Drop structures and weirs are small dams placed across a waterway to provide for changes in gradient, slow water velocities and reduce erosion by . Water flow is directed through the weir into a stilling basin where the energy of the flow is dissipated.
Critical factors in the success of such structures are the proper engineering of the drop structure itself to withstand hydraulic pressure and to prevent outflanking. Many weir structures will require a stilling basin.
Structures may vary from low gabion walls to very large earth dams lined with mattresses. They are classified according to the shape of the downstream face at the center of the flow.
The most common weirs are vertical structures. The downstream face of a vertical weir is flush. These structures are often used on small streams, usually in a system of weirs. High vertical weirs require a stilling basin which may be created by constructing a scour apron and counter weir from gabion mattresses.
Stepped weirs differ slightly from vertical weirs. The addition of stepped downstream faces provides for some energy to dissipation at each level. Stepped weirs are appropriate for small structures in waters without heavy sediment loads. Stepped structures are often constructed with some degree of batter.
Where larger structures are required or bearing capacity of soils is limited, sloped weirs are most appropriate. Sloped weirs are ramped on both the upstream and downstream faces. As with vertical structures, sloped weirs may require a stilling basin.
See Project Profiles featuring drop structures/ weirs
Download drawings of a typical drop structures/weirs
Review the Design Guide or Assembly Guide for more information about using Modular Gabion Systems in your application