Two Dimensional Model for Backwater Geomorphology: Darby Creek, PA

Abstract

Predicting morphological alterations in backwater zones has substantial merit as it potentially influences the life of millions of people by the change in flood dynamics and land topography. While there is no two-dimensional river model available for predicting morphological alterations in backwater zones, there is an absolute need for such models. This study presents an integrated iterative two-dimensional fluvial morphological model to quantify spatio-temporal fluvial morphological alterations in normal flow to backwater conditions. The integrated model works through the following steps iteratively to derive geomorphic change: (1) iRIC model is used to generate a 2D normal water surface; (2) a 1D water surface is developed for the backwater;(3) the normal and backwater surfaces are integrated; (4) an analytical 2D model is established to estimate shear stresses and morphological alterations in the normal, transitional, and backwater zones. The integrated model generates a new digital elevation model based on the estimated erosion and deposition. The resultant topography then serves as the starting point for the next iteration of flow, ultimately modeling geomorphic changes through time. This model was tested on Darby Creek in Metro-Philadelphia, one of the most flood-prone urban areas in the US and the largest freshwater marsh in Pennsylvania.the river increases gradually upstream to form a smooth transition between a quasi-normal flow and standing water, forcing an alteration of the hydraulic conditions. The segment of the river affected by this response-which might exceed hundreds of kilometers in low slope rivers-is called the backwater zone [12].The shift in BW hydraulics causes the water surface slope to decrease independent of the bed slope, resulting in a gradual longitudinal alteration of the flow depth and velocity [13]. The vertical velocity distribution in BW conditions is less divergent compared to the one in a uniform flow [14]. These changes in the hydraulics of the vertical and horizontal flow velocity in BW zones consequently alter the sediment transport processes.Current approaches to BW in morphological studies are primarily based on either measured or estimated hydraulic parameters coupled with the estimated sediment transport [9,11,[15][16][17][18][19][20][21]. The studies of sediment transport in BW conditions are scarce due to the complex dynamics of the sediment transport in BW zones and the lack of field-measured data. The presence of the BW complicates different types of hydrometric measurements including the water surface slope, velocity, and discharge [19]. A lack in measured data is even more severe for transitional areas where the flow state changes from normal to a gradually varied flow [18]. Nittrouer et al. used measured cross sections in the lower Mississippi River to back calculate velocity, shear stress and sediment discharge [22]. They concluded that where the flow transitions to the BW zone, the cross-sectional area increases in the downstream in low-moderate flows, resultin...

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