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Aggradation, or sediment deposition, is a very common problem at many bridges crossing small streams in the upper Susquehanna River watershed. Modifications to the channel near a bridge crossing can lead to the extensive deposition of sediment in the bridge waterway. This partial blockage of the bridge waterway poses serious safety problems to the overall bridge structure. The most common solution for aggradation in Pennsylvania is to dredge the stream channel frequently. While this mitigation procedure is successful in clearing some of the sediment that is blocking the waterway, it also creates conditions that promote further sediment deposition at the bridge. Attempts also have been made to mitigate sediment ag‐gradation with stream restoration structures. Sediment transport and bridge hydraulics need to be investigated to determine a more sustainable solution to aggradation at bridge crossings. The main goal of the presented research was to initiate monitoring at a bridge site to measure changes in stream bed elevation. This data will then be used to develop mathematical simula‐tion models of the sediment processes at the bridge crossing. The current bridge design guidelines focus only on the transport of the water through the bridge waterway. The moni‐toring and mathematical modeling of sediment processes at bridges will help inform bridge design guidelines that account for the transport of sediment through the bridge waterway in addition to the water.
♦Monitor streambed elevations at a bridge crossing where stream restoration structures have been installed to mitigate sediment deposition.
♦Apply mathematical models with varying complexity to simulate sediment and hydraulic processes at bridge crossings.
♦Develop recommendations for considering sediment processes in the design of stream‐bridge crossings.
•Continue monitoring White Deer Creek • Resurvey stream cross‐sections periodically • Directly measure sediment movement, hydraulic, and hydrologic data with ADCP and other field equip‐ment. Anticipated uses of the collected data include:
‐Verification of hydrologic modeling of the White Deer Creek watershed, ‐Discharge measurements to be used with sediment movement data to simulate sediment trans‐
port during storm events, ‐Confirmation of 2‐D model simulation of velocity distribution at cross sections.
•Create complete mathematical models of hydraulic and sediment processes of White Deer Creek using HEC‐RAS, BRI‐STARS and FESWMS.
•Simulate sediment erosion & deposition between current and future events at bridge crossings and stream restoration structures with 1‐D and 2‐D models.
•After using field data to validate simulation capability, develop hypothetical models to aid in the develop‐ment of bridge design guidelines that consider sediment and hydraulic processes.
We would like to thank Shanon Burkland with the Union County Conservation District for her help with gathering data for the restoration pro‐ject. We would also like to thank Michael Baker Jr Undergraduate Research fund for providing funding in the summer of 2009 as well as the Katherine Mabis McKenna Summer Environmental Internship Program for funding in the summer of 2010. We would like to thank Benjamin Hayes of the Bucknell Environmental Center for helping with data collection and analysis and for the use of his data collection equipment.
The research focused on the lower section of White Deer Creek at the Old Route 15 bridge crossing in White Deer, PA. White Deer Creek is a tributary to the West Branch of the Sus‐quehanna beginning near Lavonia, PA and ending at White Deer, PA. The White Deer Creek watershed is approximately 47 square miles with a majority of it being forested. In the past, a large amount of deforestation and logging has caused the creek to become unstable, from an engineering standpoint. The current “clear water” conditions in the watershed have caused a large amount of gravel‐ and cobble‐sized sediment to begin moving downstream in the channel (Hayes and Kochel 2010). This large amount of sediment had been building up near the bridge opening, Figure 1, and causing the need for increased maintenance.
In 2004, a restoration project was designed for the lower portion of White Deer Creek, near the old route 15 bridge crossing, to restore the creek and mitigate the sediment problems that occurred around the bridge. As built in 2006, this project involved the installation of two rock vane weirs, one upstream and one downstream of the bridge, along with channel bank stabilization using large riprap boulders. The weirs were installed to create a faster sec‐tion of the channel to “carry” the sediment through the bridge opening and not allow it to deposit in the vicinity of the bridge. The riprap created a non‐erodible channel bank so that the channel could not change its course near the bridge or erode the bank farther into the nearby property.
Several models were investigated to determine which models would best fit our site conditions. Many models have different approaches to predicting sediment transport near bridge crossings. HEC‐RAS (Brunner 2002) was the first model selected to predict one‐dimensional hydraulics and sediment transport in the stream. This model is free to the public and was recently updated to in‐clude sediment modeling . The HEC‐RAS model is the most commonly‐used model in engineering design affecting rivers or streams and is therefore very accessible to design engineers and an im‐portant tool when considering the development of design guidelines. The second program selected was BRI‐STARS, BRIdge Stream Tube model for Alluvial River Simula‐tion (Molinas 2000). This program was developed by the Federal Highway Administration to aid in the hydraulic modeling of bridge crossings and is also offered free to the public. BRI‐STARS is a semi‐two dimensional model for hydraulics and sediment transport as it considers cross channel variation with stream tubes as well as downstream variation of hydraulic and sediment transport parameters.
