1. CATEGORY

1.0 – River Training

2. DESIGN STATUS

Level II

3. ALSO KNOWN AS

Buried windrow revetment, riprap fill-trench, setback revetment.

4. DESCRIPTION

Trench fill revetments are constructed by excavating a trench along the top of the bank and placing stone riprap in the trench. As the bank erodes, the stone is undercut and "launches" down the bank line, resulting in a more gradual, protected slope. Earth removed for excavation of the trench may be used to cover the riprap, thus completely concealing it until it is launched (USACE, 1981).

5. PURPOSE

Trench fill revetments are installed to provide lateral stability to a channel by allowing natural erosive processes to form a gentle slope on the streambanks in reaches where some soil loss can be tolerated. The self launching stone provides a less intrusive technique for stabilization when compared to riprap installations on the entire bank, and eliminates the need for smoothing or flattening the slope.

6. PLANNING

7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Excavation of the trench requires disturbance of a corridor along the top bank, and therefore this measure is not recommended for sites where the top bank supports valuable riparian vegetation. Terrestrial habitat values of steep eroding banks where trench fill revetments are normally placed are usually improved, especially if the revetment formed by the launched stone and the top bank area disturbed by excavation is allowed to revegetate. Benthic aquatic habitat at the bank toe may be improved by the addition of stable substrate (Shields et al., 1995).

8. HYDRAULIC LOADING

Permissible shear and velocity for trench fill revetments is related to the size of rock used in construction. Other factors, such as the angularity of the stone, the thickness of the layers of stone, and the angle at which the faces of the stone structure are constructed also come into play. Detailed guidance for sizing stone for bed and bank stabilization structures is beyond the scope of this guideline, and many approaches are available (See Special Topic: Designing Stone Structures). However, the Maynord (1995) equation gives a D₅₀ stone size for an angular stone riprap revetment of 0.875 m (2.9 ft) if the near-bank vertically-averaged velocity is 3.5 m/s (11.5 ft/s), and flow depth = 1 m (3.3 ft), and stone is placed on a bank slope of 1V:1.5H. Use of riprap larger than this is unusual.

9. COMBINATION OPPORTUNITIES

Stone is sometimes piled along the top bank in a linear ridge called a windrow revetment, instead of burying it in a trench.

10. ADVANTAGES

• Eliminates underwater excavation.

• Trench-Fill Revetments are simpler to design than a revetment placed on an actively eroding streambank (Biedenharn, et. al., 1997).

11. LIMITATIONS

• Requires greater stone volume due to uncertainty in the launch process.

• Requires noncohesive bank to properly function.

12. MATERIALS AND EQUIPMENT

Stone size and application rates for demonstration sites along the Missouri River constructed in the late 1970s ranged from 10.4 to 17.9 t/m (3.5 to 6.0 T/ft) of bank protected. Stone sizes used for windrow revetments along larger rivers have included 90 and 180 kg (200-lb and 400-lb) maximum size gradations (Henderson and Shields, 1984).

13. CONSTRUCTION / INSTALLATION

Construction is usually conducted from the top of bank, and care should be exercised in operating heavy equipment at the top of steep banks (Keown, 1983). In addition, geotechnical analyses are recommended to examine the impact on bank slope stability of adding weight in the form of riprap fill. After erosion undermines the trench and stone begins to launch, stone may be added on an “as-needed” basis until the bank stabilizes. Alternatively, if the bank reaches a stable configuration without launching some of the stone, it may be excavated for use elsewhere. Site-specific conditions determine the amount of stone required to reach equilibrium.