Traektoriâ Nauki = Path of Science. 2020. Vol. 6, No 9 ISSN 2413-9009 Morphological Adjustment of Kahang River Consequent to Construction of Kahang Dam Muhammad Nda 1, 2, Mohd Shalahuddin Adnan 1, Mohd Azlan Bin Mohd Yusoff 1, Gideon Jiya Shabako 2, Yakubu Musa 2 1 Universiti Tun Hussein Onn Malaysia Parit Raja, Batu Pahat, Johor, 86400, Malaysia 2 The Federal Polytechnic, Bida KM 1.5, Doko Road, Bida, Niger State, Nigeria DOI: 10.22178/pos.62-4 Abstract. In a tropical country like Malaysia, the availability of research works to monitor channel changes as a response to situating a dam over an observation period may help to establish new management LСC Subject Category: TD1-1066 strategies. These investigations will constitute a vital approach in assessing the changes to channel morphology downstream due to the Received 25.08.2020 presence of a dam and its operations. In the present study, the effects Accepted 28.09.2020 of Kahang Dam on Kahang River morphology, in particular, the Published online 30.09.2020 downstream section was investigated and analyzed between January 2017 to January 2018. The rainfall pattern of the study period, the flow regime concerning the period, and geomorphological channel Corresponding Author: adjustments were carried out during the field surveys over a 400 meters Muhammad Nda reach. Different responses were leading to high flow and low flow [email protected] regimes during periods with and without any interference due to dam operations. Similarly, the river channel was characterized by adjustments and realignments, causing modifications to the river © 2020 The Authors. This article morphology. Changes in cross-sectional area and local incision were is licensed under a Creative Commons also noticed as a direct response to sediment deficit and bank erosion Attribution 4.0 License process. Keywords: Kahang River; kahang Dam effects; Downstream morphological adjustment. INTRODUCTION and for this reason, they cannot be used to deter- mine the morphological response of a particular Generally, river flows are closely related to chan- river. Dams cause a combination of reduction of nel flood control and navigation [1]. The river flow and accumulation of sediment. Initially, riv- course evolution has distinctive features and is in- fluenced by multiple factors, including upstream ers seem to be more resilient to decreased flow and show signs of sediment deposition, but as flow, channel boundary, hydrological patterns, and downstream water level [2]. After the intro- time passes, sediment trapping in the upstream duction of a dam for water impoundment pur- reservoir reverses this tendency and the poses, it is essential to adjust to the changes as re- riverbank and bed begin to erode, resulting in channel incision [5]. If the incision progresses, the gards the hydrological conditions and other fac- main river channel is isolated from the flood tors of the downstream section [3]. The impact of dams on downstream river morphology has al- plains because high flows are no longer able to ready been discussed in a variety of studies offer- flood them [6]. Most of the sand bed rivers have ing a list of river morphological reactions, with [4] been observed to react to dams in this way. presenting a detailed literature review on the ef- Channel morphology, consisting of dimensions of fect of dams on downstream river morphology. width, depth, and slope, which are a function of These studies do not differentiate rivers based on discharge regime and sediment contribution. Any their bed material and controlled flow patterns, adjustment in discharge and sediment Section “Technics” 2001 Traektoriâ Nauki = Path of Science. 2020. Vol. 6, No 9 ISSN 2413-9009 concentration below the dam is expected to alter MATERIALS AND METHODS the existing balance or equilibrium, thereby caus- Malaysia is located near the equator characterized ing channel form adjustment [7]. Sediment con- by high solar energy all through the year. The air centration is a primary factor associated with the is mostly moist and commonly covered with downstream impact of the dams. Sediment con- clouds all year round. Johor is in the southern re- centration is considered to be vital in ensuring a gion of Peninsular Malaysia and is divided into balance between natural and anthropogenic fac- eight administrative districts. Kluang, where the tors by changing channel form [8]. Water flowing study area is situated, is a district in the centre of from dams downstream significantly decreases the state of Johor, located on coordinate the suspended sediment concentration, and this 2.0301 °N, 103.3185 °E. This area received about sediment deficiency is projected to result in an in- 2100–2500 mm of rainfall every year, with an av- creased rate of channel bed and bank erosion [9]. erage temperature of 28 °C. Nevertheless, the relationship