Adichai Pornprommin

Non-linear turbulence models for multiphase recirculating free-surface flow over stepped spillways

The flow over a stepped spillway is free surface, multiphase and turbulent. Moreover, a recirculation region occurs at each step of the spillway. The flow characteristic is hence complicated and difficult to predict. In previous work on stepped spillways (Chen et al. Journal of Hydraulic Engineering, ASCE, 128 (7), 683–688 and Xiangju et al. Science in China Series E: Technological Sciences, 49 (3), 674–684), only the linear turbulence model was applied which gave low accuracy. Furthermore, there are still no optimised model constants for this kind of flow. The present work therefore aims to apply the large eddy simulation (LES), the non-linear and the modified non-linear turbulence models to simulate the flow over the spillways with various slopes and step heights. It is found that the modified model is able to predict the flow close to the LES but requires less computational time by a factor of up to 4. Moreover, the modified model gives higher accuracy than the LES for the cases of the spillways with higher slopes and step heights.

Channelization on plateaus with arbitrary shapes

[1]xa0The initiation of channels and the factors that control their spacing is one of the most fundamental problems in geomorphology. Here we present an analysis of channelization that is downstream driven, so called because channelization is initiated at some point away from the divide where the Froude critical flow is achieved. By using two novel techniques, the “frozen time approach” and the “momentary stability concept,” we are able, for the first time, to investigate downstream-driven channelization that occurs on a plateau with an evolving base profile. The “frozen time approach” assumes that the time evolution of the base state can be ignored in the linearized equations, whereas the “momentary stability concept” assumes that an unsteady base state becomes momentarily unstable to a disturbance when the growth of the disturbance is faster than the evolution of the base state. The analysis shows that an arbitrary slope profile is momentarily unstable and incised by channels before it evolves into a steady base state if the topographic curvature at the location where the Froude critical flow is achieved is sufficiently small. As the curvature decreases further, dominant channel spacing increases slightly in a purely erosional case, while clear dominant channel spacing tends not to appear in an erosional-depositional case. It is also found from the analysis that in both cases, the channel spacing is on the order of 1000 times the Froude critical depth. The results are compared with field observations and experiments, and they show satisfactory agreement.

Inception of stream incision by seepage erosion

[1]xa0Because seepage erosion is generated by complex interactions with other processes, associated stream incision process is not well understood. In this study, some fundamental characteristics of incipient incision by seepage erosion were investigated by laboratory experiments and linear stability analysis. The experiments were conducted with various sediment layer depths and gradients. With similar discharges in the experiments, incision spacing decreases with increasing depth of the sediment layer and with increasing gradient, whereas incision width increases with increasing sediment layer depth. A linear stability analysis was performed using the Dupuit-Forchheimer equation and an expression of the planimetric retreat of the scarp. The retreat velocity of the scarp consists of two terms: (1) a power law function that describes the specific discharge in excess of a critical discharge and (2) a diffusion-like function that describes the incision edge shapes, in which the retreat rate is enhanced or reduced by the convexity and concavity of the edges, respectively. This analysis shows that the characteristic incision spacing becomes infinitely small when the effect of the edge shapes is excluded. Using the experimental data of incision spacings, the values of the diffusion-like coefficient in the second term were estimated. Since the weight of a failure block and hydraulic pressure are the driving forces in the slope stability analysis, a relationship was found between the diffusion-like coefficient and the combination of the two forces.

Initiation of Channel Head Bifurcation by Overland Flow

Channel head bifurcation is a key factor for generating complexity of channel networks. Here, we investigate incipient channel head bifurcation using linear stability analysis. Channel heads are simplified as circular hollows, toward which surface sheet flow accelerates in the radial direction. Sinusoidal perturbations in the angular direction with different angular wavenumbers k are imposed on the bed, and their growth rates Ω~ are computed. Because the channel head radius R~c is extending over time, the base state (circular hollow in the absence of perturbations) also evolves continuously. With the use of the momentary stability concept, the evolving base state is defined as momentarily unstable to the imposed perturbation if the disturbance is growing faster than the evolution of the base state. It was found that in the range of sufficiently small R~c, bifurcation cannot be initiated. As R~c increases, bifurcation starts to be possible with k≈ 3–5. A higher k implies bifurcation with a narrower channel junction angle (θ = 2π/k). The average junction angle of the Colorado High Plains for the smallest drainage area is about 85∘ with a standard deviation of 35∘ [Solyom and Tucker, 2007]. Our predicted angles (75∘-120∘ ) agree qualitatively with the observed angles. Finally, we propose a simple criterion to compute the threshold R~c for the onset of bifurcation.

Water footprint of maize in thailand

Thailand is the third largest net virtual water exporter in relation to crop trade with the volume of 47 billion m3/yr. Thus, one of the reasons of water scarcity in Thailand is possibly due to the export of the agricultural sector. Maize is one of twelve economic field crops of Thailand, which is essential to the stock feed industry. In this study, water footprint of maize in Thailand was calculated in details by using the monthly production statistics, daily Meteorological date and crop coefficient experimented in Thailand. There are 3 steps in the computation as follows: 1. Calculate the maize evapotranspiration 2. Evaluate the crop water use by the soil water balance method in root zone 3. With the crop yield and fertilization data, water footprint is calculated. The results of this study show that the country-averaged water footprint of maize in Thailand equals to 1,132 m3/ton comprised of 894 m3/ton green and 237 m3/ton grey water footprints. Since maize cultivation is mostly in the rain-fed area (99.2%), the blue water footprint in this study. Is negligible Udonthani province has the highest water footprint of maize (1,368 m3/ton), while Phichit the lowest (953 m3/ton). Comparing with the results of Mekonnen and Hoekstra (2010), this study shows the similar value of the country-averaged green water footprint of maize. However, the grey water footprint was found to about double of previous study.