A /uniFB02oodplain is an area of land close to a stream or river that extends from its banks to the valley walls or high mountains surrounding it. It is frequently inundated during periods of high runo/uniFB00. Along with the main stream, a large volume of /uniFB02ow passes through this /uniFB02oodplain during /uniFB02ood discharge. The term "/uniFB02oodplain /uniFB02ow" refers to this redundant discharge. When studying the hydromorphodynamics of a river from bank to bank, it is necessary to understand the /uniFB02ow distribution throughout the main stream and /uniFB02ood plain. The main stream’s water level and discharge are critical parameters for estimating river bank erosion. As a result, the /uniFB02oodplain may operate as a sediment sink during periods of high /uniFB02ow. River hydraulic and hydrodynamic processes are intricate. There are numerous river modeling tools and approaches available for evaluating the distribution and generation of river /uniFB02ow over /uniFB02oodplain and main-stream. This study examines such two methods for generating river /uniFB02ow over the /uniFB02ood plain and in the main stream. The approaches involve the use of a one-dimensional momentum equation and a two- dimensional modeling tool (Mike/two.pnum/one.pnumC) to generate main-stream and /uniFB02ood plain /uniFB02ow. The Kalni-Kushiyara river is chosen as a case study in order to determine its validity. /one.pnum Introduction A /uniFB02ood plain is a region that is /uniFB02ooded on a regular basis by water coming from a local river or from the river network within a watershed Sarker et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum); Sarker (/two.pnum/zero.pnum/two.pnum/one.pnumb). The majority of /uniFB02oodplains are produced through deposition inside river meanders and overbank /uniFB02ow. There is a knowledge gap on the hydraulics of overbank /uniFB02ows. Researchers now know the most of what we know about water /uniFB02ow in /uniFB02oodplain areas from experimental or statistical studies. The discharge associated with the bank-full stage might well be calculated using a stage-discharge rating curve constructed from collected data. When /uniFB02ood plain con- veyance is substantial in comparison to channel conveyance, the stage discharge rating curve will exhibit a noticeable break at bank-full stage Dottori et al. (/two.pnum/zero.pnum/zero.pnum/nine.pnum); Koussis (/two.pnum/zero.pnum/zero.pnum/nine.pnum); Reitan and Petersen-Øverleir (/two.pnum/zero.pnum/one.pnum/one.pnum); Morlot et al. (/two.pnum/zero.pnum/one.pnum/four.pnum). This arrangement arises because, in alluvial streams, stage is a unique function of discharge Sarker (/two.pnum/zero.pnum/two.pnum/one.pnuma,d,c). As a result, estimating a rating curve is considerably simpler. However, com- plicated scenarios often emerge as a result of the sporadic and unexpected occurrence of /uniFB02oods, especially big ones, the short duration of overbank /uniFB02ows, and the inherent risks associated with taking measurements during /uniFB02ood discharges. Due to the complexity of /uniFB02ow conditions, measurements of /uniFB02ow may vary during severe events. While calculating the hydraulics of /uniFB02ood /uniFB02ows on a real river is very complex, it is important for practical reasons such as assessing a channel’s and /uniFB02oodplain’s conveyance capacity or forecasting /uniFB02ood route. The aim of this article is to provide a practical, precise, and predictable tool for /uniFB02ood /uniFB02ow separation that is based on the momentum equation. /two.pnum Study Area The Kalni River has a substantial /uniFB02ood plain, and bathymetric data are available that are suitable for our techniques. The Kalni-Kushiyara River system starts in India, on the banks of the Barak River. The Barak River /uniFB02ows through Assam, Manipur, and Mizoram in India. The river then enters Bangladesh near Amalshid, where it divides into the south-/uniFB02owing Kushiyara River. The river is known as the Kalni River downstream 1 of Markuli. In a tiny section of the Kalni River, the /one.pnumD and /two.pnumD-momentum techniques are used. Following that, comparisons of these two approaches are discussed. The Kalni-Kushiyara River management project provided all data for this section of the Kalni River (see /uniFB01gure /one.pnum). Figure /one.pnum: Area of study for the Kalni River segment. /three.pnum Methods In this section we aim to explains and formulation of our proposed /one.pnum-D method and the governing equation, that has been adjusted to generate discharge using only water level, cross-sectional, and roughness inputs for a small reach. This technique adopted convective acceleration and a change in the slope of the water surface. The governing equation is numerically solved by discretizing cross-sections relative to the water level (see details in Reza et