Kuniaki Miyamoto

Evaluation of strips of centipede grass for sediment load reduction

Reddish sediment runoff from agricultural fields results in coastal environmental problems in Okinawa, Japan. Recent studies have demonstrated the effectiveness of strips of centipede grass (Eremochloa ophiuroides (Munro) Hack.), a perennial turf grass, in reducing the sediment loads from farmlands. However, sufficient information has not been provided to determine the appropriate strip specifications in the grass strip design. This study evaluated centipede grass strips for reduction of reddish sediment runoff from farmlands in Okinawa, Japan. A numerical model simulating the reddish sediment transport in the grass strip was constructed to determine the sediment removal efficiency of the strip. The model was verified using data obtained from field plot experiments with the grass strips under natural conditions. The sensitivity analysis of the model showed that the length of the grass strip (i.e. the dimension of the strip in the direction of flow) and unit inflow discharge have a great effect on sediment removal efficiency. The sediment removal efficiency obtained from the model simulation increased with the length of the strip and the increment of the efficiency decreased with the length of the strip. Therefore, these results indicate that the effective and efficient length of a centipede grass strip is 3 m for the reduction of reddish sediment loads under typical farmland conditions in Okinawa.

Modeling of Sediment Transport in Surface Flow with a Grass Strip

Reddish sediment runoff from land areas during rainfall causes environmental problems in coastal areas of the Okinawa region, Japan. Sediment delivered to the coastal areas causes water pollution, sedimentation and degrades the coastal ecosystems and fisheries resources. Agricultural fields are a major source of sediment runoff in the region (Yoshinaga & Onaga, 1993; Minami et al., 2002). Nakasonoe et al. (1998) reported that sediment from agricultural fields accounts for 70% of the total sediment runoff in the region. Countermeasures for runoff in agricultural fields are promoted as an important issue in the Okinawa region. Countermeasures so far proposed include terrace work, drainage canals, sediment ponds, grass strips, cover cropping, mulch farming, contour farming, deep tillage, crop rotation and green manure (Hudson, 1995; Morgan, 1995). The grass strip countermeasure involves installing grass bands at the downstream end of an agricultural field to reduce the amount of non-point source pollutants, such as sediment and nutrients from an agricultural field into the stream (Dillaha et al., 1989). Grass strips are currently installed by prefectural governments assisted by the Ministry of Agriculture, Forestry and Fisheries as public work projects for water conservation, and also as a measure to help conserve agricultural land, water and the environment in rural areas. Grass strips can also be installed by farmers themselves as one of agricultural activities. There are various factors affecting how effectively grass strips reduce sediment runoff, such as flow rate of inflowing water, sediment properties, slope conditions and features of the grass (Haan et al., 1994). When installing grass strips as a countermeasure for sediment runoff, various conditions must be taken into consideration. In designing grass strips, it is necessary to know the quantitative relationships between the various factors influencing grass strips and their effects on reduction of sediment runoff, and to determine appropriate and reasonable parameters for installing grass strips. In previous studies, Sugawara et al. (2001), Osawa et al. (2005) and Shiono et al. (2005) conducted field experiments in Ishigaki Island and the northern part of the Okinawa Main Island, and reported their results on the effects of grass strips for reducing reddish sediment runoff. Shiono et al. (2007) also reported the influence that the length of grass strips in the direction of flow and particle sizes of sediment flowing into grass strips have on the ability of grass strips to reduce reddish sediment runoff. These results were obtained from field

RUN-OFF ANALYSIS METHOD OF SURFACE-SUBSURFACE WATER FLOW USING TRIANGLE SCHEME

Two-dimensional runoff analysis with rect-linear finite differential scheme or one-dimensional runoff analysis with more complex finite differential scheme has been studied. We propose triangle scheme that covers ground surface to make a numerical model of the ground surface topography and strata structure. The ground surface is covered by point-polygon system and the vertexes of a polygon are defined at the gravity centers of triangles surrounding the point. The topography and strata structure are explained by the elevation of ground surface and boundary of each stratum. The depth is defined at the point and the flux is defined at the gravity center of the triangle. Based on the arrangement of variables, two-dimensional runoff analysis can be done. We apply kinematic wave theory to the triangle scheme and show an example of calculation.

Mechanics of Sediment Transport and Sediment Laden Flow. 4. Governing Equations for Sediment Laden Flows.

これまで3回にわたって土石流、掃流状集合流動、掃流砂を含む流れについて解説してきた。まず、土石流は典型的な固液混相流のひとつであり、それを単一の連続体で近似する場合の流れの力学的なフレームとその解析法について解説した。さらに、その理論を掃流砂にまで応用し、その結果と従来の掃流砂に関する解析法との比較を行った。両者には確かに類似の関係式を見いだすことができるものの、その基となっている力学的なフレームに関する理解はかなり異なっていることを示した。このことは、流砂現象一般を固液混相流として統一的にとらえることの可能性を示唆するとともに、今後、流砂現象に対して、固液混相流としての支配方程式がどのように構築されるべきかについて十分考察する必要があることを示している。そこで流砂現象がどのような支配方程式に基づいて説明されてきたかを整理概観し、その課題について若干言及してみたい。

Model of Pyroclastic Flow and Method of Forcasting the Doposit area

A gravity flow of granular bodies is considered as a model of main body of pyroclastic flow. Characteristics of the granular flow can be described by Kanatanis constitutive equations, which are obtaind in consideration of the energy loss caused by the only interparticle friction by the collison between the particles. Using above model, the 1991 pyroclastic flow at Mt. Unzen-dake were reproduced by the numerical simulation. The result of calculations roughly agree with the actual phenomena.