Potential Environmental Impacts Caused by Sedimentation at the Tambakboyo Retention Basin D.I.Yogyakarta to Fulfill the Needs For Watershed

The use of the Tambakboyo Retention Basin for more than 10 years has led to sedimentation at the bottom of the reservoir due to precipitated particles that are transported along with the water flow. Sedimentation of this reservoir can have several impacts on the environment. This study aims to analyze the possible environmental impacts that could occur in the Tambakboyo Retention Basin through the results of an analysis of the sedimentation rate and volume of water availability that has been carried out in previous studies. The results of previous studies indicate that the sedimentation rate in the Tambakboyo Retention Basin is included in category I or very light. However, several impacts may occur if there is no management, including reduced reservoir water holding capacity, flood hazard, excessive soil enrichment, lack of water so that the reservoir is no longer useful, and can even damage the reservoir. Thus, sediment management efforts need to be carried out, including repairing open land (greening), dredging sediments in the reservoir, and building sediment traps upstream of the reservoir.


Introduction
There are several reservoirs in the Special Region of Yogyakarta, one of which is the Tambakboyo.The location is in Tambakboyo Hamlet, Depok District, Sleman Regency, Yogyakarta.Utilizing the downstream Tambakboyo and Buntung Rivers, authorities constructed this reservoir to store excess water during the rainy season and to manage water resources for conservation purposes.Authorities constructed the reservoir from 2003 to 2007, giving it an inundation area of 7.8 hectares and a reservoir capacity of 400,000 cubic meters.Based on the purpose of its use, this reservoir is used to meet the needs of irrigation and raw water in Sleman and its surroundings, tourism, and fishing activities.However, until now, the Tambakboyo Reservoir has not been optimally utilized, especially for irrigation and raw water needs, primarily because of land conversion for housing purposes, leading to its transformation into a retention basin.
The operational age of the Tambakboyo Reservoir has been over 10 years.With various uses carried out around the reservoir, sediment may have accumulated at the bottom of the reservoir.[1] Moreover, some of the drain gates at the Tambakboyo Retention Basin have deteriorated due to rust, as they have never been operated since construction.This situation raises the potential for sedimentation in the relatively large reservoir, which occurs when soil particles from erosion are transported by water flow and enter the reservoir.Thus, the sediment accumulation in the reservoir can also cause siltation and reduce the amount of water storage.
This condition has been studied in evaluating the operation and maintenance of the Tambakboyo Reservoir in 2019.Based on the evaluation results listed in [2], it is known that there has been a decrease in storage capacity from 400,000 m 3 to 300,285 m 3 and a reduction in the area of the storage reservoir from an area of 7 Ha to 5.6 Ha.Based on the following statements, this study presents the results of the analysis of the sedimentation rate and water availability volume carried out in previous studies.Furthermore, an analysis is carried out on the possible environmental impacts in the Tambakboyo Reservoir.These studies can be the basis for decision-making steps in sustainable reservoir management.

Reservoir Sedimentation Rate
Sedimentation rate calculations can be carried out generally using the USLE method by analyzing four calculation factors, including the rain erosivity factor (R), the soil erodibility factor (K), the length and slope factor (LS), and the land cover factor (CP).Furthermore, the analysis results are multiplied by the river basin's area.The USLE equation is formulated as in equation 1 [3].
Numerous researchers have conducted studies utilizing the USLE method at the Tambakboyo Retention Basin [1][2] [4].The results of the analysis of land erosion in the Tambakboyo Retention Basin using the USLE method is 1,906.91m 3 /year with a maximum sedimentation rate of 7.6 cm/year [4].Kementerian PUPR [2] also conducted an operational and maintenance (OP) evaluation in 2019, which explains that the remaining dead storage in the Tambakboyo Retention Basin is 300,285.53m 3 , of which the initial design was 427,349.02m 3 with an elevation of +147.Thus, it is known that the volume of sediment collected from 2007-2019 was 127,063.49m 3 with a sediment rate value of 10,588.62m 3 /year, equal to 11.25 tons/ha/year, which is included in the erosion hazard level I or very light erosion.Then, [1] stated that the calculation results obtained that the sediment rate in the Tambakboyo Retention Basin catchment was 28,697.201tons/year or equivalent to 13,044.18m 3 /year where the soil density used was 2.2 tons/m 3 .Based on the sediment rate in Tambakboyo Retention Basin, it is included in the classification of hazard level I or very light erosion.So, based on previous literature, it is known that there has been an increase in the rate of sedimentation in Tambakboyo Retention Basin since 2007-2020.
The sedimentation rate measurement method can also be analyzed using the MUSLE (modified universal soil loss equation) method as done by [5].In his research, the amount of soil loss in 2010-2020 was 64,897.9tons/year, with an erosion rate of 3.41 tons/ha/year.The amount of sediment transported per day is 0.1 m 3 /day.

