LONGITUDINAL STONE TOE WITH SPURS

Useful for Erosion Processes:

	Toe erosion with upper bank failure
	Scour of middle and upper banks by currents
	Local scour
	Erosion of local lenses or layers of noncohesive sediment
	Erosion by overbank runoff
	General bed degradation
	Headcutting
	Piping
	Erosion by navigation waves
	Erosion by wind waves
	Erosion by ice and debris gouging
	General bank instability or susceptibility to mass slope failure

Spatial Application:

	Instream
	Toe
	Midbank
	Top of Bank

Hydrologic / Geomorphic Setting

	Resistive
	Redirective
	Continuous
	Discontinuous
	Outer Bend
	Inner Bend
	Incision
	Lateral Migration
	Aggradation

Conditions Where Practice Applies:

Generally, longitudinal stone toe with spurs is applicable to the same areas which longitudinal stone toe protection is appropriate (see Technique: Longitudinal Stone Toe). The spurs have the added environmental benefits of scour pool formation, edge effects etc. And the spurs can successfully redirect the higher flows away from the bank. The use of this practice should be restricted to reaches with a stable bed and where toe erosion is the primary erosional process. The practice may be used where development of pool habitat and cover is needed. Construction of longitudinal stone toes reduces disturbance of the banks and channel and allows steep, high banks to fail behind the toe to create a stable bank angle. The channel must, however, be wide enough to accommodate spurs. If the stream is not wide enough or the spurs are relatively long, undesirable erosion may occur on the opposite bank.

In addition, stone toe is well-suited for banks experiencing general slope instability. The toe is placed far enough from the top bank so that failure of the bank produces a stable angle (Figure 1). The stone provides additional loading and armoring of the toe to ensure that the newly formed bank is not undermined. Establishment of vegetation on the new bank just landward of the stone toe is essential for long term success.

Longitudinal stone toe can be applied in some situations where the bankline needs to be built back out into the stream, where the existing stream channel needs to be realigned, where the outer bank alignment makes abrupt changes (scallops, coves, or elbows), or where the stream is not otherwise smoothly aligned.

Complexity:

Moderate. Design is simple, usually consisting of layout and specification of a certain rate of stone application for the toe (for example, 1,000 kg/m (666.7 lb/ft)). Spur design is slightly more complex. Local conditions that require filter layers or working around existing infrastructure may complicate matters.

Design Specifications / Typical Drawings:

Spurs may be incorporated in initial design or added to existing toe. Design details (angle, crest elevation, spacing, length) may follow typical values used for Spur Dikes (angle may vary from perpendicular to approach flow to ~15° upstream, spur spacing should be about 1.5 times their length, and length should be about 1/3 the baseflow width), but crests should not be higher than stone toe crests. A review of design guidelines is found in HEC-23 (Lagasse et al. 1997). Kuhnle et al. (1999) studied the relationship between the geometry of a single spur placed in a straight movable-bed flume and concluded that scour hole size (and thus pool habitat volume) is greater for longer spurs, and present empirical formulas for predicting spur scour hole depth and volume. Spurs placed on sand beds devoid of gravel may subside as sand is washed from beneath the stone (Shields, Cooper, Knight, 1995). This problem may be addressed by placing filter fabric or a filter layer of finer stone underneath the stone spur.

Longitudinal stone toe side slopes should be equal to the angle of repose. Typically stone toe applied at a rate of 3 metric tons of stone per lineal m (1 T/ft) of protected bank will have a height of approximately 1 m (3 ft). Stone toe constructed with 6 metric tons per m (2 T/ft) stands approximately 1.5 m (5 ft) tall, whereas 1.5 metric tons/m (0.5 T/ft) is approximately 0.6 m (2 ft) tall.

Longitudinal stone toe must be keyed deeply into the bank at both the upstream and downstream ends (terminal key trenches) and at regular intervals along its entire length (tiebacks). On small streams, 25- 30 m (75-100 ft) spacing between keys (tie-backs) is typical, while on larger streams and smaller rivers, one or two multiples of the channel width can be used as a spacing guide. Excavation of trenches for keys provides a good opportunity for deep planting willow (Salix spp) posts or poles (Figure 3). The toe itself does not need to be keyed into the streambed because of its ability to "self-launch". However, in areas where the bed of the stream is uneven or deep scour holes are evident, the crest of the structure should be constructed to a specific elevation.

The terminal key trenches at the upstream and downstream ends should be excavated into the bank at an angle of approximately 30° with the primary flow direction and of sufficient length that flows will not be able to get around them during the design storm. A gentle angle is important for the end keyways, often referred to as "refusals", because it allows for smooth flow transitions coming into and flowing out of the treated reach. The terminal keys or "refusals" oriented at 90° to the bank have resulted in many failures at the downstream end of the structure, due to flow expansion at that point (Derrick, personal communication, 2000).

Longitudinal Stone Toe Typical Drawing
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Longitudinal Stone Toe with Spurs Typical Drawing
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