Simulate Overland Flow Research Articles

Finite Element Watershed Modeling: One‐Dimensional Elements

A procedure for calculating the spatial and temporal distribution of a direct runoff hydrograph from an ungauged watershed is presented. Nodal values of hydraulic roughness and slope are used to avoid kinematic shock. This formulation allows the simulation of overland flow over spatially variable surfaces typical of natural watersheds. The procedure is compared to an analytic solution for a simple case of a watershed composed of two planes. Broader application to watersheds with spatially variable parameters is possible because of the flexibility of the finite element. The Galerkin formulation of the finite element method is used to solve the kinematic wave equations. The method of characteristic and the finite element method were compared on a two‐plane system with an abrupt change in slope. The numerical and analytical considerations are presented to enable practicing engineers to adopt this method for the calculation of flow depth and discharge rate in a distributed manner throughout a watershed domina...

Removal of PCBS from wastewater in a simulated overland flow treatment system

Overland flow systems have been proposed as an efficient alternative for the removal of organic pollutants from aqueous waste streams. A simulated wastewater containing polychlorinated biphenyls (PCBs) was added to a model overland flow treatment system. Movement, biodegradation and volatilization of the PCBs were determined in the system using 14C-labeled PCBs and PCB congener-specific, high resolution gas chromatography. PCB removal approached 100% in this system. Movement of PCBs in the soil was minimal with removal occurring primarily in the upper sections of the slope. Mineralization of PCBs was low (⩽ 2% of the PCBs added) over the time frame of this experiment.

Model for Flood Propagation on Initially Dry Land

A model is presented for the simulation of shallow water flow and, specifically, flood waves propagating on a dry bed. The governing equations are transformed to an equivalent system valid on a deforming coordinate system and are solved by a dissipative finiteelement technique. A second‐order difference scheme is employed for the integration in time. The implicit nonlinear equations resulting from the weak formulations are solved by the Newton‐Raphson method, and the set of linear algebraic equations generated is solved by a frontal algorithm. A deforming grid generation scheme is introduced in the dissipative finite‐element formulation to account for the effects of the propagating or receding wave fronts on dry land. The accuracy and stability of the model is examined by comparing the model results with observed data from an experimental field test. Results of trial runs for the simulation of overland flow are also presented.

Estimating Runoff Peak Rates from Flat, High-Water-Table Watersheds

ABSTRACT SIX methods of estimating stormwater runoff peak discharge are evaluated as to their performance on watersheds of Florida's flatwoods resource area. Characteristics of flatwoods watersheds include extremely flat relief, sandy soils, dynamic shallow water tables, and scattered wetlands. Data collected by the U.S. Geological Survey and South Florida Water Management District from five small (8 to 1450 ha) agricultural watersheds (improved and unimproved pasture) served as the basis of evaluation. Runoff peak rate estimation techniques ranged in approach from empirical formulas to an overland flow simulation model. Among the established methods examined, best results were achieved using the overland flow technique. Three of the six peak rate estimation methods (the SCS graphical method, the CREAMS hydrologic model equation, and the SCS triangular hydrograph method) were modified to improve their performance on flatwoods watersheds..

UPLAND SOIL EROSION SIMULATION FOR AGRICULTURAL WATERSHEDS1

ABSTRACT: A soil erosion simulation model that considered the physical conditions of agricultural watersheds and that interfaced with the modified USDAHL‐74 watershed hydrology model was developed. The erosion model simulates the detachment and transport of soil particles caused by raindrop impact and overland flow from rill and interrill areas. The model considers temporal and spatial variation of plant residue, crop canopy cover, snow cover, and the moisture content of surface soil as modifying factors of the erosive forces of raindrop impact and overland flow. The hydrology model simulates overland flow and some of the physical parameters that are used in the erosion model. The simulation is executed in the time interval determined by the rainfall rate or snowmelt rate. The erosion model compares the transport capacity of the overland flow and the sediment loaded in the overland flow to determine the fate account for the free soil particles that have already been detached and are readily available to be transported by the overland flow. The model was tested with data from two small agricultural watersheds in the Palouse region of the Pacific Northwest dryland. The model was calibrated by trial‐and‐error to determine the coefficients of the model.