Availability of Reservoir Water
Calculation of water availability in the Tambakboyo Retention Basin by analyzing the inflow discharge calculation.This analysis utilizes data on a reservoir area of 78,000 m 2 and a reliable discharge for 10 years, from 2010 to 2019, with an 80% probability.Next, the calculation of the amount of evaporation that occurs in the reservoir is carried out by taking into account the following data: 1.
Wind Velocity (U) = 0.07 m/s The amount of evaporation that occurs in the Tambakboyo Retention Basin is stated in Equation 2.

𝐸 = 𝑁 𝑢 (𝑒𝑠 − 𝑒𝑑)
(2) IOP Publishing doi:10.1088/1755-1315/1311/1/012001 The study of volume analysis of water availability in the Tambakboyo was conducted by [6] and [7].[6] also explained that the water availability of the Tambakboyo Retention Basin occurs throughout the year from January to December, with the highest amount occurring in February at 1,989.29 L/s and the lowest occurring in December at 36.12 L/s.On the other hand, [7] explains water availability in the Tambakboyo Reservoir, presented in Table 1

Result and Discussion
Based on previous research, it was stated that sedimentation in the Tambakboyo Dam was classified as very light.However, the analysis results also show that water availability in the Tambakboyo Basin is decreasing.This decrease is made clear by statements [6] and [7], which explain that water availability in the Tambakboyo Embung can only meet needs in certain months due to the lack of catchment capacity of the Tambakboyo Embung when the rainy season arrives.This incident reduced the availability of Tambakboyo reservoir water for irrigation [8].
Another impact of increased reservoir sedimentation is the danger of flooding and excessive soil enrichment (eutrophication) [9].In essence, the construction of reservoirs is intended to accommodate excess water to be used for irrigation activities and as a source of raw water.However, the presence of sedimentation in the reservoir causes a reduction in water storage capacity, causing flooding.Furthermore, if viewed from a quality point of view, the sediment in the reservoir can be a storage place for organic matter, which causes fertility in the soil.This excessive increase in soil fertility can occur due to increased nitrogen and phosphate content in the waters.In the research of [10], it was explained that the Tambakboyo Dam could not accommodate pollutants in the form of nitrates and phosphates based on class II water quality standards according to [11].This condition is also in line with the research of [12] explaining the results of the Tambakboyo Retention Basin water quality analysis, namely, the levels of nitrite (NO2) and nitrate (NO3) have exceeded the quality standards set by [13] according to water class II.The results [12] showed that nitrate was 10 mg/L, and nitrite was 0.8 -0.91 mg/L.
Based on the following description, management efforts are needed at the Tambakboyo Reservoir to reduce the possibility of environmental impacts occurring.[14,15] provided several recommendations for sediment handling, including repairing open land (greening), dredging sediments in the reservoir, and building a sediment trap upstream of the reservoir.In addition, to protect the body of the reservoir against fluctuations in the water level, rocks that are resistant to weathering (rip-rap) are installed on the upstream slope of the dam.

Conclusions
The Tambakboyo Reservoir has experienced very light sedimentation.However, if this sedimentation is not managed, it can cause environmental impacts, including reduced reservoir water storage capacity, flood hazard, excessive soil enrichment, lack of water so that the reservoir is no longer useful, and even damage to the reservoir.So, sediment management efforts need to be carried out, including repairing open land (greening), dredging sediment in the reservoir, and building sediment traps upstream.

Table 1 .
. Discharge the availability of retention basin water every month (m 3 /s)