The influence of a dam on the downstream degradation of a river bed: case study of the Tigris River

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Hydrology in Mountainous Regions. II - Artificial Reservoirs; Water and Slopes (Proceedings of two Lausanne Symposia, August 1990). IAHS Publ. no. 194, 1990.

1 Hydrology in Mountainous Regions. II - Artificial Reservoirs; Water and Slopes (Proceedings of two Lausanne Symposia, August 1990). IAHS Publ. no. 194, The influence of a dam on the downstream degradation of a river bed: case study of the Tigris River Thair M.AL-Taiee Assistant Lecturer, Saddam Dam Research Center, Mosul University, Mosul, Iraq ABSTRACT Changes in Tigris river bed profiles at Mosul station for different period during the years 1985, 1986, 1987 and 1988 after Mosul dam construction, in addition to the measurements of suspended sediment concentration from the downstream river were used to study the effect of water storage in Mosul dam on the degradation processes occurred in the downstream Tigris river 's bed due to the high velocity and tractive force of the released clear water from the reservoir. From the comparison of Tigris river profiles, surface water levels of the river associated with the analysis of grain size distribution of the bed load material and water temperature variation it was concluded that Mosul dam effect on the degradation of Tigris river bed was clear due to its retaining percent of 95.5 of suspended sediment load in the reservoir.also it was observed that about twenty five centimeters in an average was degraded from Tigris river bed in addition to the increment of the (Dso) of river bed material from 8 mm.to 17mm. after four years of Mosul dam construction. INTRODUCTION There are many negative side effects of the dam construction on the downstream river 's regime and on its hydraulic structures.before dam construction the river was in a state of equilibrium characterized by a clear low water and a flood was loaded with suspended matter.as the water stores behind a constructed dam, it looses most of the material suspended in it. The increased water velocity of pure water in front of the dam starts to degrade the bottom and the banks of the downstream river to compensate for the loss of the material deposited in the reservoir. Minor degradation below dams is sootimes desirable and beneficial since it increases channel capacity and improves drainage of adjoining lands. Tremendous quantities of bed materials have been removed from the channel below Hoover'dam.Measurements reported by Borland and Miller,(1964) indicated that about 11,559,320 cubic metres have been removed from the channel for a dist- 153

2 ThairM. Al-Taiee 154 ance of 148 kilometers ailes below the dam during the period 1935 to Observation Bade at lake Mead indicated that when the clarified water was released from the dan on the Rio grand, the flow was carried away from the bed more fine materials than coarse particles (lane, 1953). Several modern dans in India of relatively low height have failed as a result of erosion at the toe of the structure due to the sediment carried away fron the bed by the clarified water (Jogtefcar,1951). The rate of degradation below reservoirs depends upon the type of material in the channel and the hydraulic characteristics of the outflow.these degradation processes will eventually reduce the water level and increase the hydraulic pressure on the structures located downstream of the dam such as bridges. The present case study for the above mentioned problem was applied to observe the effect of Mosul dam on the degradation of Tigris river bed. The dam was located on Tigris river 60 kilometers north of Mosul city, (Fig 1). The dam construction completed in 1985 and started to store water behind in the beginning of FIG. 1 Location nap.

3 155 A dam and downstream degradation of the Tigris METHODOLOGY A Tigris river profile in the entrance of Mosul city was selected in the present work as a case study of the present problem. This river profile was located within Mosul discharge Measurement station which was installed 150 metres upstream of an old Mosul bridge. The field estimates of degradation are based on observations of drops in the bed level of the studied river profile and the decreases in water levels. Field suspended sediment measurements, discharge measurements, measurements of surface water slope and cross section profiles are periodically carried out after dam construction for the years 1985 till the end of the year 1988 by the ministry of Irrigation (1988). Sediment samples from the Tigris river bed at Mosul station were analyzed for grain size distribution by Nedeco (1976),Najib (1980) and Hayawi (1987). Water temperatures from the upstream and downstream of Mosul dam were measured and recorded too. RESULTS AND DISCUSSION It is natural that such a complex scheme like a dam should have some effects. Some of those are positive whilst others are negative.an important one of those negative effects are the degradation processes which are the by-product of flowing clear water from the dam to the downstream river. This clear water has trapped its suspended sediment material in the upstream reservoir before releasing from the dam. The above mentioned problem was applied on Mosul dam trying to study its effect on the downstream Tigris river specially near one of the oldest hydraulic construction in Mosul city (old Mosul bridge). Three important factors and indications were used in the present work to satisfy and discuss the effect of releasing clear water from Mosul dam on downstream river. One of those indications was the sediment concentration measurements in downstream river for the periods before and after Mosul dam construction.the other two factors were the cross section profiles of Tigris river for different periods and the water temperature differences between the upstream and downstream behaviour of erosion and deposition processes. For the sediment concentration, the mean annual suspended load transported by Tigris river prior to construction of the dam have been amounted to about tons. The maximum observed sediment concentration prior to construction of Mosul dam was 6725 ppm. whilst the minimum was 29 ppo. giving the ratio of (230-1), (ministry of Irr. 1988). After the construction of the dam, the mean monthly concentration of suspended sediment in the downstream river dropped to a range between 85 ppm. and 38 ppm. It is clear from the above mentioned numbers that since construction of the dam (1985),95.5% of the total suspended sediment load carried by flood was deposited in Mosul reservoir. This

