Dam gateway protection30 November 2011
A device to protect spillway gates from breakaway barges would be an asset to navigation projects. E. Allen Hammack, Richard L. Stockstill, Mark A. Hopkins, and Jane M. Vaughan from the US Army Corps of Engineers give an insight into the design concept and analysis for a navigation dam gate guard
Accidents on navigable waterways in the US can cause barge tows to break up and, subsequently, allow individual barges to be carried downstream by the current. As a breakaway barge approaches a navigation structure, its path is essentially determined by the flow patterns around the lock and dam. A primary concern is that a barge will travel to the dam, pass between spillway gate piers, and either strike a gate or become jammed.
Either way, the result can be the loss of gate control and perhaps the loss of a navigable pool. Hite (2008) reports on recent closures of US Army Corps of Engineers navigation projects attributed to tow/barge accidents. These accidents have been costly to the towing industry due to closures and to the government due to expensive structural repairs. Examples of accidents that have occurred in the last decade include events on the:
• Ohio River at Belleville Locks and Dam (Figure 1) and at Montgomery Lock and Dam both in January 2005.
• Smithland Locks and Dam in April 2005.
• Lock and Dam No. 2 on the Red River in December 2004.
• Cheatham Lock and Dam on the Cumberland River in March 2002 (Figure 2).
Removing the barges from the gates can be a difficult, time-consuming, and expensive operation. Designers and operators of locks and dams need a means of arresting breakaway barges and avoiding their impact on critical structural and mechanical components. A device to protect spillway gates from breakaway barges would be an asset to the Corps of Engineers’ navigation projects.
Barge impact loads
The key to developing a sufficient device to absorb the impact of a runaway barge is accurately determining the loads that the barge will impart during impact. The total load in the direction of impact is comprised of the impact force and the drag force the currents induce on an arrested barge. The impact force is expected to be significantly larger than the drag forces, so the maximum impact force is taken as the design load.
A physically-based method of analysing the interaction of floating objects with riverine and navigation structures is described by Stockstill et al (2009). This method integrates a two-dimensional (2D) depth-averaged hydrodynamic model and a three-dimensional (3D) discrete element model (DEM). The combined models compute the detailed interaction of barges and dams under the influence of the river current. The mechanical details of the modelling system are presented by Stockstill, and this system of coupling a discrete element model with a multidimensional flow model is useful in estimating forces on hydraulic structures.
Other design considerations
The initial focus of the gate guard design was determining how to withstand the barge’s impact load, but other factors contributed to the final concept. Rather than requiring an immediate stop, the guard could undergo a controlled deformation as the barge is brought to rest.
To avoid the accumulation of debris or ice, any structure added to a navigation dam could not rest in or on the pool. However, it could be positioned in such a way that allowed rapid deployment. The method of deployment was then considered given the magnitude of barge impact loads and the short warning before collision. Remote deployment, perhaps from the control house of the structure, was considered critical to increase safety of personnel.
Navigation projects that have large pool variations throughout the year require that the structure be positioned for barge impact at lower pools, but not so low that at high water the barge simply floats over it. Modifications to the concept design will be required for sites having extremely large pool variations. Design details such as overall dimensions and mounting locations will need to be developed on a project-specific basis.
The device shown in Figure 3 is a set of cables supported by tetrahedra placed on the spillbay piers of a navigation dam. When not deployed (see Figure 4) the tetrahedra sit on top of the piers and have no effect on the flow approaching the spillway gates or the flow near the structure nor will they collect debris. During deployment the tetrahedra rotate forward until they come to rest on the piers (Figure 4). The device remains in front of the piers until the barges have been captured.
Two cables connect each adjacent tetrahedron. A vessel floating towards a spillway gate will initially impact the lower cable, which is designed to absorb the majority of the vessel’s longitudinal momentum. After impacting the lower cable, the vessel may start sliding over the bottom cable. The likelihood of this behavior is increased if the front of the barge – because of the rake – makes initial contact. If this sliding occurs, the upper cable will limit the vertical movement until the vessel stops. These two cables work in conjunction to catch the vessel as it moves towards the spillway gate.
