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Manual, Sediment Committee, Hydraulics Divi bors, and Coastal Engineering Division, ASCE, sion, ASCE, New York, 1975. San Francisco, 1975. 24. Task Committee for Preparation of Sedimentation 28. D.E. Overton and M.E. Meadows. Stormwater Mod Manual. Sediment Transportation Mechanics: eling. Academic Press, New York, 1976. Sediment Discharge Formulas. Journal of Hy 29. M.T. Tseng and others. Evaluation of Flood draulics Division, Proc., ASCE, Vol. 97, No. Risk Factors in the Design of Highway Stream HY4, April 1971. Crossings. Office of Research and Development, 25. W.A. Thomas and A.L. Prashuhn. Mathematical FHWA, Rept. FHWA-RD-75-54, June 1975. Model of Scour and Deposition. Journal of Hy 30. V.R. Snyder and K.V. Wilson. Hydraulics Design draulics Division, Proc., ASCE, Vol. 103, No. of Bridges with Risk Analysis. Office of Re HYB, Aug. 1977. search and Development, FHWA, Jan. 1980. 26. H.H. Chang and J.C. Hill. Minimum Stream Power 31. Design of Encroachments on Floodplains Using for Rivers and Deltas. Journal of Hydraulics Risk Analysis. FHWA, Hydraulic Engineering Division, Proc., ASCE, Vol. 103, No. HY12, Dec. Circular 17, 1981. 1977. 27. Y.H. Chen and D.B. Simons. Mathematical Model- ing of Alluvial Channels. Proc., Symposium of JUblication of this paper sponsored by Committee on Hydrology, Hydraulics, Modeling Techniques: Vol. 1--Waterways, Har- and Water Quality.
Abridgment Stream Channel Grade Changes and Their Effects on Highway Crossings
STEPHEN ARNE GILJE
Stream channel degradation and aggradation are significant hydraulic problems affected, whether in meters or kilometers, is dic at river crossings. Degradation is lowering of the streambed, independent of tated by the physics of the situation involved. scour caused by obstructions or constrictions. Rapid or long-term degradation Degradation or aggradation that is significant is usually due to a significant change in normal sediment-transport relations. enough to be of engineering concern is caused by Aggradation is stream infilling and occurs when more sediment is supplied to a major changes in the river environment. stream than the stream is capable of transporting. Problems caused by stream degradation are far more common than those caused by aggradation. If actions HIGHWAY PROBLEMS DUE TO GRADE CHANGES are to be taken to protect a highway crossing against grade changes, early rec ognition of these hazards is imperative. Techniques for determining whether a crossing is experiencing degradation or aggradation are observation of stream The extent of stream degradation and aggradation in characteristics (geomorphology), anticipation of gradation changes based on the United States is demonstrated by a 1978 Federal watershed activities, and measurement of pertinent stream dimensions. Streams Highway Administration (FHWA) research study · in areas of high sediment yield are most prone to grade changes. Severe grade Data from 224 sites that were experiencing various changes are often due to human intervention in natural stream processes. The hydraulic problems were assembled and carefully problems associated with degradation and aggradation warrant special attention analyzed. Thirty-nine of these sites (17.4 percent) because protective measures effective against local hydraulic hazards are inef had undergone changes in streambed elevation. fective for protection against grade changes. Degradation is the lowering of a stream channel; therefore, a problem at crossings is the exposure of footings, pilings, and foundations (see Figure 1). Degradation is lowering of a stream channel caused Undermining of channel banks or highway fill results by a significant change in normal sediment-transport in failure of bridge approaches, revetment, and relations. It is independent of scour created by other countermeasures. Undermining of channel banks isolated obstructions or constrictions. Aggradation causes general stream instability and exacerbates occurs when more sediment is supplied to a stream debris problems (5). Channel instability is often a than the stream is capable of transporting. To deal clue to future degradation problems; on the other with grade changes, it is necessary to understand hand, some instability problems (channel cutoffs) what causes them and how to recognize their features. result in degradation. Degradation alters crossing The processes of streambed grade changes have conditions so that hydraulic hazards that under de been defined differently by various authors (.!_-1.l • sign conditions pose no significant threat to the The differences in definition stem from differences integrity of crossings become critical. For ex in the limitations on temporal and spatial perspec ample, local scour considered acceptable under de tive; as a result, grade changes are sometimes con sign conditions could cause bridge failure when fused with scour and fill. superimposed on a degraded channel. The pervasive nature of grade changes is often Aggradation--general infilling of a stream described in terms of a long time and great dis channel--causes a reduction in the flow area avail tance. This is an accurate description of many case able at crossings (see Figure 2). In extreme cases, studies but fosters the misconception that all grade the flow area is less than that necessary for design changes progress similarly. When basic stream discharges, which results in overtopping of the forming factors are altered, the stream response is roadway or bridge deck. During flooding enough hor to change channel geometry. The direction of izontal or turning movement can occur to cause change, whether vertical or lateral; the rate, bridge damage. Aggradation increases stream insta whether in seconds or in decades; and the distance bility because excessive sediment carried by an ag- 8 Transportation Research Record 896
