Sediment Discharge Field Test using a Burrowing-Type Sediment Removal Suction Pipe with a Water Level Difference of 12 Meters

  When a dam is constructed in a river, sediment that flows with river accumulates in the reservoir. This can cause the problems of decreased effective capacity of a dam and changes in the downstream riverbed environment. In consideration of this, River and Dam Hydraulic Engineering Research Team is developing a technology called "the burrowing-type sediment removal suction pipe" that suctions sediment like a vacuum cleaner using the water level difference provided by the reservoir. Our developing aims is to make it applicable to many reservoirs at low cost as a sediment discharge technology as supplies sediment accumulated in the reservoir to the downstream river, with the goals of extending the reservoir´s life and improving the downstream environment (Fig. 1 and Photo 1).

  As a result of our developing thus far, we confirmed that, if four pipes with a diameter of 30 cm are used, the technology is less expensive than dredging and other conventional technologies. We further confirmed a performance that it can discharge sediment accumulated in approximately 10,000 m3 (equivalent to approximately twenty 25-meter elementary school swimming pools) in about 50 hours with a water level difference of 2.4 m. As our next step, we conducted a field test using a sabo dam with a 12-meter water level difference. The objective of the field test was to verify functions and find out issues for the practical application of this technology at high water level differences as similar level as actual dams.

(Contact: Dam and Appurtenant Structures Research Team)

Onsite Observations toward the Development of a Feed culture Reefs for Fish in the Northern Fishing Ground in the Sea of Japan

Figure-1 Feed culture reef

1. Purpose of this study

  In recent years, fishery production in Japan has decreased to about 40% of its peak. Around Hokkaido, catches of fishes that migrate in fishing grounds in the northern part of the Sea of Japan, such as Alaska Pollock, and flounder, have been markedly decreasing. In this critical situation, expectations for attempts to develop artificial reef that contribute to the protection and proliferation of target fishes not only in coastal zones, but also in deeper sea areas including those on continental shelves, have been increasing. To develop artificial reef in deep sea areas, it is necessary to qualitatively and quantitatively clarify the effectiveness of such artificial reef in deep sea areas. We estimated that one role for artificial fish habitats is to increase food sources for fishes, including plankton and benthic organisms (i.e., feed culture). With the ultimate goal of increasing the amount of resources for target fishes, we have been conducting research on "feed culture reefs" (Fig. 1) to increase food sources for fish. This document introduces an artificial reef that was installed in a relatively deep sea area (water depth of about 90m) and the onsite observations around this reef.

2. Feed culturing of artificial structures

Figure-2 Artificial reefs and the arrangement
of the observation points

  Currently, onsite observations have been performed at an artificial reef installed at a depth of about 90m in an area about 10km off the coast of Rishiri Island, Hokkaido, which is in a northern fishing ground in the Sea of Japan (Fig. 2). To clarify how the artificial structure affects benthic organisms (organisms that live in the sea bottom mud), the biomass of benthic organisms was analyzed by collecting mud from the sea bottom using a mud sampler. Observation points from A0 to A5 were set in a radius of about 250m from the center of the area where the artificial reef was installed.

  To enable sampling at the 90m-deep location, where work by divers is difficult, sampling was done by a remotely operated vehicle (ROV) equipped with a mud sampler (Figure-3). By moving the ROV very close to the reef modules that make up the artificial reef, the state of the habitat for benthic organisms was investigated.

  The results of the onsite observation are shown in Figure-4. The distribution of benthic organisms at and around the artificial reef is shown. The area with artificial reef modules (the area within the double circles in Figure-2) is defined as the "reef area," and the area beyond that area is defined as the "control area." The reef area had greater amounts of living benthic organisms than the control area had. The categorization of the benthic organisms showed about 80% of them to be annelids at all observation points. Annelids are important food for flounder. The observations showed that the artificial structures promoted the cultivation of fish feed.

Figure-3 ROV with a mud sampler

Figure-4 Observation points and amounts
of benthic organisms

3. Conclusion

  In future research, we will continue to collect data on the effectiveness of the artificial reef in the target sea area, and elucidate the mechanism of culturing for each fish species in artificial reefs installed offshore. Therefore, we will propose a method to estimate the effect of installing artificial reefs.

(Contact: Fisheries Engineering Research Team, CERI)