The analysis of degradation and aggradation of the riverbed: A case study of the Winongo River

The Winongo River, with a soil base, makes it possible to experience sedimentation events. In rivers with groundsill for retaining sediment rate, the flow can occur, grinding sediment (degradation) or aggradation on the riverbed. This study was conducted in Winongo River to see the effect of groundsill on riverbed degradation and aggradation levels. This research was carried out using the MPM, Engelund & Hansen, and Laursen-Copeland equations in HEC-RAS 6.3.1 by dividing Winongo River into 71 points with a spacing of ± 500 m, including nine with groundsills. Based on the effect of the groundsill on the riverbed, the cross sections at its upstream and downstream tend to experience quite different riverbed elevation changes. Conditions in the upstream groundsill tend to experience aggradation while the downstream experiences degradation. The three equations modelling results show that MPM and Engelund & Hansen equations are closer to field conditions than the Laursen-Copeland equation. The use of D50 sediment to model changes in riverbed elevation is more likely to experience degradation because the average grain size of D50 in this study sample was 0.82 mm.


Introduction
Rivers are natural or artificial waterways that carry water and sediment from upstream to estuaries.Apart from functioning as a collector and diverter of water, rivers also have an important role in transporting sediments such as sand, silt and gravel.This sediment comes from soil erosion around rivers or from tributaries that empty into the main river.
Yogyakarta Province is traversed by many rivers, one of which is the Winongo River.The Winongo River has an elongated shape from north to south with a length of about 41.3 km, has its headwaters in the northern part of Yogyakarta Province, and empties into the Opak River in the southern part of Yogyakarta Province.In addition, because of the tropical climate, Yogyakarta Province also has high rainfall, especially during the rainy season.
Changes in river morphology occur mainly after extreme rains during the rainy season and are influenced by the flow of water and existing sediments as well as the topography of the river itself [1].High rainfall is directly proportional to the amount of river flow discharge.The higher the rainfall, the 1311 (2024) 012011 IOP Publishing doi:10.1088/1755-1315/1311/1/012011 2 greater the river flow discharge.A large flow discharge increases the possibility of erosion.Changes in river morphology occur due to erosion that erodes the riverbed and banks and causes degradation.Meanwhile, excess sediment originating from erosion in the upper reaches of the river can cause aggradation (rise in the riverbed) in the lower reaches of the river.
Large degradation and aggradation of the riverbed and river cross sections can result in flooding and is a very decisive factor in the collapse of water structures such as bridges, weirs and groundsills [2].Erosion on riverbanks due to river flow and rain can reduce the life of retaining walls, which should last a long time, and even the age of the groundsill could be reduced by half than planned.If this condition occurs periodically, it can endanger the surrounding conditions [3].The research results of [4] showed that the Winongo River experienced aggradation due to material from the eruption of Mount Merapi and bank erosion, which caused changes in the morphology of the river, damage to the retaining walls, and the potential to cause collapse.
A solution is needed to determine the effect of extreme rainfall on the occurrence of aggradation and degradation on the riverbed of the Winongo River.To interpret the background conditions, a simulation using the HEC-RAS software is required.HEC-RAS is used because it has the ability to simulate both permanent and non-permanent water flow on the water surface profile in natural and artificial canals and rivers.In addition, HEC-RAS can also carry out sediment transport analysis, bottom moving analysis, and water quality analysis and assist in the hydraulic design process [1].
In the research of [5], an analysis was carried out using five methods to predict changes in the morphology of the Welang River.As a result, the Meyer-Peter Müller (MPM) and Engelund & Hansen methods show more accurate simulation results, and there is a change in the morphology of the Welang River.In the study of [6], calibration of the sediment load was carried out using several methods, and the results showed that the Meyer-Peter and Müller (MPM) equation gave the value that most closely matched the data in the field.Therefore, the MPM, Engleund & Hansen, and Laursen-Copeland methods were chosen to carry out further simulations.This study is also conducted to find out which equation that shows the most similar result to the field condition.

