Manning Roughness Coefficient Research Articles

Discharge estimation combining flow routing and occasional measurements of velocity

Abstract. A new procedure is proposed for estimating river discharge hydrographs during flood events, using only water level data at a single gauged site, as well as 1-D shallow water modelling and occasional maximum surface flow velocity measurements. One-dimensional diffusive hydraulic model is used for routing the recorded stage hydrograph in the channel reach considering zero-diffusion downstream boundary condition. Based on synthetic tests concerning a broad prismatic channel, the "suitable" reach length is chosen in order to minimize the effect of the approximated downstream boundary condition on the estimation of the upstream discharge hydrograph. The Manning's roughness coefficient is calibrated by using occasional instantaneous surface velocity measurements during the rising limb of flood that are used to estimate instantaneous discharges by adopting, in the flow area, a two-dimensional velocity distribution model. Several historical events recorded in three gauged sites along the upper Tiber River, wherein reliable rating curves are available, have been used for the validation. The outcomes of the analysis can be summarized as follows: (1) the criterion adopted for selecting the "suitable" channel length based on synthetic test studies has proved to be reliable for field applications to three gauged sites. Indeed, for each event a downstream reach length not more than 500 m is found to be sufficient, for a good performances of the hydraulic model, thereby enabling the drastic reduction of river cross-sections data; (2) the procedure for Manning's roughness coefficient calibration allowed for high performance in discharge estimation just considering the observed water levels and occasional measurements of maximum surface flow velocity during the rising limb of flood. Indeed, errors in the peak discharge magnitude, for the optimal calibration, were found not exceeding 5% for all events observed in the three investigated gauged sections, while the Nash-Sutcliffe efficiency was, on average, greater than 0.95. Therefore, the proposed procedure well lend itself to be applied for: (1) the extrapolation of rating curve over the field of velocity measurements (2) discharge estimations in different cross sections during the same flood event using occasional surface flow velocity measures carried out, for instance, by hand-held radar sensors.

EVALUATION OF THE EVALUE MODEL FOR ESTIMATING MANNING'S ROUGHNESS IN FURROW IRRIGATION

ABSTRACTManning's roughness coefficient is a key parameter in the design and evaluation of furrow irrigation. Despite its importance, estimation of this parameter in surface irrigation is difficult, particularly in furrow irrigation. In this study, the EVALUE model was used for estimating Manning's roughness coefficient in furrow irrigation. The EVALUE model is based on volume balance and has been developed for estimating Manning's roughness coefficient in surface irrigation. EVALUE estimates both infiltration parameters and Manning's roughness coefficient simultaneously. The main input for the model is depth of flow along the furrow at different times. Evaluation of EVALUE was performed by comparing measured and simulated advance and recession data using SIRMOD software. The estimated values of Manning's roughness coefficient ranged between 0.02 and 0.102. The estimated values of Manning's roughness coefficient by EVALUE were somewhat different from the suggested SCS values, particularly during the first irrigation, but comparable with the results shown by other studies. Copyright © 2011 John Wiley & Sons, Ltd.

