Overland Flow Velocity Research Articles

Reliability analysis of design discharge for mountainous gully flow

A probabilistic equation of design discharge for a mountainous gully was derived in terms of reliability probability. Its derivation was based on the rational formula, the rainfall-intensity-duration formula, and the First-Order Second-Moment method (FOSM) by considering the uncertainty of parameters, such as the annual rainfall, the runoff coefficient, the watershed area, the sediment concentration, the length and velocity of overland flow, and the horizontal length and elevation difference of a gully, involved in the formulas. The probabilistic equation was also applied to the probabilistic design discharge for a debris-flow-prone gully in Taiwan.

Overland flow velocity and roughness properties in peatlands

Overland flow is an important component of peatland hydrology. Hydrological models of peatlands are being developed that require estimates of flow velocity and its controls. However, surprisingly little is known about overland flow velocities in peatlands. Some peatlands have also been drained using open ditches, and these need to be incorporated into flow models. This paper presents field data on the velocity of overland flow and drain flow in upland peatlands. The relationships between flow velocity, vegetation cover, slope, and water depth are explored. Sphagnum provided a significantly greater effective hydraulic roughness to overland flow than peatland grasses. In all cases, a significant break in process occurred for flows with water depths of around 1 cm so that there were two components of the roughness curve. This is consistent with partial submergence theory for very shallow flows where resistance increases with depth as the soil surface first becomes fully submerged. While each surface cover type should be considered separately, the results also suggest that a first‐order estimate of Darcy‐Weisbach roughness and mean velocity can be based on a single parameter for each surface cover. This paper presents an empirical overland flow velocity forecasting model that can be applied to peatlands. The model combines the partially submerged component for flows with water depths below 1 cm with the fully submerged component for flows with depths up to 5 cm, which are representative of the depths of flows that occur across peatlands.

Effects of hedgerows on sediment erosion in Three Gorges Dam Area, China

The Three Gorges Dam Area refers to the river section from Chongqing to Yichang on the Yangtze River, which has a drainage area of 75,098 km 2, and involves 19 cities and counties. Contour hedgerows have been used in this area to control soil erosion and to improve hillslope stability in the catchment of this river section. Five experimental hedgerow plots were established in 1994 in order to study the effects of hedgerows on erosion control. During the period of 1994-1997, runoff and soil loss data were collected on these test plots, including the chemical and physical properties of soil and related topographical data. The results indicate that: (1) after 4 years of cultivation and crop planting, soil fertility increased dramatically in the hedgerow plots. Soil organic matter, total nitrogen, and total phosphorus contents in the hedgerow plots were 5-9 times higher than that in the control plot. In each of the hedgerow plots, soil structure became more stable, the quantity of granules larger than 0.02 mm increased and those finer than 0.02 mm decreased; (2) All hedgerow plots showed a major effect on reducing soil loss and surface runoff; (3) Overland flow velocity along the upper portion of the hedgerow plots was greatly reduced due to hedgerow resistance, which explains the significant decrease in soil losses in hedgerow plots, despite the fact that the hedgerow plots and the control plots had the same total runoff.

Variability of shallow overland flow velocity and soil aggregate transport observed with digital videography

AbstractField experiments at Tiramoana station 30 km north of Christchurch, New Zealand using an erosion plot 16·5 m long, 0·6 m wide, and with a slope of 14–14·5° on rendzina soil aimed to measure the variability of flow velocity and of soil aggregates transport rate in shallow overland flow. Discharge/cross‐section area ratio was used to estimate mean velocity, and high‐speed digital video camera and image analysis provided information about flow and sediment transport variability. Six flow runs with 0·5–3·0 L s−1 discharges were supercritical with Froude numbers close to or more than 1. Mean flow velocity followed Poiseuille law, float numbers were more than 1·5 and hydraulic resistance was an inverse proportional function of the Reynolds number, which is typical for laminar flows. Hence actual velocity varied through time significantly and the power spectrum was of ‘red‐noise’, which is typical for turbulent flow. Sediment transport rates had even higher variability, and soil aggregates transport was a compound Poisson process. Copyright © 2008 John Wiley & Sons, Ltd.

