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Publication Number: FHWA-HRT-05-072
Date: July 2006

Assessing Stream Channel Stability At Bridges in Physiographic Regions

 

3. FIELD OBSERVATIONS

Numerous stream-bridge intersections were observed across the United States to develop and test the stability assessment method. The streams were to reflect a broad range of stream types and physiographic regions; thus, 57 site visits were conducted in 13 physiographic regions and subregions, including Pacific Coastal, Basin and Range, Trans Pecos, southern Rocky Mountains, Great Plains, Central Lowlands, Interior Lowlands, Ozark-Ouachita Plateau, Appalachian Plateau, Valley and Ridge, Piedmont regions, and Atlantic Coastal Plain.

In addition to collecting observations at streams covering a variety of erosion issues, sizes, and physiographic regions, the following criteria also were used in selecting appropriate sites:

The data for each of the streams are summarized in tables 5-7. Streams that are named N# in table 5 are unnamed on topographic maps. Table 5 provides the locations and global positioning system (GPS) coordinates of the bridges, the physiographic Province, land use, and stream classification. Each of the channels was classified according to the Montgomery-Buffington scheme. The Montgomery-Buffington scheme does not include engineered or altered channels. However, this method is still useful as a basic descriptor of the primary processes in the stream (e.g., transport versus response) and is, therefore, included. To include altered streams, the USACE method (10) and a simple observation of channel pattern (based on both field observation and aerial photos) also were used to classify or categorize the stream types. The resulting stream type is provided as a combination of these methods in table 5. The Rosgen classification method was not considered because it is unnecessarily data intensive for the purposes of assessing channel stability in the vicinity of a bridge. Table 6 provides the bed and bar material, the percent of sand (Fs), and any controls observed in the banks or on the bed. Table 7 provides observations made on the banks, including vegetation, bank material, bank height, and any erosion characteristics. In the next section, observations made in each of the physiographic regions are described.

Table 5. River data summary.
River Map Location GPS Location Physiographic Province M-B/USACE Class* Land Use Channel Pattern
N. Latitude (deg) W. Longitude (deg)
1. Saline
U.S. Rt. 183 22.5 kilometers (km) N of Hays, KS 39.0973 99.3055 Great Plains D/MA Cultivated Meandering
2. S. Fork Solomon R. U.S. Rt. 283 1.6 km S of Hill City, KS 39.3506 50.8457 Great Plains D/MA-BR Cultivated Meandering to braided
3. N. Rush Cr. State Route (S.R.) 71, S of Limon, CO 39.0609 103.7035 Great Plains D/MA Managed/grass Meandering
4. Arkansas R. S.R. 291, N of Salida, CO 38.6127 106.0618 Rocky Mtn. R/MA Natural/ cultivated Meandering
5. Tomichi Cr. S.R. 114, 12.9 km E of Gunnison, CO 38.5202 106.7852 Rocky Mtn. R/MA,MO Cattle pasture Meandering
6. Murietta Cr. Main St., Temecula, CA 33.4924 117.1499 Pacific Coastal D/MA-BR, MO Suburban Meandering
7. Jacalitos Cr. Jayne Ave., 9.7 km E of U.S. Interstate (I)-5 exit to Coalinga, CA 36.1369 120.2771 Pacific Coastal D/MA-BR Cattle grazing Meandering to braided
8. Dry Cr. Dry Creek Road, CA 38.4114 122.4513 Pacific Coastal C/MT- MA Wooded Irregular
9. Dutch Bill Cr. Bohemian Hwy., 9.7 km E of Oakville, Napa Valley, CA 38.4239 122.9569 Pacific Coastal R-C/MA-MT Wooded Irregular
10. Buena Vista Cr. S.R. 58 1.6 km E of Buttonwillow, CA 35.3995 119.5321 Pacific Coastal B/MA-BR Cattle grazing Meandering to braided
11. Mojave R. 1st Ave., Barstow, CA N/A N/A Basin and Range B/BR Rural/industrial Braided

*C = cascade, S = step pool, P = plane bed, R = pool-riffle, D = dune-ripple, B = braided, MT = mountain torrent, MA = meandering, MO = modified, S.R = State Route, Cr. = Creek, R. = River

Table 5. River data summary, continued.
River Map Location GPS Location Physiographic Province M-B/USACE Class* Land Use Channel Pattern
N. Latitude (deg) W. Longitude (deg)
12. Rt. 66 Wash Rt. 66, E of Ludlow, CA 34.7160 116.1053 Basin and Range B/AR Natural Arroyo/braided
13. Sacramento Wash S.R. 68, 27.4 km E of Lauchlin, AZ 35.2250 114.2800 Basin and Range R-B/MA-BR Rural/mining Meandering to braided
14. Rio San Jose S.R. 6, 1.6 km S of I-40, Exit 126, NM 34.9675 107.1749 Trans Pecos R/MA,MO Grazed/natural Meandering, channelized
15. Rio Puerco S.R. 6, 24.2 km E of Los Lunas, NM 34.7966 106.9905 Trans Pecos R/MA,MO Grazed/natural Meandering, channelized
16. W. Elk Cr. E. Third St. (Rt. 66), W of Elk City, OK 35.4119 99.4522 Central Plains R/MA,MO Suburban, cattle Meandering, channelized
17. Beaver Cr. U.S. Rt. 183, ~1.6 km N of Arapahoe, OK 35.5941 98.9602 Central Plains R/MA,MO Agricultural, cattle Meandering
18. Brush Creek U.S. Rt. 62, 1.6 km E of Jacktown, OK 35.5068 96.9862 Central Plains R/MA Agricultural, grazed Meandering
19. Unnamed U.S. Rt. 62, 1.6 km E of Boley, OK 35.4862 96.4594 Central Plains R/MA Natural, grazed, rural Meandering
20. Little Skin Cr. U.S. Rt. 64, 1.6 km W of Muldrow, OK 35.3982 94.6211 Ozark-Ouachita Highlands R/MA Natural, rural Meandering
21. Unnamed U.S. Rt. 64, at Dyer, AR 35.4964 94.1363 Ozark-Ouachita Highlands R/MA, MO Rural Meandering, straightened
22. Little Cypress Cr. S.R. 59, 1.6 km S of I-40, Exit 35 35.3402 89.5018 Coastal Plain R/MA Agricultural, rural Meandering

*C = cascade, S = step pool, P = plane bed, R = pool-riffle, D = dune-ripple, B = braided, MT = mountain torrent, MA = meandering, MO = modified, S.R = State Route, Cr. = Creek, R. = River

