Trend of bed load material and bed load transport in Sungai Kenau for year 2013 until 2020

A study was carried out to determine the trend of bed load material characteristic and bed load transport rate in Sungai Kenau at Sungai Lembing, Kuantan, Pahang between year 2013 until 2020. Sediment is a material that can be moved and deposited to the new location through the river. This study will be focusing in bed load transport as it is the key component to determine the sediment transport rate in the river. Two sampling station have been chosen to measure the bed load transport in year 2013 until to 2020. Schocklitsh and Duboys equation was used to predict sediment transport load and it was compared with measured sample from Helly-Smith equipment. The collected sample was analysed to determine the trend of bed load material and bed load transport in Sungai Kenau for year 2013 until 2020. According to Duboys calculation the highest bed load transport rate is Sample 1 with 0.013220 lb/sec and the lowest is on Sample 5 with 0.002310 lb/sec both of the samples are taken on the same station but on the different day. Meanwhile according to Schocklitsh calculation the highest bed load transport rate is Sample 7 with 3.79374 lb/sec and the lowest is on Sample 5 with 1.98018 lb/sec both of the samples also taken on the same station but on the different day. It can be sure that the higher flow rate of the river the higher bed load transport rate. It can be obtain that according to Wentworth Scale the median grain size in Sungai Kenau were between 0.68 mm to 2.6 mm where the bed material can be classified as coarse sand to the very fine gravel.


Introduction
Rivers plays and provided many important things that human needed. History show that the first human civilization started near the river as the river provides food, transport and shelter. A river, that commonly freshwater is a watercourse that flowing towards ocean, lake and another river. Water from the river is a basic natural resources and very essential in many human activities. Most of the human economic activities are located in the river basin as it provided placed for agriculture industry, hydro power generation, navigation, tourism, mining and irrigation. Even human are also having threat with the flood problem, siltation, sand deposition and bank erosion of river [1]. The process of sediment transport is referred when there is water flowing down on a slope in river channels that contain erodible bed that may scour the loose particle inside the river bed such as sand, rock and organic material the moved it to the downstream [2]. Sediment particle can be defined as whenever there is contact between the moving fluids with erodible bed and the local shear stress exerted by the flow to the boundaries of the channel is strong enough to disturb and immersed the friction force and the weight factor of the bed material [2]. The velocity of the flow, the particle size, and the relative densities of the sediment and the fluid will recognize whether the bed material moves as bed or  [3]. Over a pass few decades researcher start to notice that mechanism of sediment transport in open channel is the one of the branches of modern alluvial hydraulic subjected to in depth investigation. The sediment transport study is to create an estimation of the amount of sediment material with only specific flow can carry, thus this attract a number of researcher to investigate about this issue such as Colby (1964), Englund-Hansen (1967) and Einstein (1950) have proposed graphical and analytical procedure to calculate the sediment discharge in the stream. Study of sediment transport is very important in hydraulic construction such as planning and design water reservoir, river training and bed deformation, sediment yield estimation for river basin, design and maintenance of stable irrigation channels and beach erosion and coastal sediment transport [3]. Sediment can be classified in the three forms which is suspended load, bed load and solution load [4]. Sediment characteristics can greatly affect the rate and pattern of the river condition. There are some factor that can affect the condition of the river bodies such as the chemical composition of sediment, particle size, fall velocity and any other factors. One of the most important properties when study about moving sediment, is particle size [5]. Particle size can greatly effects the calculation of the transport rate and bed material. As the solid particles has various type of size and classification when moving along the natural stream or any water bodies thus this classification has been recorded and accepted by the organization and widely used as the source of information by hydraulic engineers [6]. Human activities near the river such as mining, housing and facilities construction is one of the factor that contribute to the increasing rate of sedimentation. Increasing rate of sediment rate inside the river will cause the river become shallow thus will lead to other problem such as flooding and destroy habitat. The moving sediment can give negative impact to the water quality in water bodies as it act as transport mechanism of hazardous material such as heavy metal, pesticides and organic material this also known as contaminated sediment [7]. This hazardous material can give effect to the environment and human.

Research Objectives
There are three objective has been obtain from this research which are to identify the Characteristic of sediment in Sungai Kenau at Sungai Lembing,to identify the sediment concentration effect toward the environment and lastly is to calculate bed load transport rate using the Duboys and Schoklitsch equations.

Scope of Study
This research is conducted on September 2013 until February 2020. The location of study area is in Sungai Kenau at Sungai Lembing, Kuantan, Pahang. The scope of study in this research is about effect from the human activities near the river to the sedimentation rate at Sungai Kenau that are currently facing problem due to mining and housing development activity. All of this activity can contribute to the increasing of sedimentation rate in the river and will give worse effect to the river ecosystem at Sungai Kenau. The increase in water demands causes more conflict between the human system and the river ecological system [8]. Figure 1 show the view of Sungai Kenau at station 1 for sampling datawhile Figure 2 show the view of Sungai Kenau at station 2 for sampling data.