Thair M. Al-Taiee 156 siltation process in the reservoir brought into clear degradation downstream river of the dam due to the releasing of clear water with high tractive force and high resistance.

4 Thair M. Al-Taiee 156 siltation process in the reservoir brought into clear degradation downstream river of the dam due to the releasing of clear water with high tractive force and high resistance. Average monthly records of suspended sediment concentration befor and after dan construction were shown in Table (1). TABLE 1 Average Monthly Sediment concentration (ppn) in Tigris River at Mosul station Before and After Daa Construction. Period/year J F M A M J J A S O N D Variation of Grain Size Bed Material Bed material size in the downstream reach from Mosul dao exerts considerable influence on the nature, degree, rate and extent of degradation. Decrease in peak discharges and its sediment concentration after dam closure due to flood storage, reduces the ability of the river to transport the coarsest fractions of the bed material. However, reduced sediment loads downstream from the dam allows the regulated flows to erode fine material from the bed and banks. This leads to progressive coarsening of the bed material and the development of surface armour layers which protect the subsurface layers from erosion. The analysis of the field measurements and observations made on the Tigris river indicated that when the treated water released from Mosul dam to the river the flow carry away from the bed more fine particles than coarse particles.figure (2) shows the increase in the size of the Tigris river bed material at Mosul station for different periods before and after dam construction. One of the important indications of the river degradation is the water surface slope and the water surface elevation of the river at certain site. It was found that the water surface slope at Mosul river station for the periods before and after the daa construction (1980, 1987) was 4.7 x 10-"* and 4.85 x 10 - * m/m respectively. (Figure 3) shows the discharge rating curve at Mosul station before and after dam construction.(ministry of Irrigation,1988). It was clear from the figure that the average water level drop in the river after Mosul daa construction was about twenty five centimeters which gave an indication that a river bed degradation occurred. Number of Tigris river profiles at Mosul station were dependant as the second factor in the observation of river degradation. These river profiles represent different

5 157 A dam and downstream degradation of the Tigris «60 " 50 * * Nedeco 1976 Najib Hay cwi 1987 '/" /" / / r-v \/ ''' DQrticlf <;i7p ~5amr uruvel fine med coais. càais finè I fine jmed" eoar- coafee FIG. 2 Grain-size distribution of Tigris river bed material at Mosul station o -c c40 01 o «30 a </> tn "355 ÂÔED~ 5000 KB) 3, Discharge m/s- FIG. 3 Tigris River rating curve at Mosul station. discharges and period after dam construction, (Figure 4). An observation of the Tigris river profiles for the years 1985 and 1988, the effect of sediment absence in the downstream river due to Mosul dan construction was clear During the year 1985, the maximum degradation occurred was forty to fifty centimeters when the river discharge increased from 285 m 3 /s during December 1984 to 2044 m 3 /s during March 1985.While during the year 1988(the abnormal flood season),

