|
Abstract:
|
Superelevation transition is often used to help balance the centrifugal forces on vehicles through curved roadway sections . Such transitions have regions with near -zero cross -slope as the pavement cross -section rotates from a negative to positive grade . For drainage of roadway surfaces , regions with near -zero slope constitute 'irregular topography' . This condition promotes extended stormwater runoff drainage path lengths and may result in excessive splash from vehicles and hydroplaning . A critical concern is the effect of longitudinal slope on stormwater drainage through superelevation transition . The overall goal of this study is to provide design guidance on longitudinal slope at superelevation transitions through application of a numerical simulation model of highway drainage . Sheet flow on urban pavement surfaces is very shallow , typically measuring a depth less than one centimeter . For modeling of such flow conditions , any small discontinuity or over -simplification of the surface geometry may result in failure in the flow computation . The kinematic wave approximation to the full Saint -Venant equations is often used in many surface and subsurface water models due to its simplicity in application . However , this model fails when backwater effects , ponding , or flow on reverse slope occurs in the local scale . Furthermore , due to the complexity in the surface geometry and the existence of drainage systems , the kinematic wave model is not sufficient for modeling urban stormwater runoff . On the other hand , the full dynamic wave (DW ) model usually requires more computational effort . The long computation time of DW model often compromises the accuracy of the model , making the model practically inefficient . In this study , an algorithm was developed to properly represent the irregularly shaped roadway surfaces near superelevation transition areas with unevenly spaced curvilinear grids based on the geometry profile provided by a roadway design software package such as MicroStation CAD . With this accurately defined geometric representation , a nonlinear hydrodynamic diffusion wave model for hydraulic analysis developed in this research estimates the flow depth and runoff volume on the pavement surfaces . The model computes the flow responses for rising hydrographs using a preconditioned general Conjugate Gradient method . Kinematic boundary conditions developed for the open boundaries at the upstream and downstream boundaries compute the boundary values explicitly at each time step . The result of a numerical experiment shows that the spread and concentration of sheet flow is closely related to the transition in cross slope , longitudinal slope , rainfall intensity , and the width of the road . The characteristics of the sheet flow on superelevation transition areas are analyzed to find the optimal longitudinal slope . It is found that the longitudinal slope in the range of 0 .3 % -0 .4 % is the optimal slope at superelevation transition areas which minimizes the depth of stormwater runoff . An example application of the model on a rural highway in Texas is also presented . It is found that a significant amount of stormwater may exist on traffic lanes at the superelevation transitions tested . The predicted ponding depth exceeds the minimum value for potential hydroplaning , and the pattern of the flow concentration may cause differential drag forces on traffic vehicles . |