Meandering Compound Channel Research Articles

Experimental Observations of Floodplain Vegetation, Bedforms, and Sediment Transport Interactions in a Meandering Channel

AbstractFloodplains provide important ecological, hydrological, and geomorphic functions within river corridors. During overbank flows, complex hydrodynamic conditions occur as water exits and re‐enters the channel and interacts with hydraulically rough floodplain vegetation. However, the extent to which floodplain vegetation influences channel‐altering hydrodynamic forces and thus bedform topography and sediment transport is poorly understood. We address this knowledge gap and present the results of flume experiments where we measured bedform topography under varied floodplain vegetation conditions at two overbank flow relative depths. The experiments were conducted in a 1‐m wide meandering compound channel inset in a 15.4 long, 4.9‐m wide basin. The channel bed was a mobile sand‐and‐gravel mixture with a median sediment size of 3.3 mm, and sediment transport occurred only within the channel. We tested bare and vegetated floodplain conditions with 2.7‐cm diameter rigid emergent vegetation elements at spacings of 3.0 and 12.1 units m−2. We performed a moving‐window analysis of topographic surface metrics including skewness, coefficient of variation, and standard deviation, as well as topographic patch analysis of area and contagion to measure changes in bedform heterogeneity as flow depth and vegetation density were varied. Our results show that both greater density vegetation and larger flows can increase bedform topographic heterogeneity. These findings suggest that floodplain vegetation and natural hydrologic regimes that include overbank flows can enhance stream habitat complexity. Designing for the effects of established vegetation conditions and prioritizing floodplain vegetation planting may be useful for river managers striving to achieve successful biomic river restoration.

The effect of building arrangement on the flow characteristics in meandering compound channels

Due to the fact that most of the cities and villages are built on the banks of the rivers, the hydraulic characteristics of water flow always undergo changes during floods, so it is necessary to study the interaction of the flood flows of meandering compound channel in these areas. In this paper, the effect of building arrangement on the flow characteristics in meandering compound channels was investigated by using laboratory models. For this purpose, a smooth floodplain (without building) and three different types of building arrangement on the floodplain including structural obstacles perpendicular and parallel to the floodplain flow and checkered structural obstacles have been used. The tests have been conducted at three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. All experiments were performed under a quasi-uniform and fully developed flow condition. The measurement data were recorded at different sections along the meandering main channel. Stage-discharge curves, Manning's n, velocity distribution, boundary shear stress and Darcy-Weisbach friction factor are determined over 12 test runs. The results of this research show that the arrangement of buildings has a great effect on the longitudinal velocity of the main channel. At a relative depth of 0.49, in a meandering compound channel with the arrangement of structural obstacles perpendicular to the floodplain flow, the dimensionless longitudinal velocity of the main channel compared to the smooth floodplain increases by 113%, but with the arrangement of structural obstacles parallel to the floodplain flow, the dimensionless longitudinal velocity increases by 27%. Also, the building arrangement is effective in reducing the discharge capacity of the meandering compound channel, so that in a building density of 18.5%, structural obstacles perpendicular to the floodplain flow reduce the discharge capacity of the channel by 50% compared to the smooth floodplain. In the channel with structural obstacles arranged perpendicular to the floodplain flow, Manning's roughness coefficient increases sharply with increasing the relative depth, but in the channel with the structural obstacles parallel to the floodplain flow, Manning's roughness coefficient decreases with increasing the relative depth. In the channel with the arrangement of structural obstacles perpendicular to the floodplain flow, the bed shear stress of the main channel is higher than the other building arrangements, so that at the relative depth of 0.49, the average maximum bed shear stress in all sections increases by 42% compared to the smooth floodplain.

