FLOOD06847

• County: Statewide
• Basin: Statewide
• Title: Bridge Scour and Stream Instability Countermeasures
• Date: 7/1/1997
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

Design Guideline 6

Introduction

Toskanes

Toskanes are concrete armor units which are used as a replacement for riprap (see Figure
6.1). In cases where rock can't be found in suitable sizes, concrete armor units have the
advantage that they can be constructed to meet the design size, mass, and number of units
required to provide protection. Concrete armor units have been used in coastal applications
where very large riprap would be required to resist the impact forces generated by waves.

Background

The Pennsylvania Department of Transportation (PennDOT) contracted with Colorado State
University (CSU) in 1992 to investigate a concrete tetrapod as a countermeasure for local
scour at bridge piers. The purpose of the research was to develop guidelines for selection
and placement of cost-effective tetrapod sizes to mitigate pier scour. A literature review of
concrete armor units used in coastal and river protection works led to the selection of the
Toskane as the primary concrete armor unit for which guidelines where to be developed.
The Toskanes were modified from those used in coastal applications by removing the
pointed corners from the hammerheads, increasing the length and cross section of the
beam, and including reinforcing steel in the beam.

Figure 6,1 CSU Toskanes

Design Guidelines

Hydraulic tests to evaluate the performance of
Toskanes were conducted in an indoor flume and
two outdoor flumes at CSU. Over 400 test runs
were conducted. These tests included random
and pattern placement of Toskanes tested to
failure around piers and abutments, determination
of protective pad radius, determination of pad
height (comparing installations in which the top of
the pad was level with the bed and installations in
which the pad protruded above the bed),
comparison of gravel and geotextile filters, number
of Toskanes per unit area, and effect of angle of
attack on Toskanes at a round nose pier. The
data were analyzed and using dimensional
analysis the significant parameters were
determined. The following design guidelines
reflect the results of the research conducted at
CSU (Fotherby and Ruff 1995):

1. Determine the velocity:
a. Calculate the average velocity of the river directly upstream of the bridge
(approximately 3 m upstream). Consider the number of substructure
elements in the flow at the bridge cross section. If constriction could be
significant, increase the approach flow velocity accordingly.

6.3