Table 5. River data summary, continued.
River Map Location GPS Location Physiographic Province M-B/USACE Class* Land Use Channel Pattern
N. Latitude (deg) W. Longitude (deg)
23. Unnamed U.S. Rt. 70, 10.5 km W of Jackson, TN 35.6148 88.9909 Coastal Plain R/MA Agricultural, rural Meandering
24. Unnamed U.S. Rt. 79, 3.2 km SE of Milan, TN 35.8910 88.8105 Coastal Plain R/MA Agricultural, rural Meandering
25. Honey Run S.R. 76, 0.8 km E of White House, TN 36.4779 86.6397 Interior Low Plateau P/MA Rural Slightly meandering
26. South Fork S.R. 84 just NE of intersection with Rt. 357 37.5430 85.7685 Interior Low Plateau P/MA Agricultural, rural Meandering
27. East Fork S.R. 55 at intersection with U.S. Rt. 62, 3.2 km S of Bloomfield, KY 37.8816 85.3029 Interior Low Plateau P/MA Agricultural, rural Meandering
28. Unnamed U.S. Rt. 60 3.2 km E of I-40, Exit 101, ~ 9.7 km W of Sterling, KY 38.0443 83.9933 Interior Low Plateau P/MA Rural, grazed Meandering
29. McKnown Cr. U.S. Rt. 119 N at Robinson Rd., 2.4 km S of Walton, WV 38.5934 81.3792 Appalachian Plateau P/MA Rural Meandering
30. Wolf Run U.S. Rt. 199 N, 0.8 km S of Gandeville, WV 38.6896 81.3886 Appalachian Plateau R/MA Rural Meandering
31. Alligator Cr. S.R. 765, S of Punta Gorda, just S of US Rt. 41 26.8884 82.0213 Coastal Plain D/MA Suburban Meandering
32. Peace R. S.R. 70, 1.6 km W of Arcadia, Florida 27.2213 81.8766 Coastal Plain D/MA Suburban Meandering

*C = cascade, S = step pool, P = plane bed, R = pool-riffle, D = dune-ripple, B = braided, MT = mountain torrent, MA = meandering, MO = modified, S.R = State Route, Cr. = Creek, R. = River

Table 5. River data summary.
River Map Location GPS Location Physiographic Province M-B/USACE Class* Land Use Channel Pattern
N. Latitude (deg) W. Longitude (deg)
33. Blackrock Run Stringtown Road off S.R. 25, N of Butler, MD 39.5437 76.7331 Piedmont R/MA Suburban, agricultural, natural Meandering
34. Indian Run Benson Mill Rd, off S.R. 25, N of Butler, MD 39.5691 76.7445 Piedmont R/MA Natural, agricultural, rural Meandering
35. Middle Patuxent R. S.R. 108, N of Columbia, MD 39.2290 76.9173 Piedmont R/MA Natural, suburban Meandering
36. Hammond Branch Stephens Rd., 2.4 km N of Laurel, MD 39.1318 76.8449 Coastal Plain R/MA Suburban, agricultural Meandering
37. Atherton Tributary Seneca Dr., Columbia, MD 39.1871 76.8629 Piedmont R/MA Suburban Meandering
38. Stocketts Run Sands Rd., 4.8 km SW of Davidsonville, MD 38.8831 76.6638 Coastal Plain R/MA Natural, rural Meandering
39. Mill Stream Branch S.R. 213, just S of Centreville, MD 39.0401 76.0722 Coastal Plain R6D/MA, MO Agricultural, rural Meandering
40. Kent County Tributary S.R. 446 (Broadneck Rd.), SW of Chestertown, MD 39.2039 76.1235 Coastal Plain R6D/MA Agricultural Meandering
41. Morgan Creek Kennedyville Rd., 1.6 km E of Kennedyville, MD 39.2969 75.9845 Coastal Plain D/MA Agricultural Meandering
42. Little Elk Cr. Little Elk Rd, 1.2 km N of PA-MD line, 2.4 km S of Hickory Hill, PA 39.7271 75.9078 Piedmont P6R/ MA Agricultural, rural Meandering

*C = cascade, S = step pool, P = plane bed, R = pool-riffle, D = dune-ripple, B = braided, MT = mountain torrent, MA = meandering, MO = modified, S.R = State Route, Cr. = Creek, R. = River

Table 5. River data summary.
River Map Location GPS Location Physiographic Province M-B/USACE Class* Land Use Channel Pattern
N. Latitude (deg) W. Longitude (deg)
43. Big Beaver Cr. Kuntz Mill Rd., off U.S. Rt. 222, 6.4 km N of Quarryville, PA 39.9411 76.2204 Piedmont R/MA Agricultural Meandering
45. Roaring Run PA Rt. 445, E of Nittany, PA 40.9810 77.5442 Ridge and Valley S/MT Natural Step-pool
44. Buffalo Run Fillmore Rd. (S.R. 3008) near State College, PA 40.8595 77.8769 Ridge and Valley P/MA, MO Agricultural/ rural Meandering/ straightened
46. Potter Run S.R. 144 at Potters Mills, PA 40.8013 77.6257 Ridge and Valley S/MT Natural/rural Step-pool to meandering
47. Bentley Cr. S.R. 4013 (Berwick Turnpike), about 1.6 km S of Bentley Creek, PA

N/A

N/A

Glaciated Appalachian Plateau R/MA, MO Rural residential Meandering to braided
48. N 48 S.R. 58, 1.6 km W of Allegheny R., PA, S of I-80, 41.1340 79.6944 Appalachian Plateau P, M/MT Natural/rural Plane bed, meandering
49. Reids Run S.R. 68, S of I-80 at Reidsburg, PA 41.1469 79.4020 Appalachian Plateau P, M/MT Natural, agricultural, rural Plane bed, meandering
50. Piney Creek S.R. 66, 0.8 km S of Limestone, PA 41.1281 79.3277 Appalachian Plateau P, M Rural, agricultural Plane bed, meandering
51. Little Sandy Cr. S.R. 3011 at East Branch Station, PA 41.0327 79.0509 Appalachian Plateau P, M Agricultural, rural Meandering
52. Trout Run S.R. 310, 1.6 km S of Reynoldsville, PA 41.0787 79.9016 Appalachian Plateau P, M Natural, agricultural, rural Meandering

*C = cascade, S = step pool, P = plane bed, R = pool-riffle, D = dune-ripple, B = braided, MT = mountain torrent, MA = meandering, MO = modified, S.R = State Route, Cr. = Creek, R. = River

