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19 3.1 Introduction This chapter outlines the main concepts employed to mit- igate abutment scour and introduces the criteria to be con- sidered when selecting and designing countermeasure concepts. The concepts are evaluated in a preliminary man- ner here, and the merits of selected concepts are investigated in subsequent chapters that present the results of the labora- tory phase of this study. A point to be emphasized is that effective protection against scour, along with good maintenance and repair of waterways, in concert with regular monitoring, are key con- siderations for reliable design-life performance of a bridge waterway. Bridge waterways often are fitted with various scour-protection methods that go a long way to mitigate scour concerns. However, it is quite common for bridge waterways to require maintenance and repair because of damage, or possible impending damage, caused by one or more scour processes. For example, adjustments in upstream channel alignment owing to changes in land use, the abrasive impacts of large flows, and head-cut advance along the down- stream channel may result in wear and tear of bridge water- ways. Figure 3-1 illustrates another situation that may result in the failure of riprap placed on a spill-through abutment. The present chapter outlines and illustrates concepts for scour protection and repair of bridge waterways. Waterway maintenance and repair entails undertaking one or more of the following remedial actions: 1. Approach-channel control. If the damage is attributable to a troublesome approach flow, such as that caused by channel shifting, the approach flow must be controlled to realign its passage through the bridge waterway. 2. Downstream-channel control. If the damage is caused by troublesome changes in the condition of the channel downstream of the bridge, these conditions must be miti- gated so that the bridge is no longer affected by them. It is usual for some channel-control structures to be placed so as to ensure that the adverse condition does not affect the bridge waterway. In western Iowa, for example, a common concern in this regard is the movement upstream of a head-cut or knickpoint in the channel bed. Such knick- point or head-cut migration may expose the foundations of a bridge and cause embankment failure. 3. Armoring of bridge opening. If the foundation of an abutment or a pier is about to be exposed, or an approach embankment or riverbank to be eroded, these locations need to be armored with riprap stone or some other pro- tective surface. 4. Bridge modification. Sometimes it is necessary to modify a bridge to enable better passage of flow through a bridge waterway. The bridge modification may be needed because of a change in the approach channel or to improve an inadequate initial design. 5. Drainage control. Flow draining along the sides of an approach embankment must be discharged into the water- way without eroding the waterway. 3.2 Approach-Flow Control Approach-flow control is intended to guide the approach flow directly through the bridge opening, so that the flow does not expose the bridgeâs piers, abutments, and approach embankments to scour. Flow-control methods seek to streamline the flow through a bridge waterwayâin other words, to minimize a bridgeâs obstruction to flow. It is usual that, for example, approach-flow remediation to reduce the angle between the major horizontal axis of a pier and the approach flow is warranted. There are several options for approach-flow control. These will be further discussed in the literature review section, but are summarized here. The options vary in accordance with the extent to which the approach flow has to be aligned and guided through the bridge opening. In most cases, the layouts C H A P T E R 3 Countermeasure Concepts and Criteria
of flow-control structures are determined on a site-by-site basis. The layouts sometimes require investigation by means of a hydraulic laboratory model or a two-dimensional nu- merical model. Flow control can be summarized as typically requiring the use of one or more of the following structures for the purpose indicated: 1. Guidebanks are fitted to bridge abutments in order to guide flow locally through a bridge opening. Guidebanks are used in situations where a wide flow approaches a bridge opening at an awkward angle or has to be âfun- neledâ through the bridge opening. Guidebanks are often used for guiding floodplain flow through an opening or for guiding flow through bridge openings in broad, braided channels. 2. Hardpoints ensure that channel alignment is maintained in situations where the approach channel may otherwise tend to shift laterally. 3. Spur dikes are fitted to force the realignment of a channel and/or to increase flow velocities. Channel realignment may be needed when an approach channel is shifting later- ally. Increased flow velocities may be needed in situations where a channel has widened, flow velocities decreased, and the approach channel is aggrading. Channel aggrada- tion may reduce the flow area of the bridge opening. 4. Bendway weirs or barbs are fitted to stop lateral shifting of a channel and thereby to redirect the channel optimally through a bridge opening. 