A structural finite-element analysis was performed to determine the internal loads on each member of the structure as well as its deflection during impact. Truss elements were used to represent each member of the tetrahedra. Each tetrahedron in the device is a space truss with an external load applied at one vertex. The member sitting on the spillbay pier was assumed to be rigid. The barge was assumed to impact a single cable at a single point (its midspan) and not rotate after contact, so that the structure completely absorbed the impact. The impact force was divided equally between the two closest tetrahedra.
The cable must be strong enough to withstand the total impact force of the barge while transferring that load to the tetrahedra. A cable can be chosen such that its rupture strength (a readily available property) is sufficiently robust to withstand the impact. Hammack et al (2011) provide details of the structural analysis. They describe the stress calculations and discuss factors of safety against failure. One of three methods of failure – cable rupture, axial failure of the truss members, and buckling of the truss members – is considered most likely for a barge impact on the lower cable.
Cheatham Lock and Dam were selected as the demonstration project for developing a concept design. Cheatham Lock and Dam are located on the Cumberland River near Ashland City, Tennessee (67.5km downstream of Nashville). The dam has seven tainter gates each of which are 18.3m wide by 8.2m tall. The Cheatham project was chosen partly because in 2002 a gate was damaged by a breakaway barge, the gates are as wide as typical Corps navigation dams (15.2m and 18.3m wide gates are most common), the Cumberland River is not extremely wide (resulting in a reasonable flow modeling effort), and the operations personnel were receptive to engaging with the researchers.
A 2D depth-averaged flow simulation was performed using the AdH flow solver. The flow model reproduced the navigation conditions associated with an accident at Cheatham Lock and Dam on 20 March 2002 where 11 barges piled onto the piers and spillway gates and damaged one gate. On that day the total river discharge was 3454.7m3/sec, with 2775 m3/sec passing over the spillway with the gates fully-opened and 679.6 m3/sec going through the powerhouse. The model flow domain extended from the dam upstream 640m and included the powerhouse trash boom and lock guard wall piers
The flow patterns in the approach to Cheatham Lock and Dam are shown on Figure 5. The velocities directly in front of the spillway piers were about 2m/sec. The DEM simulated a single, loaded jumbo barge (59.4mx10.7m) drafted at 2.7m. A time series showing pictures of the barge impacting the gate guard’s cable is provided in Figure 6. The picture in Figure 7 shows the barge position at maximum deflection of the gate guard cable. Note that the cargo remains in the barge throughout the impact.
The gate guard concept design was sized for an impact force of 750,000 lbf striking the lower cable at midspan. Each cable is composed of 150 commercially-available strands of steel cable. The tension in each cable at maximum impact is about 1.1M lbf. The trusses are comprised of cylindrical steel members. Table 1 lists the sizes of the cable, the sizes of the truss members, and the properties for the material used for the cable and truss members. Table 2 summarises the results of the structural analysis.
With the given impact load and location, the structure (tetrahedra) deflects much less than an inch. If the gate guard is intended to be less rigid during impact, then the truss members should be modified in a way to lower the effective stiffness of the entire truss. If such modifications are made, though, the factors of safety must be monitored closely because the buckling force in particular is very sensitive to the truss member geometry. However, with the current geometry the cable is much more likely to rupture than the truss members are to fail.
E. Allen Hammack, Research Mechanical Engineer, U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199, Email: Allen.Hammack@usace.army.mil
Richard L. Stockstill, Research Hydraulic Engineer, U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199, Email: Richard.L.Stockstill@usace.army.mil
Mark A. Hopkins, Research Physicist, U.S. Army Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, NH 03755-1290. Email: Mark.A.Hopkins@usace.army.mil
Jane M. Vaughan, Engineering Technician, U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199. Email: Jane.M.Vaughan@usace.army.mil
ADH is a state-of-the-art ADaptive Hydraulics Modelling system. It is capable of handling both saturated and unsaturated groundwater, overland flow, three-dimensional Navier-Stokes flow, and two- or three-dimensional shallow water problems.