HYDROLOGIC AND HYDRAULIC DESIGN OF CULVERTS AND BRIDGES

Abstract

Bridges and culverts are important structures on highways. Hydrological and Hydraulic design of such structures is an important activity for the civil engineers. The aim of present study is to study in detail the hydrologic and hydraulic aspects of such design. For designing the bridges or culverts, one of the most important hydrologic parameter is to estimate the design peak flood that is desired to pass off safely without harming the• structure.. To estimate peak runoff from adjoining areas in an ungauged catchment, there are several methods suggested in the literature. In the present study, the SCS method and Kinematic wave method are described in detail. SCS method compute the volume of surface runoff for a given rainfall event from small agricultural, forest, and urban watersheds. Once the volume of surface runoff is estimated. It is important to estimate peak runoff ,so synthetic unit hydrograph is used to compute peak runoff but also hydrograph of runoff. In Kinematic wave method, water surface profile and surface runoff hydrograph under varying rainfall excess, have been obtained using method of characteristics. The varying rainfall excesses are divided into small time interval and the average value of the rainfall excesses are considered to remain uniform in each respective time intervals. The time variant infiltration values are calculated by using- Green and Ampts infiltration equation. From these time variant infiltration values, the varying rainfall excesses are found out and these values are used to find the water surface profile and surface runoff hydrographs. In culverts, there are two basic types of flow control - inlet control and outlet control. In hydraulic design, depending on the type of flow control, the headwater elevation is estimated by assuming dimensions of culvert. ►:4:1 Bridges enable streams to maintain flow conveyance and to sustain aquatic life. In order to minimize the risk of failure, one must recognize and consider the hydraulic requirements of a stream crossing during the development.- Following aspects of hydraulic design of a bridge are considered in the present work: - Estimation of water surface profile (computation of backwater) - Estimation of scour potential In estimation of water surface profile, backwater depends on bridge constriction, piers, skew, and eccentricity. Depending on backwater, scour potential is estimated. After estimation of all components of scour, one has to consider the economic aspect, and decide if bridge should be expanded or constricted.