확산파에 기초한 분포형 유출모형의 개발 및 적용

분포형 유출모형에 대하여는 컴퓨터의 발달과 지리정보시스템의 구축 및 관련정보의 제공이 활성화되면서 최근 많은 연구가 진행되고 있다. 이러한 분포형 유출모형은 대상유역을 보다 세분 요소화하여 계산하는 이론적이고 물리적인 기반의 모형이다. 본 연구에서는 토지피복 상태에 따라 결정되는 매개변수와 2차원 확산파 방정식에 기초하여 지표면에서의 유출량을 계산하는 모형을 개발하였다. 기존에 연구되었거나 개발 중인 유출모형은 대부분 Manning-Strickler의 평균 유속공식과 Manning 조도계수를 이용하여 유속과 유량을 산정하고 있다. Manning 조도계수는 사용상의 편의성 때문에 보편적으로 사용하고 있으나, 차원이 일치하지 않고 추정 시 모호한 문제점이 있다. 이러한 문제를 개선하기 위해 본 연구에서는 Manning-Strickler식뿐만 아니라 차원이 일치하는 Chezy의 평균유속공식을 적용하여 유출모형을 개발하였다. 또한, Chezy의 마찰계수를 적용하기 위하여 조고의 함수로 표현되는 지수형 마찰계수 산정식을 도입하였다. 따라서 모호한 조도계수의 개념을이용하지 않고 거친 정도를수치화하여 물리적인 의미를 가진마찰계수를 산정하고 적용 가능성을 검토하였다. 본 연구에서는 개발된 모형을 부채꼴 실험유역과 장방형 실험유역 및 실제유역인 안성천유역을 대상으로 6개의 사상을 적용하여 그 적용성을 확인하였다. According to the improvement of computer's performance, the development of Geographic Information System (GIS), and the activation of offering information, a distributed model for analyzing runoff has been studied a lot in recently years. The distribution model is a theoretical and physical model computing runoff as making target basin subdivided parted. In the distributed model developed by this study, the volume of runoff at the surface flow is calculated on the basis of the parameter determined by landcover data and a two-dimensional diffusion wave equation. Most of existing runoff models compute velocity and discharge of flow by applying Manning-Strickler's mean velocity equation and Manning's roughness coefficient. Manning's roughness coefficient is not matched with dimension and ambiguous at computation; Nevertheless, it is widely used in because of its convenience for use. In order to improve those problems, this study developed the runoff model by applying not only Manning-Strickler's equation but also Chezy's mean velocity equation. Furthermore, this study introduced a power law of exponential friction factor expressed by the function of roughness height. The distributed model developed in this study is applied to 6 events of fan-shape basin, oblong shape test basin and Anseongcheon basin as real field conditions. As a result the model is found to be excellent in comparison with the exiting runoff models using for practical engineering application.

Basin irrigation design with longitudinal slope

Abstract The aims of this paper are to analyze theoretically the influence of the longitudinal slope of a surface irrigation field on the uniformity of irrigation and to provide practical tools to design, analyze and manage surface irrigation systems with longitudinal slope and blocked end. An example is shown where a 20% savings in water is obtained by giving the field the optimal slope. In 1982, Clemmens and Dedrick published a practical set of dimensionless graphs to level-basin design and analysis (with no slope). This article generalizes those graphs taking account the existence of field slope. So, Clemmens and Dedrick's graphs are a particular case of obtained results. The analysis is based on solving one-dimensional free surface Saint-Venant equations including infiltration, applying the dimensional analysis to reduce the number of variables involved. Saint-Venant equations are solved with the finite differences method, applying the full hydrodynamic model and the zero-inertia model. Two computer programs are used: WinSRFR and POZAL (a specific software that calculates the optimal cutoff time). The result is a set of three-dimensional graphs that show the relationships of field slope, irrigation uniformity and the rest of the involved dimensionless variables, related to infiltration parameters, Manning roughness coefficient, cutoff time, inflow rate and field length and width. The graphs could be useful in practice to determine the optimal slope of a field, the inflow rate or the length and width of a field, achieving substantial savings of water in surface irrigation.

DETERMINATION OF MANNING ROUGHNESS COEFFICIENT FOR BORDER IRRIGATION SYSTEM

Manning roughness coefficient is one of the most important factors affecting the border irrigation design. The purpose of this study is to determine the Manning roughness coefficient values for border irrigation in a clay loam soil with compacted and un-compacted soil under continuous and surge flow. The determination was based on the volume balance equation in the form of a differential equation that was solved with the forward finite difference. The results indicated that, soil surface compaction had its effect on n values of the first irrigation and had not any effect for the next irrigations. For continuous flow, at first irrigation, the values of n were 0.1915 and 0.164 for un-compacted and compacted soils respectively. At the second irrigation, the n values decreased by about 44.51% and 49.82% compared to the first irrigation for compacted and un-compacted soil respectively. However, after the second irrigation, the seedling of wheat resulted on increases the n values. The minimum value, 0.083, of n was achieved at the second irrigation for surge flow. While, the maximum value, 0.25, of n was found at the seventh irrigation for continuous flow. Also, by using surge flow, the n values were decreased compared with those for continuous flow without vegetation crop or with wheat crop.