Table 5. River data summary, continued.
River Map Location GPS Location Physiographic Province M-B/USACE Class* Land Use Channel Pattern
N. Latitude (deg) W. Longitude (deg)
53. Pootatuck R. Walnut Tree Hill Rd. near Sandy Hook, CT 41.4376 73.2702 New England P-R, MA Suburban, natural Meandering
54. Mill R. Judd Rd., N of Easton Reservoir, CT 41.3017 73.2760 New England P, MA, MO Natural Meandering, straightened
55. Aspetuck R. Silver Hill Rd. at Easton, CT 41.2596 73.3249 New England P, MA, MO Natural, rural Meandering, straightened
56. W. Br. Saugatuck R. Stonebridge Rd. at Wilton, CT 41.1949 73.3875 New England P, MA Natural, suburban Meandering
57. Mianus R. June Rd., N of Merritt Pkwy, Stamford, CT 41.1048 73.5867 New England P, MO Natural, suburban Meandering, straightened

*C = cascade, S = step pool, P = plane bed, R = pool-riffle, D = dune-ripple, B = braided, MT = mountain torrent, MA = meandering, MO = modified, S.R = State Route, Cr. = Creek, R. = River

Table 6. River channel data.
River w/y Bed Controls Bank Controls Fs (%) Bed Material Bar Type Bar Material Bar Width Bar Vegetation
1. Saline

20

None Abutments 80 Sand Alternate Sand 1/2 W Grasses
2. S. Fork Solomon R.

17

None Abutments 100 Sand Alternate Sand 1/3 W None/grass
3. N. Rush Cr.

15

None None 80 Sand None None N/A N/A
4. Arkansas R.

14

Gravel and cobble armor Boulders, gravel armor, bridge abutments 30 Cobbles Point bars, midchannel Very coarse gravel-cobbles 1/5 W Minimal
5. Tomichi Cr.

19

None Right bank riprapped 50 Very fine gravel Midchannel (d/s only) Unknown < 1/5 W Shrubs
6. Murietta Cr.

9

None

None 100 Sand Alternate Sand 1/2 W Heavily vegetated
7. Jacalitos Cr.

23

None

Riprap at both bridge abutments 90 Sand Irregular/ combo Coarse sand > 1/6, collectively very wide Minimal
8. Dry Cr.

11

Boulders and occasional bedrock

Occasional bedrock and abutments 10 u/s Cobbles

None u/s;

Alternate/ irregular d/s

Sand 1/2-2/3 W None
9. Dutch Bill Cr.

10

None u/s

Bridge abutments 50 u/s Fine gravel Point bars Gravel 2/3 W None
10. Buena Vista Cr.

7.5- 15

None

None 100 Sand Irregular Sand 1/2 W None
11. Mojave R.

6.7

None

Piers, abutments 100 (fine) Sand Braided Sand Wide None

Fs = portion of sand, u/s = upstream, d/s = downstream, W = width, R. = River, Cr. = Creek

Table 6. River channel data, continued.
River w/y Bed Controls Bank Controls Fs (%) Bed Material Bar Type Bar Material Bar Width Bar Vegetation
12. Rt. 66 Wash

10

Boulders Boulders, bridge protection, abutments 70 Sand, gravel Irregular Sand, gravel Moderate None
13. Sacramento Wash

20

None Bridge protection, piers, abutments 75 Clay, silt, sand, gravel Irregular Sand and gravel Moderate None
14. Rio San Jose

5

None Riprap, piers, abutments 70 Clay, silt, sand None N/A N/A N/A
15. Rio Puerco

16

None Clay/silt cliffs, piers, bank stabilization 20 Silt, clay, sand Alternate, point Silt, clay Wide Grasses
16. W. Elk Cr.

9

Water line, debris None 100 (silt) Silt None N/A N/A N/A
17. Beaver Cr.

< 4

None Bridge protection, soil blocks 100 (silt) Clay, silt Irregular Clay/silt Moderate None
18. Brush Creek

5

Boulders from riprap Bridge protection, bank stabilization 100 (silt, sand) Clay, silt, sand Irregular Sand, silt, little gravel None Narrow
19. Unnamed

15

Boulders Boulders, bridge protection, cliffs 100 Silt, sand Irregular Very fine sand Wide None, grasses

Fs = portion of sand, u/s = upstream, d/s = downstream, W = width, R. = River, Cr. = Creek

Table 6. River channel data, continued.
River w/y Bed Controls Bank Controls Fs (%) Bed Material Bar Type Bar Material Bar Width Bar Vegetation
20. Little Skin Cr.

12

Beaver dams, debris Bedrock, bridge protection

40

Sand, gravel Alternate bars Gravel Grasses, shrubs Moderate
21. Unnamed

5

Debris Bars, debris

80

Silt, sand, gravel Only under bridge Sand N/A N/A
22. Little Cypress Cr.

9

Few boulders Bridge abutments and pier

80

Sand w/ silt, gravel Irregular Sand Narrow None
23. Unnamed

9

Bridge protection Debris, bridge protection

100

Sand Irregular Sand Wide None
24. Unnamed

9

Grade control Bridge protection, piers

100

Sand Irregular d/s only (u/s pooled) Sand Wide None
25. Honey Run

12

Outcrop Piers, bars

30

Gravel Alternate Gravel Narrow Grass/none
26. South Fork

9

None Piers

30

Gravel None N/A N/A N/A
27. East Fork

9

Boulders, gravel armor Bridge protection

50

Sand, gravel None N/A N/A N/A
28. Unnamed

12

Occasional bedrock, boulders Occasional bedrock, abutments

20

Sand, gravel, cobbles None N/A N/A N/A
29. McKown Cr.

6

Gravel armor Bridge protection

10

Gravel None N/A N/A N/A

Fs = portion of sand, u/s = upstream, d/s = downstream, W = width, R. = River, Cr. = Creek

Table 6. River channel data, continued.
River w/y Bed Controls Bank Controls Fs (%) Bed Material Bar Type Bar Material Bar Width Bar Vegetation
30. Wolf Run

5-8

Gravel armor, occasional bedrock, grade control at bridge Clumps of failed bank material

10

Gravel Irregular (mostly from failure material) Gravel Moderate None
31. Alligator Cr.

24

None Confined left bank concrete wall or rock lined

18

Silt None N/A N/A N/A
32. Peace R.

