Journal of Hydraulic Engineering

The 2,000‐year‐old Roman aqueduct of Nîmes functioned for about eight centuries, but once it was no longer properly maintained, it suffered from severe incrustation. That problem, by no means confined to antiquity, resulted from two principal causes: (1) the chemical composition of the water, high in calcium and low in magnesium; and (2) the very low gradients of this gravityflow conduit. Calcium carbonate deposits formed at the rate of more than a millimeter per annum. This paper: (1) describes the topographical reasons for the gradients and addresses the probable flow conditions in the unencumbered channel on the basis of Manning's formula; (2) presents a likely mechanism of deposit formation that fits equilibrium and thermodynamic considerations; and (3) shows the results of a numerical analysis that relates deposit growth to flow characteristics. The findings are consistent with the archeological relics and the historical traditions.

An equation for the exponent in the power‐law velocity distribution is formulated by relating the rate of energy dissipation due to turbulent shear of the sediment bed‐layer to the increased rate of fluid‐shear energy dissipation produced by the moving sediment. The suspended‐sediment concentration distribution corresponding to the power‐law velocity profile is calculated and simplified. The relation derived for the power‐law exponent is found to be in good agreement with data from a large group of flume and river flows. The powerlaw velocity profile and the corresponding concentration distribution are shown to be in excellent conformity with the distributions measured by Vanoni and Brooks.

The rainfall‐frequency atlas published by the National Weather Service (NWS) has been extensively used in the storm drainage structures design. The users of such maps should recognize the fact that these maps are the result of many processes involving smoothing, translation, and interpolation. Rainfall depths extracted from these publications can only be regarded as representative values associated with uncertainties. Quantification of the uncertainty associated with the rainfall isopluvial maps in the NWS Technical Paper Number 40 is made in this paper. The method applies first‐order analysis in conjunction with Chen's general rainfall intensity‐duration‐frequency formula. Examples of performing point error analysis and developing error maps are given.

A mathematical model for defining the bed topography and the bed material size distribution in uniformly curved bends of alluvial rivers is developed, in which the coarsening of sediment size toward the outside bank is predicted and the bed topography is calculated coupling with the lateral sorting. The major agency for the sorting is that coarser grains feel a larger ratio of transverse gravitational force to fluid force than finer grains. It is revealed that the sorting reduces the depth of scour considerably in the outer area of bends. The model is tested with a laboratory test, and it is applied to an actual river, both of which support the current theory.

A series of field surveys was conducted at Fresh Creek Basin, a tributary of Jamaica Bay, New York, the site of a proposed storage facility for combined sewer overflow (CSO). Data collected during storm events of various intensities indicate that stratification usually occurs during CSO discharge. During all storm events, including intense storms that may cause plug flow, salinity intrusion occurs rapidly with a rising tide. A laboratory model study reproduced the proposed prototype CSO storage facility. It was found that plug flow can be induced in a long channel by placing baffles (lateral obstructions forcing flow beneath or over them) in the flow field. A series of under baffles was found to be more effective than a single baffle; alternating under and over baffles were found to promote stratification. An existing two‐dimensional, unsteady, laterally averaged mathematical model was applied as a predictive tool.

Conventional methods of energy dissipation by single‐fall hydraulic jump‐type stilling basin, roller bucket, or ski‐jump bucket may not be suitable for high dam spillways because of very high velocities at the end of the spillway, inadequate tailwater depths, and/or site conditions. A suitable spillway alternative can be a cascade of falls with a stilling basin below each fall. This spillway can be constructed on the side of the valley downstream of the dam. Relationships for cascading stilling basins have been developed and demonstrated by the numerical example of the 714.76‐ft (218.00‐m) high Tehri dam spillway on the Bhagirathi River in the Ganga Valley of the Central Himalayas.