Carlos V. Alonso

Effect of bias adjustment and rain gauge data quality control on radar rainfall estimation

Thirty major storms that passed over Goodwin Creek, a small research watershed in northern Mississippi, were analyzed to assess the bias between radar rainfall estimates at rain gauge locations and the gauge amounts. These storms, each contributing at least 10 mm of storm total rainfall, accumulated approximately 785 mm of rain, which corresponds to about half the average annual rainfall amount for the area. The focus of this study was to demonstrate the importance of (1) bias adjustment of the radar rainfall estimates and (2) the quality control of the rain gauge data used for bias adjustment. The analyses are based on Memphis Weather Surveillance Radar—1988 Doppler radar data, tipping-bucket rain gauge data, and raindrop spectra information collected within the Goodwin Creek catchment. Because of measurement and rainfall estimation uncertainties, radar observations are often combined with rain gauge data to obtain the most accurate rainfall estimates. Rain gauge data, however, are subject to characteristic error sources: for Goodwin Creek, malfunctioning of the tipping-bucket rain gauges was frequently caused by biological and mechanical fouling, and human interference. Therefore careful quality control of the rain gauge data is crucial, and only good quality rain gauge information should be used for adjusting radar rainfall estimates. By using high-quality gauge data and storm-based bias adjustment, we achieved radar rainfall estimates with root-mean-square errors (RMSE) of approximately 10% for storm total rainfall accumulations of 30 mm or more. Differences resulting from radar data processing scenarios were found to be small compared to the effect caused by bias adjustment and using high-quality rain gauge data.

Experiments on headcut growth and migration in concentrated flows typical of upland areas

Experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas. In a laboratory channel, packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow. The rainfall produced a well-developed surface seal that minimized surface soil detachment. During overland flow, soil erosion occurred exclusively at the headcut, and after a short period of time, a steady state condition was reached where the headcut migrated at a constant rate, the scour hole morphology remained unchanged, and sediment yield remained constant. A fourfold increase in flow discharge resulted in larger scour holes, yet aspect ratio was conserved. A sediment bed was deposited downstream of the migrating headcut, and its slope depended weakly on flow discharge.

Simulating ephemeral gully erosion in AnnAGNPS

Ephemeral gully erosion can cause severe soil degradation and contribute significantly to total soil losses in agricultural areas. Physically based prediction technology is necessary to assess the magnitude of these phenomena so that appropriate conservation measures can be implemented, but such technology currently does not exist. To address this issue, a conceptual and numerical framework is presented in which ephemeral gully development, growth, and associated soil losses are simulated within the Annualized Agricultural Non-Point Source (AnnAGNPS) model. This approach incorporates analytic formulations for plunge pool erosion and headcut retreat within single or multiple storm events in unsteady, spatially varied flow at the sub-cell scale, and addresses five soil particle-size classes to predict gully evolution, transport-capacity and transport-limited flows, gully widening, and gully reactivation. Single-event and continuous simulations demonstrate the models utility for predicting both the initial development of an ephemeral gully and its evolution over multiple runoff events. The model is shown to recreate reasonably well the dimensions of observed ephemeral gullies in Mississippi. The inclusion of ephemeral gully erosion within AnnAGNPS will greatly enhance the models predictive capabilities and further assist practitioners in the management of agricultural watersheds.

Conservation practice effects on sediment load in the Goodwin Creek Experimental Watershed

Water quality and aquatic habitat due to unstable stream channels and high sediment concentrations during storm runoff events are major environmental concerns on the 2,132 ha (5,266 ac) Goodwin Creek Experimental Watershed in north Mississippi. Effects of enrolling erodible lands in the Conservation Reserve Program (CRP) and instream grade stabilization structures were evaluated using measured rainfall, runoff, and sediment concentration data and model simulations. Signatures of naturally occurring radionuclides indicated that 78% of the total sediment load originated from channel sources. The change of land to a CRP-like state (reducing cultivated land from 26% to 8%) reduced erosion and runoff from fields and thus decreased total sediment concentration by 63% between 1982 to 1990. Simulations using the Fluvial Routing Analysis and Modeling Environment model indicated that mean sediment yields would increase from 15% to over 200%, depending upon location in the watershed, if in-channel structures were not present. The combined effect of the grade control structures and the change of lands to a CRP-state was to reduce sediment yields by 78% near the outlet of the watershed.

Modeling long-term soil losses on agricultural fields due to ephemeral gully erosion

It is now recognized worldwide that soil erosion on agricultural fields due to ephemeral gullies may be greater than those losses attributed to sheet and rill erosion processes. Yet it is not known whether the common practice of repairing or obliterating these gullies during annual tillage activities exacerbates or mitigates soil losses over long time periods. Here, a numerical model is used to demonstrate the potential effects of annual tillage on cumulative soil losses from four geographic regions plagued by ephemeral gullies as compared to no-till conditions where the gullies are free to grow and evolve over time and space. Historical precipitation data and field measurements were compiled for specific sites in Belgium, Mississippi, Iowa, and Georgia, and the model simulated ephemeral gully development and evolution during the growing seasons over a continuous 10-year period. When agricultural fields are not tilled annually, the simulations suggest that gullies attain their maximum dimensions during the first few years in response to several relatively large runoff events. During subsequent runoff events, the gullies no longer erode their channels significantly, and soil losses due to gully erosion decrease markedly. When agricultural fields are tilled annually, the ephemeral gully channels are reactivated, thus causing significant soil losses each year in response to runoff events. Over the 10-year simulation, the modeling results suggest that erosion rates in these four geographic regions can be 250% to 450% greater when gullies are tilled and reactivated annually as opposed to the no-till condition. These results reveal that routine filling of ephemeral gully channels during tillage practices may result in markedly higher rates of soil loss as compared to allowing these gullies to persist on the landscape, demonstrating a further advantage of adopting no-till management practices.

