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B-1 Identifying and delineating banklines is often dependent on the physical scale of the channel, the scale of the aerial photo on which the bankline is being delineated, the resolution and sharpness of the photo, and the density of the vegetation along the bankline. On good quality aerial photos, bankline features are more easily identified on large channels than on small channels. When focusing on a particular location, the optical resolution is generally poorer and the details are less discern- able on aerial photos taken at higher altitudes than on those taken at lower altitudes. Sharp, high-resolution aerial photos allow the user to zoom in or focus closely on a particular loca- tion to more easily identify bankline features. Vegetation types and density play a critical role in accu- rately delineating a bankline, especially when sun angles cre- ate significant shadow effects. In addition, the top of a bank on a color aerial photo is easily identified by the differences in color among the bank, the vegetation, and the water, whereas the top of a bank on a black and white aerial photo can be much more difficult to identify. Because of the difficulty that can be encountered when delineating a bankline and determining the radius of a bend, the following guidelines are provided using a variety of exam- ples for a number of common natural conditions. BANKLINE DELINEATION Banks that are naturally devoid of significant vegetation generally occur in arid or semi-arid climates, such as the desert Southwest and the high plains of the western United States, or in areas where riparian forests are nonexistent or poorly devel- oped. Agricultural activities and urban development tend to remove vegetation along a river corridor, often close to or right up to the edge of the channel. In these cases, the bankline is easily delineated. On a black and white aerial photo, the top of the bank along an unvegetated bankline stands out in contrast to the bank slope and the channel bed or water. The bank slope, when lit by the sun, appears much lighter than the top of the bank, and water in the channel can either be darker or lighter than the top of the bank, depending on lighting conditions and the sediment load of the flow in the channel. When the bank slope is shaded, the shadows provide a sharp contrast among the top of the bank and the bank slope, water, or channel bed (see Figure B.1). Even during near-bankfull flows, the break between the top of bank and the water surface is defined by a sharp change in contrast (see Figure B.2). However, along many rivers and streams, vegetation often partially or completely obscures the banklines, and, depending on the time of year that the aerial photography mission was flown, the vegetation can make delineation of the banklines difficult at best. Aerial photographs obtained in late fall and winter are generally better for delineating banklines because leaf-off conditions exist over much of the country. Photos obtained during the spring and summer months are difficult to use because bank features are obscured by dense leafy vegeta- tion that overhangs and obscures the bankline. When the bankline is only partially visible, the bankline can be delin- eated by connecting the observable portions of the bank as shown in Figure B.3. Because not all aerial photography is flown in the fall and winter when leaf-off conditions exist, in some photos dense vegetation can make bankline identification difficult and bankline delineation less accurate. In these cases, exposed sec- tions of the bank should be identified where possible for use in delineating the bankline. It will then be necessary to examine the banklines over the entire photo to determine the general extent of vegetation overhang. In places where the vegetation ends and exposes the bankline, the approximate amount of overhang can be determined. The bank along obscured sec- tions of the channel edge can be delineated by drawing a line at a distance from the edge of the vegetation equivalent to the approximate amount of the overhang. These concepts are shown in Figure B.4. If the sun is not directly overhead at the time the photo is acquired, the crown of the vegetation at the edge of the bank can be used to locate the bankline approximately (see Figure B.4). At different sun angles, one half of the crown of a tree appears shaded while the other half is in direct sun. The con- trast can be used to approximate the center of the tree, which can be used to approximately locate the bankline, assuming that the tree is located directly on top of and at the edge of the bank and is not leaning out over the channel an apprecia- ble amount. The channel width in obscured reaches should be compared to widths in unobscured reaches to ensure that the obscured reach widths are generally consistent with unobscured reach widths. In some cases, high flows or overbank flooding in combi- nation with overhanging vegetation can also obscure the bankline and make delineation of the bankline difficult. In these cases, the vegetation may aid in identifying the general location of the bankline by changes in contrast as shown in Figure B.5. Where a channel is small compared with the size of the vegetation crown, the banklines and a significant portion of the channel may be completely obscured by overhanging vegetation, as shown in Figure B.6. In these cases, delin- eating the banklines may be nearly impossible, and it may be necessary to define the channel and conduct a meander migration analysis using the channel centerline (see Figure APPENDIX B DELINEATING BANKLINES AND BENDS
B-2 Figure B.1. The Ouachita River during low flow near Arkadelphia, Arkansas: (a) arrows show the top of the bank; (b) the top of the bank is delineated by a dotted line. (a) (b)
B-3 Figure B.2. The Wabash River during high flow near Newport, Indiana: (a) arrows show the top of the bank; (b) the top of the bank is delineated by a dotted line. (a) (b)
B-4 Figure B.3. The Big Black River during low flow near Bovina, Mississippi: (a) arrows show the top of the bank; (b) the top of the bank is delineated by a dotted line. (a) (b)
