Numerical assement and evaluation of a stream monidification strategy to address sediment aggradation issues at Dean Road bridge in Dale County, Alabama

Abstract

Severe aggradation in bridge structures is a serious issue since it may lead to overtopping episodes during peak flows or increased risk of serious structural damage. Aggradation processes may be caused by changes in watershed land use or clear-cutting near streams among others causes. In order to predict aggradation and degradation in the river, filed data were collected and hydrologic and hydraulic models are built. Field monitoring data, such as rainfall, water depth, water velocity and sediment concentration were collected through sensors and water samples. These field measurements were used in the model as inputs to enable building and calibration of the model. River modeling tools, such as HEC-RAS and SRH-2D, are useful in computing characteristics of stream flow, such as velocities and stages, if aggradation processes take place. While the field monitoring data (rainfall data) were used as the model input of hydrologic model (HEC-HMS), the discharge of the hydrologic model is used as the input of the hydraulic model. These models can also be used to assess proposed changes in the stream characteristics to minimize aggradation issues, including stream modification with the objective of increasing flow velocities near the roadway-stream crossing. This work presents results of using HEC-RAS 5 and SRH-2D to estimate the impacts of aggradation and proposed geometric changes in a stream in Southeast Alabama. One specific point (under the bridge) with monitoring data was selected from the two models and the results are compared. Results indicate important impacts of geometric details in the stream flow characteristics, and indeed point to the benefits of using a 2D modeling approaches to evaluate the impacts of stream modifications to reduce aggradation issues.