between river damming and the commonly assumed dominance For this study, the cross-section of the river will be of downstream erosion is not consistent. It has divided into elemental strips of equal width using a graduated line, which will be pegged at the two been found to differ with time following the clo- ends of the river cross-section. Changes to river sure of the dam, for instance, a study done on the morphology that is caused downstream the river Red River in Vietnam by [10] and the distance due to dam regulation are measured using cross- from the dam as documented by [11] in the Me- section repeated survey technique. Cross-section kong River, suggest that suspended sediment con- of the river channel is benchmarked at each end in centration was found to recover as the distance such a way that it can be identified and resur- from the dam increased. veyed [16]. Starting from the dam stilling basin, Several kinds of research have been carried out firm wooden pegs of 1.2 m to 1.5 m in length were focusing on the consequences of introducing a driven into the ground at 25 m interval. The pegs dam on a river as regards the downstream sedi- were mounted at the two sides (right- and left- ment deficit and flow reduction with little work hand side) of the riverbank until the 400 m reach. done on the aspect of the river morphological The width is measured across the river channel at change, for example, findings arising from a re- 90° to the channel at about 0.2 m above the water view paper by [12] on sediment load change in the level. The start and finish point for width meas- Yangtze River. Similarly, studies by [13, 14] pre- urement is the active river state (not considering sented the effects of damming as regards sus- the floodplain). The river depth is measured at the pended sediment trapping and flow reduction for midpoint of each benchmark (mounted peg) the Yangtze River and Mekong River, respectively. along the river cross-section using a ranging pole. Besides, authors [15] carried out a study down- The change in the area and subsequently total sur- stream Kahang River on bedload sediment con- face area of the river morphology was calculated centration variabilities due to Kahang Dam. starting from January 2017 to January 2018 (i.e., Therefore, further studies are needed to examine January, March, May, July, September, November the response of river morphology downstream all in 2017 and January 2018) as represented by dam reaches considering many factors such as Equation 1 and 2 respectively. The measurements sediment transport, flow regime, hydrological were taken once in two months to see the mor- pattern, channel bed, and bank erosion. phological adjustment of the river channel over time. The present study aims to explore the morpholog- ical adjustments at the downstream reaches of Kahang Dam by considering the dam operations, 퐶퐴 = 푤푑 (1) flow regime, and rainfall pattern. The findings will where 퐶퐴 = 푐푟표푠푠 − 푠푒푐푡표푛푎푙 푎푟푒푎 (푚2); provide an insight into how the factors, as men- tioned earlier, contribute to changes in Kahang 푤 = 푎푐푡푣푒 푤푑푡ℎ 표푓 푡ℎ푒 푐ℎ푎푛푛푒푙(푚); River morphological adjustment. Additionally, the 푑 = 푚푑푝표푛푡 푑푒푝푡ℎ 표푓 푡ℎ푒 푐ℎ푎푛푛푒푙(푚). findings can be used to deduce appropriate strat- egies for protection and management for dam downstream reaches. 푛 푅푆퐴 = ∑푖=1 퐶퐴푖 (2) where 푅푆퐴 = 푇표푡푎푙 푅푣푒푟 푠푢푟푓푎푐푒 푎푟푒푎 (푚2); Section “Technics” 2002 Traektoriâ Nauki = Path of Science. 2020. Vol. 6, No 9 ISSN 2413-9009 and collected for one-year data (January 2017 – 퐶퐴푖 = 푐푟표푠푠 January 2018). − 푠푒푐푡표푛푎푙 푎푟푒푎 (푚2)푓표푟 푒푎푐ℎ 푐ℎ푎푛푎푔푒 (. 푒. 퐶ℎ 25 … … 퐶ℎ 400) RESULTS AND DISCUSSIONS The effects of dams on the river morphology are Velocity measurement. The flow rate (velocity) studied because dams tend to retain sediments measurements were equally taken at the same and induce flow reduction downstream of its host. midpoint were the depth measurements were In the beginning, rivers appear to show elements taken during the cross-sectional survey using the of sediment deposition, but with time this ten- Electric magnetic water current and speed meter. dency disappears as most sediments are trapped The current meter is an instrument made up of in the reservoir upstream, and the riverbed starts the following; current speed sensor, LCD system, to respond with channel incision as a result of and a post holder. Flow measurement was taken eroding. Continuous river channel incision will by lowering the current sensor to a depth 0.6 d be- force the main river channel to shrink and discon- low the water, and the corresponding flow rate nected from its floodplains because inundation (Vm/s) displayed on the LCD system after 30 sec- caused by high flows is no more. This phenome- onds and reading recorded [17]. These measure- non is the typical response of most rivers to the ments were taken at the same time with the river dam.