Concurrent use of finite element and finite volume methods for shallow water flows in locally 1-D channel networks

SUMMARY A numerical model is developed for shallow water equation in locally 1-D channel networks. The model concurrently uses the standard Galerkin finite element method for the continuity equation and the finite volume method with an upwind scheme for the momentum equation. The surface gradient method is consistently employed. A minimum treatment is given for channel junctions so that application to multiply connected channels do not require any special consideration The model is capable of computing different types of transcritical flows, wet and dry flows, and flows with complex source terms. Standardized test problems and laboratory experimental data are used for verifying the model. Applicability of the models is validated in a multiply connected channel network draining hydromorphic farmlands located in a West African savanna, and Manning's roughness coefficient is identified, so that the steady solution is consistent with field observations. Unsteady simulation demonstrates that the model is capable of stably reproducing shifts of hydraulic jumps in flows of sub-millimeter water depths. Copyright © 2011 John Wiley & Sons, Ltd.

Role of Land–Water Classification and Manning's Roughness Parameter in Space-Borne Estimation of Discharge for Braided Rivers: A Case Study of the Brahmaputra River in Bangladesh

The proposed Surface Water and Ocean Topography (SWOT) mission will provide global, space-based estimates of water elevation, its temporal change, and its spatial slope for terrestrial water bodies. Using derivations of water slope from the Shuttle Radar Topography Mission (SRTM) elevation data, river bathymetry and Manning's equation, the potential of SWOT for discharge estimation of large braided rivers in humid climates, such as the Brahmaputra river, was found to be promising (Jung , in Earth Surface Processes and Landforms, 2010). In this study we extend the work on assessing SWOT for braided rivers to understand the sensitivity of two river hydraulic parameters to discharge estimation: 1) section factor (AR2/3) derived from land-water classification and in situ river bathymetry and 2) Manning's roughness coefficient. For braided rivers, the first parameter, is intimately dependent on how braided rivers are classified of the multiple channels (water) and in-stream braided bars (land) that consequently dictates the accuracy of wetted perimeter and area of flow estimation from water elevation data. We show that the use of the minimum water elevation data at a river cross section minimizes estimation of section factor which consequently minimizes outlier discharge estimation reported in the Jung study. We also show that by treating roughness coefficient “flexible” as a calibration parameter, discharge estimation from SRTM elevation data can be further improved through trial and error manual optimization. Our sensitivity study illustrates the value of treating section factor and roughness coefficient as calibration parameters for data assimilation systems that use SWOT observables to estimate river discharge in braided rivers.

Depth‐averaged, spatially distributed flow dynamic and solute transport modelling of a large‐scaled, subtropical constructed wetland

AbstractConstructed wetlands are being utilized worldwide to effectively reduce excess nutrients in agricultural runoff and wastewater. Despite their frequency, a multi‐dimensional, physically based, spatially distributed modelling approach has rarely been applied for flow and solute transport in treatment wetlands. This article presents a two‐dimensional hydrodynamic and solute transport modelling of a large‐scaled, subtropical, free water surface constructed wetland of about 8 km2 in the Everglades of Florida, USA. In this study, MIKE 21 was adopted as the basic model framework. Field monitoring of the time series hydrological and chloride data, as well as spatially distributed data such as bathymetry and vegetation distribution, provided the necessary model input and testing data. Simulated water level profiles were in good agreement with the spatio‐temporal variations of measured ones. On average, the root‐mean‐square error of model calibration on annual water level fluctuations was 0·09 m. Manning's roughness coefficients for the dense emergent and submerged aquatic vegetation areas, which were estimated as a function of vegetation type, ranged from 0·67 to 1·0 and 0·12 to 0·15 s/m1/3, respectively. The solute transport model calibration for four monitoring sites agreed well with the measured annual variations in chloride concentration with an average percent model error of about 15%. The longitudinal dispersivity was estimated to be about 2 m and was more than an order of magnitude higher than the transverse one. This study is expected to play the role of a stepping stone for future modelling efforts on the development and application of more advanced flow and transport models applicable to a variety of constructed wetland systems, as well as to the Everglades stormwater treatment areas in operation or in preparation. Copyright © 2010 John Wiley & Sons, Ltd.

