THE KEY TO STABILITY IS THE KEY

Introduction

This topic deals with bank and flank keys. Toe scour keys are also required for many techniques, and those should be designed with the depth of anticipated scour in mind. Some excellent resources for determining scour depths are:

CHANLPRO¹, Maynord, S., Hebler, M., and Knight, S. (1998). User’s manual for CHANLPRO, PC program for channel protection design, Technical Report CHL-98-20, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. (pdf)

Richardson, E.V., & Davis, S. R. (2001) Evaluating scour at bridges: Hydraulic Engineering Circular No. 18, Fourth Edition, FHWA NHI 01-001, Federal Highway Administration, Washington, D.C. http://199.79.179.19/OLPFiles/FHWA/010590.pdf retrieved November 2004

Richardson, E.V., Simons, D.B., & Lagasse, P.F., (2001) River Engineering for Highway Encroachments, FHWA NHI 01-004, HDS-6 (Hydraulic Design Series Number 6), Federal Highway Administration (Office of Bridge Technology), Washington D.C., and the National Highway Institute, Arlington VA, December 2001. http://199.79.179.19/OLPFiles/FHWA/010589.pdf retrieved November 2004.

Protection of flanks

Many streambank protection projects fail at the upstream or downstream end. Local flow acceleration or 'expansion' at the downstream end of the protection project often leads to local scour that progressively undercuts the protection and eats its way upstream. At the upstream end, erosion associated with impinging flow sometimes results in flow above or behind the protective measures and eventual failure. Keys consist of rock riprap or other heavy granular material buried in deep trenches dug at 30° angles (not perpendicular) to flow (see Figure 1). Most continuous techniques that fail do so at the upstream or downstream ends, and the failure is usually a result of improperly aligned keys. Keys oriented perpendicular to flow result in accelerated erosion, expansion and 'eddies' at the downstream end. Always orient keys at a 30° angle to the direction of flow (D. Derrick, personal communication, 2002).

All bank protection systems must be keyed into the bank. Never let the stream scour deeper than, or flank (get behind), bank protection works. Key dimensions should be sized to the stream and stream flows. The volume of the stone per unit length within the key should be greater than or equal to the volume of stone per unit length contained within the river training structure.

Design considerations include:

• Soil types.

• Flow velocities.

• Flood crests, durations and recurrence intervals.

• Active failure areas.

• Vegetation.

• Height of bank.

• Exposed key rocks in banks can be angled to help redirect flow.

• Crests of keys should be flush (even) with the bank, as this will decrease turbulence and chance of scour (see Figure 1). If key rock is exposed on the surface of the bank, it can be angled (using Bendway Weir theory, flow will be redirected perpendicular to the axis of the key) to help redirect flow on the bank back toward the stream.

• Keys at the upstream and downstream ends of continuous longitudinal features should not be at a 90° angle to the feature, but at a 30° angle.

Definitions:

1.    Key:  Consists of rock, stone, or other immovable material buried in the bank in deep trenches at the upstream and downstream ends of the structure.
2.    Tie backs:  Periodic keys that ‘tie’ the structure into the bank; used most frequently with indirect, continuous methods (eg., Longitudinal Stone Toe).
3.    Key roots:  Extensions of keys or tie backs that extend into the overbank areas.

Figure 1. Plan and Cross-section views of Keys

Types of Keys

• Type A Key:  Stone dumped on the bank toe or piled up against the existing bank.
• Type B Key:  The bank is excavated at the toe with stone used as fill in the excavated area.
• Type C Key:  The trench is excavated from the toe, up-bank to a certain elevation (Q-1, Q-2, or Q-10 heights are examples), or to within 1-2 ft of top bank, then backfilled with stone.  If not keyed to the top of bank, keyways can be left open, and excess stone from the project added to increase key height.
• Type D Key:  A Type D Key is a Type C key with the addition of a bankhead (also called root, key root, keyway, stone root, or dig-in) excavated perpendicular (into the bank) from the top of the Type C key, then filled with stone.  A conservative rule-of-thumb for small to medium sized streams is that the length of the key root should equal the sum of the maximum bank height plus the maximum scour depth.
• Standard designs on larger rivers:  Structures on the Red River (base flow channel width varies from 215-300 m (700-1000 ft)) have 30 m (100 ft) key roots into the bank, and on the Mississippi River (500 m to 800 m (1,600 ft to ½ mile) wide), the key roots are 90 m (300 ft) long.

¹ Note: Netscape users may need to access this file directly from the folder on the CD. (HTML > WorksCited > CHANLPRO >CHANLPRO.exe)