Journal of Hydraulic Engineering

If they are negatively buoyant (denser than ambient water), inflows to lakes and reservoirs sink (plunge) below the water surface after entering. The location of the plunge line or maximum penetration into the lake or reservoir is related to the ratio of inertial to buoyant forces, and lake or reservoir geometry. Experiments have been conducted in diverging horizontal laboratory channels, and the plunging phenomenon has been reproduced. For flows separating from the channel wall the relationship $x_p/B_0=0.52F_0^4$ has been found to describe the maximum horizontal penetration $x_p\text{.}$ The basic form and exponent of this relation was developed from jet flow theory, and the coefficient was experimentally determined. $F_0$ is the inflow densimetric Froude number and $B_0$ is the inflow channel width. Experimental data for $x_p/B_0$ are also reported for attached flows. In this case the distance to plunging can be expressed as $x_p/B_0==(0.74F_0-0.5)(\tan \delta {)}^{-1}\text{.}$

A negatively buoyant inflow to a lake or reservoir sinks to the bottom and forms an underflow. In this process, the “plunging flow” entrains ambient water and becomes diluted. The dilution alters the buoyancy and hence the flow behavior of the underflow. Since literature on entrainment by underflows does not include the “plunging process” itself, great uncertainty exists when the initial conditions of underflows have to be estimated. Therefore, the density driven plunging phenomenon is experimentally investigated in a horizontal diverging channel. Entrainment rates are determined from velocity and temperature measurements and related to channel diffuser angle, inflow densimetric Froude number, and inflow channel aspect ratio. The experiments are conducted at inflow channel Reynolds numbers larger than 10,000. Experimental diffuser angles are varied from 3° to 180°, inflow densimetric Froude numbers from 1.8 to 21 and channel aspect ratios from 0.5 to 1.8. The information is of use as input to one‐dimensional stratified lake or reservoir water quality models.

Measurements of turbulent fluctuations of horizontal and vertical components of velocity and of suspended solids concentration in the region of 0.50 to 1.25 m above the bed have been made in the upper reaches of the Conwy and Tamar estuaries. The charcteristics of the turbulence for the velocity components were found to be similar to those for laboratory flume data for appropriate steady and homogeneous conditions. The characteristics of the turbulence fluctuations of suspended solids concentration were found to be in some ways similar to those of the turbulent fluctuations of the horizontal and vertical components of velocity and salinity. The relative velocities are strongly dependent on relative depth and particle gradient Richardson number. The intensity of the suspended solids concentration fluctuations increases with the suspended solids concentration vertical gradient. The momentum and suspended solids flux correlation coefficients decrease with increasing gradient Richardson number, with the vertical suspended solids flux correlation coefficient decreasing more rapidly than the horizontal suspended solids flux correlation coefficient. These effects are tentatively explained by the concept of turbulent fluctuations being progressively replaced by wave‐like fluctuations in stable conditions.

Quasi‐two‐dimensional turbulence was investigated experimentally in an apparatus consisting of two closely spaced vertical parallel walls. A jet of salt water was injected downward between the walls. Entrainment of fluid by the turbulence produced an expanding buoyant jet that was many times wider than the wall spacing. Photographs were taken of the turbulent motion in the confined region. The volume flux in the plume was measured using a direct method for a range of source flow from the limit of a jet to that of a plume. The variation of volume flux with distance in the plume followed closely the relationship derived using a linear spreading hypothesis. Despite the anisotropic nature of the turbulent motion and the frictional retardation of the plume by the walls, the spreading rate of the confined plume was remarkably similar to that of a line plume, which is a two‐dimensional flow. Flow visualization revealed significant differences in the turbulent structures for cases of jet and plume.

In a previous study, it was noted that the addition of six years of data to the Nevada Test Site daily precipitation data base resulted in significant changes in the calculated values of annual average precipitation. The goal of the investigation described here was to examine the effect of the period of record available for analysis on estimates of annual precipitation and to discuss the effect of changing estimates of annual precipitation on estimates of regional groundwater recharge.

Two empirical precipitation disaggregation models are developed for use in continuous hydrologic modeling. The first model is developed for use in disaggregating hourly rainfall data into 20‐min intervals. The second model is developed for use with disaggregation time intervals from 1–30 min. Use of such models allows a more precise prediction of watershed response than may be obtained using a one‐hour time step or a smaller time step with average rainfall values.

A hysteresis loop between the suspended-sediment discharge and the water discharge makes the prediction problem of sediment yield more difficult. An exponential relation (water - sediment curve) Q_s = Q^b_o is used to compute the suspended-sediment discharge Q_s generated by the estimated overland runoff component Q_o. a and b are constant parameters to be established from observed data of suspended-sediment discharge and the computed overland flow. The established result is found to be in good agreement with observed data.

The conventional and the more refined difference schemes of the Muskingum method are evaluated from the considerations of diffusion analogy principle, and as numerical schemes for replacing the Muskingum analytical solution. It has been found from the first consideration that the coefficients of both schemes are same in terms of channel and flow characteristics, and thus they result in the same outflow hydrograph for a given inflow hydrograph and the channel reach. However from the second consideration it is seen, that for practically realizable input shapes, the conventional difference scheme reproduces the analytical solution more closely than the more refined difference scheme.

There is no agreement on the best way to develop flood frequency curves and synthesize runoff peak flow rate data, and no precise procedure which applies to all cases of flood frequency statistics. Consequently it is prudent to check the data to see whether a given flood frequency curve is apppopriate. One such check is a statistical evaluation of the rainfall depth-duration frequency data corresponding to the critical duration of the catchment as associated with each annual peak flow rate. A chi-squared test applied to this data is useful in checking how ``rare'' or ``probable'' the rainfall data are with respect to the model frequency, thereby checking how probable is the flood frequency curve. It is interesting that such a check is sensitive to the precise definition chosen for the ``T-year event.''