Study Area
The study was conducted at Sungai Lembing, Kuantan, Pahang. It was concentrated on the bed load transport of the collected data at the selected station. The study locations are located at coordinates of 3.9146° N, 103.0327° E near the Sungai Lembing food court. This location was selected because Sungai Lembing was once a famous mining site in peninsular Malaysia. It was the largest tin mining site in 1990 to 1998. Sungai Lembing is a one of river bodies in Kuantan district, while Sungai Kenau is one of the branches of Sungai Lembing. Even though the tin mining activities at the site doesn't active anymore, the tailing from the mining site still can give effect to the river body morphology and ecosystem near them [9].

Methodology
Comparing between four year which is year 2007, 2013, 2019 and 2020 will show the bed load transport rate using the bed load equation and Helly-smith equipment in Sungai Kenau at Sungai Lembing. Two sampling point represent the difference between the bed load transport and bed load particle in each sample. The required data that collected on site are bed material sample and flow characteristic of the river such as width, depth and velocity of the flow. The sample then stored and dried in the oven for 24 hours. Sieve analysis test was conducted by divided the sample into 500 grams for each sample and put it on mechanical shaker about 15 minutes. The sizes of sieve trays used in this research were 10.0 mm. 6.3 mm, 5.0 mm, 3.35 mm, 1.18 mm, 0.6 mm, 0.3 mm, 0.15 mm and 63μm. Each type of sediment represents the different size that characterise from the Udden Wentworth class [10] table as reference of sediment particles classification. All the data that obtain from the laboratory and on-site measurement are important to calculate the bed load transport. Helly-Smith equipment was used to determine the measured bed load transport immediately in the site. Figure 3 shows the flow chart of methodology from the sampling until data evaluation   Figure 4 shows the Particle size distribution curve in Sungai Kenau for each sample. The particle size distribution of each sample of bed load material that taken in Sg.Kenau are plotted after doing sieve analysis test. From the figure above shows the particle size distribution of each sample taken. From the graph it shows that the median grain size 50 is 0.68 mm for Sample 1 on 4 th September 2013, 1.  In the Udden-Wentworth scale there is a detail about the size range of sediment that already named in the class such as (boulder, cobble, pebble), for the sand type (very coarse sand, medium sand, fine sand) and for the finer sediment (silt, clay, mud). The bed load material size and type classification are very important because it will affect the value of bed load transport rate.

Bed load Discharge Equation
Bed load transport or bed load discharge can be determine using two different bed load equation which is Schocklitsch and Duboys equation. This method is known as prediction method

The Duboys equation
The formulation in predicting sediment discharge has been started since 1879 by Du Boys who is introduce the tractive force approach in his bed load sediment formula. Duboys (1879) introduced the tractive force or bed shear stress which was an entirely new concept. DuBoys (1879) developed the following formula

The Schoklitsch equations
From analysis of the experimental data of Gilbert (1914), Schocklitsh (1914) showed that the nonuniform nature of shear stress distribution in a channel cross section. Because of the average shear stress in Du Boys model is a poor criterion for the determination of bed load transport rate. (2)

The percentage different equations
Percentage different equation was use to compare the different value of the data between each sample.
(3)   Data on the Table 4 shows the relationship between flow rate and predicted bed load discharge using Duboys and Schoklitsch equation. Firstly for Duboys equation for sample 1 on 4 th September 2013, the bed load discharge is 0.013220 lb/sec when the flow rate is 3.55 ft 3 /s. then for sample 2 on 5 th September 2013, the bed load discharge is 0.008300 lb/sec, when the bed load 5.289 ft 3 /s. Sample 1 and 2 shows the highest flow rate due to rainy condition. The increasing upstream flow has increase the flow rate in the station. Sample 5 and 6 on 13 th November 2019 shows the lowest bed load discharge which is 0.002310 lb/sec and 0.003070 lb/sec. the flow rate on that day also decrease than sample 1 and 2 which is 1.040 ft 3  show the highest sediment rate between all of the sample because the sample were taken after the rainy weather at the station even though the flow rate is the lowest, assumed that the bed material from the upstream already reached the station thus it can greatly affect the calculation.    Table 7 contain data for Predicted bed load using Schoklitsch and Duboys equation and Measured Bed Load. As shown in the table only data 7 and 8 are available this is because past researcher doesn't take part in using Helly-Smith bed load sampler. From that we can see using Duboys equation proven to give more accurate and same data with the measured bed load than the Schoklitsch equation. For Sample 7 and 8 on 21 th February 2020 Duboys equation shows the bed load transport for that day is 0.007890 lb/sec and measured bed load shows 0.01464 lb/sec while Schoklitsch equation shows 3.79374 lb/sec. there are large gap between the data shows in Schoklitsch equation. As using measured data there are high possibilities to get false data due to human errors. Duboys equation also suitable to calculate bed load discharge in Sungai Kenau as it applicable for gravel type sediment.

Conclusions
In conclusion, based on the median grain size, the sediments or bed load material were classified as gravel type which is applicable for Duboys and Schoklitsch equation as the prediction method of bed load transport rate in Sg. Kenau. From the discussion it shows that the flow rate and river velocity give great influence on the bed load discharge as there are great comparison between the lowest and the highest flow rate of velocity on the data. In other word the higher velocity flow, the greater the bed load discharge. Sediment particle size also important to determine the size of material that take place inside the bed river. Increasing sediment rate in the river will cause flood problem and disturbance on natural habitat.