20

None

Gravel armor (bank protection), bridge riprap, debris

100

Very fine sand Irregular/ combination Very fine sand Wide, 1/2 W Grasses, reeds, trees
33. Blackrock Run

5.5-17

Bedrock

Bedrock, abutments

65

Sand, gravel Irregular Sand, gravel Narrow None
34. Indian Run

5

Debris Bridge riprap, abutments, fiber logs, debris

50

Sand, gravel, cobbles Irregular Sand, gravel Wide Grasses
35. Middle Patuxent R.

16

Gravel armor Bedrock, gabions, riprap at bridge

20

Sand, gravel Mid, point Sand, gravel Narrow None
36. Hammond Branch

7

Boulders Bridge protection, abutments, riprap

60

Sand, gravel None N/A N/A N/A

Fs = portion of sand, u/s = upstream, d/s = downstream, W = width, R. = River, Cr. = Creek

Table 6. River channel data, continued.
River w/y Bed Controls Bank Controls Fs (%) Bed Material Bar Type Bar Material Bar Width Bar Vegetation
37. Atherton Tributary

16

Bedrock, boulders Boulders, abutments

40

Sand, gravel, boulders Irregular Sand Narrow None
38. Stocketts Run

16

None Abutments

50

Sand, gravel Point Sand, gravel Moderate None
39. Mill Stream Branch

13-16

None Abutments, debris

100

Sand Alternate Sand Narrow None
40. Kent County Tributary

3

None Abutments, concrete slabs

80

Silt, sand, gravel Irregular Silt, sand Wide Grasses
41. Morgan Cr.

10

None Riprap, abutments

80

Silt, sand None N/A N/A N/A
42. Little Elk Cr.

20

Boulders Bedrock, boulders, bridge protection, abutments

20

Sand, gravel, cobbles, boulders Point Gravel, cobbles Narrow None/grasses
43. Big Beaver Cr.

20-29

Gravel armor (d/s) None

20

Silt, sand, gravel, cobbles Midchannel (u/s), point (d/s) Silt (u/s), sand, gravel (d/s) Wide None
44. Buffalo Run

10

Gravel armor, u/s weir Bridge protection

30

Sand, gravel None N/A N/A N/A
45. Roaring Run

20

Boulders, riprap from failed bridge protection Bridge protection, bank stabilization

10

Gravel, cobbles, boulders Alternate (d/s only; u/s step-pool) Gravel, cobbles Moderate None

Fs = portion of sand, u/s = upstream, d/s = downstream, W = width, R. = River, Cr. = Creek

Table 6. River channel data, continued.
River w/y Bed Controls Bank Controls Fs (%) Bed Material Bar Type Bar Material Bar Width Bar Vegetation
46. Potter Run

15

Gravel/cobble armor Bridge abutments, bank stabilization

30

Sand, gravel, cobbles None N/A N/A N/A
47. Bentley Cr.

32

None Abutments

60

Gravel Point, midchannel Gravel 1/4-2/3 W None
48. N 48

15

Gravel/cobble armor Boulders, abutments, bank stabilization

10

Gravel, cobbles None N/A N/A N/A
49. Reids Run

15

Gravel/cobble armor, boulders Boulders, abutments

10

Gravel, cobbles, boulders None N/A N/A N/A
50. Piney Creek

10

Gravel armor Bridge protection, abutments, bank stabilization

< 20

Gravel, cobbles Point bar Gravel Moderate None
51. Little Sandy Creek

18

Gravel armor Abutments

< 20

Gravel, cobbles, boulders Midchannel Gravel Grasses Narrow
52. Trout Run

10

Gravel armor Bridge protection, abutments

30

Gravel, cobbles None N/A N/A N/A
53. Pootatuck R.

28

Bedrock, boulders, gravel armor Boulders, abutments

10

Gravel, cobbles, boulders Irregular Gravel, cobbles Narrow None

Fs = portion of sand, u/s = upstream, d/s = downstream, W = width, R. = River, Cr. = Creek

Table 6. River channel data, continued.
River w/y Bed Controls Bank Controls Fs (%) Bed Material Bar Type Bar Material Bar Width Bar Vegetation
54. Mill R.

13

Gravel armor, beaver dam d/s Abutments

50

Sand, gravel Alternate d/s Sand, gravel Narrow None
55. Aspetuck R.

19

Gravel riffles Boulders, bridge protection, bank stabilization

60

Sand, gravel Alternate d/s (minor) Sand Narrow None
56. W. Br. Saugatuck R.

45

Boulders Boulders, abutments, debris jams, islands

40

Sand, gravel, cobbles Midchannel, islands Sand, gravel, cobbles Moderate Shrubs, trees
57. Mianus R.

13

Gravel armor (d/s of dam) Boulders, abutments, bank stabilization

40

Gravel, cobbles None N/A N/A N/A

Fs = portion of sand, u/s = upstream, d/s = downstream, W = width, R. = River, Cr. = Creek

Table 7. River bank data.
River Material Bank Angle (deg) Bank Height (m) Vegetation Erosion Location Bank Failure Locations Exposed or Bare Banks
1. Saline R. Silty clay loam 30-60 0.9 Grass, deciduous trees; single dense and continuous band; healthy, vertically oriented None None Minimal
2. S. Fork Solomon R. Sand 30-60 0.6 Grass, reeds; no trees on channel banks; but in flood plain Opposite and behind bars; adjacent to structures No mass wasting Frequent, but no cohesion
3. N. Rush Cr. Loam/sand 60-80 0.9 Grasses General fluvial; outside meander bends Outside meanders Outside meanders high, vertical
4. Arkansas R. Sand, gravel, cobbles, silt 30-60 1.8 Grass, shrubs, deciduous and coniferous trees, healthy, vertical Along straight reaches Minor Minor
5. Tomichi Cr. Silty clay Vertical 0.9 Grass, few deciduous trees (riparian, not bank) Minor None None
6. Murietta Cr. Sandy silty clay 60-80 1.5-2.4 Reeds, sparse deciduous trees well back from bank in flood plain, vertical General, along channel banks General; bank slides visible Frequent, but sand so collapse
7. Jacalitos Cr. Sand, gravel 30-60 (Vertical at outside bends) 0.9 Grass, deciduous trees General fluvial, outside meanders Outside meanders, recent slides Occasional
8. Dry Cr. Silty clay Vertical 0.6-4.6 Dense deciduous trees; healthy, diverse, some leaning 30E Outside meander; upstream of structure General where too steep Minor
9. Dutch Bill Cr. Silt Vertical 0.9-6.1 (incised) Deciduous trees, some in channel (lateral migration) Fluvial outside meander Minimal Some u/s of right abutment