Sensitivity of the hydrological response to the variability of rainfall fields and soils for the Gard 2002 flash-flood event

Summary In the general context of field experiment design, this paper presents a modeling study that quantifies the respective impact of rainfall estimation and soil variability on the simulated discharge for an extreme event in southern France. The CVN distributed hydrological model, built within the LIQUID® modeling platform is used. The method is illustrated for two medium sized catchments, Saumane (99 km2) and Uzes (88 km2) using raingauges and two radar estimates. The soil properties are extracted from an existing soil database provided for the whole region. The model parameter specification uses available observation and a priori hydrological knowledge. No parameter adjustment is performed. For model evaluation on the regional scale, simulated maximum peak discharges are compared with post-flood estimations for 32 catchments. The area of these catchments ranges from 2.5 to 99 km2 and model results are satisfactory. Then, the study focuses on the Saumane and Uzes catchments. A sensitivity analysis highlights the role of the Manning roughness coefficient on the simulated hydrographs dynamics. The impact of the bottom boundary condition of the infiltration and water redistribution module is also shown for the gauged Saumane catchment. Then the impact of rainfall input and soil spatial variability is presented. The results show that (i) the use of radar data is necessary to properly simulate the flood dynamics; (ii) although radar volume-scanning strategy has been shown to give more accurate results on a pixel/gauge comparison of the rainfall estimations, it is not necessarily the case when catchment averaged amounts are considered, especially for catchments in mountainous areas; (iii) the impact of the variability in soil properties on the simulated discharges is of the same order of magnitude as the impact of differences in rainfall estimation; (iv) the flood dynamics presents two phases: the first one, mainly controlled by the soil properties and the second one, since the soils are saturated, controlled by the rainfall variability. Therefore, uncertainties on both observations need to be mitigated in order to improve flash-flood understanding.

ESTIMATION OF MANNING ROUGHNESS COEFFICIENTS ON PRECAST ECOLOGICAL CONCRETE BLOCKS

Agricultural irrigation and drainage canals in Taiwan are generally constructed using concrete. Concrete is used for the safety of conveyance efficiency. It also reduced loss and is structurally safe. The irrigation ditches and soil revetments in Taiwan fitted with a strong concrete lining. The smooth nonporous concrete surface makes the biology perch difficult and causes a monotonous flow. For these reason, an artificial materials-precast ecological concrete blocks (PECBs) was developed in this research. This was done in order to modify both the traditional concrete lining and the natural field materials in canal construction. Precast ecological concrete blocks (PECBs) were tested by hydraulic model experiments. These experiments used four kinds of height of projection under the condition of unaffecting conveyance efficiency. The experimental results show that the Manning roughness coefficients of PECBs, in a range of are greater than that of piling up cobblestone of concrete as side protection. The results also indicate that the Manning roughness coefficients are in inverse proportion to the discharges and in proportion to the bed slopes. The relationship between the roughness coefficient and surface roughness were estimated after regression analysis. The analysis of hydraulic characteristics and estimation of the Manning roughness coefficients, through this hydraulic modeling experimental study, can be used as a reference for the improvement in construction of irrigation and drainage canals.

Parameter Importance and Uncertainty in Predicting Runoff Pesticide Reduction with Filter Strips

Vegetative filter strips (VFS) are an environmental management tool used to reduce sediment and pesticide transport from surface runoff. Numerical models of VFS such as the Vegetative Filter Strip Modeling System (VFSMOD-W) are capable of predicting runoff, sediment, and pesticide reduction and can be useful tools to understand the effectiveness of VFS and environmental conditions under which they may be ineffective. However, as part of the modeling process, it is critical to identify input factor importance and quantify uncertainty in predicted runoff, sediment, and pesticide reductions. This research used state-of-the-art global sensitivity and uncertainty analysis tools, a screening method (Morris) and a variance-based method (extended Fourier Analysis Sensitivity Test), to evaluate VFSMOD-W under a range of field scenarios. The three VFS studies analyzed were conducted on silty clay loam and silt loam soils under uniform, sheet flow conditions and included atrazine, chlorpyrifos, cyanazine, metolachlor, pendimethalin, and terbuthylazine data. Saturated hydraulic conductivity was the most important input factor for predicting infiltration and runoff, explaining >75% of the total output variance for studies with smaller hydraulic loading rates ( approximately 100-150 mm equivalent depths) and approximately 50% for the higher loading rate ( approximately 280-mm equivalent depth). Important input factors for predicting sedimentation included hydraulic conductivity, average particle size, and the filter's Manning's roughness coefficient. Input factor importance for pesticide trapping was controlled by infiltration and, therefore, hydraulic conductivity. Global uncertainty analyses suggested a wide range of reductions for runoff (95% confidence intervals of 7-93%), sediment (84-100%), and pesticide (43-100%) . Pesticide trapping probability distributions fell between runoff and sediment reduction distributions as a function of the pesticides' sorption. Seemingly equivalent VFS exhibited unique and complex trapping responses dependent on the hydraulic and sediment loading rates, and therefore, process-based modeling of VFS is required.