al. (/two.pnum/zero.pnum/one.pnum/four.pnum)). This numerical solution and data management were carried out by an application developed in VBA (Visual Basic for Applications). A two-dimensional model is also adjusted from collected /two.pnum-D morphological model from the IWM library in connection with the Kalni-Kushiyara River management project. Finally, the extracted discharge was calibrated against the /two.pnum-D discharge of selected study portion using statistical test (t-test Sarker (/two.pnum/zero.pnum/two.pnum/one.pnumb)). /three.pnum./one.pnum /one.pnum-D Governing Equation This approach is governed by the /one.pnum-D momentum equation (see /one.pnum), the di/uniFB00erential form of momentum equation can be simpli/uniFB01ed for small longitudinal direction only. This momentum equation is composed of three types of forces (Pressure, Gravity, and Friction (see details in Reza et al. (/two.pnum/zero.pnum/one.pnum/four.pnum); Sarker (/two.pnum/zero.pnum/two.pnum/one.pnumd))), each of which produces two distinct types of acceleration (Convective acceleration and Local acceleration Sarker (/two.pnum/zero.pnum/two.pnum/one.pnumd)). This simpli/uniFB01ed /one.pnum-D momentum equation can be written as: @Q @t+@ @t(Q2 A) +gA@h @xgA(SSf) = 0 (/one.pnum) Carrying out the above mentioned steps with regard to the momentum correction factor, water surface slope compared to the datum, and making use of both discharge and cross section characteristics, the equation simpli/uniFB01es to: @z @x+1 2g@ @x( Q2 A2) +1 g@(Q A) @t+Q2 K2= 0 (/two.pnum) /two.pnum wherex=longitudinal distance along the reach, t=the time coordinate, z=z0+hvertical distance of water surface from the horizontal datum, z0is the vertical distance of bed from the horizontal datum, = momentum correction factor, A=cross-sectional area and K=conveyance of the section. This calculation works nearly exactly when the water level /uniFB02uctuates substantially over time. The Momen- tum Correction Factor ( ) is contingent on the overall cross sectional conveyance characteristics. There are an large number of tiny parts in each cross section. K=NX j=1Kj=1 NNX j=1AjR2 3 j (/three.pnum) whereKj=Conveyance of the tiny area of each cross section, Aj=Area of the small pocket, Rj= Hydraulic radius of the small pocket and n=Manning’s roughness. While the momentum coe/uniFB03cient can be estimated as =1 U2AZA 0u2dA=A K2NX j=1K2 j Aj(/four.pnum) /three.pnum./two.pnum Solution of the /one.pnum-D Governing Equation It is anticipated that no noticeable discharge is going to enter or leave the region that is separated by the gap between the two adjacent portions. That means that if cross sections for a straight stretch are straight and su/uniFB03ciently near, then the equation /two.pnum may be reduced. Thus, modest longitudinal discharge /uniFB02uctuation is a possible explanation for the variance in discharge seen in this instance Reza et al. (/two.pnum/zero.pnum/one.pnum/four.pnum); Dottori et al. (/two.pnum/zero.pnum/zero.pnum/nine.pnum). (huhd) +Q2 2g( u A2u d A2 d) +xuxd 2(1 K2u+1 K2 d)Q2+ (xuxd)@Q A @t= 0 (/five.pnum) The solution reduces to a quadratic equation, representing a single unknown discharge, when the local acceleration is ignored in equation /five.pnum. It may be written as: Figure /two.pnum: The /uniFB02ow chart of algorithms. /three.pnum (huhd) +Q2 2g( u A2u d A2 d) +xuxd 2(1 K2u+1 K2 d)Q2= 0 (/six.pnum) Numerical solutions are used to solve the equation for di/uniFB00erent water level variations while implementing a Visual Basic application. This /uniFB02owchart shows the solution algorithm: Only by calibrating the Chezy’s Cfor given water level, cross-section, and chainage data upstream and downstream can one /uniFB01nd the discharge within the reach Reza et al. (/two.pnum/zero.pnum/one.pnum/four.pnum). This method is shown in the next section. /three.pnum./three.pnum /two.pnum-D Model (MIKE /two.pnum/one.pnumC) Setup Two–dimensional model set up consists of selection of model domain, generation of computational grid, generation of model bathymetry, generation of the water level-discharge boundaries and selection of sim- ulation period. MIKE /two.pnum/one.pnumC combines the hydrodynamic model and morphological model in mobile bed condition. In this model, mass conservation is done in both longitudinal and transverse direction and momentum conser- vation equation using Renolds Depth Average Navier-Stokes formula which calculates momentum transfer in both direction. The surveyed bankline of Kalni-Kushiyara river has been used to generate computation curvilinear grids (shown in /uniFB01gure /four.pnum). The Kalni-Kushiyara River bathymetry was obtained by superimposing the surveyed bathymetry data on the curvilinear grid. The speciality of curvilinear grid is to simulate the streamline curvature with the /uniFB02ow and as well as sediment movement in both