Yuji Toda

Numerical study on turbulent flow and honami in and above flexible plant canopy

Abstract The waving of flexible plant termed honami is caused by large-scale vortices which are induced by inflectional instability of flow field. The movement of plant in turn may affect the turbulent flow field via resistance to flow, turbulence production and dissipation. The present study treats the interaction of turbulent flow and honami employing 2D-LES, in which a concept of “plant grid” is used to treat the movement of plant in addition to the ordinary LES grid. The drag due to the plant is formulated in proportion to the square of the relative velocity between flow and motion of plant in momentum equations for turbulent flow field. In transport equation of turbulent energy, the additional terms of turbulent production and dissipation due to motion of plant are included. The computational result indicates that the honami reduces the periodicity of vortex generation, and the velocity fluctuation, therefore, becomes irregular if compared with the case for which the plant is assumed to be rigid. It was also found that the honami repeats generation and disappearance.

Sediment-associated nutrient transport during floods in a gravel river

For riparian vegetation, sediment-associated nutrient transport during flood is one of the major agencies for controlling the nutrient environments of the soil. This paper describes the sediment-associated nutrient transport during flood in a gravel river by field observation and numerical computation. A field observation was performed to know the effects of flood flow on the soil environment of flood plain in a gravel river. In the observation, the grain size distribution and the chemical composition of the flood plain soils were measured before and after the flood. The results of the observation show that the amounts of particulate nutrients in the flood plain soil decrease during fairly large flood, because the fine sands which include the nutrients abundantly were removed by the flood flow. A series of numerical computations for the transport of suspended sediments and the associated transport of nutrients during flood was performed by varying the peak discharge of the flood. The result of the computation on the reduction of the particulate nutrients in the flood plain soils agrees well with those observed in the field. It was found that the particulate nutrients on the flood plains decrease if the discharge of the flood exceeds about 500 m3/s in the observation area.

Geomorphological and riparian vegetation responses following a low-head dam removal: a study based on literature review

ABSTRACT More and more low-head dam structures have deteriorated in recent years. Unlike the large dam, the geomorphological and ecological impacts of a low-head dam removal have not been quantified with insufficient monitoring data on pre- and post-removal. Therefore, this study intends to identify the low-head dam removal impacts on geomorphology and riparian vegetation based on previous studies. The characteristics of stored sediment in the impoundment play a critical role for geomorphological responses on a low-head dam removal creating a knickpoint and promoting a headcut migration. These geomorphological changes often form a new floodplain and create enough room for riparian vegetation establishment. The river geomorphology after a low-head dam removal can be a state of quasi-equilibrium within about a decade. In this state, a newly formed floodplain tends to be colonized by riparian vegetation in many low-head dam removal cases. After a decade to several decades, the riparian vegetation in the floodplain often develops to tree plants.

Numerical modeling of interspecific competition between filamentous and nonfilamentous periphyton on a flat channel bed

Interspecific competition between filamentous and nonfilamentous periphyton usually results in predominance of filamentous periphyton in natural shallow river streams. The mechanisms of this interspecific competition must be known to understand the growth characteristics and primary productivity of stream periphyton communities. In the present study, a numerical simulation model is presented to describe the growth and interspecific competition between filamentous and nonfilamentous periphyton on a flat channel bed, in which the processes of the interspecific competition are an integral part of the modeling of growth strategies. The temporal growth of each kind of periphyton is calculated by balancing primary production, immigration, respiration, and detachment. For the modeling of primary production and respiration, transportation of nutrients and other substances inside the biofilm is estimated using diffusion equations including the effect of turbulent diffusivity near the channel bed surface. In estimating the immigration rate, different immigration functions for filamentous and nonfilamentous periphyton are used in order to represent the different effect of turbulent flow on the immigration strategy of each type of periphyton. A series of numerical computations under different hydraulic conditions shows that the degree of dominance of filamentous periphyton increases with time, and the net primary production of periphyton increases with increasing friction velocity.

Integrated flood countermeasures in the upper and middle Chao Phraya River Basin

Flooding is a major natural hazard in the Chao Phraya River Basin (CPRB) because of its geography and the strong influence of seasonal monsoon rains. After the catastrophic flood of 2011, Thailand has not developed any major flood defenses, except for emergency measures. To minimize fatalities and economic losses, integrated flood countermeasures are necessary for flood control in the CPRB. A consideration of the widest possible set of flood countermeasures is needed to develop new alternatives that provide the information for overcoming severe flooding. The objectives of this study are (1) to investigate whether integrated flood countermeasures in the upper and middle CPRB can overcome severe flood events, (2) to identify the best solution from a set of flood countermeasures for each severe flood event, and (3) to explore the optimal operation of retention areas and dams for sustainable flood management in the upper and middle CPRB.

STUDY ON PRODUCTION AND DETACHMENT OF PERIPHYTON GROWING ON FLAT CHANNEL BED

Laboratory experiments for the kinetics of periphyton growth were conducted to find out the effect of hydraulic characteristics on the production and the detachment of periphyton, in which the amount of periphyton and the rate of detachment were measured. The results of the experiments show that the growth rate of periphyton becomes large in the case turbulence near the channel bed becomes large. It was also found that the detachment of periphyton mainly depends on the physiological action of periphyton.A numerical simulation model of the growth of periphyton was presented to describe the production and detachment in details. The result of the calculation shows that the biomass of the filament type periphyton is mainly determined by the balance between production rate and respiration rate.

FIELD OBSERVATION ON WATER QUALITY AND PRODUCTION OF ATTACHED ALGAE AT RIFFLES AND POOLS

From view points of ecology, it is important for river engineers to mainfest the hydraulic and ecological characteristics of riffles and pools. A field observation was performed at riffles and pools in Tama-river in Tokyo. The hydraulic characteristics, the amount of attached algae and water the qualities were measured using various devices. It was found that the production rate of the attached algae at riffls is larger than that at pools. The primary production of observed riffles and pools are very large, yielding temporal variation of dissolved oxygen during day-time.

STUDY ON HYDRAULIC CHARACTERISTICS AND PRODUCTION RATE OF CHLOROPHYLL-A AT RIFFLES BY FIELD OBSERVATION

瀬の流れと水質及び生物生産に関する現地観測を東京都奥多摩地域の秋川で行った. 河床礫や水表面波形の空間分布を様々な計測機器を用いて計測した結果, 大礫の平均径や標準偏差が水表面のテクスチャに影響を与えることが明らかになった. 同時に, 瀬の日射環境, 溶存酸素量や一次生産量を計測した結果, 溶存酸素量の日変化は主に河床の藻類の光合成によって引き起こされること, 観測対象とした瀬の河床付着藻類は2.49～6.65g・O2/m2/day程度の値を示し, 他の水域と比較して高い一次生産力を有することが明らかになった.