6 Thair M. Al-Taiee , APril 1988 Q =4666 m/s 207- y 9 way o o APril 205- e «' J un S S Distance -meter- Fig. 4 Tigris River profiles at Mosul station for different periods. the degradation depth reached ninty to one hundred centineters when the river discharge increased fron 530 irvs druing Feb. to 4666 n 3 /s during April. The sane degradation process occurred during the year 1987 in which the degradation depth reached thrity centineters when the discharge increased fron 450 to 1600 n 3 /s. These observations of the degradation processes gave an indication that although, there is a proportional relation between degradation depth and discharge, there is also an inverse relation with the sediaent concentration. Figure (5)shows clearly that Tigris river bed was degraded and refilled during the years 1985,1987 and 1988 after daa construction due to subsequent high and low flood crest as clearly shown through the river profiles at April and June These degradation and deposition processes were extended and continued around the piers of the old Mosul bridge.these processes show that some engineering protection around the piers of the bridge may be required such as filling the degraded cavities around the foundation of the

7 159 A dam and downstream degradation of the Tigris ê5 ëb î5ô iéo îffi 2to Distance _ meter- Mav 1987 Q= 1603 m/s.nqv.j987 _0_EZ60_n?/s_. Oct Q = 449 m7s ÏBÔ ÎS Î5Ô 200 ' Distance - meter APril =4666 m/s S ~5 85 ÎÔO ÎS) léo Distance - meter_ FIG. 5 Tigris River profiles at Mosul station for the years 1985, 1987 and piers with large size rock riprap which gives more stablization to the piers and push away the vortices formed there. Water Temperature Effects It has been noted that the movement of bed material in mov-

Thair M. Al-Taiee 160 able bed can be affected by the temperature of the water, (Walter, 1963). That is to say the change in viscosity of water affect the fall velocity of a particle of sediment.

8 Thair M. Al-Taiee 160 able bed can be affected by the temperature of the water, (Walter, 1963). That is to say the change in viscosity of water affect the fall velocity of a particle of sediment. This fall velocity essentially determines or controls the ability of water to put into suspension or motion certain sizes and quantities of sediment. A preliminary analysis to some of the laboratory experiments showed an increase in the energy gradient with a decrease in water temperature for the same rate of bed movement,(walter 1963). It is known that the stored water behind a dan,releases to the downstream river from a large depths in reservoir in which it keeps water in a cool temperature. This cool water will aid in the degradation processes of the river bed. Water temperature differences between the upstream and downstream of Mosul dam was observed,(table 2). This decrease in the water temperature downstream of the daa added a significant factor to the mentioned reasons for Tigris river degradation. TABLE 2 Water Temperature in C of Tigris River For Different Periods. Oct. Nov. Jan. Feb. Mar. May. June. Period upstream of dam downstream of dam REFERENCES Borland, W.M and Miller, C.R. (1964) Degradation below reservoir. Handbook of Applied Hydrology, V.T. Chow. Hayawi,G.A.M.(1987)A hydraulic model to study and train Tigris river between Ninavah bridge and AL-Hurriya bridge at Mosul city. Thesis presented to the University of Mosul at Mosul.College of Engineering, Irrigation Department. Jogtekar, D.V. and Wadelar, G.T.U951) The effect of weirs and Dams on the regime of rivers. International Association of Hydraulic Research. Bombay. Lane, E.W and Borland, W.M.(1953) River bed scour during floods. American Society of Civil Engineers, Journal of Hydraulic Division. Vol. 5. Ministry of Irrigation (1988).Seasonal Reports,Mosul Irrigation Office,Iraq. Najib, Y.E.(1980) Characteristics of Tigris river at Mosul. Thesis presented to the University of Mosul at Mosul, College of Engineering, Irrigation Department. Nedeco (1976) Navigation study Tigris river, Mosul-Baghdad Reach, Republic of Iraq, Planning report. Walter C.Carey (1963) Effect of Water temperature on the river bed configuration. Paper No. 31 In The Proceeding of the Federal Inter-Agency of Sediment Conference.Miscellaneous Publication No.970. Agricultural Research Service.

MATHEMATICAL MODELING OF FLUVIAL SEDIMENT DELIVERY, NEKA RIVER, IRAN. S.E. Kermani H. Golmaee M.Z. Ahmadi

MATHEMATICAL MODELING OF FLUVIAL SEDIMENT DELIVERY, NEKA RIVER, IRAN. S.E. Kermani H. Golmaee M.Z. Ahmadi JOURNAL OF ENVIRONMENTAL HYDROLOGY The Electronic Journal of the International Association for Environmental Hydrology On the World Wide Web at http://www.hydroweb.com VOLUME 16 2008 MATHEMATICAL MODELING