Journal of Hydraulic Engineering

All lakes created on natural rivers are subjected to reservoir sedimentation. The construction of a dam significantly modifies the flow conditions of natural streams inside and downstream of an artificial lake. The sediment concentration is often high during the flood season, and the entering flow shows a greater density than the ambient fluid. Suspended load can therefore be carried along the reservoir bottom to the dam in the form of turbidity currents. This paper presents research results that help to better understand this physical phenomenon, which contributes to reservoir sedimentation. It is based on in situ measurements, a laboratory scale model of turbidity currents and numerical flow simulations. The study of a thousand-year flood in the Luzzone Reservoir in the Swiss Alps using the developed computer model revealed the potential of such a tool. In particular, the impact on the sediment deposits was analyzed. A valuable evaluation of the incidence of such a turbidity flow is presented and its effects are compared to observations. Significant progress has been made in understanding the importance of turbidity currents in reservoir sedimentation.

Movable bed experiments from a flume into a basin were conducted to study the development of alluvial deltas. The experimental setup was aimed at the bed-load mode. Results showed that the development of the delta can be divided into three stages. In the first stage, the delta progressed mainly in its length, and a successfully derived equation described the shape of the delta in this stage. In the second stage, the delta developed mainly in its width. The length, width, front thickness, and central thickness of the delta were used to scale the geometric similarity. The plane geometry of deltas can be described using Gaussian functions. Transverse and longitudinal profiles fit the hyperbolic and linear function, respectively. The length, width, and thickness of the delta deposition at the end of the second stage have their own maximum values, R_c, B_m, and Z_f, respectively, for each case. The total volume V of sediment deposition could be simply related to these maximum values by V = αR_cB_mZ_f + β with shape parameters α and β around 0.178 and 174. The third stage was characterized by the interactions between the delta development and stream-channel variations. The ratios of the delta slope to the channel slope were closely related to the occurrence condition of the stream channel. Finally, the temporal variations of stream channels were investigated qualitatively.

Correct estimation of sediment volume being carried by a river is very important for many water resources projects. Conventional sediment rating curves, however, are not able to provide sufficiently accurate results. Artificial neural networks (ANNs) are a simplied mathematical representation of the functioning of the human brain. Three-layer feed-forward ANNs have been shown to be a powerful tool for input-output mapping and have been widely used in water resources problems. The ANN approach is used to establish an integrated stage-discharge-sediment concentration relation for two sites on the Mississippi River. Based on the comparison of the results for two gauging sites, it is shown that the ANN results are much closer to the observed values than the conventional technique.

Results of laboratory experiments carried out to determine the effective surface roughness for shallow overland flow as a function of the runoff rate, roughness element height, and underlying soil condition are presented. This work was conducted to extend the understanding of the mechanics of shallow overland flows based on the relationships between the Reynolds number, Froude number, and surface resistance over a wide range of conditions. Results exhibit a strong dependence of the effective roughness on the ratio between the depth of flow and the height of the roughness element, and a strong inverse relationship was found between Manning's n (and the Darcy-Weisbach friction factor f) and the Froude number. Three distinct subcritical flow regimes were identified: (1) Submerged flow (R < 300); (2) sheet flow (R > 1,200); and (3) transitional (partially submerged) flow (300 < R < 1,200). Analysis and synthesis of laboratory data allowed relationships between the Reynolds and Froude numbers, and Manning's n, which are suitable for practical engineering applications, to be established in this study.

The dilution and plume trajectory of the tee diffuser has been investigated via the collection of experimental data for a wide range of ambient current conditions. A new dilution equation in which the stagnation effect between ambient current and diffuser discharge is assumed to be a function of the ratio of the ambient momentum to the discharge momentum, m_r, is proposed modifying the conventional theory of Adams that significantly underpredicts mixing for large m_r. A simple equation for the plume trajectory including the dependency of the momentum ratio is also derived by dimensional analysis. Experimental results on the near field dilution show that when m_r < 1 the dilution decreases with m_r, whereas when m_r > 1 it increases with increasing m_r, and approaches the stagnant water dilution for very large values of m_r. The equation is applied to aid the preliminary design of a diffuser discharging heated water from a power station in Korea.

The use of computational fluid dynamics to simulate physical phenomenon, such as flow through open channels, has become more prolific as computational resources and numerical algorithms have improved. With current algorithms, the designer defines the shape of the channel geometry, and the code calculates the fluid dynamic state within the channel. For these computational tools to be effectively used in the design process, the designer should be able to define the desired flow characteristics and the computational tool should generate the channel geometry that yields the closest match. In this paper, a finite-element simulation code, which solves the viscous, 2D, shallow water equations, is modified to become an efficient, robust design tool by inclusion of discrete sensitivity analysis and the Gauss-Newton optimization algorithm. This technique is demonstrated for design of supercritical flow channels with the objective of producing constant depth flow downstream of the channel transitions.

A 3D numerical model with an orthogonal curvilinear coordinate in the horizontal direction and sigma coordinate in the vertical direction has been developed. This model is based on POM (Princeton Ocean Model). In this model a second moment turbulence closure submodel is embedded and the stratification caused by salinity and temperature is considered. Furthermore, to adapt to estuary locations where the flow pattern is complex, the horizontal time differencing is implicit with the use of a time-splitting method instead of the explicit method in POM. This model is applied to the Pearl River estuary, which is the largest river system in South China, with Hong Kong at the eastern side of its entrance. The computation is verified and calibrated with field measurement data. The computed results mimic the field data well.