Albert Molinas

Transport of sediment in large sand-bed rivers

A sediment transport equation based on universal stream power is presented for the prediction of bed-material concentrations in large sand-bed rivers. The universal stream power, which is derived from the energy concept, has the advantage of eliminating the energy slope as a parameter. The energy slope, which is in the order of 10-5 for large rivers, is a major source of uncertainty in measurements. The analysis shows that relationships derived from flume experiments with shallow flows cannot be universally applied to large rivers with deep flows. Also the use of dimensionless homogeneous parameters in an equation is not sufficient to ensure its applicability to flow conditions where flow depths are several orders of magnitude larger. The comparisons between computed and measured sediment concentrations indicate that the commonly used Engelund and Hansen, Ackers and White, and Yang equations which were developed using mainly flume experiments are not applicable for large rivers with flow depths and Reynolds numbers up to 100 times larger than those found in flumes. The Toffaletis method which was developed mainly from field data gives reasonable predictions of sediment transport rates for large rivers. Using the proposed equation, the computed sediment transport rates are in much closer agreement with the actual measured values in large and medium rivers.

Effects of Cohesive Material Properties on Local Scour Around Piers

Local pier scour has been the topic of numerous research studies on noncohesive alluvial materials. However, because of the complexities involved in cohesive-material scour, very little effort has been devoted to the study of scour around bridge piers situated in cohesive materials. The parameters that affect the scour mechanism at bridge piers located in saturated and unsaturated clayey soils are identified, and prediction equations for quantification of the effects of some of the cohesive properties on the resulting local scour are developed. The scour depth predictors developed from the analysis of the experimental data express pier scour in terms of the approach flow conditions, initial water content, compaction, and soil shear strength of the cohesive bed materials. These predictors are related to scour in noncohesive alluvial material to derive factors that express clay scour as a percentage of sand scour. At bridge locations where exact soil properties are unknown, these estimators may be used to provide bounds to scour estimations.

Choking in water supply structures and natural channels

Techniques are presented to predict the choking phenomenon which occurs in natural and man made waterways with constriction features. The techniques use the energy equation, the momentum principle, and the continuity equation in one dimensional flow derivation; both the form and the friction losses are considered in the study. The different loss coefficients assumed and introduced in the governing equations are extracted and grouped in functional relationships to predict the choking occurrences. Governing equations are transformed to relate a threshold value of Froude number, termed as the limiting Froude number, to the geometry of the constriction feature and to the discharge coefficient for different flow regimes including subcritical and supercritical approach flow conditions. Governing flow equations in constricted channels result in a Froude number termed as the working Froude number. By comparing the limiting and the working Froude numbers, occurrences of choking phenomenon can be predicted. Theoret...

Energy losses and threshold conditions for choking in channel contractions

This paper presents a study on the energy losses and threshold conditions for choking in short, lateral contractions in subcritical open channel flows. A theoretical equation to predict the limiting opening ratio for choking is derived from the conservation of energy and continuity principles. This equation accounts for the critical flow conditions in the contraction and the local energy losses. For the computations of energy losses, an expression for the energy loss coefficient is developed based on a total of 186 sets of choking experiments conducted in the past by various researchers. It is shown that the proposed equation for energy loss coefficient represents the experimental data from various sources within 5% discrepancy. The analysis shows that for flows under choking conditions the energy losses between the upstream section and the critical section in the contraction are mainly governed by the opening ratio, σ, and are also affected by the encroachment structure shape, the inlet angle, α, and the relative contraction length, L*. The energy loss coefficient can vary from 1.56 to 0.27 for σ changing from 0.12 to 0.87. It can also be reduced up to 60% for encroachment structure shape changing from sharp-corner to rounded-corner conditions; and up to 75% for a going from 90° to 30° and L* from 1.33 to 0.

Debris Flow Simulation for Highway Cross Culverts

A study on the design of highway cross culverts in an area of potential debris flow along the Snowmass Canyon segment of SH-82 in Colorado is presented. Considering the characteristics of the drainage network and the highway cross culverts, FHWA’s BRI-STARS model is enhanced to simulate surface profiles for debris flows. Appropriate sizing of highway cross culverts is provided on the basis of the results of the model simulations for various approaches utilizing both clear-water and debris flow techniques. It is shown that the traditional clear-water modeling results in considerably undersized culverts for debris flows. The modified BRI-STARS model was verified using field data and was shown to be an effective tool in the design of cross culverts to pass debris flows across highways.