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3.1 INTRODUCTION Initial screening is performed to determine whether a channel is straight, meandering, anabranched, or braided. The methodology presented here applies only to meander- ing channels; braided and anabranched streams are screened out. The planform characteristics of meandering streams are classified using a form of the Brice (1975) method that has been modified in the Handbook to cover the geomor- phic stream types that users will likely encounter in the field. Secondary screening is then applied to identify sta- ble meanders based on the degree of channel width unifor- mity. This allows the hydraulic engineer to screen out stable meanders that generally pose little or no threat through lateral migration. 3.2 INITIAL SCREENING Alluvial streams can be classified according to their patterns as single-thread, braided, or anastomosing (see Figure 3.1). The appearance of the stream when viewed on aerial photo- graphs is used to screen out braided and anastomosing chan- nels, which are beyond the scope of the methodologies presented in the Handbook. Only channels identified as meandering will be analyzed beyond the initial screening. 3.3 MEANDER CLASSIFICATION A number of morphological classification schemes for allu- vial rivers were evaluated (Brice, 1975; Schumm, 1977; Schumm and Meyer, 1979; Schumm, 1981; Montgomery and Buffington, 1997; Rosgen, 1994). The utility of different meth- ods was investigated with the aim of producing a classification system that has the following attributes: ⢠It is simple and directly applicable to the meanders encountered in the field. ⢠The classification provides a rational basis for screening out stable and highly unstable patterns. ⢠It is in pictorial format, requiring only a map, aerial pho- tograph, or visual inspection to apply. ⢠Its application does not require field data (e.g., sediment sampling). ⢠It requires minimal training and/or instruction for end users. 14 Based on these criteria, the Brice (1975) method was found to be the most appropriate. However, using the Brice classes to stratify the large data set used in developing the methodology to predict channel migration under NCHRP Project 24-16 revealed that not all the channel classes that Brice identified would commonly be encountered by hydraulic engineers work- ing in the field. Hence, a modified Brice (1975) method was developed. This consists of nine channel categories that were optimized for use in bend classification and secondary screen- ing for meander migration prediction, as shown in Figure 3.2. 3.4 SECONDARY SCREENING Before predicting migration rates for selected bends along a meandering stream, it is necessary to identify whether mean- dering is stable or active. The method employed for this pur- pose is that of Brice (1982), who was able to discriminate qual- itatively between stable and laterally active channels on the basis of the degree of width variability along the course of the stream. Through extensive historical documentation and field observations of streams, he discovered that channels that do not vary significantly in width are either static or relatively sta- ble, whereas channels that are wider at bends migrate actively. In Briceâs study, highly sinuous, equal-width streams were the most stable, equal-width streams of lower sinuosity were slightly active, and wider-at-bend streams had the highest migration rates. Testing Briceâs hypothesis against the larger database assembled for NCHRP Project 24-16 validated this screening procedure. As a result, secondary screening of meandering channels using the Brice width-variability criterion can be used to differentiate between stable meandering streams and chan- nels with actively migrating bends. Bends of stable mean- dering streams can be considered sufficiently stable that bend migration is unlikely to pose a threat to bridges or highway structures; no further analysis of these streams is required. In the modified classification scheme (see Figure 3.2), equal-width rivers in classes B1 and G1 are static or relatively stable and are screened out on this basis. Equal-width, deep, or incised channels of Class A are also screened out. Class A channels may not migrate because they are deeply inset into geologically competent materials such as bedrock, or they may be actively cutting through erodible materials but not migrating laterally to any significant degree. Class F streams are compound channels with a âwanderingâ low-water channel, wide bars, and back channels. Wandering CHAPTER 3 SCREENING AND CLASSIFICATION OF MEANDERING STREAMS
15 streams feature lateral activity that is sporadic and spatially disorganized. Lateral migration in such situations is highly unpredictable, and wandering streams should be screened out as potentially so unstable and unpredictable that further eval- uation would not be likely to produce a meaningful prediction of meander migration. Migration rates for meander bends on rivers classified as any of the remaining categories (B2, C, D, E, and G2) are amenable to prediction by photogrammetric comparison techniques (see Chapters 5 and 7). Illustrated examples of the application of the classification and screening techniques are presented in Chapter 8. Figure 3.1. Alluvial channel pattern classification. Degree of anabranching pertains to anastomosing channels. SOURCE: Thorne, 1997 (modified from Brice, 1975)
16 Figure 3.2. Modified Brice classification of meandering channels. MODIFIED BRICE CLASSIFICATION SCREEN A SINGLE PHASE, EQUIWIDTH CHANNEL INCISED OR DEEP B1 SINGLE PHASE, EQUIWIDTH CHANNEL B2 SINGLE PHASE, WIDER AT BENDS, NO BARS C SINGLE PHASE, WIDER AT BENDS WITH POINT BARS D SINGLE PHASE, WIDER AT BENDS WITH POINT BARS, CHUTES COMMON E SINGLE PHASE, IRREGULAR WIDTH VARIATION F TWO PHASE UNDERFIT, LOW-WATER SINUOSITY (WANDERING) G1 TWO PHASE, BIMODAL BANKFULL SINUOSITY, EQUIWIDTH G2 TWO PHASE, BIMODAL BANKFULL SINUOSITY, WIDER AT BENDS WITH POINT BARS * * * * NOTE: WHERE SCREEN = * , CLASS FALLS OUT DUE TO IMPLICATIONS OF CONSIDERABLE STABILITY OR EXCESSIVE INSTABILITY