1차원 부정류 모형을 활용한 낙동강 하류의 홍수 특성

Rainfalls would increase the discharges or stages of tributary channels in natural watersheds, which in turn augment the magnitude of main stream stages. Rising of water surface elevation in main streams can affect and damage the human activities because of the possibilities of the breakdown or overflow of the embankment. Therefore it is necessary to establish the structural or non-structural alternatives for the sake of prevention or treatment of those disasters. Many mathematical models to analyze the flood flows in natural watercourses have been proposed as the non-structural alternatives so far. In this study one of the such models, FLDWAV developed by NWS(National weather Service), is applied to the downstream reach of Nakdong river. Model calibration is performed on various Manning's roughness coefficients at the gauging stations. The simulation results are compared well with hydrological estimations of flood discharges considering the effects of multipurpose dams upstream of control points.

Modelling intercrop management impact on runoff and erosion in a continuous maize cropping system: Part I. Model description, global sensitivity analysis and Bayesian estimation of parameter identifiability

SummaryIn order to evaluate the long‐term impact of intercrop management on runoff and erosion in a continuous maize cropping system a new model was developed, based on the continuous plot‐scale runoff model of Laloy & Bielders (2008). This model is called the Continuous Runoff and Erosion Hillslope model with DYnamic soil Surface properties (CREHDYS). This paper details the erosion and crop growth and decay model components and presents a thorough global variance‐based sensitivity analysis (GSA) with regard to the erosion prediction, followed by a Bayesian parameter identifiability assessment. As compared with the classical local univariate sensitivity analysis, a GSA is able to deal with the typical non‐linearity of process‐based hydrological and erosion models. The most influential parameters were the Manning's roughness coefficient followed by the saturated hydraulic conductivity of wheel track cells and the median particle size of the material. The soil aggregate stability and soil‐cohesion parameters were found to be almost non‐influential. Most of the results of the parameter identifiability procedure were in close agreement with the GSA. Indeed, the parameter uncertainty seemed to be proportional to the degree of influence, with Manning's coefficient being the most precisely identified, whereas soil and wheel track cohesion parameters showed the largest uncertainty. Exceptions were the soil aggregate stability and the Green‐Ampt soil matric potential. The uncertainty associated with the former was surprisingly low given its low level of influence whereas the uncertainty associated with the latter is partially explained by its negative correlation with the soil saturated hydraulic conductivities of overland flow and wheel track cells.

Determination of the Manning roughness coefficient influenced by vegetation in the river Aa and Biebrza river

The aim of this study was to investigate the variation of channel bed roughness in two rivers, as important parameter in hydraulic modelling especially with regard to flood control. The universities of Ghent (UG) and Antwerp (UA) are conducting scientific research in the river Aa in Belgium and the Biebrza river in Poland in order to better understand the phenomena involved and to come to a more accurate determination of the different parameters influencing flow. In this paper, the determination of the roughness coefficient ‘n’ from the Manning equation is used. This coefficient is not easy to determine and is varying constantly. It is influenced by the meandering character of the river, the bed material and the average grain size, the channel bed forms, the channel obstructions, the geometry changes between sections and the vegetation in the channel. Furthermore, due to these parameters, the roughness of the channel is not equally distributed over the channel, the banks and the floodplains. So, using literature data does not always lead to satisfactory results, due to the different situation in the field (Werner et al. J Hydrol 314:139–157, 2005). Therefore, measurements are necessary to determine the variation of the Manning coefficient. The Manning coefficient is a function of the discharge, but will also vary over the time due to the mentioned influences. In a multidisciplinary research project on the fundamental exchange processes in river ecosystems, hydraulic measurements were performed on a regular base in the river Aa. During these measurement campaigns, velocity and discharge measurements were carried out in multiple cross-sections. Once a month, the discharge and the water levels were measured at the upstream and the downstream end of the test stretch. On the river Biebrza, similar intensive measurement campaigns took place along a 6 km stretch in the upstream part of the river. An accurate determination of the Manning coefficient according a seasonal variation is an important tool in hydraulic modelling.

