Journal of Hydraulic Engineering

Hydrologic modeling requires accurate estimation of daily areal rainfall. In many parts of the world, this task is complicated by sparse rain‐gage networks and unreliable or missing data. This paper presents a simple, mathematically elegant, and economical algorithm that exploits the linear nature of multiquadric hyperboloid surface‐fitting equations to derive rain‐gage weights for calculating areal rainfall over polygonal approximants of catchments. In addition, gages are classified according to their statistical characteristics using the covariance biplot. This graphical method identifies the records of those gages that have similar statistical properties, those gages that may be unreliable, and highlights possible outliers in the rainfall record. The concept of spatial multicollinearity (a form of overfitting, or redundancy, defined by the geometry of the gages and the catchment boundary) is introduced and is used to help select a suitable set of gages as candidates for the spatial integration. The approach is illustrated on the Slangspruit catchment in Natal, South Africa.

Economical design of hydraulic networks has been a subject of research for some time. Piping has been the main focus of this research because it represents the greatest percentage of the investment in a network. Pipe cost increases with size, but in addition, depends on the material chosen and the pipe class (thickness). However, optimization methods currently available usually take into account only the pipe diameter, as the variable that relates pipe cost to hydraulic capacity. In this paper a method for optimal design achieving greater savings in the investment in irrigation systems by considering factors other than pipe size alone is proposed. The method identifies optimal strategies for use of pressure‐reducing valves (PRVs) in order to reduce pressure in the downstream pipes, thereby reducing wall thickness and investment cost in those pipes. PRVs are common elements in irrigation networks so optimal strategies for location represents an opportunity to achieve considerable savings in investment cost. The method is based on a dynamic programming formulation and is demonstrated by application to an example network.

In this study, a general relationship for incipient motion of the sediment below a jet is first developed. Then, for the particular case of a pipe outlet, an experimental program has been conducted to test the resulting equation and to supply the necessary coefficients. For visual observation of sediments, the half‐jet technique was used, which made it possible to conduct a large number of tests in a relatively short period of time. The methods of sizing riprap, for a no‐scour plunge‐pool energy dissipator, based on incipient motion and incipient failure, are presented. Other topics investigated and discussed in this paper include effects of gradation and thickness of riprap as well as criteria for a gravel filter. A unified relationship for various riprap gradations is proposed by using $D_{30}$ for the incipient motion and $D_{90}$ for the incipient failure as the characteristic particle size.

Supercritical expansion flow in nearly horizontal rectangular channels is actually based on the Rouse wall geometry. As the transition structure becomes extremely long, a revised design involving significant shortening is presented. The revised design is based on separate observations of the flow in the abrupt expansion, and two alternative designs, i.e. the modified and the reversed Rouse wall curves. The modified Rouse wall curve is found to give no improvement over the abrupt expansion and is thus dropped. The reversed Rouse wall curve yields better flow conditions than the abrupt expansion and is significantly shorter than the original Rouse curve. The semiempirical approach accounts for the effects of streamline curvature and viscosity. The free surface is shown to be governed by the Froude similarity law; the velocity field involves significant Reynolds effects in addition. Thus, the latter may not be analyzed by a generalized approach. The surface of flow may further be analyzed when using a correctly distorted coordinate system. Thus, insight is given in the surface profiles along the expansion axis and the side wall. Design recommendations based on the reversed Rouse wall curve end the paper.

In open channel flows, an important design parameter is the amount of entrained air. The presence of air within the flow increases not only the bulk of the flow, but also the transfer of atmospheric gases (e.g. oxygenation). Further aeration of high‐velocity flows may prevent or reduce cavitation damage. The present paper reviews the characteristics of self‐aerated flows on spillways and chutes, including uniform flows and gradually varied flows. First, the uniform flow conditions are presented with new prototype results. Similarities with suspended‐sediment flows and extremely rough flows are developed and the interaction between air bubbles and turbulence is discussed. Then, the basic equations for gradually varied flows are developed using the same method as Wood in 1985. The results are applied to chutes and tunnel spillways and are compared with experimental data.

FLO‐2D is a two‐dimensional finite difference model that simulates clear‐water flood hazards, mudflows, and debris flows on alluvial fans and urban floodplains. Interactive flood or mudflow routing between channel, street, and floodplain flow is performed using a uniform grid system to describe complex floodplain topography. A quadratic rheological model, developed from field and laboratory mudflow data, enables appropriate simulations of flooding conditions ranging from clear water to hyperconcentrated sediment flows. Computer‐aided design (CAD) graphics of predicted time‐sequenced flood depths automates the delineation of flood hazards. Replication of the 1983 Rudd Creek mudflow in Utah demonstrates the capability of the model.

Sand and gravel raining from a river bed results in irregular pits on the river bed. The migration of the pits might potentially threaten the safety of downstream bridge piers and other in‐stream hydraulic structures. Models were developed to simulate the movement of pits for predicting the effect of migrating pits on in‐stream structures. In view of random characteristics inherently residing in hydraulic and hydrologic processes, it is essential for an engineer to assess the overall uncertainty features of a hydraulic model output subjected to its stochastic input parameters. As an illustration, this paper analyzes the uncertainties of a pit migration model recently proposed by Lee et al. (1990) using three methods including the first‐order variance estimation method, point estimation technique, and Latin hypercubic sampling. Comparisons of merits and limitations of these methods are also made.

An aqueduct is a canal conveyance structure utilized to cross a river when the bed level of the canal is clearly above the river high flood level. One such aqueduct structure is a reinforced concrete rectangular flume having a width usually less than that of the canal for economic reasons. The ratio of the width of the flume to that of the canal is termed flume ratio. At present, the only criterion for choosing the flume ratio is that the Froude number of flow in the flume should not exceed a specified value in the subcritical range. Using the specific energy diagram, three approaches are described for hydraulic design of the flume. Equations are presented for prediction of flow conditions in the flume, bed elevation of the flume and head loss due to the structure. The three approaches have been compared for their use in actual practice.