The operation, performance, costs of, and benefits from ten on-farm reservoirs (OFRs) in use for drought alleviation through supplementary irrigation in Rajabarihat ofGodagari Thana of Rajshahi District were studied for two wet seasons in 1994-95.On average, the OFRs are 15 years old, 1.5 m in depth, and 0.07 ha in area.The average area occupied bythe OFRs is about14% of the service area and 10% of the catchment area.Analysis of rainfalland storage data shows that in an average year the OFRs are expected to be completely full bythe beginning of September. In two out of ten years OFRs are expected to be about75% fullby the beginning of October when the droughts are expected.A 20% increase in yield wasobserved between the irrigated and non-irrigated farms during the study period.Analysis ofcosts of and benefits from supplementary irrigation using OFR water shows that the net presentvalue (NPV), internal rate of return (IRR), and benefit-cost ratio (BCR) are Tk.13000, 35%, and 1.5 respectively. Considering 80% dependable rainfall, the design standard for constructingan OFR in the study area is presented, and it was observed that the area requirement for OFR construction can be reduced by 45% through proper design.

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A sedimentation model of a dam reservoir can be used not only for analyzing the cycle of dam renewal, but also for obtaining information about the quality of irrigation water from a dam reservoir. The aim of this report is to examine one of the simplest models with parameters that characterize sedimentation in dam reservoirs. For this purpose, data from the Jatiluhur dam reservoirs (storage volumes 811×10⁶-3.0×10⁹m³) in Indonesia and the Anou dam reservoir (storage volume 9.8×10⁶m³) in Japan were applied.
We developed two hypotheses. One is that sedimentation into a lake Dottom is proportional to the load of suspended solids into dam reservoirs (case L1). The other is that it is proportional not to load into the reservoirs but to load from the reservoirs (case L2). Even though case L2 is well known in the literature, we added case L1 as we thought the actual circumstances would he somewhere between the two.
First we compared the settling rate of suspended solids in case L2. Settling rates at Tatiluhur were found to be 9-68- (supposed 435)m/y. In the literature, the settling rates in the eight Great Lakes (storage volumes 25×10⁶-11×10⁹m³) were 8-447m/y. However, settling rates of the Anou dam reservoir were as large as 9055-57909m/v.
In case L1, the settling rates were given according to the results in case L2, the ratio of water depth, and the water remaining time. The given settling rates were 8 32 (supposed 53)m/y for Jatiluhur, 8-246m/y for the Great Lakes, and 94-95m/y for Anou. The rates for the Anou dam reservoir were not so large.
Thus, one of the characteristics of Anou dam reservoir is that the settling rate is very large (case L2) or that the sedimentation into a lake bottom is proportional to the load of suspended solids into dam reservoirs (case L1). For the Anou dam reservoir, both of the results seem to be reasonable when we remember the steep bed slope of the incoming river and the amount of sedimentation at the connection with the incoming river. Therefore, we are expecting that this sedimentation model is effective when both cases are applied.

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Based on the many pioneer works on earth pressure on buried pipes, it isconfirmed that arching around the culvert increases the vertical pressures on the top of a culvertand reduces the horizontal pressures on the sides of a culvert. Because of the practitioners'interest in a more rational structural design of buried conduits, much attention has been paid tothe vertical load imposed on the top of a conduit than on the sides. In cases of culvertsconstructed under fill dams, however, the present authors infer that the conditions (of reducedstresses) adjacent to the sides of a conduit are critical to the danger of hydraulic fracturing ata stage when the horizontal stresses exerting on both sides of a culvert become lower than thereservoir water pressure. Based on this inference, the authors further postulate that hydraulicfracturing is the most likely cause of leakage (mostly concentrated leakage) and of failure ofmany low dams along the outlet conduit especially after heavy rain. Two case studiessupplemented by FEM analysis are presented and the results clearly confirm the authors'inference. Some countermeasures to cope with this situation are also recommended.

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