Experimental assessment of velocity and bed shear stress in the main channel of a meandering compound channel with one-sided blocks in floodplain

Meandering rivers have an important role in transporting runoff from rainfall, especially during floods. With many cities and villages along rivers and the construction of water-management structures in floodplains, the flow pattern of the meandering rivers changes in time of floods. Here the flow behaviors in meandering compound channels with smooth floodplains and one-sided building floodplains are investigated using laboratory experiments. The experiments have been performed at three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. The results show that the block density in a floodplain has a significant effect on flow behavior in the main channel. So that, with the increase of the relative depth, the ratios of main channel discharge compared to the total discharge (Qmc/Q) decreases in the bends with minimal extension of blocks floodplain, the crossover areas and the bends with maximal extension of blocks floodplain. For a given Dr, the significant increase in the Manning's n values as the density of the blocks increases (approximately 30%). With increasing the blocks density from zero to 23.7%, the maximum Darcy–Weisbach friction factor in sections CS1 and CS7 increases by 110% and 2%, respectively, which shows that increasing the density of blocks does not have a great effect on f in the section CS7. Under one-sided blocked floodplain conditions, the maximum longitudinal velocity in the main channel in the section with maximal extension of the blocks in the floodplain (CS7) is greater than the section with minimal extension of blocks in the floodplain (CS1). At the relative depth of 0.49, the maximum value of depth-averaged velocity in sections CS1-4 decreased by an average of 52% compared to the smooth floodplain. The maximum shear stress on the bed at the apex section for all channels occurs near the convex side and the bed shear stress at all sections decreases with decreasing relative depth and in the section with minimal extension of blocks floodplain (CS1) and cross-over sections. The maximum shear stress of the bed decreases with increasing the block density and these changes are more near the convex side than the concave side.

Closure to “Determination of Discharge Distribution in Meandering Compound Channels Using Machine Learning Techniques” by Abinash Mohanta, Arpan Pradhan, and K. C. Patra

Closure to “Determination of Discharge Distribution in Meandering Compound Channels Using Machine Learning Techniques” by Abinash Mohanta, Arpan Pradhan, and K. C. Patra

Discussion of “Determination of Discharge Distribution in Meandering Compound Channels Using Machine Learning Techniques” by Abinash Mohanta, Arpan Pradhan, and K. C. Patra

Discussion of “Determination of Discharge Distribution in Meandering Compound Channels Using Machine Learning Techniques” by Abinash Mohanta, Arpan Pradhan, and K. C. Patra

Numerical study of flow characteristics in compound meandering channels with vegetated floodplains

Large eddy simulations were conducted to simulate the flow in compound meandering channels whose main channel sinuosity was 1.381. Then, the floodplain vegetation was generalized using the momentum equation coupled with the drag force formula. The mean flow pattern, secondary flow, coherent structure, turbulence characteristics, and lateral mass and momentum transport with and without floodplain vegetation with relative depths (Dr) of 0.3–0.5 were studied. Results showed that the floodplain vegetation enabled the flow of the main channel to be more concentrated. The maximum average velocity in the cross section of the main channel increased by 100% and 30% when the relative depth was 0.3 and 0.5. Under the influence of floodplain vegetation, the secondary flow cell transformed greatly with the change in relative depth. When Dr < 0.3, the vegetation caused the vortex center of the secondary flow to move closer to the concave bank side, and the secondary flow distribution presents a flow pattern not flooding the floodplain. When Dr > 0.3, the spatial change in the secondary flow was not obvious. In addition, the floodplain vegetation did not change the large-scale vortex that was separated from the boundary layer of the convex bank side. Meanwhile, the floodplain vegetation increased the overall turbulence intensity, turbulent kinetic energy, and Reynolds stress of the main channel, and it increased the range of lateral mass exchange of the inbank flow and the mean and turbulent transport flux of each cross section.

Validation of Computational Fluid Dynamics Approach of Lateral Velocity Profile Due to Curvature Effect on Floodplain Levee of Two-stage Meandering Channel

Rivers play an integral part in the sustenance of life on earth. The river or natural channel’s sinuous nature causes erosion of its outer wall and silt deposition on its inner wall. These changes in the terrain make it vital to design the meandering compound channel. Levees, both straight and meandering, can be constructed on both sides of the channel to guard the surrounding lands. For doubly meandering levee alignment, the momentum transfer between the main channel and the adjoining floodplains affects the velocity distribution across the channel cross-section. So, hydraulic engineers must investigate this complex flow phenomenon in the case of a doubly meandering compound channel. This study investigates the flow in meandering compound channels using numerical and physical modelling, both of which are important in understanding non-uniform flow and its behaviour. As a result, a research is presented on the sharing of velocity over the cross section of the main channel and the floodplains of meandering compound channels. The geometry of meandering compound channels, having straight as well as sinuous floodplain levees, is chosen for this study. Unlike the experimental models, numerical hydraulic models are remarkably economical. So, in this research, the numerical hydraulic model is adopted to determine the different flow characteristics of compound meandering channels. In this paper, a complete three-dimensional and two-phase Computational Fluid Dynamics (CFD) model of the meandering compound channels is analyzed making use of the RNG K-ɛ turbulence model as well as Volume of Fluid (VOF) methodology. A comparison study between numerical results and experimental results are represented, considering the two different experimental data of compound meandering channel having the main channel meandered with sinuosity (ratio of channel length to the down valley length) of 1.37 and were flanked by straight floodplains as well as meandering floodplains having sinuosity 1.06 on both sides. By error analysis, it is observed that the numerical results agree with that of the experimental ones.