5. Vanes are an alternative to spur dikes, bendway weirs, or barbs for use in improving approach channel alignment. Note that, in progressing from channel-control structures 1 through 5 outlined above, the repair effort entails dealing with a widening channel to ensure that the channel passes centrally through the bridge waterway. Additional channel-control methods include: 6. Removal of vegetation and sloughed riverbank material, which is a mundane but very important requirement for ensuring that the bridge opening does not become clogged and that flow within the waterway does not get deflected adversely toward a pier or abutment. 7. Bridge widening or shifting, which is an option when channel control is infeasible. For some bridge sites and approach channels, the most technically and fiscally feasi- ble option is to add a span to a bridge. This option becomes attractive if the bridge opening should be in- creased in area to reduce flow velocities in the opening and if an abutment has experienced damage to the extent that it has been largely washed out. 3.3 Criteria Bridge waterways often are fitted with various scour- protection methods that may largely mitigate scour concerns. This section outlines and discusses the main issues associated with scour countermeasures and the principal criteria required for acceptable performance of a countermeasure method that substantially reduces scour at or near a bridge abutment. The main issues that arise with respect to scour counter- measure selection touch on aspects of technical effectiveness, constructability, durability and maintainability, aesthetics, and environmental impact. These issues comprise the essen- tial considerations underlying selection criteria for scour countermeasures. The literature review chapter discusses the range of options for scour countermeasures to mitigate or lessen scour at bridge abutments. The most common conventional options involve riprap placed in various configurations, and flow guidance by means of guidebanks at the approach to a bridge. In addition, newer countermeasures that show promise are being used, such as alternative materials for armor units (e.g., geobags and cable-tied blocks) and newer forms of structures that work to modify flow behavior (e.g., vanes and purpose- built walls). Whether the countermeasure option being con- sidered is well tried and conventional or is novel, it should be subject to a set of criteria that address the factors associated with implementing the countermeasure. The criteria for selecting a countermeasure usually encom- pass the following set of considerations: â¢ Technical effectiveness (including no substantial adverse effects), â¢ Constructability, â¢ Durability and maintainability, 20 Figure 3-1. Erosion of channel bank in the outlet of a side drain has made this abutment more prone to scour.
â¢ Aesthetics and environmental issues, and â¢ Cost. It is prudent to mention that scour control can be a tricky process. When protecting an abutment against scour or pro- tecting the entire bridge, it is possible that the scour problem is simply shifted elsewhere or that another problem may result. Therefore, when considering possible scour-protection concepts, it is important to evaluate the likely consequences of the protection method. For instance, scour protection of an abutment may concentrate flow locally and aggravate scour at an adjacent pier, and adjustment of the angle at which a channel approaches a bridge opening may result in bank erosion a short distance downstream of the bridge. In the context of bridge waterways, it cannot be assumed that, once set along a prescribed orientation and bed elevation through a bridge waterway, a river or stream channel will hold to its course. 3.4 Technical Effectiveness The technical effectiveness of a countermeasure method is the first consideration in deciding whether to employ the method. The primary measure of technical effectiveness of a counter- measure is its capacity to substantially reduce scour and thereby prevent abutment failure as well as failure of an adjoining com- ponent of a bridge waterway, notably a pier or riverbank. As mentioned above, a further consideration is that the counter- measure does not contribute to a scour problem elsewhere. Experience, especially as obtained from the extensive test- ing conducted for the present project, shows that no counter- measure totally eliminates scour at a bridge waterway. It is common for a countermeasure to reduce scour depth, but also to shift the location of deepest scour. A scour counter- measure may be considered effective if it accomplishes the following goals: â¢ For spill-through abutments, scour is limited such that the embankment side slopes around the abutment structure (e.g., standard stub abutment) do not erode hydraulically or collapse owing to a geotechnical instability. â¢ For wing-wall abutments, the scour depth is limited to be above the top of the pile cap (if the abutment is on a pile foundation) or the top of the footing slab (if the abutment is on a slab footing). â¢ The countermeasure does not inadvertently cause or worsen scour at another location in the bridge waterway. As mentioned at the outset of this report, channel width spanned by a bridge has a significant bearing on the selection of scour countermeasures. Accordingly, it is useful to reiter- ate briefly here that the scour-protection methods applied to short bridges usually are limited to armoring of the bridge waterway and the approach channel. The comparative nar- rowness of a bridged channel makes the use of certain channel- control methods infeasible, as is mentioned subsequently. For narrow channels (as defined in Chapter 2), channel con- trol normally is limited to the use of riprap armoring, possi- bly along with stubby structures like rock hardpoints. The debris-blockage risk for bridges over narrow channels is exac- erbated by scour countermeasures involving the use of rela- tively elaborate or long structures, such as spur dikes or flow-guidance vanes. 