On the use of the diffusive wave for modelling extreme flood events with overbank flow in the floodplain

Summary Catastrophic flood events are frequently chosen as reference events, for modelling the impact of scenarios of river bank remediation to prevent inundations. However, these rare events are poorly understood due to the poor hydrological knowledge available, and due to the lack of data. A model for routing flood events adapted to the few available data during historical extreme events was developed. The model subdivides the floodplain into cells linked to the main channel, and the diffusive wave model with overbank flow was used to route floods in the main channel and through the inundation floodplain. The linearized equations are resolved by an implicit numerical scheme to ensure stability and convergence. The flood event of October 1958 on the Herault river, which caused one of the largest inundations Southern France during the XXth century, was studied as the application case. As input data, the model requires a Digital Elevation Model, geographical map and cross sections in order to subdivide the channel network into cells. The model parameters are the Manning roughness coefficients in the main channel and in the floodplain. The model was applied to simulate the flood event of 1958, and was validated on other flood events under similar hydro-meteorological conditions. Then, the model was applied to quantify the impact of scenarios of river bank remediation. Fourteen scenarios of construction of dams, embankments, or coupling both dams and embankments, were modelled. The model simulates hydrographs in the main channel, the water depth, the area and the volume on both right and left banks of the floodplain cells. Results show that the model developed herein, with relatively little hydraulic modelling, is well adapted to simulate extreme flood events, and is useful for environmental managers to study the impact of river bank remediation on inundations.

Application of the Soil and Water Assessment Tool (SWAT) to predict the impact of alternative management practices on water quality and quantity

Alternative land management practices such as conservation or no-tillage, contour farming, terraces, and buffer strips are increasingly used to reduce nonpoint source and water pollution resulting from agricultural activities. Models are useful tools to investigate effects of such management practice alternatives on the watershed level. However, there is a lack of knowledge about the sensitivity of such models to parameters used to represent these conservation practices. Knowledge about the sensitivity to these parameters would help models better simulate the effects of land management. Hence, this paper presents in the first step a sensitivity analysis for conservation management parameters (specifically tillage depth, mechanical soil mixing efficiency, biological soil mixing efficiency, curve number, Manning's roughness coefficient for overland flow, USLE support practice factor, and filter strip width) in the Soil and Water Assessment Tool (SWAT). With this analysis we aimed to improve model parameterisation and calibration efficiency. In contrast to less sensitive parameters such as tillage depth and mixing efficiency we parameterised sensitive parameters such as curve number values in detail. In the second step the analysis consisted of varying management practices (conventional tillage, conservation tillage, and no-tillage) for different crops (spring barley, winter barley, and sugar beet) and varying operation dates. Results showed that the model is very sensitive to applied crop rotations and in some cases even to small variations of management practices. But the different settings do not have the same sensitivity. Duration of vegetation period and soil cover over time was most sensitive followed by soil cover characteristics of applied crops.

Application of WEPP Hydrologic Simulation Model for Prediction of Rainfall and Runoff from Hilly Watersheds in Meghalaya

In the present study, agriculture on the bench-terraced land was considered as one of the main alternatives to shifting cultivation. Considering the fact that most of the watersheds in the area are ungauged and sufficient database is lacking, modelling approach was adopted to quantify the soil erosion and runoff from the fields. Water Erosion Prediction Project (WEPP)-Watershed Model ver: 2002 was selected to describe the erosion processes on hill slopes considering its state-of-the-art technology. The model was reasonably calibrated based on a comparison between measured and simulated runoff for three land uses viz; shifting cultivation, agriculture with conservation measures and undisturbed natural vegetation. Calibrated model was run for event-based prediction of runoff and soil loss. The model over-predicted the small values of runoff and under-predicted the large values of runoff. Sensitivity analysis indicated that runoff was very sensitive to the Manning's roughness coefficient, initial saturation level of the soil and effective hydraulic conductivity. Nash-Sutcliffe model efficiency (ME) and Root Mean Square Error (RMSE) ranged from 65 to 76% and 51.3 to 63.9, respectively, for all the three land uses. The conservation measures helped in reducing the sediment yield from 36.6 kg.ha-1. mm-1 of rainfall from shifting cultivation field to 8.6 kg.ha-1 .mm-1. It can be inferred that the WEPP model can be used with reasonable accuracy, to simulate different management practices on hill slope for development and management planning of a hilly watershed.