Field‐scale evaluation of infiltration parameters from soil texture for hydrologic analysis

Recent interest in predicting soil hydraulic properties from simple physical properties such as texture has major implications in the parameterization of physically based models of surface runoff. This study was undertaken to (1) compare, on a field scale, soil hydraulic parameters predicted from texture to those derived from field measurements and (2) compare simulated overland flow response using these two parameter sets. The parameters for the Green‐Ampt infiltration equation were obtained from field measurements and using texture‐based predictors for two agricultural fields, which were mapped as single soil units. Results of the analyses were that (1) the mean and variance of the field‐based parameters were not preserved by the texture‐based estimates, (2) spatial and cross correlations between parameters were induced by the texture‐based estimation procedures, (3) the overland flow simulations using texture‐based parameters were significantly different than those from field‐based parameters, and (4) simulations using field‐measured hydraulic conductivities and texture‐based storage parameters were very close to simulations using only field‐based parameters.

METHODOLOGY: A SIMPLE RAINFALL SIMULATOR AND TRICKLE SYSTEM FOR HYDRO-GEOMORPHOLOGICAL EXPERIMENTS

A simple rainfall simulator is described which consists of stand-alone sprinkling units with cone jet nozzles that spray downward. With a fall height of 4.57 m and water pressure of 67 kPa, the median drop size of the simulated rainfall is 2.40 mm. Rainfall from two units irrigating a 2 by 2.5 m plot has a total kinetic energy of 0.57 J/m2/s which is approximately 90% of the equivalent energy of natural rainfall at the simulation intensity of 72.4 mm/h. To simulate overland flow, perforated trickle pipes were designed that permit overland flow rates to be readily adjusted and closely controlled by simply varying the input water pressure. In field experiments in southern Arizona, rates between 572 to 1400 cm3/s were generated from a 19-mm pipe. Given their low cost, simplicity, and portability, the rainfall simulator and trickle system should be attractive to researchers working in a wide variety of geomorphic environments.

Overland Flow and Sediment Transport Simulation on Small Plots

ABSTRACT Amathematical model, OFESSD (Overland Flow, Erosion, Sediment Routing and Surface Degradation), was developed to simulate overland flow and sediment delivery and to predict soil surface geometry changes due to erosion on small fallow soil plots. Overland flow was simulated using a dynamic form of the Saint-Venant equations with a modified version of Holtan's infiltration equation and with flow resistance characterized by Manning's equation. Two dynamic conservation of mass equations were used to simulate sediment routing and soil surface degradation. Erosion was computed using Foster's rill and interrill erosion relationships. Yalin's equation was used to compute flow sediment transport. The model did well in simulating overland flow and sediment delivery. Soil surface geometry changes were better predicted after drainage patterns were estabHshed than during their establishment. The model showed potential for use as a prediction tool, with further development

KINETICS OF NITROGEN LOSS IN SIMULATED WASTEWATER TREATMENT IN A SOIL-PLANT SYSTEM

We investigated the effect of nitrogen forms on the kinetics of N loss in a small-scale soil-plant system, simulating overland flow conditions with controlled alternate aerobic-anaerobic cycles. The substantial gaseous N losses of applied 15NH4+-N (18.47 to 30.65 percent) indicate the presence of favorable oxidation-reduction conditions for the simultaneous occurrence of nitrification-denitrification reactions. Gaseous loss through NH3 volatilization was unlikely in this study, due to slightly acidic to neutral soil conditions. The 15NO3--N accumulation in both the 15NH4+- and 15NO3--N treatments was negligible, indicating that 15NO3--N was not stable under the experimental conditions. The rate of gaseous N loss computed by a zero-order kinetics model was 3.93 μg 15N per gram per day for the 15NO3--N treatments and 2.09 μg 15N per gram per day for the 15NH4+-N treatments. The conversion of NO3--N to NH4+-N as a consequence of respiratory reduction was not evident in the study.

Efficiency of Nitrogen Removal in a Simulated Overland Flow Waste Water Treatment System

AbstractIn a simulated overland flow waste water treatment system, vertical measurements of redox potential indicated the presence of both oxidized and reduced zones that provided favorable conditions for simultaneous nitrification‐denitrification reactions. Addition of C sources substantially reduced the redox potential and enhanced the rate of nitrate reduction. Nitrogen transformation rates were examined by the use of labeled 15N under controlled laboratory conditions.Adsorption and retention of NH4+‐N in soil matrix accounted for approximately 70–90% of the NH4+‐N applied in simulated waste water. Nitrification occurred in surface oxidized soil layers, resulting in conversion of waste water N to nitrate and nitrite. The downward movement of nitrate to the reduced soil layers in the overland flow model during subsequent application of waste water lead to denitrification and assimilatory nitrate reduction.Estimation of N balance in this study indicated that the overland flow system was capable of removing 80–90% of the added NH4+‐N within the N concentration range commonly found in municipal waste waters. However, labeled 15N data showed that an average of 11–21% of the N was taken up by vegetation and approximately 5–10% was immobilized and incorporated into soil organic N. Thus, the net loss of N is much less than would be predicted by nonlabeled N mass balance calculations. Overall, only 66–80% of the added labeled N was removed from the simulated overland flow model under laboratory conditions.