Calculation of watershed flow concentration based on the grid drop concept

The grid drop concept is introduced and used to develop a micromechanism-based methodology for calculating watershed flow concentration. The flow path and distance traveled by a grid drop to the outlet of the watershed are obtained using a digital elevation model (DEM). Regarding the slope as an uneven carpet through which the grid drop passes, a formula for overland flow velocity differing from Manning's formula for stream flow as well as Darcy's formula for pore flow is proposed. Compared with the commonly used unit hydrograph and isochronal methods, this new methodology has outstanding advantages in that it considers the influences of the slope velocity field and the heterogeneity of spatial distribution of rainfall on the flow concentration process, and includes only one parameter that needs to be calibrated. This method can also be effectively applied to the prediction of hydrologic processes in un-gauged basins.

Effect of a Shallow Water Obstruction on Long Wave Runup and Overland Flow Velocity

A study is presented to examine the one-horizontal dimension effect of a shallow shelf obstacle on nonlinear long wave runup. Due to the large horizontal-vertical aspect ratio of this problem, it is not well suited for experimental analysis, and therefore this study is purely numerical. Simulations are performed for various incident wave conditions, obstacle height and widths, and final beach slopes. Many of the setups involve breaking, either through approaching the obstacle as a large breaking bore, incipient breaking on top of the obstacle, or breaking during the beach uprush. The general conclusion of this study is that, for highly nonlinear waves wave height/shelf water depth 0.5, the obstacle will always act to reduce the runup and the maximum overland velocity. However, for very small obstacle lengths, particularly for extremely large waves, this reduction may be practically inconsequential. Interestingly, for weakly nonlinear waves 0.1, due to front steepening over the obstacle, greater overland velocities can result from increasing obstacle length. Consistent with previous studies, it is found that the final beach slope is of primary importance for determining the runup.

Runoff hydraulic characteristics and sediment generation in sloped grassplots under simulated rainfall conditions

Evaluation of grass influence on soil erosion process can provide important information in soil and water conservation. The laboratory experiment was conducted to study runoff and sediment producing processes and runoff hydraulics in the grassplots with different covers (35%, 45%, 65% and 90%) and bare soil plot (control) at a slope of 15. The results showed that grass significantly reduced runoff and sediment. Compared with bare soil plot, the grassplots had a 14-25% less runoff and an 81-95% less sediment, and played a more important role in reducing sediment at the final stage of rainfall. There was a significantly negative logarithmic relationship between sediment yield rate (SDR) and cover (C): SDR = 1.077-2.911 ln(C) (R-2 = 0.999'). Sediment yield rate of grassplots decreased with rainfall duration, and decreased linearly as runoff rate increased. Overland flow velocities deceased with increase in grass cover, and the cover had greater effect on lower slope velocity than upper one. Froude numbers decreased with increase in cover, and flow regimes of all treatments were laminar and tranquil. Darcy-Weisbach and Manning friction coefficients of grassplots increased as ground cover increased. Therefore, increase in grass coverage can efficiently reduce soil loss and improve ecological environments. (c) 2006 Elsevier B.V. All rights reserved.

3D simulation of soft geo‐objects

In terms of geometry, existing approaches for 3D GIS modelling can be classified into three types: surface‐based, volume‐based, and hybrids. These approaches are suitable for representing rigid geo‐objects, but so far there are no ideal GIS modelling methods for simulation of soft geo‐objects such as flowing water and soil. In this study, a GIS flow element (FE) and GIS soft voxel (SV) were used for simulation of overland flow and soil erosion. The length of a GIS FE is directly proportional to the velocity of overland flow, while the inclination of a GIS FE represents the direction of overland flow. The GIS SV is generated by implicit functions, which can be utilized to create a smoothed surface. This study reveals that GIS FEs can realistically represent the dynamics and stochastics of soft geo‐objects, while GIS SVs can simulate deformation of soft geo‐objects. Moreover, both of these approaches can reflect geoscientific laws because the animation of 3D scenarios is controlled by geoscientific models.