Location
The research was conducted on the Winongo River, which is one of the rivers in the Special Region of Yogyakarta, Java Island, Indonesia.Its upstream is in Sleman Regency, especially around Mlati, with coordinates 7°44'10.36"E and 110°22'7.87"S.This river flows to the mouth of the Opak River in the Kretek, Bantul Regency, with coordinates 7°59'22.17"E and 110°18'47.38"S.

Data Collection
Primary and secondary data collection was carried out to meet the needs of input data in this study.a. Hydrometry Data Hydrometry is basically the process of measuring or collecting data in a river, which includes measuring the depth of the river, measuring flow rate, river flow velocity, and manning values [7].Hydrometric data were obtained through field surveys at three predetermined locations along the Winongo River based on SNI 03-2414-1991 [8].This survey considers the channel depth and flow velocity at the selected point.The survey results are in the form of cross-sectional dimensions and flow velocity.The data is then processed and used to calibrate the manning roughness number.

b. Groundsills and Cross Sections Data
Water structures, which are the physical infrastructure needed for river management, have the function of protecting and controlling the river.Based on the results of previous research and from observations in the field, there are nine groundsills.c.Sediment Data In this study, testing the calculation of sediment transport required grain gradation data from sediment samples taken in the field.After taking sediment samples during the survey, grain gradation testing was

d. Observed Discharge
The discharge data used in this study is data obtained from DPUPESDM.The data includes high discharges taken from February 15 th to March 16 th , as well as low discharges taken from August 15 th to September 13 th .The data is used in flow modelling using the HEC-RAS 6.3.1 software.Modelling is done using wet season and dry season discharge data.Furthermore, a calibration process is carried out to check the suitability of the flow depth manning number generated by the modelling with that measured during the field survey.Calibration is done by modifying the value of the manning number in the cross-section being analyzed.The goal is to achieve compatibility between the modelled data and the data obtained from field surveys.The goal is to achieve compatibility between the modelled data and the data obtained from field surveys.

Unsteady Flow Analysis on HEC-RAS 6.3.1
This analysis uses data on unstable flow conditions (unsteady flow).Calculations are made for the river segment from the estuary (downstream) from 1 st point to 71 st point to the upstream.In modelling using unstable flow, it is necessary to input boundary data on the upstream and downstream, as well as initial condition data.Each model has different boundary conditions and initial conditions.The results of the unsteady flow analysis include information about the inundation area, elevation, and flow velocity generated by the modelling.

Sedimentation Analysis on HEC-RAS 6.3.1
Sediment modelling analysis on HEC-RAS 6.3.1 uses quasi-unsteady flow analysis.This analysis aims to determine changes in the base elevation of the Winongo River over a year by inputting tidal data and simulating it on HEC-RAS 6.3.1.In this simulation, the sediment transport equations used are Laursen-Copeland, Meyer-Peter Müller (MPM), and Engelund & Hansen, as can be seen in Table 1.Input sieving analysis data is based on the percentage of passing with the sediment data facility in HEC-RAS 6.3.1, then enter condition data in the form of grain size gradation data and equilibrium load (sediment load balance) with the defined bed gradation facility [9].At this stage, it is important to enter river and groundsill geometry data.In addition, at this stage, the roughness values of manning, elevation and station, downstream reach length, and main channel bank station need to be entered.In the inline structure data menu, there are options including weir/embankment (groundsill), gate (weir) and culvert.In this study, weir/embankment is used because it is an editor menu for groundsill buildings.
Table 1.Riverbed sediment transport equations

Sediment Grain Data
The sediment grain data were obtained by taking the material directly from the riverbed of Winongo River to examine.Sediment data was taken from 24 points along the Winongo River, as the graphs are shown in Figure 2 to Figure 5.

