be 70 mg/l = 70 g/m3 of water. Divided by an average density of carbonates and evaporates of 2.6 g/cm3 = 27 cm3 of dissolved rocks in one m3 of floodwater and that calculates to 0.0025% m3 of chemical load per m3 of floodwater. This amount of dissolved loads is strongly negligible when compared to a solid sediment load of 2.22% weight percent of
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Table 1. Precipitation water average weighted composition in different areas of Jordan (EC in µS/cm, all others in meq/l) [15] [16] .
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Table 2. Flood flow composition of the Plateau wadis (EC in µS/cm,
in mg/L and all others in meq/L).
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Table 3. Flood flow composition along wadis pouring into the Jordan Rift Valley (EC in µS/cm,
in mg/L and all others in meq/L).
the floodwater quantity (Table 4), making only 1/888 of the latter or 0.0035 of it.
Therefore, chemically dissolved quantities of erosional or weathering products of rocks play only a very negligible role in the erosional rates of rocks and that shows that solely eroded solids transported along river or wadi courses reflect the quantities of the erosion processes.
4.2. Sediments in Dam Lakes
The quantity of deposited rocks in dam lakes in m3 and as a percentage of the dam capacity, the annual sedimentation rates as well as the size of the catchment area, and the average long-term precipitation are listed in Table 4. This information together with the information on the average annual flood flow amounts
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Table 4. Dams, construction, accumulated sediments in their lakes, annual sedimentation rates, catchment area and area’s average precipitation [14] .
allow to calculate the sediment loads per m3 of floodwater and to calculate the erosion rates transported as solids along wadis discharging into the dams as listed in Table 5.
In Table 5, the average sediment loads as percentages of m3 of sediments per m3 of floodwater are calculated. They show a range of 0.88% to 3.0% of sediment load (m3) transported by each m3 of floodwater. These rates depend on the characteristics of the catchment area of precipitation, the geologic formations cover, slopes and topographic characteristics, and land use; e.g.: urbanization, managed agriculture, or mining.
4.3. Topographic Effects on Erosion Rates
Certainly, the configuration of a catchment area plays a very prominent role in the erosion process. Some catchments are quite flat others are very steep; some are steep in their highest areas others are steep in their lowest areas and so forth. Nonetheless, all the catchments we are dealing with extend from the highlands of Jordan and end in the Jordan Rift Valley owning very similar topographic configurations. However, the elevation differences between the highest areas and the lowest areas are different and that means different gradients of topographic sloping. Generally, the highest 10% - 15% and the lowest 10% - 15% of a catchment area in the studied catchment areas represent the main differences between the studied catchments. However, the highest 90% and the lowest 10% or the highest 80% and the lowest 20% can also well represent the topographic slopes of areas with minor differences. To account for this topographic factor in the calculation of the erosion rates, the elevations of the highest 15% and the lowest 15% in a catchment area are found to better represent the topographic configuration than the highest and lowest points of a catchment (Table 6). Figures 2(a)-(h) show the topographic elevations of the discussed 8 dams in Jordan.
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Table 5. Average annual flood flow amounts, accumulated sediments, and sediment loads per m3 of water and calculated erosion rates transported as solids along wadis discharging into dams (Calculated based on [14] ).
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Table 6. Calculated elevations of the highest 15% and the lowest 15% of the catchment areas of the studied dams, their differences, and the difference in elevations related to the extent of the dam catchment area.
4.4. Effects of Types of Rocks
The rock types covering the catchment areas play a major role in the quantity of eroded and transported materials e.g.: the very high erosion rate in Wadi Kafrain catchment can easily be referred to the outcropping easily erodible friable sandstones covering around 20% of its catchment area [18] . In addition, Wala catchment is partly covered to ~15% by easily erodible naturally combusted limestone [19] .
Table 7 lists the erosion rates in relation to prevailing rock cover of the catchment areas, their urbanization and slopes.
4.5. Effects of Urbanization
Urbanization plays a positive role in reducing the sediment loads of floodwater,
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Figure 2. Elevation map for (a): Wala dam, (b): Wadi Al-Arab dam, (c): Tannur dam, (d): Shueib dam, (e): Mujib dam, (f): Karak dam, (g): El Kafrein dam, and (h): King Talal dam.
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Table 7. Correlation of the erosion rates (ascending order) and the prevailing rock types in the catchment area and the difference in topographic elevation between 85% and 15% of the catchment area (flatness).
because built-in areas reduce the erosion of rocks and soils. That is due to increases in cemented and asphalted areas related the urbanization processes. It would not be easy in the course of this study to quantify the effect of urbanization on erosion rates, because of the interplay of the other factors affecting erosion.
4.6. Explanation of the Erosion Rates in the Different Catchments
The erosion rates of the different catchment areas are explained as follows:
· King Talal dam catchment: intermediate precipitation, high urbanization rate of around 25%, erosion resistant rocks covering the catchment area, and intensive agricultural activities can explain the very low erosion rate.
