Walking from building to building in late March, one of our engineers noticed some interesting patterns in puddles of melting snow. As the water ran off the snow piles, it carried with it fine sediment—a perfect, small-scale example of what sediment transport looks like. Naturally, he took out his phone and captured a few quick videos to share.
Sediment transport is a naturally occurring process, the results of which we see all around us. On geomorphic time scales, the shapes of our rivers, coastlines, even canyons and mountains are made in large part due to erosion, sediment transport, and deposition. All pretty cool things. Now, if we talk about this in terms of an engineering time scale, sediment transport can mean very different things. Sediment can be beneficial, and sediment can have adverse impacts on infrastructure. To our clients, scaled physical and numerical modeling of sediment transport is typically looked at for one of three reasons.
Improving O&M Costs
Simply put, sediment and man-made machinery can be a problematic mix that can damage equipment or imped process flows. In the example of cooling water intakes for power plants for instance, sediment can be the cause of multiple problems. Bed load sediment (generally the coarser sediment) can settle in the intake, causing flow to be blocked. If the coarse sediment, which is more abrasive, reaches the pump, it can cause damage to the pump impellor and contribute to piping erosion). Finer, suspended sediment can affect the cooling water system performance, specifically with the pump intake structure, heat exchanges or pipes with low velocities.
By modeling these structures at scale, Alden can advise clients on optimal intake and pump designs, as well as identify ways to mitigate sediment-related problems where they are most likely to occur. The end result is a system with lowered operating and maintenance costs.
Read about Smithland Hydroelectric project on the Ohio River where similar studies have helped.
Reducing Environmental Impacts
Removing sediment, e.g. dredging programs, is a common and important aspect of engineering design. But the reality is, sediment removal is also expensive. With dredging, other problems can get dug up, like where to put the removed sediment or what happens when you disturb contaminated legacy sediment. Clients must consider not only the monetary factors associated with sediment removal, but also its environmental impact.
The truth is, you can never truly eliminate sediment, especially in bodies of water where dredging frequently happens. But by modeling it, you can better predict where sediment goes. Where does bed load sediment drop out? Does it cause scouring? Physical, and even numeric modeling in some cases, that can inform designs to help reduce adverse sedimentation, it’s impact on the surrounding environment— including water quality, and the large expenses associated with sediment removal.
Optimizing Beneficial Uses of Sediment
In the case of the major undertakings of the Louisiana Coastal Protection and Restoration Authority, sediment and sediment transport provides a net benefit, especially if deposited in targeted areas. Coastal Louisiana is experiencing unprecedented land loss, and with it, loss of natural defenses against storms, ways of life, not to mention the negative economic impacts. The CPRA has a number of programs, projects, and initiatives aimed at reversing this trend through coastal protection and restoration, including sediment diversion projects which rely on the mechanics of sediment transport to accomplish the goal of rebuilding land.
We have been hard at work building a 1:65 scale physical model of the Mississippi River to help CPRA project teams optimize the design of a river sediment diversion inlet and conveyance channel. The sole purpose of these structures is to aid in rebuilding land in Barataria Bay. The models will verify and evaluate how much sediment gets into and transports down a diversion channel and how the area downstream of the outfall will respond. In addition, the models will help determine if there are any undesirable effects to the way in which sediment moves, including unintentional settling or scouring. These structures are quite expensive and the modeling component of the project will help validate a design that works.
Sediment can affect many other areas than those mentioned above: it can be a big problem for reservoirs, and is a concern for storm water designs; it can impact boat docking or weaken civil infrastructure. We have helped clients resolve these design engineering problems and then some. One thing is for certain, sediment transport is all around us so slow down and take some time to look around. You may be surprised by what you see.
Got sediment? Contact us to learn more about how we can help.
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