Manning Values
The purpose of calibration using hydrometric data is to ensure the water level and flow rate generated by the HEC-RAS 6. level between the simulation results and the field does not match, it is necessary to change the manning number.The calibration process is carried out under conditions of high discharge, low discharge and moderate discharge.Calibration locations were carried out at three different points, namely upstream (Cross 690), middle (Cross 524), and downstream (Cross 273).The data obtained during field calibration is in the form of cross-sectional shape and channel velocity.The data is then processed into a crosssectional area and flow rate.The calibration results show that the average manning rate in the upstream section is 0.022, in the middle section is 0.026, and in the downstream section is 0.025.The manning values are within the range corresponding to a riverbed made of soil.

Unsteady Flow Analysis
After obtaining the appropriate manning values, unsteady flow and quasi-unsteady flow (sediment) simulations were carried out.The data used in this research comes from several sources.Cross-section data were obtained from BBWSSO-DIY, while discharge data were secondary data obtained from DPUPESDM-DIY.There are two debit data used in this study, namely high discharge from February 15th to March 16th and low discharge from August 15th to September 13th.The data is then inputted into unsteady flow data as boundary conditions and flow hydrograph.In the initial conditions, a value of 120 is used for high discharge and 160 for low discharge.In unsteady flow data, the normal depth value of 0.0025 is used for the downstream boundary conditions.

Quasi-unsteady Flow Analysis
In this study, quasi-unsteady flow modelling was carried out to analyze the degradation and aggradation of the river bed using locally distributed sediment samples.There were 24 sediment samples taken from the Winongo River, and each sediment sample represented several cross sections upstream and downstream of the location.Two measured discharge data are used in this modelling, namely high discharge from February 15th to March 16th and low discharge from August 15th to September 13th.
Locally distributed sedimentation modelling uses input parameters in the sediment data menu, including initial conditions and transportation parameters.The MPM, Engelund & Hansen, and Laursen-Copeland equations were used to analyze the channel bed elevation using locally distributed sediments.These parameters are entered in the define/edit bed gradation menu, and the maximum depth is filled with 1m.
Based on the results of modelling on HEC-RAS 6.3.1, the riverbed changes.

Groundsill Addition for Bank Protection
This study uses quasi-unsteady flow modelling with locally distributed sediments and finds changes in elevation along the Winongo River.The results showed that the riverbed tends to decrease or degrade.This is in line with the research results of [10], which state that subsidence in the river bed can have an impact on damage to water structures.One way to deal with changes in riverbed conditions is to build groundsill.During modelling, there was a significant decrease in the riverbed.The greatest degradation was recorded in the Winongo River, with a depth of up to 1 m in all three equations used.This study shows that even though there are nine groundsills, it is still possible for riverbank collapse to occur due to riverbed degradation.This degradation can have significant negative impacts.When riverbeds degrade, water flow can be disturbed and cause excessive erosion.Such erosion can destroy vegetation, erode riverbanks, and cause land loss.This finding is supported by research conducted by [11], which states that degradation of the riverbed can cause riverbank landslides and loss of the river's ecological function.The same thing was also found in a study by [12], which showed that degradation of the riverbed can cause collapse and cracks in river banks.Therefore, it is necessary to take action to control the degradation of the riverbed and maintain the stability of the riverbanks in order to prevent further negative impacts.

Conclusions
Based on research using locally distributed sediments with HEC-RAS 6.3.1 modelling on the Winongo River, there are several important findings.(1) There are differences in the trend of changes in riverbed elevation upstream and downstream of the groundsill.On the upstream groundsill, aggradation occurs, while on the downstream groundsill, there is degradation.This is caused by the influence of the groundsill as a transverse structure that holds the sediment flowing along the river.(2) Out of three equations used in modelling (MPM, Engelund & Hansen, and Laursen-Copeland), the MPM equation gives results that are more in line with field conditions than the Engelund & Hansen and Laursen-Copeland equations.This shows that the MPM equation is more accurate in modelling sediment transport, riverbed elevation changes, and sedimentation patterns.

Figure 9 .
Figure 9. Upstream and downstream of groundsill on Cross 459.8 Note: The minus sign (-) indicates degradation

Table 2 .
Average degradation and maximum aggradation values of Winongo riverbed