3.5 Constructability To be practicable, a countermeasure method must be con- structible. Though some countermeasures may seem to have technical merit in laboratory tests or when sketched out on paper, they may not be constructible. Accordingly, they can- not be implemented. This criterion is especially significant when considering retrofitting a countermeasure to an exist- ing abutment for which site access is very difficult, or when the countermeasure requires a high degree of precision in its placement, but site conditions (e.g., swift flowing water) hamper precise placement. Many rivers flow perennially, and their beds are never dry to allow for easier construction. The construction and placement of a countermeasure in flowing water is a common difficulty that limits the use of some countermeasure methods. Not only is it not always possible to plan the construction schedule for periods of low flow in the river, but physical access to regions beneath the bridge deck can be limited. Locating large equipment under an existing bridge with small clearance is not always feasible. 3.6 Durability and Maintainability The use of durable construction materials is important for the success of a countermeasure, at least with respect to the intended working life of the countermeasure. Any material used as part of a countermeasure method must be able to withstand the potentially severe natural conditions of the river. Depending on bridge location, these conditions may include loads and abrasions owing to contact with water flow, ice, and woody debris. Moreover, scour-induced changes in local bathymetry of the channel may alter structural loads on the countermeasure (e.g., as the countermeasure becomes more exposed to flow or readjusts its disposition). Corrosion and decay may weaken some countermeasure methods (for example, rust and rotting are also common modes of failures of steel cable and wood, respectively). Materials such as stone, steel cable, concrete, geotex- tiles, and wood are sometimes used for countermeasures. 21
Suitably durable stone should be used. Not all stone is suit- able for use as riprap, and some stone has a shorter work- ing life than does other stone; for example, dolomite is not as durable as granite. Steel cable, such as is used in cable- tied blocks, is subject to rust unless coated with epoxy or another protective material. Concrete is durable, but is sub- ject to cracking, spalling, and the corrosion of the internal rebar. Geotextile fabrics can be obtained in various thick- nesses and, therefore, can be quite durable, but lose strength with time. Wood has been used to support or shore up piers and abutments, but is subject to rotting when repeatedly wetted and dried, as occurs in rivers with fluctu- ating water levels. Care must be taken, therefore, to select durable materials for countermeasure construction that are commensurate with the intended life of the countermeasure. The ongoing costs of maintaining a countermeasure can be substantial and, therefore, should be taken into consideration from the onset of the design process. When considered over the entire design life of the bridge, maintenance costs can be more than the initial construction costs. To be kept in mind is the concern that certain scour countermeasures may be dif- ficult to maintain, especially for durations beyond the intended nominal design life of the countermeasure. Some commonly expected maintenance activities include the replacement of dislodged riprap stone and care of grassed side slopes of guidebanks. Ensuring adequate drainage can also pose a maintenance problem for earthfill embankments forming the approach to an abutment or used to form guide- banks. Cable-tied blocks that form large mats for scour pro- tection can be displaced by the uplifting of the upstream row of blocks, and the entire mat might then be moved as a result. Steel cables can corrode, and concrete blocks can be cracked or chipped. Geobags can be torn or lose strength with age. Consideration of maintenance is a real concern in counter- measure selection and design. It is important that the state of a countermeasure method be checked regularly, especially immediately following a major flood flow. One further con- clusion from the present study is that many countermeasures themselves suffer damage while mitigating a serious scour condition at an abutment. 3.7 Aesthetics and Environmental Issues While normally somewhat of a secondary consideration, but of high value regarding public acceptance, the aesthetics of the appearance of a countermeasure method can be an important criterion in countermeasure selection. The degree of importance may depend on the urgency of protecting an abutment or on local attitudes. It is a criterion to be evaluated during countermeasure selection and design; countermea- sures that impair the appearance of the bridge waterway may not be suitable for long-term use. Pertinent environmental issues in abutment countermea- sures include the disruption of the ecology by blocking fish passage along the river, and increased water levels (and thereby flood levels) upstream of a bridge because of flow blockage by a countermeasure. Some countermeasures, such as a low weir or grade-control wall, have the potential to block the migration of fish and amphibians along a stream or river. For example, a check dam extending across the entire width of the river can halt bed degradation attributable to head- cutting, thereby averting the threat of abutment. Other than to point out environmental concerns as a criterion for coun- termeasure selection, the present report does not address environmental aspects of bridge waterways. 