An experimental evaluation of the blocks in floodplain on hydraulic characteristics of flow in a meandering compound channel

• Flow behavior in meandering channels with 2-sided blocks in floodplains examined experimentally. • The presence of blocks in floodplains significantly enhances Q mc / Q ). • In cases with blocks densities of 6.2% and less, the values of Manning's n decrease by increasing Dr. • For all cases of blocks densities, the maximum bed shear stress occures near the convex side of CS1. • With high blocks density, apparent lateral velocity gradients seen in central region of CS3-CS5. In this research, the main flow behaviors in the meandering compound channels with different densities of two side blocks on the floodplains were investigated by using laboratory experiments at the three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. The results show that the blocks density on the floodplain has a significant effect on the flow behavior in the main channel. So that the smooth channel efficiently conveys 100% more water on average than that conveyed in the case with maximum blocks density. For the case with smooth floodplains, the Q mc /Q ratios of main channel discharge to total discharge increases continuously by reducing the flow depth, reaching 47% for Dr = 0.29 (Dr is the relative depth). By contrast, Q mc /Q in the channel with maximum blocks density peaks at 70% (>50%), showing that the main channel controls the capacity of compound channel conveyance because the blocks occupy most of the cross-sectional area of the floodplain when Dr = 0.29. In cases with blocks densities of 6.2% and less in floodplain, the Manning n decreases by increasing the relative depth (Dr). In cases with blocks densities of 9.7% and 23.7%, the Manning n increases by increment the relative depth. In addition, in cross-over sections of the case with smooth floodplains, a steep gradient of velocity in the vertical direction appears from the dense contour lines around the bankfull level near the right-hand sidewall indicating that a considerable effect occurs from the upstream flow. Instead, for channel with maximum blocks density, an apparent lateral velocity gradient was located in the central region in cross-over sections. The maximum shear stress of the bed at the apex section for all channels occurs near the convex side and the amount of bed shear stress at all sections decreases with reducing the relative depth. In the cross-over sections, due to the increase of the flow angle between the main channel and the floodplain, the effect of the secondary flow is stronger and the flow velocity decreases more, so the Darcy–Weisbach coefficient increases more in this area.

Experimental study on flow mechanisms in a sine-shaped meandering compound channel

Hydrodynamic processes in compound channels are closely related to the cross-sectional geometry of the river. The cross-sectional shape of natural meandering rivers is always irregular, with a curved main channel (MC) and one or two floodplains (FPs), and most are confined within levees. The geometric shape of the cross-section of such rivers changes greatly in a flood, which influences the exchange of momentum and energy between the MC and FPs. Many previous studies have focused on flow mechanisms in compound meandering channels of relatively regular and simple cross-section, which is not representative of natural rivers. An experimental study of a sine-shaped meandering compound channel of irregular shape and with levees was thus conducted. Using detailed experimental data, the development of streamwise flow, secondary flow and turbulent shear stresses over the cross-sections were analysed. Special emphasis was placed on the momentum exchange between the MC and FPs. The results showed that flow transport in the MC plays a leading role and the momentum exchange between the MC and FPs is more intensive for a high water level. The formation, development and decay of secondary flow in compound meandering rivers are closely related to the momentum exchange between the FPs and the MC. Finally, transverse flow plays a dominant role in the process of momentum exchange and the effects of turbulence are negligibly small.

