Characteristics Q Sediment Motion Meandering River Shear Stress Researching the behavior of the river.
Especially at the bend, where the morphology of the river is not always
straightforward. Flow velocity high water and grinding at river bends occur at
different points. This research was conducted on five adjacent bends on the Sidoan River section. This study examined the condition of
riverbed sediment, knowing the stability of riverbed sediment granules based on
shearing velocity, and stability of riverbed sediment granules based on shear
stress. The method used in this study is geometric measurement. Q
## 1. INTRODUCTIONResearching
the behavior of the flow at the bends of the river is very important because
the morphology of the river is not always straight so that the flow-through can
be affected by the bend. The meandering river is a dynamic system and very
nonlinear. Sidoan River has a groove that bends. as
shown in Figure 1 below that on the outside of the bend there is a scouring
which is certainly caused by the presence of centrifugal force flow. At
certain times large discharges and flow velocity are prone to overflow and
scouring at river bends. Scouring of sediment and deposition on rivers can
occur at different points, of the many bends found on the Sidoan
River, and used five adjacent bends as a research object. This
study examined the condition of riverbed sediment at five bends, knowing the stability
of riverbed sediment granules based on sliding velocity.
According
to Rinaldi [1] Researching the bend, especially about the
depth of the scouring around the bridge abutment, stated that the scouring
occurred due to the changing flow pattern, other researchers [2] researching the amount of scouring that
occurs at the bends of the river, especially when there are pillars or
abutments at the bend, this study was conducted in the laboratory, while
another research examines the relationship between discharge and river
meandering of geometry [3]. Marttotorang [4] observing the effect of riverbed width on
flow patterns that occur in each section due to changes in flow velocity, with
increasing flow velocity in the cross-section, sediment transport becomes
large. Other research on bedload sediment [5],[6] conducted in the laboratory and
the field. the results of research on the velocity and number of sediment
particles show a logarithmic relationship to the average size of the sediment,
the base material transport rate of the river width unity increases with
increasing depth of the geometric equation function, and other studies [7],[8] the movement of sediment particles
is also greatly influenced by the shape of the river basin. Analyze
the distribution of velocity and sediment in the channel bends, especially
single bends [9],[10],[11] flow velocity increases on the inside of the
bend when passing through the start of the bend, and velocity decreases at the
end of the bend. Other research on the relationship between riverbed material
characteristics and its geometry shows that the middle part of the river has
the most content in coarse-grained soil (d≥0.150 mm) while on the
riverbanks the most content is fine-grained soil (d≤0.150 mm) [12],[13]. Purnama
[9] the distribution trends of the
suspended sediment concentration increase towards the bottom of the channel and
decreases towards the water surface with the distribution trend getting erect
with a more uniform value towards the water surface. The discharge and flow
velocity are directly proportional to the increase in the volume of scouring
that occurs [13]. ## 2. MATERIALS AND METHODS## 2.1. HYDROLOGYThis
research begins by collecting hydrological data, with this data calculated the
rain intensity is the height of rain or the volume of rain per unit of time.
The amount of rain intensity varies, depending on the length of rainfall and
the frequency of its occurrence, and frequency analysis. This frequency
analysis is intended to determine the relationship between the magnitude of
extreme events and the frequency of the occurrence of these events, frequency
analysis is needed. Frequency analysis for rainfall, in general, can be done
using the frequency analysis of the Log Pearson III Distribution. Meanwhile, to
determine which empirical frequency distribution method fits the existing data
sample, statistical testing is required. Rain
intensity for a short duration was estimated using the Mononobe
formula [14]. (1) ## 2.2. HYDROGRAPHAccording to Triatmodjo [15], a hydrograph is a
curve that gives a relationship between flow parameters in the form of flow
rate or water level with time. The hydrograph consists of a rising limb, peak,
and recession limb, shown in Figure 2.
## 2.3. AMOUNT OF DISCHARGETo determine the amount of river discharge based on rain. it is necessary
to examine the relationship between rain and river flow. The amount of flow in
the river is determined mainly by the amount of rain, the intensity of the rain, the area of the rain area. the length of time of
the rain, the area of the
river, and the characteristics of the watershed [16]. ## 2.4. METHOD OF NAKAYASUEquation
of
HSS Nakayasu and Figure
3 [15].
(2)
## 2.5. RIVER MORPHOLOGYHay [17]assesses that the
behavior of the river is meandering as shown in Figure 4.
Morisawa
[19] formulated that the classification of bends
is the ratio between the length of the flow of the meander and the length of
the axis of the bend. SI = the
length of the river flow at the bend divided by the length of the axis of the
bend, if SI<1.05 = straight river. SI>1.5 = meandering river, and
0.5>SI<1.5 = sinuous river. ## 2.6. FACTORS AFFECTING THE DEPTH OF SCOURShields’s
diagram states the relationship between R). Shields’s graph is often used to
evaluate the critical shear stress, as in Figure 5, by knowing the Reynolds
number (_{e}R) of grain or
grain diameter (_{e}d), then the critical
shear stress value (τ)
can be seen. If the bottom shear stress of the flow is above the critical value,
then the sediment grains move, or in other words: _{c}τ_{ο}˂ τ
the bottom grains moved._{c}
Shields’s
graph defined the initial
motion to be the following equation: (3) (4) Shear velocity: (5) Shear stress: (6) ## 2.7. SIMULATION OF HEC-RAS 5.0HEC-RAS is an integrated package. designed for interactive use in
multi-tasking environments. The HEC-RAS is an integrated package. designed for
interactive use in multi-tasking environments. This system is designed to
provide two-dimensional river modeling using steady flow, steady flow, and
sediment transport calculations based on a geometric representation of the
river network. Provides steady-flow surface water profile calculations for
river networks with sub-critical. Supercritical, or mixed flow regimes [21]. HEC-RAS has
four components of a one-dimensional model: steady-flow water level profile
count, unstable flow simulation, sediment transport count, and water quality
counts. ## 2.8. BEDLOAD ROUGHNESSSediment
grain size classification refers to the Scale American Society of Testing Material (ASTM) Table 1. Roughness
coefficient [22]. (7) Roughness
coefficient [23].