Spatial patterns of soil erosion and deposition in two small, semiarid watersheds

This work investigates spatial patterns of hillslope erosion and sediment yields in a semiarid ecosystem considering influences of vegetation, slope, rocks, and landscape morphology. The 137Cs inventories were measured on one shrub and one grassed watershed in southeastern Arizona. Calculated mean erosion rates in eroding areas were 5.6 and 3.2 t ha−1 yr−1, and net erosion rates for the entire watershed, including depositional areas, were 4.3 and nearly 0 t ha−1 yr−1 for the shrub and grass watersheds, respectively, over the past four decades. Differences in hillslope erosion rates between the two watersheds were apparently due to vegetation: while on the shrub site, runoff pathways were unobstructed, on the grass site, runoff was obstructed by vegetation patches and litter. Hillslope erosion rates within the watersheds were not correlated to slope gradient or curvature but were correlated to rocks in the upper soil profile. These results are interpretable in terms of slope‐velocity equilibrium wherein overland flow velocities became independent of slope gradient because of differential rock cover, which evolved as a result of preferential erosion of fine material on the steeper slopes prior to 137Cs deposition. Watershed morphology and channel incision controlled sediment yield. Most of the eroded soil was deposited in swales of the grassed watershed. Most of the soil eroded in the shrub watershed was exported from the watershed outlet by way of a well‐incised channel system. The study shows that measurement of sediment yield from a watershed can be a poor indicator of erosion taking place within the watershed.

Investigation of Soil Erosion from Bare Steep Slopes of the Humid Tropic Philippines

Abstract At the Visayas State College of Agriculture (ViSCA) on the island of Leyte in the Philippines, hydrologic and soil-loss measurements were recorded for 32 erosion events over 3 yr on three 12-m-long bare soil plots with slopes of approximately 50%, 60%, and 70%. Measurements included rainfall and runoff rates at 1-min intervals, total soil lost per event from the plot, rill details when observed after an erosion event, and soil settling-velocity characteristics. Storm events are characterized by high rainfall rates but quite low rates of runoff, because of the consistently high infiltration rate of the stable clay soil (an Oxic Dystropept). Both observation and modeling indicated that overland flow is commonly so shallow that much of the soil surface is likely to be unsubmerged. For the 70% slope plot, half the events recorded mean sediment concentrations from 100 to 570 kg m−3. A somewhat constant hydrologic lag between rainfall and runoff is used to estimate a Manning’s roughness coefficient n of about 0.1 m−1/3 s, a value used to estimate velocity of overland flow. Possible effects of shallow flows and high sediment concentrations on existing erosion theory are investigated theoretically but are found to have only minor effects for the ViSCA dataset. A soil erodibility parameter β was evaluated for the data whenever rilling was recorded following an erosion event. The values of β indicate that, except for events with higher stream powers, other erosion processes in addition to overland flow could have contributed to soil loss from erosion plots in a significant number of events.

The influence of both process descriptions and runoff patterns on predictions from a spatially distributed soil erosion model

AbstractSpatially distributed, process‐based erosion models have been built to predict both erosion and deposition rates and patterns. However, predictions by these models often do not meet our expectations. There are several possible reasons for this, which can be subdivided into three groups: (i) process descriptions within the models may be incomplete or incorrect, leading to structural errors (model error); (ii) parameterization is difficult leading to a high degree of uncertainty regarding model inputs (input error); and (iii) the spatial implementation of the model (the spatial discretization and the routing methods used) may be inadequate. Hitherto, most studies of erosion model performance have focused on input parameter uncertainty: the effects of process description and spatial implementation of the model have often been neglected. In this study an attempt is made to compare some aspects of the effects of process description, parameter uncertainty and model implementation on model performance. A series of model runs was carried out to evaluate to what extent the predictions of the spatially distributed erosion model LISEM are affected by the description of the hydraulic resistance, the parameter values used to characterize hydraulic resistance as well as the routing algorithm used to generate the runoff pattern.The process description used for overland flow resistance and velocity has a clear effect on the erosion and deposition rates predicted by the model. However, relatively minor variations in a single parameter value (hydraulic roughness) appear to affect model output more strongly than variations in the process description itself. The use of different routing algorithms leads to notably different erosion and deposition patterns, but total erosion and deposition rates are much less affected. Copyright © 2005 John Wiley & Sons, Ltd.

