How streamflow affects bridge project
Bridge Answer Man
Question: How does the Grand Avenue bridge project monitor the rising river levels, and what is the impact of the high water on the construction?
River flow is always a hot topic, especially in a town with two rivers. An accelerated bridge construction method was chosen for this project as a way to minimize impacts to downtown and the Hot Springs area. Using this construction method meant that more infrastructure needed to be placed in the river to access bridge piers and other components of bridge construction. Thus the design of the causeways — work platforms build out from riverbanks — is a critical element of the project. The daily river flows affect the construction activities on the causeway and dictate how and when crews can work in the river.
Early in the project, CDOT engaged local water consultant AECOM to create a three-dimensional model of river hydraulics. Flows dating back 75 years and the contour or shape of the riverbed was used to create this computer model. Known as a Computation Fluid Dynamics (CFD) model, it uses cutting edge 3D modeling to help determine the design of both the existing south causeway and the future north causeway.
The 3D CFD model is able to show the velocity of the water at very specific points in the river. This is a much greater analysis than the typical one-dimensional study, which can give only average velocities at a particular cross-section of the river. The model looked at upstream conditions, critical flow at the causeway and subcritical flow. With this information, AECOM was able to predict how and where the waves or “hydraulic jumps” would occur above and below the Grand Avenue bridge, as well as the effect the waves would have on river users, river safety, the existing bridge and upstream properties.
AECOM monitors the seasonal runoff, snowpack, predicted run-off peaks and reservoir releases to develop a weekly river report. The construction team then makes scheduling decisions based on water levels. For example, the north causeway (planned for construction in 2017) is not permitted to be built until water levels are low. If the three-day river forecast exceeds 21,500 cubic feet per second (the 10-year flood event), the contractor will begin to remove portions of the causeway. The latest projections call for river flows to peak Tuesday or Wednesday afternoon at 10,500 cubic feet per second, well below this threshold.
CDOT is also monitoring the existing bridge pier for potential scour conditions on the riverbed with transmitters that were buried in the river bottom near the bridge this spring. The transmitters are small, buoyant devices that will float up to the water surface and notify the project team if scour were to occur. This provides a continuous, real-time, cost-effective way to monitor potential scouring of the pier.
As required by the Clean Water Act, the Army Corps of Engineers issues a 404 permit anytime discharge or fill material is placed in navigable waters, including wetlands. The Grand Avenue bridge project required a 404 permit that has conditions based on certain river flows, so monitoring and projecting the peak runoff is critical.
With peak runoff expected any day now, all eyes are on the river runoff and daily river conditions. In next week’s column, I will take an in-depth look at the 404 permit.
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