Experimental and numerical analysis of velocity distribution in a compound meandering channel with double layered rigid vegetated flood plains

Vegetation is one of the major topographic features that is encountered along and across the margins and flood plains of many rivers systems. This vegetation creates a most complex flow mechanism in the compound river bed channels; therefore, a detailed analysis is required to observe the flow and vegetation interactions to understand the hydrodynamic aspects in the river systems. This paper studies the effect of double-layered rigid vegetation in a meandering channel on the flow characteristics at two relative depth conditions, of 0.34, 0.45 which creates an alternate emergent and submerged flow situation. The three-dimensional velocity distribution was captured using micro-ADV. The concept of two relative depth conditions allowed us to capture record and classify the velocity zones between the short and tall vegetation. Compared to the flow in the main channel, flood plains registered relatively lower velocity values due to the resistance offered by the vegetation along the flood plains, which consequently led to the increase in main channel flow velocities. Velocities compared to the non-vegetated meandering channels; the highest velocity readings were recorded at the centerline of the main channel. Numerical analysis was also conducted using the CFD codes in fluent. The vegetation geometry is modelled as cylindrical dowels of diameter 10 mm and two-variable heights of 7.5 cm and 15 cm at two relative depth flow conditions. The experimental results were numerically compared using the k-ϵ model along with grid sensitivity tests. The final simulated numerical results were found to be close and in good agreement with experimental values.

The Effect of Building Arrangement on the Flow Characteristics in Meandering Compound Channels

The Effect of Building Arrangement on the Flow Characteristics in Meandering Compound Channels

Assessment of Shear Stress Distribution in Meandering Compound Channels with Differential Roughness Through Various Artificial Intelligence Approach

Assessment of Shear Stress Distribution in Meandering Compound Channels with Differential Roughness Through Various Artificial Intelligence Approach

Influence of Floodplain Flooding on Channel Siltation Adjustment under the Effect of Vegetation on a Meandering Riverine Beach

Flooding in a sediment-laden floodplain is affected by riverine beach vegetation and the shape of a meandering compound channel. The laws of water and sediment exchange and the deposition distribution in beach troughs are very complex. These factors play a significant role in the formation and development of secondary suspended rivers, in the adjustment of the beach horizontal gradient, and even in the evolution of the flood control situation. In this study, we used a combination of experimental simulation and theoretical research to carry out a generalized model test of floodplain flooding evolution, analyzed the lateral distribution characteristics of sediment-laden flow and sediment factors in a meandering compound channel under the conditions of beach vegetation, and revealed the pros and cons of beach vegetation on the adjustment of the beach and channel siltation. The model test results of the flooding in the floodplain in the compound channel with meandering vegetation showed that the main stream was not only concentrated in the main channel but also appeared near the foot of the left and right bank levees and formed flood discharges along the embankment. As the riverine beach siltation was mainly concentrated at the riverine beach lip, the vegetation on the riverine beach had a significant effect on slowing down the flow velocity. Whether it was a row or full vegetation on both sides of the bank, this played an important role in the stability of the main channel. When there was no vegetation on the riverine beach, the main channel was easy to move. The arrangement of full vegetation on the riverine beach had a uniform effect on the velocity distribution of the riverine beach, which reduced the phenomenon of excessive velocity at the foot of the riverine beach and increased the velocity effect in the main channel. These results will provide a theoretical basis for the utilization of riverine beach areas and river management in the lower Yellow River and have a great significance for enriching the basic theory of water and sediment movement and promoting the integration of hydraulics, river dynamics, and ecology.

Gene-expression programming for calculating discharge in meandering compound channels

Computation of flow in natural channels is the key to the solution of numerous engineering problems particularly at the time of flood. Therefore, different approaches have been incorporated for accurate estimation of flow discharge in compound meandering channels. The present work focuses on developing an empirical model for estimating the flow of meandering compound channels using gene-expression programming (GEP) by considering four non-dimensional parameters viz Relative Depth, Sinuosity, Coherence Parameter, and Discharge Ratio (Qvdm/Qbf). The model developed for the meandering compound channels is best suited for sinuosity in the range of 1.04–2. To measure adequacy of the model, the results of the developed model are compared with various discharge estimation methods such as the Single Channel Method (SCM), Divided Channel Method (DCM), Meander Belt Method (MBM), Modified Divided Channel Method (MDCM), Interacting Divided Channel Method (IDCM), and Coherence Method (COHM). Results indicate that the proposed GEP model predicted the ratio of total discharge in compound channel (Qt) to the bank-full discharge (Qbf) in main channel satisfactorily with root mean square error of 0.15 and 0.19 for training and testing dataset, respectively. The developed model is also used to validate with higher sinuous channels having sinuosity exceeding 2.0. Finally, the model was successfully validated with natural flood conditions of River Baitarani, India (sinuosity of 1.334) giving the minimum errors in terms of mean absolute square error of 0.885 and 0.882 for the two overbank flow depths of 7.5 and 8.63 m, respectively.

