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Development of the leather tanning industry in the village of Banyakan, Piyungan, Bantul had a negative impact on the environment. The industrial waste from the leather tanning plant mixed with Cr metal enters the Opak River and is distributed to various places. River water is used as irrigation water which carries heavy metals that result in the accumulation of soil and plants, as well as another biota. The purpose of this study was to determine the profile and level of chromium accumulation in plants along the Opak River. The study was conducted at 6 stations in 6 different districts consisting of samples of irrigation water, sediment, stem roots, leaves, and rice plants. Heavy metal analysis on the sample was carried out using the AAS (Atomic Absorption Spectrophotometry) method. The results of the analysis of heavy metal concentrations that are distributed to the environment show that the plants that are the highest are 2,470 ppm, then sediment is 1,161 ppm, and irrigation water is 0.235 ppm. The highest accumulation of heavy metal Cr in the planting section of rice was consecutively in the grain > leaves > roots > stems. Results of the analysis of the BCF value of rice plant were categorized as a moderate accumulator plant, while the analysis of the TF value of rice plant was categorized as a phytoextraction plant. Health risk analysis HQ > 1, which means that rice from all stations is hazardous to health. Rice and water samples exceeded the established quality standard threshold, except for sediment which was still at a critical level.

This research conducted in Opak River, in Yogyakarta. This river has broad (river flow area) 27,04 km2, and 20,11 km in length. The research objective is to make a planning about effective rainfall in 50 years and found the Opak Sand Pocket design. This research used methods with direct-survey in location, and collected secondary data from related agencies. From the data, it was obtained the value of precipitation the design, discharge flood design, dimensions of building hydrolic design (Main Dam, Sub Dam, Apron), and analysis the effectiveness of sand pocket in reducing the sediment that has happened. Based on the results of the research, discharge maximum ( Q50 ) that occurs in the river of 202,77 m3/s. So it can be calculated that sand pocket designed will have wide of apron 54,04 m, with total high of Main Dam 6 m, total high of Sub Dam 6 m, length of apron 10 m, thick of apron 0,96 m. Based on the ability of sand pocket in reducing the rate of the sediment that is happened, the building has effectiveness until 90,20 % in reducing bed load based on the calculation. Therefore, it can be argued that the building has been effective to reduce the speed of sediment occurring in Opak River.Penelitian dilakukan di Kali Opak, yang terletak di Yogyakarta. Kali Opak memiliki luas DAS 27,04 km2 , dengan panjang 20,11 km. Tujuan penelitian merencanakan besar curah hujan efektif 50 tahunan di Kali Opak dan menemukan desain Sand Pocket. Metode yang digunakan dalam penelitian adalah dengan melakukan survey langsung di lokasi, serta mengumpulkan data-data sekunder dari instansi terkait. Data-data tersebut kemudian diperoleh nilai curah hujan rancangan, debit banjir rancangan, dimensi desain hidrolis bangunan (Main Dam, Sub Dam, Apron, dan bangunan pelengkap), serta analisis efektifitas Sand Pocket dalam meredam laju sedimen yang terjadi.Berdasarkan hasil penelitian, diketahui debit maksimum (Q50) yang terjadi pada sungai tersebut sebesar 202,77 m3/s, maka dapat diketahui bahwa Sand Pocket yang direncanakan akan memiliki lebar pelimpah dengan dimensi 54,04 m, dengan tinggi total Main Dam sebesar 6 meter, tinggi total Sub Dam sebesar 4 meter, panjang Apron sepanjang 10 m, tebal 0,96 m. Bangunan tersebut cukup efektif untuk meredam laju dari sedimen yang terjadi pada Kali Opak dengan kemampuan Sand Pocket dalam mengurangi laju sedimen yang terjadi, diperoleh hasil perhitungan bahwa bangunan tersebut memiliki efektifitas hingga 90,20 % dalam mengurangi bed load.

This study examines the fluvial readjustment of a Javanese river impacted by the major eruption of the Merapi volcano (Indonesia) in October and November 2010. The basin of the Opak River, located on the southern flank of the Merapi, was subject to substantial sediment input related to massive pyroclastic deposits that were remobilized by numerous lahars during the year after the eruption. Two study sites were equipped in order to evaluate the morphodynamic evolution of the riverbed of the Opak River. Topographic surveys, bedload particle marking, and suspended sediment sampling revealed an important sediment mobilization during efficient flash floods. Surprisingly, no bed aggradation related to the progradation of a sediment wave was observed. Two years after the eruptive event, marked bed incision was observed. The Opak River readjustment differs from that of other fluvial systems affected by massive eruptions in two ways. Firstly, local population extracted the sand and blocks injected by the eruption as they represent a valuable economic resource. Secondly, several dams trapped the major part of the sediment load remobilized by lahars.