longitudinal and transverse direction. To solve the mass balance accurately, the /two.pnum-D model was done in several sub-models. Bathymetry of the model contains the cross section of river including /uniFB02ood plain. The upstream discharge and down- stream water level boundary was collected from /one.pnum-D North-East regional model of IWM. The simulation period has been considered from /one.pnum-January /two.pnum/zero.pnum/one.pnum/one.pnum to /three.pnum/one.pnum-December /two.pnum/zero.pnum/one.pnum/one.pnum. /four.pnum Input Data Figure /three.pnum: U/S Cross-section. /four.pnum Figure /four.pnum: D/S Cross-section. In order to investigate the ability of the suggested tool for the separation of /uniFB02ood plain /uniFB02ow from observed discharge, we have picked a short reach (which was described in the previous section) that will allow us to test it. We have chosen one u/s cross section and one d/s cross section from /uniFB01gure /three.pnum and another from /uniFB01gure /four.pnum. Additional information in the form of a cross sectional water level hydrograph can be seen in /uniFB01gure /five.pnum. Figure /five.pnum: Water Level Hydrograph. /five.pnum /five.pnum Results and Discussion Figure /six.pnum showed how the bank to bank discharge simulated using our proposed method compared to the bank to bank discharge from the MIKE /two.pnum/one.pnumC. It is important to point out that a signi/uniFB01cant linear relationship (slope = 1:08) was observed from their comparison with R2value of 0:99, in addition, ttest results show a signi/uniFB01cant correlation with a 95% con/uniFB01dence interval Sarker (/two.pnum/zero.pnum/two.pnum/one.pnumb). Hence, one can separate /uniFB02ood plain /uniFB02ow from the observed discharge depicted in the /uniFB01gure /six.pnum. Therefore, our proposed technique can be used where bank to bank /uniFB02ow and /uniFB02ood plain /uniFB02ow separation is necessary due to hydrologic or morphological purposes. Figure /six.pnum: Discharge Comparison. Multiple R /zero.pnum./nine.pnum/nine.pnum/eight.pnum/two.pnum R Square /zero.pnum./nine.pnum/nine.pnum/six.pnum/four.pnum Adjusted R Square /zero.pnum./nine.pnum/nine.pnum/six.pnum/four.pnum Standard Error /two.pnum./eight.pnum/seven.pnum/nine.pnum/nine.pnum Observations /two.pnum/nine.pnum/one.pnum/three.pnum ANOVA df SS MS F Signi/uniFB01cance F Regression /one.pnum /six.pnum/eight.pnum/three.pnum/nine.pnum/five.pnum/six.pnum/four.pnum./nine.pnum/five.pnum/two.pnum /six.pnum/eight.pnum/three.pnum/nine.pnum/five.pnum/six.pnum/four.pnum./nine.pnum/five.pnum/two.pnum /eight.pnum/two.pnum/four.pnum/six.pnum/three.pnum/three.pnum./seven.pnum/three.pnum/zero.pnum/five.pnum /zero.pnum Residual /two.pnum/nine.pnum/one.pnum/one.pnum /two.pnum/four.pnum/one.pnum/four.pnum/four.pnum./zero.pnum/two.pnum/zero.pnum/three.pnum /eight.pnum./two.pnum/nine.pnum/four.pnum/zero.pnum Total /two.pnum/nine.pnum/one.pnum/two.pnum /six.pnum/eight.pnum/six.pnum/three.pnum/seven.pnum/zero.pnum/eight.pnum./nine.pnum/seven.pnum/two.pnum Coe/uniFB03cients Standard Error t Stat P-value Intercept -/two.pnum./nine.pnum/zero.pnum/three.pnum/two.pnum /zero.pnum./zero.pnum/seven.pnum/nine.pnum/seven.pnum -/three.pnum/six.pnum./three.pnum/nine.pnum/nine.pnum/zero.pnum /two.pnum./zero.pnum/seven.pnum/eight.pnum/two.pnumE-/two.pnum/three.pnum/nine.pnum MIKE/two.pnum/one.pnumC-Q /one.pnum./zero.pnum/eight.pnum/one.pnum/one.pnum /zero.pnum./zero.pnum/zero.pnum/one.pnum/one.pnum /nine.pnum/zero.pnum/eight.pnum./zero.pnum/nine.pnum/three.pnum/four.pnum /zero.pnum Lower /nine.pnum/five.pnum% Upper /nine.pnum/five.pnum% Lower /nine.pnum/five.pnum./zero.pnum% Upper /nine.pnum/five.pnum./zero.pnum% -/three.pnum./zero.pnum/five.pnum/nine.pnum/six.pnum -/two.pnum./seven.pnum/four.pnum/six.pnum/eight.pnum -/three.pnum./zero.pnum/five.pnum/nine.pnum/six.pnum -/two.pnum./seven.pnum/four.pnum/six.pnum/eight.pnum /one.pnum./zero.pnum/seven.pnum/eight.pnum/eight.pnum /one.pnum./zero.pnum/eight.pnum/three.pnum/four.pnum /one.pnum./zero.pnum/seven.pnum/eight.pnum/eight.pnum /one.pnum./zero.pnum/eight.pnum/three.pnum/four.pnum Table /one.pnum: Regression statistics of discharge comparison. /six.pnum /six.pnum Conclusion and Recommendation Morphological and hydrological knowledge of watershed can be signi/uniFB01cantly understood by measurement of the bank-to-bank /uniFB02ow and /uniFB02ood plain /uniFB02ow. Hence, this program has signi/uniFB01cant potential in the context of Morphology and hydrology of watershed. The consideration of local acceleration along with new numer- ical scheme can perfectly mimic the natural separation of /uniFB02ood plain /uniFB02ow. Furthermore, power spectral analysis Sarker (/two.pnum/zero.pnum/two.pnum/one.pnume) of these extracted discharge could be helpful to understand the accuracy and nature of the /uniFB02ood that can be subjected to further study.