Enhanced Channel Division Method for Estimation of Discharge in Meandering Compound Channel

Accurate prediction of shear force distribution along the boundary in open channels is a key to the solution of numerous hydraulic problems. The problem becomes more complicated for meandering compound channels. A model is developed for predicting the percentage of shear force at the floodplain (%Sfp) of two-stage meandering channels using gene-expression programming (GEP) by considering five dimensionless parameters viz. the width ratio, relative depth, sinuosity, bed slope, and meander belt width ratio as the inputs in the model. Basing on the %Sfp, the apparent shear force along the division lines of separation in compound channels is selected for discharge calculation using the conventional channel division methods. An Enhanced Channel Division Method (ECDM) is introduced to calculate discharge by assuming interface line at main channel and floodplain junction. A modified variable-inclined (MVI) interface is suggested having zero apparent shear determined from flow contribution in the main channel and floodplain. The MVI interface is further used to calculate discharge in the meandering compound channels. Performance of the GEP model is tested against other analytical methods of calculating %Sfp. Error between the observed and calculated discharges using the MVI interface is found to be the minimum when compared to other interface methods. The enhance channel division method is successfully applied for validating the two available overbank discharge values for the river Baitarani at Anandapur (drainage area of 8570 sq. km), giving the minimum errors of 0.31% and 1.02% for flow depths of 7.5 m and 8.63 m, respectively.

Discharge estimation in wide meandering compound channels

ABSTRACT A model for stage–discharge relationship in two-stage wide meandering channels, devised by statistical analysis of new experimental boundary shear data and some published shear data, is presented. By evaluating the measured boundary shear percentage on floodplains of unequal length on either side along with corresponding wetted area percentage for sinuous channels, it is seen that different but particular empirical relationships exist between shear percentage and percentage flow area due to various flow mechanisms occurring on floodplains lying on either side of the bend. The empirical relationships obtained for left floodplain shear is an exponential function while that for total floodplain is a power function. Expressions for average zonal velocity and hence for zonal discharge are then estimated by 1D analysis to compute the H–Q rating curve for such channels. This method is validated through its application to different published data sets with discharge prediction error lying close to ±10% on most occasions. The method when compared with some other well-known 1D approaches is found to perform reasonably well as evident from error analysis. The method when tested for predicting flow distribution, the error percentage is again within 10% for available limited data series.

Velocity distribution characteristics in meandering compound channels with one-sided vegetated floodplains

To discuss the effects of asymmetrically vegetated floodplains on the velocity distribution characteristics and discharge capacity of a meandering main channel, in this paper, continuous river bend models with smooth floodplains and one-sided vegetated floodplains were designed in a generalized way. Three-dimensional instantaneous velocity data were obtained with an Acoustic Doppler Velocimeter (ADV), and the velocity distribution characteristics and discharge capacities of the generalized models were compared. Compared with the smooth floodplain conditions, the one-sided vegetated floodplain conditions are associated with lower cross-sectional average velocities, lower ratios of main channel discharge to total discharge near the bending apexes with minimal extension of vegetated floodplain, but higher ratios near the bending apex with maximal extension of vegetated floodplain. The deflection degrees of depth-averaged velocities in the meandering main channel are related to the cross-sectional position, relative depth and floodplain vegetation. The depth-averaged velocities near the bending apexes are inclined towards the main channel concave sides, those in the upstream crossover area are generally inclined towards the vegetated side, and those in the downstream crossover area are obviously inclined towards the smooth side. For a given relative depth, the deflection degrees near the bending apexes are smaller than those in the crossover areas. The deflection degrees near the bending apexes with minimal extension of vegetated floodplain are smaller than those under smooth floodplain conditions, but the deflection degrees near the bending apex with maximal extension of vegetated floodplain are obviously larger than the corresponding values under smooth floodplain conditions. Near the bending apexes, the longitudinal depth-averaged velocities are higher close to the main channel convex side and lower adjacent to the concave side. When the relative depth is small, one-sided vegetated floodplain conditions feature larger transverse gradients of the cross-sectional longitudinal depth-averaged velocities near the bending apexes with minimal extension of vegetated floodplain. When the relative depth is large, the transverse gradients near the bending apexes with minimal extension of vegetated floodplain are all much smaller than the corresponding values of the smooth floodplain conditions. However, near the bending apex with maximal extension of vegetated floodplain, there are no significant differences concerning the transverse gradients of the cross-sectional longitudinal depth-averaged velocities under the both floodplain conditions. One-sided vegetated floodplain conditions significantly influence the longitudinal velocity distributions in the meandering main channel. For a given relative depth, the longitudinal velocities at the cross-sectional positions under one-sided vegetated floodplain conditions are lower than those under smooth floodplain conditions to varying extents.