· Wadi Karak dam catchment: the weathering resistant rocks covering the catchment area and the high urbanization can explain the very low erosion rate.
· Wadi Al-Arab dam catchment: the solid rocks composed of chert and limestone, high urbanization rate, and intensively managed agricultural development can explain the low erosion rate.
· Wadi Shueib dam catchment: the solid rocks composed of hard dolomites and limestones, high urbanization rate, and intensively managed agricultural development can explain the low erosion rate.
· Tannur dam catchment: the heavy open cast mining activity of phosphate and gypsum rocks in addition to frequent frost formation in this high latitude area can explain the relatively high erosion rate.
· Mujib dam catchment: the barren nature of the catchment and the very low urbanization rate can explain the relatively high erosion rate.
· Wala dam catchment: the barren and easily erodible combusted rocks covering the catchment area can explain the high erosion rate.
· El Kafrain dam catchment: the very high slope, the high precipitation, and the friable sandstone rocks covering the area can explain the very high erosion rate.
4.7. Comparison with Other Studies
Studies on erosion rates dealt with erosion rates of rocks such as granite 2.5 - 5.5 m/106 yr., basic rocks 3 - 9 m/106 yr. [2] , and kimberlite 3.5 - 10 m/106 [3] . Basin-wide erosion rates, according to [21] , using cosmogenic 10Be methods, ranged from: 5 - 60 m/106 yr (in Sierra Nevada, USA, 15 - 60 m/106 yr, in the Smoky Mountains, USA, 14 - 37 m/106 yr, in the Blue Mountains, Australia, 10 - 20 m/106 yr and in Sri Lankan Mountains 5 - 11 m/106 yr). [1] measured 20 - 40 m/106 yr in carbonate terrains. In the Namib Desert. [2] measured all over erosion rates of landscape of 1 - 16 m/106 yr. The obtained erosion rates for the different catchment areas in this study ranged from 13 - 212 m/106 yr (Table 5). In the case of Jordan, it seems that the calcareous types of rocks covering most of the studied catchment areas, the weakly cemented sandstones present in some catchment areas, the very steep topography and the barren rocks are major factors leading to the high erosion rates.
Worth mentioning here is that, natural erosion processes cannot be controlled by alleviating their causative elements (Rocks, weather, topography etc.) nor by their impacted elements (slopes, outcropping rocks, soils). However, some soil conservation measures can reduce the transportation of eroded rocks, but that do not reduce erosion itself.
5. Temporarily Changing Erosion Rates, the Case of the Dead Sea Retreat
Since about five decades, the Dead Sea has been retreating because of diversions of its feeding waters within its catchment area. It drooped from around 390 mbsl (surface area of 1020 km2) in the sixties of the last century to 420 mbsl in 2005 (surface area of 635 km2) to reach approximately 500 mbsl in 2050 (surface area of 520 km2) [22] . Figure 3 shows the change in water level in the Dead Sea between 1972 and 2019.
Around 320 km2 of the sea area turned into fest land changing the evacuated area from depositional terrain to erosional terrain (Figure 4 and Figure 5) that change created a temporary disequilibrium in the erosion/sedimentation system. This example tells us much about what has been taking place during the last few million years of the life of the Dead Sea and its ancestral lakes such as Samra and Lisan Lakes.
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Figure 3. Changing in Dead Sea water level between 1972 and 2019 [22] .
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Figure 4. Former depositional area and present erosional area due to the recession of the Dead Sea.
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Figure 5. Head ward erosion in the immediate surroundings of the Dead Sea shore caused by the retreat of the Dead Sea (at around 500 m east of its present shore).
Erosion rates reflect weathering rates in the respective catchment area. Changes in the base level elevation (lake, sea, playa) play a major role in erosion and sedimentation rates, but that have only little to do with the physical and chemical weathering processes except the role of exposure of additional rock parts to weathering and erosion.
6. Conclusions
In this study, erosion rates of domain rocks are calculated based on dams’ catchment areas and the sediment loads transported by flood flows into dams’ lakes. The study results show that the chemically, via floodwater transported quantities of materials are negligible compared to the solid materials transported by the water. It calculates erosion rates ranging from 0.013 to 0.212 mm/yr for the different catchment areas.
The slopes of the catchment areas affect the erosion rates and that is exposure of rocks to weathering processes (85% of area’s size elevation divided weathering rates by 15% area’s size elevation). Weathering rates of the different rock types covering a catchment area are another factor affecting erosion rates. For example, weathering rates of friable sandstone or weakly consolidated marls are higher than those of dolomites or silicified limestones. The high percentage coverage of friable sandstone in the Kafrain catchment or the combusted limestone in the Wala catchment may result in strongly increasing the weathering rates and hence increasing the quantity of eroded materials from the catchment area of a wadi or a river course.