title: Wave-frequency flows within a near-bed vegetation canopy

authors: Henderson, Stephen M.

subjects: Science & Technology

abstract: We study water flows and wave dissipation within near-bed pneumatophore canopies at the wave-exposed fringe of a mangrove forest on Cù Lao Dung Island, in the Mekong Delta. To evaluate canopy drag, the three-dimensional geometry of pneumatophore stems growing upward from the buried lateral roots of Sonneratia caseolaris mangroves was reconstructed from photogrammetric surveys. In cases where hydrodynamic measurements were obtained, up to 84 stems per square meter were observed, with stem heights <  0.6 m, and basal diameters 0.01–0.02 m. The parameter α =(frontal area of pneumatophores blocking the flow)/(canopy volume) ranged from zero to 1.8m−¹. Within-canopy water velocity displayed a phase lead and slight attenuation relative to above-canopy flows. The phase lead was frequency-dependent, ranging from 0 to 30 degrees at the frequencies of energetic waves ( >0.1 Hz), and up to 90 degrees at lower frequencies. A model is developed for wave-induced flows within the vertically variable canopy. Scaling suggests that acceleration-induced forces and vertical mixing were negligible at wave frequencies. Consistent with theory, drag-induced vertical variability in velocity scaled with ꓥ = Tw/(2πTf) , where Tw=wave period, Tf=2/(CD₍ₐ|ᵤ|₎) is the frictional time scale, CD≈2 is the drag coefficient, and  |ᵤ| is a typical flow speed. For fixed wave conditions ( |ᵤ| and Tw), theory predicts increasing dissipation with increasing vegetation density (i.e. increasing a), until a maximum is reached for order-one Λ. For larger Λ, within-canopy flow is so inhibited by drag that further increases in a reduce within-canopy dissipation. For observed cases, Λ≤0.38 at energetic wave frequencies, so wave dissipation near the forest edge is expected to increase with increasing pneumatophore canopy density. However, under different wave conditions, the most dense canopies may occasionally approach the dissipation maximum (Λ≈1). Predicted dissipation by the pneumatophore canopy was sufficient to attenuate most wave energy over distances slightly less (more) than 100 m into the marsh in 1 m (2 m) water depth.

text: Wave-frequency flows within a near-bed vegetation canopy We study water flows and wave dissipation within near-bed pneumatophore canopies at the wave-exposed fringe of a mangrove forest on Cù Lao Dung Island, in the Mekong Delta. To evaluate canopy drag, the three-dimensional geometry of pneumatophore stems growing upward from the buried lateral roots of Sonneratia caseolaris mangroves was reconstructed from photogrammetric surveys. In cases where hydrodynamic measurements were obtained, up to 84 stems per square meter were observed, with stem heights <  0.6 m, and basal diameters 0.01–0.02 m. The parameter α =(frontal area of pneumatophores blocking the flow)/(canopy volume) ranged from zero to 1.8m−¹. Within-canopy water velocity displayed a phase lead and slight attenuation relative to above-canopy flows. The phase lead was frequency-dependent, ranging from 0 to 30 degrees at the frequencies of energetic waves ( >0.1 Hz), and up to 90 degrees at lower frequencies. A model is developed for wave-induced flows within the vertically variable canopy. Scaling suggests that acceleration-induced forces and vertical mixing were negligible at wave frequencies. Consistent with theory, drag-induced vertical variability in velocity scaled with ꓥ = Tw/(2πTf) , where Tw=wave period, Tf=2/(CD₍ₐ|ᵤ|₎) is the frictional time scale, CD≈2 is the drag coefficient, and  |ᵤ| is a typical flow speed. For fixed wave conditions ( |ᵤ| and Tw), theory predicts increasing dissipation with increasing vegetation density (i.e. increasing a), until a maximum is reached for order-one Λ. For larger Λ, within-canopy flow is so inhibited by drag that further increases in a reduce within-canopy dissipation. For observed cases, Λ≤0.38 at energetic wave frequencies, so wave dissipation near the forest edge is expected to increase with increasing pneumatophore canopy density. However, under different wave conditions, the most dense canopies may occasionally approach the dissipation maximum (Λ≈1). Predicted dissipation by the pneumatophore canopy was sufficient to attenuate most wave energy over distances slightly less (more) than 100 m into the marsh in 1 m (2 m) water depth.

year: 2017

title: Choosing the right microcontroller: A comparison of 8-bit Atmel, Microchip and Freescale MCUs

authors: Slade, Mel

subjects: engineering

abstract: When choosing a microcontroller there are many options, so which platform should you choose? There is little independent information available to help engineers decide which platform might best suit their needs and most designers tend to stick with the brand with which they are familiar. This is a difficult question to answer without bias if the people
conducting the evaluations have had previous experience with MCU programming predominantly on one platform. This article draws on a case study. We built three “Smart” Sprinkler Taps, small, self-contained irrigation controllers, differing only in the microcontroller unit (MCU) on the inside. We compare cost, development software quality and hardware performance from the perspective of a new user to each of the platforms.

text: Choosing the right microcontroller: A comparison of 8-bit Atmel, Microchip and Freescale MCUs When choosing a microcontroller there are many options, so which platform should you choose? There is little independent information available to help engineers decide which platform might best suit their needs and most designers tend to stick with the brand with which they are familiar. This is a difficult question to answer without bias if the people
conducting the evaluations have had previous experience with MCU programming predominantly on one platform. This article draws on a case study. We built three “Smart” Sprinkler Taps, small, self-contained irrigation controllers, differing only in the microcontroller unit (MCU) on the inside. We compare cost, development software quality and hardware performance from the perspective of a new user to each of the platforms.

year: 2011-11

title: The city of Rotorua and its meaning to Ngati Whakaue

authors: Te Awekotuku, Ngahuia

subjects: New Zealand

abstract: This paper discusses the history and settlement of the city of Rotorua as it relates to Ngati Whakaue.

text: The city of Rotorua and its meaning to Ngati Whakaue This paper discusses the history and settlement of the city of Rotorua as it relates to Ngati Whakaue.

year: 1976-08