Sudden pollutant discharge in vegetated compound meandering rivers

In this study, the effects of different vegetation densities and different relative flow depths on the longitudinal dispersion coefficient are investigated in a compound meandering channel. Simulated vegetation with three different densities was placed over the floodplain and tracer was released in line simultaneously and equally in the main channel and flood plain. Digital image processing technique was used to measure the tracer concentration along the channel by analyzing a series of sequential images of the tracer cloud. Acoustic Doppler Velocimeter was used to measure 3-D velocity components. The results showed that the depth-averaged longitudinal velocity increases in the main channel due to the presence of vegetation but it declines in the floodplain. The maximum amount of the turbulent kinetic energy and the dimensionless longitudinal dispersion coefficient (K/U⁎H) were observed at the bend apex. Moreover, as the relative flow depth increases, K/U⁎H declines in the compound meandering channel for all the vegetated cases. Additionally, the longitudinal dispersion coefficient increases up to 59% in the main channel and decreases up to 42% in the floodplain by increasing the vegetation density in a specific relative flow depth.

Discharge Estimation at the Apex of Compound Meandering Channels

A new method is proposed to calculate flow in a compound meandering channel by considering the momentum transfer between different zones. Vertical momentum transfer, at the horizontal interface of the lower main channel and the meander belt; and the horizontal momentum transfer, at the vertical interfaces between the meander belt and the adjoining outer floodplains. Modelling of this analytical method for the bend apex section is carried out, for a control volume of unit length. Calibration has been carried out for different laboratory channels and a natural river, demonstrating that the present model is capable of providing an adequate prediction of discharge in experimental as well as in field study. A constant value of 0.01 as the momentum transfer coefficient is verified to act at the interacting interfaces for the meandering channels. The zonal velocity for each subsection is determined, which facilitates prediction of discharge distribution.

Flow and its distribution in wide meandering compound channels

A new stage discharge relationship is suggested for wide meandering streams (ratio of floodplain width and that of main channel in excess of 5.5 and up to 30) with overbank flow. The boundary shear data obtained from various data series with specified range of width ratio and channel sinuosity are analysed. By regression technique, new empirical relationships are derived between the channel dimensions and amount of shear generated for different flow conditions. These new boundary shear models are then used to obtain the stage discharge relation in channels with given roughness characteristics. The present method is then validated for its usefulness by applying the same to a number of data series of small scale and large scale flume experiments of other research projects from the literature. The errors (%) in discharge estimation are within limits for such methods as per practice (i.e. around ±10% for both smooth and rough meandering channels). The method is also compared with a number of other methods from the literature by application to different data series. The RMSE (root mean squared error) value for all the data series validated is estimated as 9.6% and the method is found to be ranked third among all the methods. The method is also able to predict satisfactorily the flow distribution in case of a number of homogeneously smooth and non homogeneously roughened meandering channels of varying sinuosity with overbank flow for large scale flume experimental data sets. The mean error for three available data series is estimated as −3.4% while the RMSE error is 10.68%. Due to its ease of application and simple approach, this 1D method may serve as a preliminary design tool for assessing stage discharge curve for wide two stage channels in field conditions for application in flood estimation and flood warning cum relief measures. Such a preliminary study might better decide the need for further refinement through any exhaustive deep study; the more robust and expensive 2D or 3D analysis.