OTHER TYPES OF SCOUR

Other scour conditions can affect the bed elevation in a bridge reach. These can result from erosion related to the planform characteristics of the stream (meandering, braided or straight), variable downstream control, flow around a bend, or other changes that decrease the bed elevation. These scour conditions can occur at bridges located upstream or downstream of a confluence of two streams or the confluence of individual braids in a braided stream.

3.8.1 Discussion

In a natural channel, the depth of flow is usually greater on the outside of a bend. In fact, there may well be deposition on the inner portion of the bend at a point bar. If a bridge is located on or close to a bend, scour will generally be concentrated on the outer portion of the bend. Also, in bends, the thalweg (the part of the stream where the flow is deepest and, typically, the velocity is the greatest) may shift toward the inside of the bend as the flow increases. This can increase scour and nonuniform distribution of scour in the bridge opening. In some cases during high flow the point bar may have a channel (chute channel) eroded across it (see FHWA 2012b). This can further skew the distribution of scour in the bridge reach. Consequently, other scour conditions such as these are differentiated from contraction scour which involves removal of material from the bed across all or most of the channel width.

The relatively shallow straight reaches between bendway pools are called crossings. With changes in discharge and stage the patterns of scour and fill can also change in the crossing and pool sequence. These geomorphic processes are discussed in more detail in HEC-20 and HDS 6 (FHWA 2012b and 2001). These processes can contribute to scour in a bridge reach. They are cyclic and may be in equilibrium around some general bed elevation. There are no equations for predicting these changes in elevation. Generally, a study of the stream using aerial photographs and/or successive cross section surveys can determine trends. In this case, the long-term safety of the bridge may depend, primarily, on inspection.

Some unique scour conditions are associated with a particular channel morphology. Braided channels will have deep scour holes when two channels come together downstream from a bar or island (confluence scour). At other times a bar or island will move into the bridge opening concentrating the flow onto a pier or abutment or changing the angle of attack. In anabranching flow, where flow is in two or more channels around semi-permanent islands, there is a problem of determining the distribution of flow between the channels, and over time the distribution may change. The bridge could be designed for the anticipated worst case flow distribution or designed using the present distribution. In either case, inspection and maintenance personnel should be informed of the potential for the flow distribution and scour conditions to change.

Other scour conditions can be caused by short-term (daily, weekly, yearly, or seasonal) changes in the downstream water surface elevation that control backwater and hence, the velocity through the bridge opening. Similarly, a bridge located upstream or downstream of a confluence can experience scour caused by variable flow conditions on the main river and tributary. This scour is reversible and it is considered "scour" rather than long-term aggradation or degradation.

3.8.2 Determining Other Types of Scour

Scour at a bridge cross-section resulting from variable water surface elevation downstream of the bridge (e.g., tributary or downstream control) is analyzed by determining the lowest potential water-surface elevation downstream of the bridge insofar as scour processes are concerned. Then determine contraction and local scour depths using these worst-case conditions.

Scour in a channel bendway resulting from the flow through the bridge being concentrated toward the outside of the bend is analyzed by determining the superelevation of the water surface on the outside of the bend and estimating the resulting velocities and depths through the bridge. The maximum velocity in the outer part of the bend can be 1.5 to 2 times the mean velocity. A physical model study can also be used to determine the velocity and scour depth distribution through the bridge for this case.

Estimating scour in the bridge cross-section for unusual situations involves particular skills in the application of principles of river mechanics to the site-specific conditions. To determine these other types of scour in the bridge opening may require 2-dimensional (2-D) computer programs (for example, FST2DH (FHWA 2003b) or a physical model. Such studies should be undertaken by engineers experienced in the fields of hydraulics and river mechanics.

CHAPTER 4 – SOILS, ROCK, AND GEOTECHNICAL CONSIDERATIONS 4.1 GENERAL

Because scour is caused by the erosive action of flowing water as it excavates and removes earth materials from the beds and banks of streams, it is useful for engineers to have a background and basic understanding of the properties of earth materials as they relate to scour and erosion. In this context, hydraulic forces can be considered a load, and the engineering properties of the soil characterize the resistance to that load. This chapter provides an introduction to the physical properties of soils and rock, and the behavior of these materials at the interface between the channel boundary and the flow field. Figure 4.1 provides examples of scour in soil and rock.

a. Scour in soil (sand) b. Scour in rock (sandstone) Figure 4.1. Photographs of scour in soil and rock.

In the most general sense, earth materials can be categorized as either soil or rock. In the engineering sense, soil can be defined as any unconsolidated geomaterial composed of discrete particles with gases and liquids in between. The maximum particle size that qualifies as soil is not fixed, but depends on the uses and functions to which the material is put, and the ease with which it can be moved and placed. As a rule of thumb, for trench and footing excavations and for the construction of fill in layers, an upper (limiting) particle size for soils can be considered to be about 12 inches in diameter, a practical limit on what a worker can lift by hand (Sowers and Sowers 1970).

For engineering purposes, rock may be defined as any indurated geomaterial that requires drilling, wedging, blasting, or other methods of applying force for excavation. The minimum degree of induration that qualifies as rock has sometimes been defined by an unconfined compressive strength of about 100 to 200 psi (700 to 1400 kPa). From an engineering or functional viewpoint, the definition of rock is complicated by structure and defects, such as jointing and fractures (discontinuities). For purposes of this discussion, indurated earth materials having discontinuity spacings greater than about 4 to 8 inches (0.1 to 0.2 m) can be considered to be more rock-like, whereas materials having smaller spacings may behave more like soils, especially where this spacing frequency is in multiple dimensions.

Intermediate geomaterials (IGMs) are transition materials between soils and rocks. The distinction of IGMs from soils or rocks for geotechnical engineering purposes is made purely on the basis of strength of the geomaterials.

4.2 SCOUR PROCESSES