The two dimensional model, FESWMS‐FST2DH, is currently being considered for use. FESWMS adds more complexity to the modeling approach with two dimensional hydraulics and sediment transport in a finite element approach. FESWMS was developed by the Federal Highway Admini‐stration to simulate the movement of water and non‐cohesive sediments in rivers, estuaries, and coastal waters (Froehlich 2003).
A model has been developed using HEC‐RAS with four different geometry files (four time‐separated sur‐veyed streambeds). Figure 6 displays a cross‐section at the downstream face of the old US Rt 15 bridge that is comparing the pre‐restoration bed with the proposed restoration conditions. The proposed condition is trying to center the stream under the bridge opening. Figure 7 displays the surveyed changes to the channel profile between summer 2009 and summer 2010, both post‐restoration. A general decrease in bed eleva‐tion is observed downstream of the downstream rock weir.
Figure 8 shows the changes in a cross section near the downstream restoration structure between summer 2009 and summer 2010. The water is cutting deeper into the streambed and is causing the riprap banks to fail. Looking at the restoration structures, the upstream structure has already been filled with sediment, Fig‐ure 9, while the downstream structure, as seen in Figure 2, has begun to show signs of failure.
MONITORING:
Figure 1 ‐ Image taken downstream of the bridge, prior to Stream Restoration Project.
Figure 2 ‐ Image of the downstream rock vane weir installed as part of the restoration project.
Figure 6 ‐ Comparison of the bridge opening before (current) and after (proposed) stream restoration.
Figure 7 ‐ Comparison of the stream profile from summer 2009 to summer 2010.
White Deer Creek, PA: Implications for Understanding Bedload Transport at Bridge CrossingsWhite Deer Creek, PA: Implications for Understanding Bedload Transport at Bridge Crossings Brian P. Schultz, Graduate Assistant Jessica T. Newlin, Assistant Professor
Civil and Environmental Engineering Bucknell University, Lewisburg, PA
Stream‐bed monitoring is accomplished by surveying the channel bed with a total sta‐tion, Figure 3, both before and after a storm. This provides us with required geometric data for modeling and allows us to visually com‐pare the stream bathymetry to detect any sig‐nificant changes to be used to validate the mathematical modeling of the sediment transport processes.
Figure 3 ‐ Taken while surveying the stream in the summer of 2010.
Figure 8 ‐ Comparison of a given cross‐section downstream of
the rock weir from summer 2009 & 2010.
Figure 9 ‐ Image of the upstream rock weir.
Brunner, Gary W. (2002). HEC-RAS River Analysis System. Users Manual. Version 3.1. United States Army Corps of Engineers. Pgs. 1-11. Froehlich David C. (2003). User’s Manual for FESWMS FST2DH: Two Dimensional Depth- Averaged Flow and Sediment Transport Model. U.S. Department of Transportation. Federal Highway Administration. Pgs. 1-209.
Hayes and Kochel (2010). Impact of Logging Legacy Sediments and Geomorphic Adjustment on Stream Restoration Structures in North-Central Pennsylvania. Annual Water Resources Conference, American Water Resources Association, Philadelphia, PA. Nov 1-4, 2010.
Molinas, Albert (2000). User’s Manual for BRI-STARS. U.S. Department of Transportation. Federal Highway Administration. Pgs. 1-227
Figure 4 ‐ RiverSurveyor ADCP collecting data from the bridge at White Deer Creek (9‐30‐10).
Figure 5 ‐ Two‐person cross stream approach to data collec‐tion at White Deer Creek (Summer 2010).
DATA COLLECTION: An acoustic Doppler current profiler (ADCP) will be used to collect storm discharge measure‐ments and 3‐dimensional velocity distribution measurements. Initial measurements with the Sontek RiverSurveyor M9 ADCP have been made at White Deer Creek. Figure 4 is a photo of the RiverSurveyor being used from the bridge during the rising limb of a storm event (Sept 30 – Oct 1) that generated approximately 3.75 inches of rainfall. The White Deer Creek site also is suitable for obtaining measurements using a two‐person cross channel approach as shown in Figure 5. Therefore, data can be collected safely at locations other than the bridge during higher flow events. Figure 6 shows a screen capture of the ADCP data from a cross section upstream of the bridge at White Deer Creek collected during the falling limb of the Sept 30 – Oct 1 storm event.
Figure 6 ‐ Screen capture of the collected data at a cross section upstream of the bridge at White Deer Creek (10‐1‐2010).