What is the Purpose of the Ballast Chain in a Floating Barrier?

The ballast chain, when needed, is located along the bottom ‘hem’ or seam of a turbidity curtain or containment boom.   It comes in various materials and weights depending on conditions of the site where the silt curtain or marine boom will be deployed.   (See Fig A)
 
A floating barrier with a skirt which hangs below the surface of the water is designed to contain, separate, act as a barrier for surface debris or suspended particulate matter.  Much like hanging laundry on a clothesline, the underwater fabric will ‘billow’ in currents, swells, and tides.  In fact, they are an engineered design built to work under just these conditions. The ocean, as a living, moving, entity will create forces that continually and continuously act on the boom or barrier under the water, and the ballast chain helps to decrease the billowing angle by providing some counter-weight.
 
The purpose of the ballast chain is to offset or mitigate some of the billowing of the curtain without stopping it altogether.  Trying to hold billowing fabric completely vertical in a heaving ocean would tear it apart if it were even possible to do so.  In fact, billowing and  ‘scalloping’ of the curtain are taken into design consideration based on the selection criteria our GEI Works technical advisors use when helping clients with their site-specific solutions.  Scalloping allows for the additional length of curtain or boom to account for the pockets needed in higher current conditions.  (See Fig. B)   Ballast chain, generally weighs between .65 lbs/ft (1/4” chain) and 1 lbs/ft (5/16” chain) is most often hot-dipped galvanized steel, or can be upgraded to stainless steel for salt-water or long-term products. 
 

On boom, the ballast chain is designed so that the curtain hangs just below the floatation cells so contained (or detained) matter stays at the surface.  On turbidity curtain, longer skirts hang below the water, and the ballast chains at the bottom edge help reduce the chance that the skirt will billow completely, act like a sail, or float to the surface.  Ballast chains are designed to keep the silt curtain ‘mostly vertical,' and not completely vertical.  The ‘mostly vertical’ face is sufficient for the curtain to function as intended in all but extreme circumstances.    

The assumption is often that the ballast chain will hold the curtain skirt (the area below the waterline) completely, even rigidly, vertical in the water.  However, this does not take into consideration hydrodynamic forces on a curtain when underwater or the fact that neither a boom nor a curtain is designed to function in this manner.  The ballast chain does not act as an anchor to hold the curtain or boom in place either vertically or in position relative to shore or anchors.  The ballast chain is not meant to be utilized as a tension member, and it is not intended to be used as a connection point for other tension components such as anchor lines. As it does not act as an anchor to keep the barrier stationary – auxiliary anchoring is required.  (To find out more about anchoring, visit our Turbidity Curtain and Boom Accessories page).
 

Have questions about boom or turbidity curtain? Please don’t hesitate to contact GEI Works for your next project. Call us at +1-772-646-0597.