The Effects of Floodplain Manning's n Value on Flow Properties

The following analysis was conducted to explore what would occur if the floodplain was restored for Penn State Harrisburg’s campus stream, an unnamed tributary to the Susquehanna River tributary. As Penn State Harrisburg continues to expand, the campus stream has become washed out and eroded. In order to identify what could be done to improve the stream, a hydraulic model was required to represent different restoration scenarios and storm events. The purpose of this study is to develop a stream model using updated floodplain survey provided by Penn State Harrisburg students to demonstrate how the stream would react to restoration. HEC-RAS, geo-spatial survey points, and existing culvert information were used to create the stream model. Three different flow scenarios were chosen to represent design storms. A 200, 400, and 600 CFS flow scenario were created to show a bank full, slightly flooded, and heavily flooded conditions, respectively. Four separate models using the same Manning’s n value on the floodplain were created to represent different floodplain conditions. A Manning’s n of 0.03, 0.05, 0.10, and 0.15 were used to show the conditions of: short grass, heavy brush, large timber, and dense willows, respectively. The model demonstrated that the culvert restricted flow in all three flow scenarios which reduced the channel velocity and Froude number but increased the top width of flow just before the culvert. The highest velocity recorded was 9.77 ft/sec at river station 5 during the 600 CFS scenario with a Manning’s n of 0.15. the largest Froude number recorded was 1.01 also occurring at river station 5 but during the 200 CFS flow scenario and was independent on the Manning’s n value. Top width varied depending on the cross-sectional shape, but the widest recorded was 268.40’ which occurred just before the culvert at river station 8 during the 600 CFS event and a Manning’s n of 0.15. The model demonstrated that floodplain restoration would cause an increase in the channel velocity, and top width. Given these channel velocities, the streambed would need to be reinforced or protectively lined in order to prevent sediment transport and further washout, even if the floodplain was not restored.

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Work Title The Effects of Floodplain Manning's n Value on Flow Properties
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Open Access
Creators
  1. Brittaney A Hargenrader
Keyword
  1. Master of Science in Civil Engineering
  2. MS_CENG
License In Copyright (Rights Reserved)
Work Type Research Paper
Publication Date May 2022
Deposited April 18, 2022

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    • Master of Science in Civil Engineering, MS_CENG, The Effects of Floodplain Manning's n Value on Flow Properties
    • Master of Science in Civil Engineering, MS_CENG
    Description
    • Abstract
    • The following analysis was conducted to explore what would occur if the floodplain was restored for Penn State Harrisburg’s campus stream, an unnamed tributary to the Susquehanna River tributary. As Penn State Harrisburg continues to expand, the campus stream has become washed out and eroded. In order to identify what could be done to improve the stream, a hydraulic model was required to represent different restoration scenarios and storm events.
    • The purpose of this study is to develop a stream model using updated floodplain survey provided by Penn State Harrisburg students to demonstrate how the stream would react to restoration. HEC-RAS, geo-spatial survey points, and existing culvert information were used to create the stream model. Three different flow scenarios were chosen to represent design storms. A 200, 400, and 600 CFS flow scenario were created to show a bank full, slightly flooded, and heavily flooded conditions, respectively. Four separate models using the same Manning’s n value on the floodplain were created to represent different floodplain conditions. A Manning’s n of 0.03, 0.05, 0.10, and 0.15 were used to show the conditions of: short grass, heavy brush, large timber, and dense willows, respectively.
    • The model demonstrated that the culvert restricted flow in all three flow scenarios which reduced the channel velocity and Froude number but increased the top width of flow just before the culvert. The highest velocity recorded was 9.77 ft/sec at river station 5 during the 600 CFS scenario with a Manning’s n of 0.15. the largest Froude number recorded was 1.01 also occurring at river station 5 but during the 200 CFS flow scenario and was independent on the Manning’s n value. Top width varied depending on the cross-sectional shape, but the widest recorded was 268.40’ which occurred just before the culvert at river station 8 during the 600 CFS event and a Manning’s n of 0.15.
    • The model demonstrated that floodplain restoration would cause an increase in the channel velocity, and top width. Given these channel velocities, the streambed would need to be reinforced or protectively lined in order to prevent sediment transport and further washout, even if the floodplain was not restored.
  • Deleted Brittaney Hargenrader - Master's Signatory Page.pdf
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