Modeling Event-Based Temporal Variability of Flow Resistance Coefficient

During the storm duration, the effects of surface resistance on the developing and receding overland flow depths (hj) on different flow parameters like the overland flow resistance coefficient (Nj), flow velocities (vj), Reynolds number (Rj), and Froude number (Fj) have been studied for the six natural catchments (0.012–92.5 km2) located in different agroclimatic regions of India. The kinematic wave (KW) equations are used for simulation of overland and channel flows using explicit methods for developing numerical solutions for the estimation of flow parameters at short durations. For the complete storm duration hj-Nj relationships have been found to form hysteresis loops. To predict resistance coefficients, simple hj-Nj relationships have been worked out separately for the rising and depleting overland flow conditions. Computed hydrographs simulated by incorporating these equations in the KW model agree well with the observed ones. The overall applicability of the suggested approach is successfully demon...

GRIEROM: GRID–BASED VARIABLE SOURCE AREA SOIL–WATER EROSION AND DEPOSITION MODEL

Erosion in a variable source area and the sediment in surface runoff have a major impact on soil conservationand water quality. It is necessary to locate the soil erosion areas and the paths of sediment transport efficiently to alleviatepotential soil loss problems and protect water quality. The objective of this study was to predict temporal variations and spatialdistributions of sediment transport in a variable source area for storm events. A GRIdbased soilwater EROsion anddeposition Model (GRIEROM) is described. This model uses ASCIIformatted map data supported by the GRASS GeographicInformation Systems (GIS) and generates distributed results such as sediment concentration and flux in overland flow areas.GRISTORM was adopted for the hydrological processes. A processbased soil erosion concept was adopted to simulatesoilwater erosion and deposition. The model deals separately with rill and sheet flow and handles vegetation in terms of soilcover. The model was applied to a 170 ha watershed located in the Northern Catskill region of New York. The initial soilmoisture conditions for storm events were based on calibrated values from a Soil Moisture Routing (SMR) model and wereadjusted to include soil moisture variations within a day. Predicted flows from four storm events in 1993 were compared withobserved flows at the watershed outlet for hydrologic calibration. The average NashSutcliffe efficiency for predicting flowat the outlet using calibrated data was 0.847. Parameters related to sheet/rill flow on bare soil were more sensitive than thoseon vegetated soil. Sediment depositability (
) proved to be the most sensitive parameter, followed by the low value of soilerodibility (). The NashSutcliffe efficiencies for predicting sediment yield at the outlet for the two storms were negativebecause of additive prediction and time error of sediment concentration, respectively. The average NashSutcliffe efficiencyfor another two storms was 0.476. The temporal variations and spatial distributions of overland flow and sediment areas werepresented using GRASS. These presentations suggest that most sediment was transported to the stream from the areas thathad high overland flow velocities.

Tsunami in Papua New Guinea was as intense as first thought

Last July's tsunami in Papua New Guinea was as intense and catastrophic as news reports indicated, a scientific survey has found, and recommendations have been put forth to avert such a disaster in the future. The tsunami and the earthquake that generated it occurred July 17, 1998, and the International Tsunami Survey Team (ITST) began a weeklong investigation July 31. It was the ninth major tsunami and the most devastating the team has studied in the past 6 years. The team was able to precisely map the inundation and determine that media reports of extreme flows to fairly small sections of shoreline were accurate. Wave heights of 10 m were confirmed along a 25‐km stretch of coastline with maximum heights of 15 m and overland flow velocities of 15–20 m/s. Both are extreme measurements, given the moderate size of the earthquake and its aftershocks.The team noted that the force of a tsunami current on an object is roughly 1000 times that of a wind of the same speed.

Roughness coefficients and velocity estimation in well-inundated sheet and rilled overland flow without strongly eroding bed forms

Even with the flow of water over a soil surface in which roughness elements are well inundated, and in less erosive situations where erosional bed forms are not pronounced, the magnitude of resistance coefficients in equations such as those of Darcy–Weisbach, Chezy or Manning vary with flow velocity (at least). Using both original laboratory and field data, and data from the literature, the paper examines this question of the apparent variation of resistance coefficients in relation to flow velocity, even in the absence of interaction between hydraulics and resulting erosional bed forms. Resistance equations are first assessed as to their ability to describe overland flow velocity when tested against these data sources. The result is that Manning's equation received stronger support than the Darcy–Weisbach or Chezy equations, though all equations were useful. The second question addressed is how best to estimate velocity of overland flow from measurements of slope and unit discharge, recognizing that the apparent flow velocity variation in resistance coefficients is probably a result of shortcomings in all of the listed resistance equations. A new methodology is illustrated which gives good agreement between estimated and measured flow velocity for both well-inundated sheet and rill flow. Comments are given on the predictive use of this methodology. Copyright © 1999 John Wiley & Sons, Ltd.