LB = left bank, RB = right bank, u/s = upstream, d/s = downstream, R. = River, Cr. = Creek

Table 7. River bank data, continued.
River Material Bank Angle (deg) Bank Height (m) Vegetation Erosion Location Bank Failure Locations Exposed or Bare Banks
10. Buena VistaCr. Sand/silt

60-90

0.6 Sparse shrubs General, irregular Slides in sand Frequent
11. Mojave R. Sand

60-90

1.8 None General Continual slides Continual
12. Rt. 66 Wash Clay, silt, sand

70-90

0.6-1.8 None General Frequent Continual
13. Sacramento Wash Clay, silt, sand, gravel

60-90

0.9 Few shrubs General Frequent slides, slumps Continual
14. Rio San Jose Clay, silt, sand

60-90

1.8 Desert shrubs Outside meanders, general Outside meanders Continual
15. Rio Puerco Clay, silt

80-90

1.5 Grasses, shrubs (desert) Outside meanders, general Outside meanders Continual
16. W. Elk Cr. Clay, silt

70-90

1.5 Grasses, shrubs, few trees General Where hoof damage, steep banks Continual
17. Beaver Cr. Silt

90

2.4 Grasses, few bushes General Both banks-overheightened Continual
18. Brush Creek Silt

80

1.2-9.1 (top) Sparse trees General Minor, both banks Continual
19. Unnamed Clay, silt, sand

40-60

1.2-4.6 Sparse trees, grass, shrubs Outside meander bends RB Frequent
20. Little Skin Cr. Clay, silt

60

1.5 Grass, shrubs, trees (moderately dense) Outside meander bend, general Mostly d/s Occasional

LB = left bank, RB = right bank, u/s = upstream, d/s = downstream, R. = River, Cr. = Creek

Table 7. River bank data, continued.
River Material Bank Angle (deg) Bank Height (m) Vegetation Erosion Location Bank Failure Locations Exposed or Bare Banks
21. Unnamed Clay, silt (main), sand 70-90 0.9 Grass, shrubs, dense trees General None Minor
22. Little Cypress Cr. Clay, silt 90 0.9-3.7 Grass, dense trees General Outside bend Continuous
23. Unnamed Clay, silt, sand 80 2.4 Grass, dense trees General, outside bends General-overheightened banks Frequent
24. Unnamed Clay, silt 60-70 4.6 (levee) Grass, dense trees General None Occasional
25. Honey Run Clay, silt 60-90 0.9-1.8 (top of mass wasting) Grass, dense trees General Along RB, especially where vegetation removed Occasional
26. South Fork Clay, silt 70-80 2.4 Shrubs, sparse trees General Along both banks Frequent
27. East Fork Clay, silt 70 1.2 Grass Outside bend, general None Occasional
28. Unnamed Clay, silt 60-90 0.8 Shrubs, dense trees General (minor u/s) None Occasional
29. McKown Cr. Clay, silt 60-90 1.2 (u/s), 0.8 (d/s) Grass u/s, d/s grass on left bank, one row of trees on right General U/s only along banks Occasional
30. Wolf Run Clay, silt, gravel 60-90 1.2 Grass General, outside, inside bends Both banks Frequent
31. Alligator Cr. Silt, sand LB vertical, RB moderate 0.8 LB grass, RB dense trees LB none (concrete), RB outside meander bend (minor) None None

LB = left bank, RB = right bank, u/s = upstream, d/s = downstream, R. = River, Cr. = Creek

Table 7. River bank data, continued.
River Material Bank Angle (deg) Bank Height (m) Vegetation Erosion Location Bank Failure Locations Exposed or Bare Banks
32. Peace R. Sand Steep outside bend, moderate elsewhere 3 Dense trees Opposite bar, outside meander bends Left bank, outside bends Minor both banks
33. Blackrock Run Clay, silt, sand 50-80 0.9-2.7 Shrubs, trees Outside meander, opposite bar General-overheightened banks Occasional
34. Indian Run Clay, silt 40-80 0.8-1.8 Sparse trees LB, grass RB General, opposite obstructions RB where no vegetation Occasional
35. Middle Patuxent R. Clay, silt 40-60 1.5-1.8 Shrubs, dense trees in good condition Outside meander bends, general Limited on RB where high banks slough Frequent on both banks
36. Hammond Branch Clay, silt 60-80 1.5 Grasses, sparse trees, falling into stream, poor condition Outside meander, general Minor Continuous
37. Atherton Tributary Clay, silt 60-80 2.5-6 Sparse trees LB, dense trees RB, falling trees both banks Outside meander None u/s, RB d/s where trees removed Occasional
38. Stocketts Run Clay, silt 30-50, 80-90 outside bends 0.8-1.8 Shrubs, dense trees, falling on either side Outside meander, general None Frequent
39. Mill Stream Branch Clay, silt 40-60 0.9-2.4 Sparse falling trees LB, dense falling trees RB General None Occasional

LB = left bank, RB = right bank, u/s = upstream, d/s = downstream, R. = River, Cr. = Creek

Table 7. River bank data, continued.
River Material Bank Angle (deg) Bank Height (m) Vegetation Erosion Location Bank Failure Locations Exposed or Bare Banks
40. Kent Co. Tributary Clay (minor gravel toward bottom) 30-80 (lower where bank failed) 2.4 Grasses, sparse falling trees General Both banks, continuous Frequent
41. Morgan Cr. Clay, silt 30-80, ragged 0.9 Shrubs, sparse trees, trees upright on both banks General, ragged, irregular banks Minor Continuous
42. Little Elk Cr. Clay, silt, sand 25-34 0.8-0.9 Shrubs, trees, trees upright in good shape Outside meander bend None Occasional
43. Big Beaver Cr. Clay, silt, sand Vertical u/s, 30-40 d/s 1.2-2.1 (highly variable) Grasses u/s, trees beyond 122 m u/s Outside meander, opposite bar/obstruction Both banks everywhere for 122 m u/s Continuous
44. Buffalo Run Clay, silt Steep 0.8 Grass, shrubs General fluvial RB (ragged) Frequent on right
45. Roaring Run Clay, silt, cobbles, boulders Steep 0.8 (cliff on left) Rhododendron, trees upright, good shape Outside meander bend None Rare
46. Potter Run Clay, silt Moderate to steep 0.6 Trees in good shape on left, grass on right General fluvial on right RB along grass Where grass only
47. Bentley Cr. Silt, sand, gravel 70-90 0.9 Grass, few trees General fluvial Minimal Frequent
48. N 48 Clay, silt Moderate to steep 0.6 Very dense trees, healthy, minor grass General, outside meander bend None Minor along reach