Effect of Spatial Variability on the Simulation of Overland and Channel Flow

ABSTRACT A finite element storm hydrograph model (FESHM) was used to simulate overland and channel flow. A hypothetical watershed was used to demonstrate spatial variability of such factors as soils, landuse or cover con-ditions, topography and rainfall. Several graphical displays were created to illustrate the effect of the above factors which vary spatiotemporally. Other examples were presented to illustrate potential errors emanating from ignoring spatial variability in selected variables. Rainfall is the most sensitive variable and possibly the most difficult to obtain good areal measurement. A natural watershed was used to illustrate the effect of aggregation of soils, landuse and topographic descrip-tors. In general, the error in prediction became pro-gressively worse as the level of resolution of the data was decreased. Gross errors were noted when the area was sub-divided into one HRU and different rainfall distribu-tions were used to simulate discharge. Peak discharge varied from zero to approximately 28 mm/h.

Nitrous oxide emission from simulated overland flow wastewater treatment systems

Nitrous oxide evolution was measured from overland flow models receiving daily applications of municipal wastewater with NH 4 +-N concentration in the wastewater of 10 or 47 μgN ml −1. The amount of N 2O evolved from the system ranged from 0.07 to 1.3 mg N day −1, or 1.5–25.5 kg N ha −1yr −1, respectively. Liming the soil or increasing the NH 4 +-N in the wastewater increased the emission of N 2O from the system. The N evolved as N 2O did not exceed 1.2% of the applied NH 4 +-N, indicating N 2 to be the major denitrification end product in the overland flow wastewater-treatment system.

Nitrogen transformations in a simulated overland flow wastewater treatment system

Overland flow treatment of municipal and industrial wastewater has been proposed as an economical and effective method of removing pollutants. Properly designed and manipulated nitrification-denitrification in this technique could remove a significant amount of N. Applications of wastewater containing NH 4 − −N to a simulated overland flow model led to the disappearance of NH 4 + −N and the formation of nitrate. The N balance in simulated overland flow system was estimated by using labeled 15N. The amount of N removed in the system depends upon denitrification rates. The results of this study indicated N was absorbed by soil and applied NH 4 + −N was assimilated by the vegetation. The absorbed NH 4 + −N on the aerated surface soil mass was nitrified and converted to oxidized forms of N. The nitrate formed diffused downward to the reduced zone during subsequent wastewater applications. Some of this nitrate was denitrified to gaseous forms of N or was reduced to organic forms by assimilatory processes. Thus, the net loss of N in an overland flow system was less than would have been predicted from non-labeled N mass balance calculations.

Finite Element Simulation of Overland and Channel Flow

Finite Element Simulation of Overland and Channel Flow

Simulating Overland Flow in Border Irrigation

ABSTRACT AMATHEMATICAL model of the border irrigation process is introduced. It is based on the com-plete equations of continuity and momentum for unsteady, spatially varied flow in open channels. The equations are solved approximately by the method of characteristics. Simulated irrigations are compared to test data. Agreement between simulated and observed data is generally satisfactory. Applications of the nspdel are suggested.

Simulation of Overland Flow on Short Field Plots

Overland flow is a major component in any runoff event where the cultural practices significantly affect the watershed runoff hydrograph. A model based on the kinematic flow theory is proposed that satisfactorily predicts overland flow on very rough, short slopes. The model includes retention storage, provisions for a variable point‐infiltration rate and a variable coefficient of friction, the ‘drying up’ of the upstream end of the slope during recession, and a variable area infiltration rate that is a function of the water surface area during recession. Field hydrographs from fallow erosion‐study plots were analyzed for retention storage and estimated values of the Darcy‐Weisbach coefficient of friction. These results were used in the model to simulate hydrographs that were compared with field hydrographs to test the concepts used in the model. The model with a constant Darcy‐Weisbach coefficient of friction or Manning's n adequately describes overland flow on short erodible slopes. (Key words: Hydrology; overland flow)