Extreme inundation flows during the Hokkaido‐Nansei‐Oki Tsunami

The tsunami generated by the July 12, 1993 Hokkaido‐Nansei‐Oki Mw=7.8 earthquake produced in Japan the worst local tsunami‐related death toll in fifty years, with estimated 10–18m/sec overland flow velocities and 30m runup. These extreme values are the largest recorded in Japan this century and are among the highest ever documented for non‐landslide generated tsunamis. We model this event to confirm the estimated overland flow velocities, and we find that, given reasonable ground deformation data, current state‐of‐the‐art shallow‐water wave models can predict tsunami inundation correctly including extreme runup, current velocities and overland flow. We find that even small local topographic structures affect the runup to first‐order, and that the resolution of the bathymetric data is more important than the grid resolution. Our results qualitatively suggest that—for this event—coastal inundation is more correlated with inundation velocities than with inundation heights, explaining also why threshold‐type modeling has substantially underpredicted coastal inundation for this and other recent events.

Effect of saltating sediment load on the determination of the mean velocity of overland flow

Where overland flow velocity is measured using dye or salt tracing, the mean velocity is often determined by multiplying the velocity of the leading edge of the tracer plume by a correction factor α. Flume experiments show that a varies with both Reynolds number and sediment load. For sediment‐free flow over a sand‐covered bed, α is less than the theoretical value of 0.67 in laminar flow and increases rapidly with Reynolds number in transitional flow and more slowly with Reynolds number in turbulent flow. For sediment‐laden flow, α decreases as sediment load increases in transitional and turbulent flows. Saltating sediment extracts momentum from the flow, causing velocities near the bed to decrease and α to decrease. A multiple‐regression equation is developed which can be used to predict the value of α from Reynolds number and sediment load in transitional and turbulent overland flows.

CORRECTION FACTORS IN THE DETERMINATION OF MEAN VELOCITY OF OVERLAND FLOW

The velocity of overland flow has been conventionally measured using tracers, but it is difficult to measure the mean flow velocity directly because the centroid of the tracer plume is not easily identified. Consequently, previous investigators have measured the velocity of the leading edge of the plume and multiplied it by a correction factor α to obtain an estimate of mean velocity. An alternative method is to measure the velocity of the peak concentration in the tracer plume and multiply this velocity by another correction factor β to estimate mean velocity. To investigate the controls of α and β and develop predictive models for these correction factors, 40 experiments were performed in a flume with a mobile sand bed. Multiple regression analyses reveal that both α and β vary inversely with slope and directly with Reynolds number. The derived regression equations may be used to calculate the mean velocity of other shallow overland flows, at least within the range of slope and Reynolds number for which the equations were developed. In the experiments, slope ranged from 2.7;° to 10° and Reynolds number from 1900 to 12 600.

Technical Notes: Electronic and Visual Tracing of Leading Edges in the Measurement of Overland Flow Velocity

The correction factor used to determine the mean velocity of overland flow from the velocity of the leading edge of a dye-salt plume can be obtained either visually or electronically. The front of the dye-salt plume is concave upward from a side view. The dye edge that is identified by eye from above the flow corresponds to the base of the concave-upward front where the color contrast is most distinct. When a fine-tipped electronic conductivity probe is placed in the flow above the bed, it senses the arrival of the dye-salt mixture before the arrival of the base of the concave-upward front. Thus, the correction factor obtained using a conductivity probe underestimates that obtained by visual dye tracing.

Responses of interrill runoff and erosion rates to vegetation change in southern Arizona

Vegetation change, fi-om grassland to shrubland, has occurred over much of the Sonoran and Chihuahuan Deserts during the past century. The effect of this vegetation change on interrill runoff and erosion was examined by conducting rainfall-simulation experiments on large runoff plots on contemporary grassland and shrubland hillslopes. These experiments show that, compared to the grassland, the interrill portions of shrubland hillslopes (1) have higher runoff rates, (2) experience equilibrium runoff conditions much more frequently, (3) exhibit higher overland flow velocities, and (4) are subject to greater rates of erosion. The environmental change that has led to the vegetation change has been relatively minor, but its geomorphic impact has been substantial.