LB = left bank, RB = right bank, u/s = upstream, d/s = downstream, R. = River, Cr. = Creek

Table 7. River bank data, continued.
River Material Bank Angle (deg) Bank Height (m) Vegetation Erosion Location Bank Failure Locations Exposed or Bare Banks
49. Reids Run Clay, silt, cobbles Moderate to steep 0.9 Moderately dense healthy trees; grasses and shrubs General None Minor along reach
50. Piney Cr. Clay, silt, sand, gravel Moderate to steep 1.1 RB grass only; LB healthy trees and shrubs General Minor on RB Minor
51. Little Sandy Cr. Clay, silt Moderate 0.8 Grass within 152.5 m u/s of bridge; further u/s healthy dense trees General Along grassy areas Moderate under tree roots and along grass
52. Trout Run Clay, silt Steep 0.9 Very dense shrubs, dense healthy trees General, minor None None
53. Pootatuck R. Clay, silt, sand, gravel, cobbles Moderate 0.8 Trees, leaning on RB, good shape otherwise Minor general fluvial Some mass wasting where trees removed Occasional
54. Mill R. Silt, sand, gravel Steep 0.6 Grasses, annuals, trees. Trees leaning slightly, sparse on LB, dense on RB, good shape General fluvial None Occasional
55. Aspetuck R. Clay, silt, sand Moderate 1.1 Annuals (ferns), trees. Moderate density in good shape General fluvial None Occasional
56. W. Br. Saugatuck R. Clay, silt, sand, gravel Moderate 0.8 Annuals, shrubs, trees in good health General fluvial, opposite obstructions (significant) None Occasional
57. Mianus R. Clay, silt, sand, gravel Moderate to steep 0.9 Annuals, shrubs, trees moderately dense in good shape General fluvial None Minor

LB = left bank, RB = right bank, u/s = upstream, d/s = downstream, R. = River, Cr. = Creek

PHYSIOGRAPHIC REGIONAL OBSERVATIONS

Thirteen physiographic regions and subregions were included in the data set. A wide variety of land uses were observed in the various regions, including natural, agricultural, grazed, rural, and suburban. Width-to-depth ratios also varied widely, from 5 to 24. As expected, the largest ratios were associated with braided or semibraided channels. Very low width-to-depth ratios were associated with incised streams or those that had been engineered. Bed materials varied from very fine materials (silt and very fine sand) in the Midwest and coastal areas to coarser materials mostly associated with higher elevation streams. Bank materials varied widely. For example, the banks of the streams in the Central Plains tend to be made up of fine loess and silt deposits, which erode easily. In the Appalachian Plateau region, in contrast, the bank materials are far more cohesive and tend to be less susceptible to high erosion rates even when bank vegetation was limited.

Although stream types vary widely within any of the physiographic regions, certain common characteristics were observed. A general summary of those observations is given here. The photos of the sites are organized by physiographic region in appendix A. Many, but not all, of them are referred to in the discussion below.

Pacific Coastal

There are two striking features of stream channels within the border and lower Californian subregions of the Pacific Coastal region (see appendix A). The first is the wide diversity in types of channels. Streams range from perennial, cascading channels to arroyos. The second feature common to most streams in these subregions is the frequency of human interference and alteration. The channel bed material in most of the streams (other than first-order streams) was predominantly fine-to-medium sand, while the channel banks were sandy. The average width-to-depth ratio was 12.1. The streams tend to be very high energy; they are typically ephemeral, so they carry water only when there is rainfall. Many of these streams (and arroyos) tend to be naturally unstable, particularly in the lateral direction, and have relatively high width-to-depth ratios. Because of the high degree of channel instability and flash flooding in this region, many, if not most, of the channels in suburban to urban settings were either concrete lined or at least heavily armored with rock. Channels in the outlying areas were unlined.

Intermontane

Observations at bridge-stream intersections were collected in the Basin and Range, Colorado Plateau, and Trans Pecos subregions within the Intermontane physiographic region. Many channels within the Colorado Plateau are bedrock or semialluvial channels in which stability is a function of bedrock erosion. In the Basin and Range, however, where the climate is arid to semiarid over much of the area, the streams are ephemeral with high energy, flashy flows (see appendix A). The energy of these streams combined with the highly erosion-prone sand beds and banks creates unstable channels, particularly at bridges. Due to the high sediment load carried by these streams, the width-to-depth ratios are relatively high, with an average of 25.0. Streams in the Trans Pecos region (see appendix A) tend to have high, steep banks or valley walls, which create valley side failures and subsequent failure material to the stream. The bed materials are sand, and banks are comprised of a mix of noncohesive materials, primarily sand, with minor amounts of cohesive silts and clays. The predominant bank vegetation was desert shrubs, and mass wasting was a common form of erosion. The average width-to-depth ratio was 10.5.

Rocky Mountain System

The southern Rocky Mountains were visited in this region (see appendix A). The channels contain large bed and bank material. The streams tend to be stable, transport streams that are less disturbed by human activities than in other physiographic regions. Stream channel banks are a mix of cohesive silts and clays and noncohesive gravels and larger materials. The average width-to-depth ratio was 16.5.

Interior Plains

Three subregions-the Great Plains, Central Lowlands, and Interior Lowlands-were visited within the Interior Plains. In the Great Plains (see appendix A), vegetation in riparian areas and in the flood plains was thick, lush, and dense, except where cattle were permitted to graze. The channel beds were composed of more than 70 percent sand. Bank material was noncohesive silt, loam, and sand, but the thick vegetation helped to keep banks stable. The average width-to-depth ratio was 17.3. Erosion processes within the stream channels are primarily fluvial; observed channel banks were not sufficiently high to create significant mass wasting. Slow to moderate degradation occurred where cattle grazing was permitted.

The Central Lowlands (see appendix A) had silt and loess banks that eroded easily. Many of the streams observed had been straightened in addition to having extensive hoof and/or grazing damage. The channel beds degrade rapidly since the bed material is predominantly silt with some clay and sand. The silt banks then become overheightened, and mass failures result. High, failing banks were common even where a wider riparian buffer existed, but the rate of failure was slower (for example, see figure 7). The average width-to-depth ratio was much lower than that observed in the Great Plains, with an average of 8.3. This may be due in part to channel modifications, such as straightening.