3.8 Cost The cost of the countermeasure, or rather the cost-benefit analysis associated with the countermeasure, is a significant criterion in the selection of countermeasures. For an existing bridge, it may be the deciding consideration as to whether the existing bridge should be replaced as opposed to pro- tected. Cost-benefit estimates should include costs and benefits estimated for the design life of the countermeasure. The present project did not address the criterion of counter- measure cost. That criterion must necessarily be evaluated in accordance with the constraints prevailing in the geographic region containing the bridge being considered for counter- measure protection. The ensuing discussion, by way of example, outlines some of the considerations entailed in selecting the extent of armor blankets or the construction of spur dikes. The extent of coverage by blankets of riprap, cable-tied blocks, or geobags is an important consideration in their per- formance as a scour countermeasure. Measuring this cover- age entails consideration of cost; in general, a larger extent of coverage results in better performance, but incurs a higher cost. Better performance occurs as increased protection is provided against such failure mechanisms as edge erosion, uplifting of filter layers, or the washing away of armor units. Edge erosion effects, for instance, can pose a major problem when flow creates scour around the leading block in the case of a protective blanket. Once the flow has created scour around the upstream blocks or rocks, then uplifting can occur that will compromise the integrity of the countermeasure. It is a simple truism that consideration of the failure of a coun- termeasure method is an important aspect of assessing the technical feasibility of a countermeasure. The filter requirements for riprap blankets or cable-tied blocks are important to their effective performance and cost. The finer the pore spaces in a filter cloth, the better the 22
protection against bed material winnowing upward through the filter and riprap and thereby causing undesirable set- tling. It is necessary, to be sure, that the filter cloth must remain porous, but filter cloths with finer pores typically cost more. The largest size of filter that prevents winnowing is sought. In addition, the durability of the filter cloth is important for its long-term performance. As described subsequently in the chapters on the literature review and laboratory experiments on flow control, spur dikes are walls that extend into the waterway and direct flow away from a bank. The choice of construction material for the spur dikes can affect their performance, constructability, cost, and aesthetics. The main materials used for spur dikes are concrete, rock, and timber. Concrete and rock have the advantages of durability, but can be more costly and difficult to construct and maintain. Timber spur dikes are easier to place (for example, by means of pile driving), but may not be as durable. Additionally, to be sure, the dimensions of the spur dike (length, width, and height) are important to performance and cost. The dike with the smallest dimension that effectively reduces scour to an acceptable amount is desirable. The length of a spur dike can excessively push the flow out into the waterway and thereby cause scour on the far bank. If con- structed of rock, then the wall width and height are linked through the angle of repose of the rock. The spacing between spur dikes is important to cost and performance. A close spac- ing may increase the costs, and an excessive spacing between dikes may allow the flow to reattach to the near-abutment bank and not reduce scour. 3.9 Bridges over Narrow Versus Wide Channels For narrow channels, scour countermeasures are limited largely to channel-bank and bed-armoring concepts, rather than concepts aimed at modifying flow distribution within a channel. There is insufficient channel width to accommodate structures intended for redistributing flow within a narrow bridge waterway. A further concern with flow-guidance struc- tures located in narrow channels is that such structures may aggravate the debris blockage of a bridge waterway. In some situations, though, guidebanks are still used to direct flood- plain flow into the bridge waterway. Scour countermeasure concepts for small bridges typically entail the use of riprap, other armoring such as cable-tied blocks, and possibly hardpoints, which essentially act to locally deflect flow away from a vulnerable bank. The ensuing sections of this chapter discuss the main considerations asso- ciated with armor design for abutments. In many situations, narrow channels are not connected to floodplains. Instead, they flow as channels eroded through comparatively steep, undulating, or hilly terrain. Armoring of abutments in these situations poses a problem that heretofore has been inadequately resolved. It is a problem similarly faced when protecting so-called bottomless culverts against scour damage (Kerenyi et al., 2005). The problem concerns ade- quate placement of some form of armor cover at the upstream and downstream corners of the abutment (or bot- tomless culvert) without substantially blocking flow through the bridge waterway. 23