Test of a Method to Calculate Near-Bank Velocity and Boundary Shear Stress

Detailed knowledge of the flow and boundary shear stress fields near the banks of natural channels is essential for making accurate calculations of rates of near-bank sediment transport and geomorphic adjustment. This paper presents a high-resolution laboratory data set of velocity and boundary shear stress measurements and uses it to test a relatively simple, fully predictive, numerical method for determining these distributions across the cross-section of a straight channel. The measurements are made in a flume with a fairly complex cross-section that includes a simulated, cobble-roughened floodplain. The method tested is that reported by Kean and Smith in Riparian Vegetation and Fluvial Geomorphology in 2004, which is modified here to include the effects of drag on clasts located in the channel. The calculated patterns of velocity and boundary shear stress are shown to be in reasonable agreement with the measurements. The principal differences between the measured and calculated fields are the result of secondary circulations, which are not included in the calculation. Better agreement with the structure of the measured streamwise velocity field is obtained by distorting the calculated flow field with the measured secondary flow. Calculations for a variety of narrower and wider configurations of the original flume geometry are used to show how the width-to-depth ratio affects the distribution of velocity and boundary shear stress across the channel.

Numerical Study of the Maximum Boundary Shear Stress Induced by Raindrop Impact

The magnitude, position, and timing of the instantaneous maximum boundary shear stress caused by a raindrop on a shallow pool of water are investigated using a series of computer simulation experiments in which the Navier-Stokes equations are solved numerically. A dimensional analysis defines a set of parameters involving fluid properties and initial conditions relevant to the problem. Numerical simulations are performed over ranges of these parameters using a generic, two-dimensional, incompressible, free surface flow model. Results of the simulations are summarized in simple algebraic relationships which serve to clarify the relative importance of the parameters. The algebraic relationships also serve as an efficient substitute for the numerical model in the estimation of the magnitude, position, and timing of the maximum boundary shear stress. Laboratory measurements of water-droplet-induced shear stress are used to demonstrate the validity of the algebraic relationships.

Turbulent flow and bed pressure within headcut scour holes due to plane reattached jets

Soil erosion remains the principle cause of soil degradation worldwide, and the development and migration of headcuts in rills, crop furrows, and gullies can significantly increase soil losses on hillslopes, upland areas, and agricultural fields. Experiments were conducted to define the time-mean turbulent flow characteristics within fixed headcut scour holes typical of upland concentrated flows and to assess the distribution of these flow and pressure parameters for discrete areas of the scour hole domain. These data show that: (1) flow within headcut scour holes is analogous to plane turbulent reattached wall jets; (2) turbulence maxima are associated with the jet entry, recirculation eddies, and flow reattachment; (3) turbulent velocities are distributed asymmetrically about the free jet axis within the scour hole; (4) turbulent velocities associated with the reattached wall jet display good similarity collapse when scaled with the jet entry velocity; and (5) distributions of wall pressure near reattachment agree well with a similarity argument derived for impinging jets. This study supports the use and application of a jet impingement approach for modeling flow and soil erosion in upland concentrated flows due to headcut development and migration. Moreover, it is suggested that velocity-gradient shear, turbulent shear, and near-bed pressure gradients all are involved in soil erosion within headcut scour holes.

Near‐bed sediment concentration in gravel‐bedded streams

In this paper we present an analytical model for the concentration of suspended sediments in the proximity of the bed of gravel rivers. The model is applicable to high-gradient streams carrying sands and finer sediment particles in suspension over poorly sorted gravel beds. The analysis is carried out in terms of equations which realistically describe the velocity profile of the carrying stream and the dispersion of heavy particles in turbulent shear flows over stable gravel beds. To test our hypotheses we verified the equations using available flume and river data and evaluated their response for various combinations of bed roughness and suspended load characteristics. Our results indicate that characterization of the near-bed concentration can be formulated in terms of a velocity defect representation of the mean turbulent velocity profile and a sediment eddy diffusivity distribution developed for sand bed rivers. The near-bed concentration is shown to be rather sensitive to variations in the gravel bed relative roughness, the depth at which the concentration is evaluated above the virtual bed surface, and the ratio between the settling velocity of the suspended sediment and the bed shear velocity.

Flow near a model spur dike with a fixed scoured bed

Three-dimensional flow velocities were measured using an acoustic Doppler velocimeter at a closely spaced grid over a fixed scoured bed with a submerged spur dike. Three-dimensional flow velocities were measured at 3,484 positions around the trapezoidal shaped submerged model spur dike. General velocity distributions and detailed near field flow structures were revealed by the measurement. Clear differences were revealed between flow over fixed flat and scoured beds. Strong lateral flows were the dominant cause of the observed local scour. Shear stresses were higher for the scoured bed than in the flat bed case. Decreasing rates of scour as the scour hole developed were attributed to increases in critical shear stress in the scour holes caused by the increase in the length and magnitude of adverse slopes associated with the two main scour holes.