Roughness coefficient [24].
Manning roughness figure is a
coefficient value indicating the roughness of a channel or river bed either on the side or bottom of the channel or river. The Manning roughness value has
a relationship to the flow velocity that occurs in a section. The greater the
Manning roughness number, the smaller the flow velocity in a section, and vice
versa, the smaller the Manning roughness number, the greater the flow velocity
that occurs in a section. ## 3. RESULTS AND DISCUSSIONSRainfall
data were analyzed to obtain a design flood discharge. The design flood
discharge used is derived from rainfall data which is analyzed by frequency
analysis, then the Nakayasu synthetic unit hydrograph
(HSS) method is used to transform rain into the flow. Nakayasu
Synthetic Unit Hydrograph is a synthetic unit hydrograph model or commonly
abbreviated as HSS is a rain-discharge transformation model which is based on
the hydrograph theory of hydrograph units produced by one unit of rain due to
evenly distributed rain throughout the watershed with a certain duration, using
watershed parameters as the basis for building the model. The idea
of arranging and selecting HSS is based on limited hydrological and hydrometric
data so that the flow hydrograph is more represented by the characteristics of
the watershed itself chosen in this study because the HSS development area is very close to the research location. It is expected that the
analysis results obtained will represent and get the closest results. A flood
plain is a river control area defined based on flood discharge for at least a
50-year return period without embankments [25]. The results of the calculation of the flood
discharge by the Nakayasu HSS method in Table 2 and
Figure 6.
## 3.1. GEOMETRY ANALYSIS OF SIDOAN RIVER
The
design of flood analysis by considering the characteristics of the watershed,
first identifies several parameters, including the length of the research river
(
This
research examines the character of the river bends. River bends are classified
based on the results of field measurements processed with software HEC RAS 5.0
to get the bend character on Q
Bend 1 the
observation point is at STA. 43 path length
403 m from STA. 46 until STA. 41, the magnitude of the
bend angle 172°, axis length of the bend 348.72 m, the length of the
bend radius 338 m. Bend 2 the
observation point is at STA. 33 path length
219 m from STA. 35 until STA. 32, the magnitude
of the bend angle 93°, axis length of the bend 201 m, the length of the bend
radius 313.2 m. Bend 3 the
observation point is at STA. 22 path length
162.3 m from STA. 23 until STA. 21, the magnitude of the
bend angle 162°, axis length of the bend 112 m, the length of the bend
radius 412 m. Bend 4 the
observation point is at STA. 12 path length
402.7 m from STA. 13 until STA. 11 with the magnitude of the
bend angle 140°, axis length of the bend 233 m, the length of the bend
radius 418 m. Band 5 the observation point is at STA. 7 path length 119 m from STA. 8 until STA. 6 and with the magnitude of the
bend angle 63°, axis length of the bend 86 m, the length of the bend
radius 320 m. ## 3.2. ANALYSIS OF MEANDERINGBased on
the measurement results in the field and the calculation results obtained are
as stated in Table 3.
The
parameters of the ratio of curvature and width of the river are deflected (R/W)
can be used as a tool to predict the rate of erosion in river bends [26]. In general, the erosion behavior for river
bend conditions is stated as follows, therefore 3≤R/W≤5, thus sharp
bends are bends 1, 3, and 5, others are non-sharp bends.
Value of
∆ = (ρ
Bend that has a small angle bends 2, has a small velocity compared to bend 1, 3, 4, and 5 shown in Figure 10.
The cross-section profile shows that the riverbed at bends 1, 2, 3, 4, and
5 the elevation of the river bed outside the bend is lower than the elevation
on the inside of the bend as shown in Figure 11-15. ## 4. CONCLUSIONS AND RECOMMENDATIONSThis research was conducted on 5 adjacent
bends, using Q50 the results show that all sediment on the outside of the bend
is moving so there is a need for cliff countermeasures. the depth of flow at
the bends 1, 2, 3, 4 and 5 respectively 3.83 m, 4.32 m, 3.8 m, 4.01 m and 3.96 m. the magnitude of the
shear velocity is 0.447 m/s, 0.46 m/s, 0.43 m/s, 0.44 m/s, 0.44 m/s. Value of critical shear stress
in bends 1, 2, 3, 4, and 5 respectively 199.89 N/m², 211.8 N/m², 184.9 N/m², 193.6 N/m² dan 193.6 N/m². The highest critical
shear stress at the second bend. because in the second bend the critical shear velocity of the river bed is
large. from these results, it can be concluded that the shear velocity is
directly proportional to the shear stress. ## 5. APPENDICES
## SOURCES OF FUNDINGThis research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. ## CONFLICT OF INTERESTThe author have declared that no competing interests exist. ## ACKNOWLEDGMENTNone. ## REFERENCES
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