Streams in the Interior Lowlands (see appendix A) seemed less fragile than those in the Central Lowlands due to larger bed material (sand and gravel) and more cohesive materials in their banks (clay and silt). However, where vegetation had been removed, banks failed even when they were not greatly overheightened (see figure 8). A single row of trees in the riparian areas slowed bank failure dramatically. These streams tend to have a low width-to-depth ratio, with an average of about 10.5, and remain stable even when the surrounding land has been disturbed. This may be due to the existence of rock outcrops in the beds and banks.

Interior Highlands

In the combined Ozark-Ouachita Plateau (see appendix A), bed material was larger, containing some gravel. The bank material contains a significant percentage of cohesive clays. Natural erosion occurred at bends with increased mass wasting at bends where vegetation had been removed. Overheightened banks remained stable when more than one row of trees was in place. The average width-to-depth ratio was 8.5.

Appalachian Highlands

Within the Appalachian Highlands, the Appalachian Plateau, Valley and Ridge, and Piedmont regions were visited. In the Appalachian Plateau (see appendix A), bed material was coarser (mainly very coarse gravel to cobbles), with bank material composed of cohesive clay, silt, and minor sand. Critical bank heights appeared to be about 1.5 to 1.8 m, which result in low width-to-depth ratios. For the sites visited, the average ratio was 11.0. Watersheds are heavily forested where vegetation is undisturbed. Stream channel erosion and destabilization occurs through removal of vegetation and/or channel straightening. Overheightened banks may fail, but heal quickly if vegetation is allowed to re-establish; thus, stability tends to be fair at worst.

Stream channels within the Piedmont region (see appendix A) had cohesive banks that could stand at high angles without failure. Bank vegetation, if undisturbed, was dense and provided bank stability with about one river width of woody vegetation. Banks with angles steeper than about 60E tended to have leaning or fallen trees. The potential for debris jams is high. Occasional bedrock outcropping was noted at all streams that were visited. Bed material was sand and gravel with occasional larger material. The average width-to-depth ratio was 15.3.

The Valley and Ridge region of the Appalachian Highlands is comprised of a series of ridges separated by stream valleys. The streams in this region are often very steep, especially coming down from the ridges (see appendix A). Cascade and step-pool morphologies are common. Thus, bed materials are commonly large, such as cobbles and boulders, and often armor the bed. Banks are cohesive clays and silts with some larger materials mixed in, strongly held together by the lush vegetation found in this area. Disturbance to the banks by removal of vegetation may result in ragged, scalloped banks, but erosion of the banks is typically at a relatively slow rate. The average width-to-depth ratio was 15.0.

Figure 7. Failing banks in the Central Lowlands. Photo. This figure shows high, failing banks where silt banks became overheightened and mass failures resulted.

Figure 7. Failing banks in the Central Lowlands.

Figure 8. Failing banks in the Interior Lowlands. Photo. This figure shows banks that failed where vegetation had been removed, even when they were not greatly overheightened.

Figure 8. Failing banks in the Interior Lowlands.

Coastal Plain

The Coastal Plain covers a very large area of the Atlantic and Gulf coastal areas (see figure 6). Sites were visited along both the Atlantic and Gulf coasts (see appendix A). In both of these areas, researchers observed moderate rates of bed degradation and bank failure. A buffer of at least one river width appeared to be sufficient in most locations to keep banks stable. Where undisturbed, lush vegetation on the banks held the banks in place, resulting in excellent stability, even when banks were nearly vertical. Bed material is typically sand with minor amounts of small gravel, and banks are cohesive with clay, silt, and minor amounts of sand. Because of the cohesive banks, strong vegetative resistance, and degradation, width-to-depth ratios tended to be rather low. Streams in this region are often sluggish due to low slopes and backwater from the bays or estuaries into which they flow. Where banks or the flood plain are disturbed, debris jams are frequent. The average width-to-depth ratios were much lower in the Gulf area (9.0) than in the Atlantic area (13.5).

New England

All of the streams visited in the New England region were located in Connecticut. At all streams, the banks were heavily vegetated with large woody vegetation, providing tremendous stability to the streambanks. The bank materials typically were comprised of some cohesive materials combined with silt, sand and, in some places, gravel and larger particles. The bed materials in the New England region are considerably larger than in the Atlantic Coastal Plain to the south. The sand, gravel, and cobble beds were often armored; the width-to-depth ratios reflected this armored condition with an average value of 24. The channels were all meandering, but with beds transitional between plane beds and pool-riffle beds.

General Observations of Streams at Bridges

Channel stability is a function of levels of disturbance to the water and sediment discharges, and susceptibility of the channels to change. In every physiographic region, the disturbance that caused the greatest damage to the streams was the combination of cattle activity, vegetation removal, and channel straightening. The combined impact of these activities was worst where cattle had direct access to streams. Also, susceptibility of the channel banks to erosion significantly impacted the level of damage. Figures 9 and 10 provide examples of this combination of disturbances. All vegetation has been removed either through farming practices or by cattle grazing. The channel apparently had been straightened to provide better drainage and to maximize land for farming. Not only are cattle grazing in this area, but also they have direct access to the stream. Hoof damage is extensive. The combined disturbances have resulted in stream channel destabilization; the channel bed elevation has degraded and the banks have become overheightened and steepened. Figure 10 shows the eroding channel beneath the single-span bridge.

In many cases, maintaining a riparian buffer of an appropriate width is all that is needed to preserve channel stability. As discussed in the descriptions of the streams channels across the physiographic regions, some regions require only a single row of trees to help maintain stability, while others require a much greater width. This is due to bank materials and the susceptibility of the banks to failure. In the cases where channels are degrading because of channel straightening, cattle grazing, and urbanization effects, a vegetation buffer may not be enough to maintain stability. When the channel degrades, banks can become overheightened and fail through mass wasting. In this case, vegetation may help to slow the rate of failure, but usually cannot prevent collapse of the banks.

Figure 9. Stream impacts due to disturbances, including hoof damage, vegetation removal, and channel straightening. Photo. This figure shows an example of a combination of disturbances. All vegetation has been removed either through farming practices or by cattle grazing. The channel apparently had been straightened to provide better drainage and to maximize land for farming. Not only are cattle grazing in this area, but also they have direct access to the stream.

Figure 9. Stream impacts due to disturbances, including hoof damage,
vegetation removal, and channel straightening.

Figure 10. Impacts of disturbances at bridge (from figure 9). Photo. This is the same channel as in figure 9, shown under the bridge where the disturbances could eventually impact the bridge abutments.

Figure 10. Impacts of disturbances at bridge (from figure 9).