Case Study: Hydraulic Modeling of Runoff Processes in Ghanaian Inland Valleys

The inland valleys of West Africa are strategic in terms of food security and poverty alleviation, but scientific studies on hydrologic processes happening in these environments have not been well documented. Modeling approaches presented in this paper are an attempt to better comprehend hydraulic phenomena occurring in inland valleys. An inland valley situated in the Northern Region of Ghana is set as the study site. The inland valley comprises well-drained uplands and hydromorphic valley bottoms. There are several earthen dams across the valley bottoms, which are at the same time seasonal wetlands cultivated to rice during the rainy season. A finite volume model for the shallow water equations is developed to numerically simulate surface runoff flows in the valley bottoms during flood events. Innovation is necessitated to handle a series of different hydraulic phenomena. Flux-splitting and data reconstruction techniques are used to achieve stable computation in the complex topography of the valley bottoms. Standard problems of oblique hydraulic jump and dam break flows are used to test the accuracy of the numerical model. The Manning's roughness coefficient is determined from calibration in another Ghanaian watershed located in the Eastern Region. Using actually observed time series data of rainfall intensity, surface flows during the rainfall events are simulated in the computational domain representing the valley bottoms of the study area. Observed data of water levels in the dams are compared to predictions, and discrepancies between them are examined from the hydrological point of view. In the case of a hypothetical flood event, cascading collapses of the dams and flooding of cultivated fields are reproduced.

Simulating short‐circuiting flow in a constructed wetland: the implications of bathymetry and vegetation effects

AbstractShort‐circuiting flow, commonly experienced in many constructed wetlands, reduces hydraulic retention times in unit wetland cells and decreases the treatment efficiency. A two‐dimensional (2‐D), physically based, distributed modelling approach was used to systematically address the effects of bathymetry and vegetation on short‐circuiting flow, which previously have been neglected or lumped in one‐dimensional wetland flow models. In this study, a 2‐D transient hydrodynamics with advection‐dispersion model was developed using MIKE 21 and calibrated with bromide tracer data collected at the Orlando Easterly Wetland Cell 7. The estimated topographic difference between short‐circuiting flow zone and adjacent area ranged from 0·3 to 0·8 m. A range of the Manning roughness coefficient at the short‐circuiting flow zone was estimated (0·022–0·045 s m−1/3). Sensitivity analysis of topographical and vegetative heterogeneity deduced during model calibration shows that relic ditches or other ditch‐shaped landforms and the associated sparse vegetation along the main flow direction intensify the short‐circuiting pattern, considerably affecting 2‐D solute transport simulation. In terms of hydraulic efficiency, this study indicates that the bathymetry effect on short‐circuiting flow is more important than the vegetation effect. Copyright © 2009 John Wiley & Sons, Ltd.

Application of a Routing Model for Detecting Channel Flow Changes with Minimal Data

The discrete linear cascade model DLCM was applied for historical flow routing along the Nebraska City-Rulo section of the Missouri River in southeastern Nebraska. With the help of optimized model parameters it has been possible to identify the triggering mechanism responsible for historical changes in the stage-discharge relationship at Rulo over the 1952-2006 period, following construc- tion of Gavins Point dam on the Missouri above Sioux City, Iowa. It was found that in the second part of the past century flood celerity in the study reach slowed by about 25%, most likely caused by an increase in the Manning roughness coefficient as a result of a large increase almost a doubling in the number of wing-dykes constructed over the reach within the same period. The ease of application and minimal data requirement only discharge values at regular intervals makes the DLCM a practical tool for stream-flow analysis. It also can serve as a preliminary investigative tool for more advanced and detailed hydraulic approaches that typically require a data-rich environment and significantly greater development time.