B-5 Figure B.4. The English River near Kalona, Iowa. Yellow ovals in (a) and yellow arrows in (b) identify exposed bankline sites, and red ovals in (a) and red arrows in (b) identify overhanging vegetation sites. White arrows in (a) identify reaches where banklines are delineated using contrasts and shading of vegetation crown. The delineated banklines are shown as dotted yellow lines in (b). (a) (b)
B-6 Figure B.5. The Hatchie River near Sunnyhill, Tennessee, during high flow: (a) overbank flow in some areas obscures the bankline; (b) the approximate banklines are shown as dotted lines. (a) (b)
B.6). It should be noted, however, that this condition may be indicative of a relatively stable channel or a channel that is sufficiently small that it does not pose a significant threat from migration. Finally, in some cases where channel migration is rapid, sloughs, oxbow lakes, and chutes across point bars may make identification of a continuous bankline difficult. It will be nec- essary in these cases to identify as much of the bankline as pos- sible and then connect the breaks while maintaining a rela- tively constant channel shape. For example, Figure B.7 shows several bends with sloughs intersecting the outer bank, well- developed chutes on the point bars, and the manner in which banklines would be delineated along the bends. BEND RADIUS DELINEATION Bends come in all shapes, as indicated by the Brice mean- der loop classification diagram shown in Figure 7.4. Bends consist of four main categories of bend loop configuration: (1) simple symmetrical, (2) simple asymmetrical, (3) com- pound symmetrical, and (4) compound asymmetrical. For simple symmetrical and asymmetrical bends, the bend radius B-7 is generally easy to define by fitting a single circle to the outer bankline. For complex or oddly shaped bends, the delineation of the bend radius is more difficult and will depend on both the present and past shape of the bend and the portion of the bend that may be posing a hazard. For comparative purposes, the shape of a bend in the past will determine evaluation of the shape of the bend in the pres- ent or future (see Figure B.8). For example, if the bend has been defined by a single loop in the past, it should be evalu- ated, if possible, as a single-loop bend in the present and the future (see the yellow circles in Figure B.9). If the bend has been defined by two distinct loops in the past, it should be evaluated, if possible, as a double-loop bend in the present and the future (see the red and blue circles in Figure B.9). If the most recent bend configuration cannot be defined by a double loop, then it may be necessary to redefine the historic bend configuration as a single loop. As bends migrate or cut off, they may leave behind old chutes, sloughs, or oxbows that may be intersected by other bends, as shown in Figure B.10. When this occurs, delin- eation of the outer bank of the bend should be conducted as if the slough, chute, or oxbow were not there. Figure B.6. The Canoochee River near Claxton, Georgia. The dense vegetation completely obscures the banklines and much of the channel. The approximate channel centerline is shown as the dotted line.
B-8 Figure B.7. The Monroeville Bend on the Sacramento River near Ord Ferry, California, showing delineated banklines, sloughs (S), and chutes (C). Figure B.8. The White River site in Indiana showing the development of a simple symmetrical bend in 1937 into a compound asymmetrical bend in 1966 and 1998. S S S C C C 1998
In addition, the outer bank may be very irregular because of massive geotechnical failures or rapid retreat in areas where there are highly variable bank sediments. When this occurs, the bankline should be delineated as accurately as pos- sible, but a circle that best describes the entire bankline should be used to identify the outer bank radius (Figure B.10). IDENTIFYING CROSSINGS AND RIFFLES Crossings and riffles often occupy the same general location on a riverâthe point where the thalweg or main flow thread crosses from one side of the channel to the other. The riffle or crossing is located approximately midway between two bends, often in the area of overlap between point bars on opposite sides of the channel. In general, the spacing between riffles or crossings is about five to seven channel widths measured along the channel centerline. Figure B.11 shows the position of rif- fles (or crossings) of an idealized meandering channel. If a long straight reach is present between bends, there may be B-9 more than one riffle or crossing separated by alternate bars. The difference between a crossing and a riffle is in the size of the sediment that the channel transports. Riffles are found pre- dominantly in coarse-grained (gravel- and cobble-bed) chan- nels; they rarely occur in sand-bed channels. Riffles and crossings can be identified by changes in the brightness or contrast of the water on an aerial photograph. The riffle will appear as a bright area in the water because of reflection off of the turbulent water created by the riffle. Where riffles are present, flows may be low enough that an exposed or shallowly submerged bar (shoal) can be identified (see Figure B.12). The crossing on a sand-bed channel (see Figure B.13) may be more difficult to identify, especially during higher flows when bars may be inundated. In most instances, the crossing can be identified on aerial photos by changes in the contrast or brightness of flow in the channel, but, in some cases, it may be necessary to locate the crossing by locating the midpoint between two bends or measuring five to seven channel widths from a known crossing location. Figure B.9. The White River site in Indiana showing how the bend could be delineated by a single loop (yellow) or by two loops (red and blue). 1998
B-10 Figure B.10. Aerial photo of the Sacramento River in California showing the location of the delineated bankline (solid yellow line), the probable top of the bank (dotted yellow line), and the circles that best define the outer bank radius (red circles).
B-11 Figure B.11. Idealized meandering channel showing the hypothetical position of riffles (R) and the thalweg (dotted line). Figure B.12. Aerial photo of the gravel-bed Sun River in Montana showing the locations of several riffles identified by circles. Point Bar Point Bar R R R
B-12 Figure B.13. Aerial photo of the sand-bed Wapsipinicon River in Iowa showing the locations of several crossings identified by the circles.