Another observation that was frequently made at sites in all physiographic regions was that there was often a distinct change in channel stability upstream and downstream of the bridges. This was caused in every case by a change in property management, as it is common for a road (and, thus, a bridge) to divide property ownership. As an example, unnamed stream N 28 is wooded upstream, with a healthy wide band of upright trees keeping the banks stable (see figure 11). Immediately downstream of the bridge, all trees and other vegetation have been removed, resulting in destabilization of the banks (see figure 12).

Aerial photos were examined for each of the sites using http://terraserver-usa.com (these photos are not included in the report because they are readily available online). The photos were examined to check a larger view of the river, specifically looking at land use in the watershed and flood plain, construction areas, the extent of the riparian buffer, channel straightening, and channel pattern. In most cases, the aerial photos reinforced observations that were made on the ground. In a number of cases, the photos helped put the bridge reach into the perspective of the meander pattern, particularly where the bridge was located between meanders or just downstream of a tight meander. Old abandoned meanders also could be detected sometimes, giving an indication of previous lateral movement. Changes in channel pattern, for example from meandering to braided, can be detected on aerial photos. Examining the photos before or after visiting a site helped provide a rating, especially for the watershed condition factor.

Figure 11. Wooded land upstream of bridge. Photo. This figure is upstream of figure 12 and shows a stream that is surrounded by bushes.

Figure 11. Wooded land upstream of bridge.

Figure 12. Downstream of figure 11, vegetation removed. Photo. This figure is downstream of figure 11 and shows ragged, eroding banks due to removal of vegetation.

Figure 12. Downstream of figure 11, vegetation removed.

EFFECT OF CHANNEL INSTABILITY ON BRIDGES

Unstable channels can cause a variety of problems at bridges; however, this is not necessarily the case. For example, the Mojave River in California (see figure 13) can be considered to be a naturally unstable channel, primarily in the lateral direction, in that there is considerable lateral movement of the channel. The channel bed and banks are noncohesive fine sand that adjust readily to sudden changes in hydrology from a dry condition to flash flooding. However, the bridge at the site that was visited spans a wide section of the flood plain, thus providing room for some lateral migration. In many other sites visited, lateral migration of meanders was a potential threat to bridge abutments. In figure 14, lateral migration of a gentle meander bend has forced the channel against the left abutment. This has, in turn, caused additional local scour at the abutment and undermining of the abutments, and could result in an unstable bridge foundation. Lateral and downstream migration of this meander would have a significant impact on the left abutment.

Figure 13. Mojave River, California. Photo. This figure shows a dry channel, looking toward a bridge. This can be considered to be a naturally unstable channel, primarily in the lateral direction, in that there is considerable lateral movement of the channel.

Figure 13. Mojave River, CA.

Figure 14. Meander migration affecting right abutment, Hammond Branch, Maryland. Photo. This figure shows a river flowing toward a bridge. A meander bend upstream of the bridge turned within about 30.5 meters of the bridge.

Figure 14. Meander migrationaffecting right abutment, Hammond Branch, MD.

One of the biggest problems created by channel instability at bridges exists at single-span bridges that are only as wide as the channel. This allows for no or limited lateral or vertical adjustments of the channel. As an example, figure 15 shows a single-span bridge across a channel that is both degrading and widening. Significant widening will result in undermining of the abutment walls.

Even for channels that are unstable, the bridge may not be in danger if adequate structural redundancy is in place. Thus, an observation of channel instability is not a sufficient condition for impending structural failure. The bridge inspector must consider what impact, if any, a channel that is deemed unstable will have during the time period between inspections, especially in the event of a large hydrologic event.

Channel stabilization measures at bridges are quite common. Given the small right-of-way at most bridges, the measures typically are placed directly at the bridge and perhaps a short distance upstream or downstream. By far, the most common type of stabilization measure observed at these sites was riprap. In some cases, the riprap appeared to be effective in holding the bank in place at the bridge. In other cases, however, riprap did not appear to be effective without significant maintenance. For example, at S.R. 445 over Roaring Run in Pennsylvania, there is a high riprap wall composed of graded riprap with a median size of about 152-229 mm (see figure 16). The purpose of the wall is to prevent lateral migration of the tight meander bend just upstream of the bridge. The wall has a bank angle of about 70E. This configuration of loose, undersized riprap in such a steep arrangement has little chance of withstanding the high shear stresses imposed on it at high flows as the high gradient stream makes this tight bend. There is already evidence of riprap wall failure, as much of the stone is deposited in the stream channel just upstream of the bridge. In other cases, stabilization efforts seem to work quite well. As an example, a cross vane has been installed just downstream of the S.R. 144 bridge over Potter Run in Pennsylvania (see figure 17). The cross vane causes the flow to pool just upstream and under the bridge, slowing the high velocity and minimizing scour under the bridge and along the banks.

RELATIONSHIP BETWEEN CHANNEL STABILITY AND SCOUR AT BRIDGES

In HEC-18, scour is defined as having three vertical components: local, contraction, and bed degradation. Local and contraction scours are caused by the bridge and occur within close vicinity of the bridge. Bed degradation, on the other hand, is not caused by the bridge and may be reach-wide or even systemwide. Channel instability includes bed degradation, but also comprises other components, based on the definition given previously, such as channel widening, lateral migration, and bed aggradation. At bridges, channel instabilities can cause:

Channel aggradation in itself is not usually detrimental to the bridge structure, but it can lead to increased flooding and channel widening. At many of the bridges observed during this project, narrow, single-span bridges often were impacted more because small lateral movements of the channel could press the stream thalweg up against one abutment, increasing the local scour at that abutment.

Figure 15. Single-span bridge over unstable channel. Photo. This figure shows a single-span bridge across a channel that is both degrading and widening.

Figure 15. Single-span bridge over unstable channel.

Figure 16. Riprap stabilization wall along Roaring Run, Pennsylvania. Photo. This photo shows a high riprap wall composed of graded riprap with a median size of about 152 to 229 millimeters. The purpose of the wall is to prevent lateral migration of the tight meander bend just upstream of the bridge.

Figure 16. Riprap stabilization wall along Roaring Run, PA.

Figure 17. Cross vane downstream of bridge over Potter Run, Pennsylvania. Photo. The cross vane shown in the figure causes the flow to pool just upstream and under the bridge, slowing the high velocity and minimizing scour under the bridge and along the banks.

Figure 17. Cross vane downstream of bridge over Potter Run, PA.

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). The hydraulics and hydrology research program at the TFHRC Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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