Sediment dredging assessment of the Mrica Banjarnegara hydroelectric reservoir based on technical and economic aspects

Figures & data

Figure 1. Mrica Dam is located in Banjarnegara Region, Central Java Province, Indonesia. It was built in 1988 as a hydroelectric power plant supplying electricity to southern Java.

Figure 2. The spoil bank map and borehole points are shown on the satellite image. The existing spoil bank is the spoil bank that has been dredged by the community and consists of 19 locations. Potential spoil banks are spoil banks that have never been dredged and include 3 spots: 12, 13, and 14. No-potential spoil banks are spoil banks that are not profitable economically if the sand materials are sold.

Figure 3. Map showing the sediment dredging area at Mrica Dam.

Table 1. Sediment size classification applied in GRADISTAT software (Blott and Pye 2001, Hairan et al. 2023).

Grain size (mm/µm)	Grain size classification
64–32 mm	Very coarse	Gravel
32–16 mm	Coarse
16–8 mm	Medium
8–4 mm	Fine
4–2 mm	Very fine
2–1 mm	Very coarse	Sand
1 mm–500 µm	Coarse
500–250 µm	Medium
250–125 µm	Fine
125–63 µm	Very fine
63–31 µm	Very coarse	Silt
31–16 µm	Coarse
16–8 µm	Medium
8–4 µm	Fine
4–2 µm	Very fine
2 µm	Clay	Clay

Figure 4. Flowchart of sediment dredging assessment at PB Soedirman Dam, Banjarnegara, Indonesia.

Figure 5. The cross section of block II (BH 3–6–8) is dominated by medium sand.

Figure 6. Medium sand dominates the cross section of block IV (BH 16–18–19).

Figure 7. The cross section of block I (BH 1–2) exhibits the dominance of clayey sand.

Figure 8. Map depicting dredging and disposal sites of sediments around the Mrica Dam. The dock is located adjacent to the disposal area to save time and cost of loading and unloading dredged sediments.

Table 2. Operational expenditure data.

Electricity supply components (BPP)	Fixed cost (USD)	Variable costs (USD)	Total (USD)	Percentage (%)	BPP/m^3 (USD)
Operating cost		222,101	222,101	42.50	0.63
Maintenance cost		83,364	83,364	15.95	0.24
Employee cost	84,862		84,862	16.24	0.24
Administration		8454	8454	1.62	0.024
Depreciation	123,804		123,804	23.69	0.35
Total	208,667	313,920	522,563	100	1.49

Total operating expenses reached USD 522,563. BPP: cost of production.

Table 3. Profit-sharing calculation based on the difference between expenditure components and company income.

Expenditure components	Cost (USD)
Selling price of sand/8 m^3	22.61
Tax (20%)	4.52
Barge fuel cost (± 0.5–1 L/8 m^3)	0.23
Sand extraction fee/8 m^3	3.88
Unloading sand from pontoon to land/8 m^3	3.88
Loading fees to truck/8 m^3	3.88
Fuel cost for 3–4 L/barge (at USD 0.52/L)	1.56
Barge rental fee	1.29
Deposit rental fee	1.29
Profit sharing	–
Territorial fee/8 m^3	0.52
Total cost	17.54
Price difference for sand pump owner	1.58

Sand sales unit data are per 8 m³. The average volume of sand dredged in one day is 120 m³.

Figure 9. Satellite images of the Mrica Dam indicate a significant increase in sedimentation between 2009 and 2021.

Blott SJ, Pye K. 2001. Gradistat: a grain size distribution and statistics package for the analysis of unconsolidated sediments. Earth Surf Processes Landf. 26(11):1237–1248. doi: 10.1002/esp.261.

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Hairan MH, Jamil NR, Azmai MNA, Looi LJ, Aris AZ, Rosli MH. 2023. The analysis of large dam impacts on sediment grain size distribution in a tropical river system. Civil Eng J. 9:15–26. doi: 10.28991/CEJ-SP2023-09-02.

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