Analisis Karakteristik Butiran Sedimen di Bendung Talawaan

Abstract

ABSTRAK

              Bendung Talawaan di Sungai Talawaan, Kabupaten Minahasa Utara, mengalami permasalahan sedimentasi yang menurunkan kapasitas tampung kolam olak dan mengganggu fungsi intake. Penelitian ini bertujuan menganalisis karakteristik butiran sedimen dasar (bed load) di area bendung serta mengidentifikasi indikasi sumber sedimen yang berkontribusi terhadap penumpukan. Pengambilan sampel dilakukan dengan metode purposive sampling pada sembilan titik yang tersebar secara longitudinal (hulu, tengah, hilir) dan lateral (kiri, tengah, kanan). Analisis ukuran butir menggunakan metode analisis saringan (sieve analysis) sesuai SNI 3423:2008, dilanjutkan perhitungan diameter karakteristik (D10–D90), koefisien gradasi (Cu dan Cc), serta parameter statistik Folk dan Ward (1957). Hasil menunjukkan terjadinya fenomena downstream fining, yaitu pengecilan ukuran butir searah aliran dari pasir sangat kasar–kerikil halus di hulu (D50 = 2,07–2,37 mm) menjadi pasir sedang–halus di hilir (D50 = 0,33–0,79 mm), yang dipicu efek air balik (backwater effect) bendung. Sedimen di depan intake (Hilir Kiri) tergolong gradasi buruk terputus (gap-graded, Cc = 0,91), sedangkan sisi dalam belokan (Hilir Kanan) menjadi zona mati dengan akumulasi pasir halus tersortir baik (leptokurtic). Indeks pemilahan buruk di segmen tengah (σI = 1,17–1,37) mengindikasikan pasokan sedimen antropogenik dari penambangan pasir (Galian C), Pertambangan Emas Tanpa Izin (PETI), dan alih fungsi lahan. Disimpulkan bahwa sedimentasi bendung dipasok secara simultan oleh sumber alami lereng Gunung Klabat dan sumber antropogenik, sehingga diperlukan pembilasan hidraulis berkala, pengerukan, dan pengendalian aktivitas DAS.

ABSTRACT

             Talawaan Weir on the Talawaan River, North Minahasa Regency, suffers from sedimentation problems that reduce the stilling-basin storage capacity and disrupt the intake function. This study aims to analyse the grain characteristics of bed-load sediment around the weir and to identify the indicative sources contributing to the deposition. Samples were collected using purposive sampling at nine points distributed longitudinally (upstream, middle, downstream) and laterally (left, centre, right). Grain-size analysis followed the sieve analysis method according to SNI 3423:2008, followed by computation of characteristic diameters (D10–D90), gradation coefficients (Cu and Cc), and the statistical parameters of Folk and Ward (1957). The results reveal a downstream-fining phenomenon, in which the grain size decreases along the flow from very coarse sand–fine gravel upstream (D50 = 2.07–2.37 mm) to medium–fine sand downstream (D50 = 0.33–0.79 mm), driven by the weir's backwater effect. Sediment in front of the intake (Downstream-Left) is gap-graded (Cc = 0.91), whereas the inner bend (Downstream-Right) acts as a dead-water zone accumulating well-sorted fine sand (leptokurtic). Poor sorting in the middle segment (σI = 1.17–1.37) indicates an anthropogenic sediment supply from sand mining (Galian C), unlicensed gold mining (PETI), and land-use change. It is concluded that the weir sedimentation is supplied simultaneously by natural sources from the slopes of Mount Klabat and by anthropogenic sources, so that periodic hydraulic flushing, dredging, and watershed-activity control are required.