grading stream is prone to deposit in point and 80 percent of serious grade changes were caused by lateral bars. As these bars grow, stream sinuosity human int ervent ion i n nat ur al s tream p r ocesses (3). increases and flow is redirected into stream banks In fact, it was difficult to isolate severe gr'ide (located on the outside of meander bends across and changes that were caused naturally and were unaf slightly downstream from point bars) • The most fected by human activities. dramatic--and, fortunately, uncommon--aggradation Recurrent gravel mining or dredging, reservoir ha:i:ard occurs when a stream is filled sufficiently regulated flows. and land use changes can have con to cause floodwater to overflow streambanks and seek tinuing effects (see Figure 3). This is in contrast a new channel. In evaluating grade problems nation to natural and other human-induced causes, the ef wide, it was found that for each aggradation hazard fects of which are most severe following the impact identified there were three degradation hazards but diminish thereafter <2l • The magnitude of the identified 131. impact is easily underestimated, as is the stream In the evaluation of highway problems, more than distance affected. Many streams have degraded more than 5 m ( 8). Grade changes have pco9ressell many kilometers ;n smal l streams and hundreds of kilo meters on ma j or rivers. Figure 1. Degradation and resulting channel erosion at Middle Creek crossing of 1-80 near Lincoln, Nebraska. EARLY RECOGNITION OF DEGRADATION AND AGGRADATION
Technique s that c an r e adily be applied to determi ne whether a crossing is experiencing a grade change are observation of stream character is tics (geomor phology) , prediction of a grade change based on watershed activities , and measurement of pertinent stream dimensions. Evaluation should be based on experience with the stream in question, analysis of aerial photographs taken at different times over as long a period as possible, and field investiga tions. Hydraulic hazards can be recognized by draw ing on experience with crossings elsewhere on the stream or on streams of a similar nature. A signif icant change in t:he chaLacte:r of a st:ream--a change of any type--is often the first signal that a per vasive hydra ulic hazard is i mm ine nt (see Figure 4). Aerial photographs can be used to analyze long reaches at different periods in the past. Klingman (9) discusses the use of aerial photography in high way design. All stream sites should be evaluated in Figure 2. Aggradation on Badwater Creek at US-20 near.Shoshoni, Wyoming. the field after review of maps and aerial photo graphs.
Observation of Stream Characteristics
Degradation and aggradation are opposites. Because degradation is a more common problem, the emphasis here is on the characteristics of degrading streams. Direct observation of degradation in the early stages is difficult under perennial flow condi tions. In addition, the dynamic nature of streams normally creates channel fluctuation; which OC'n he mistaken for a progressive change. It is commonly easier to recognize the geomorphic effects of a degrading channel in the early stages than the bed change itself. An exception to this guideline is when degradation occurs as a headcut or sharp. A headcut is recognized as a local high slope area on a stream and in many cases is exhibited as rapid~ u1 Figure 3. Degradation due to gravel mining upstream and downstream on falls. This type of degradation is more common on Amite River at LA-37 near Grangeville, Louisiana. small, ephemeral, or intermittent streams that have channels composed of relatively cohesive bed mate rial or armor. Headcuts are uncommon on larger perenn ial streams or streams with erodible beds. Degradation can occur solely as downcutting (minor degradation where banks are resistant, well vegetated, reveted, or in very small streams), as downcutting resulting in bank slumping (moderate degradation and cohesive bank materials) , or as downcutting associated with severe bank erosion. All of these stream aspects are observable in aerial - photographs or, as shown in Figure 5, in the field - (note in the figure the slumping of the banks, the - erosion on the convex section o f the meander, and the toppled tree in the background). Bank erosion on both sides of a meander, disap pearance of lateral or midchannel bars, and exposure of unvegetated banks that are norma lly cove r ed ind i - Transportation Research Record 896 9 cate degradation. Evidences of degradation that can Degradation can often be subtle on a mainstream. be seen in aerial photographs are slumping and scal It is normal for some streams to be incised and loping of channel banks, an increase in vegetative exhibit bank erosion at bends and some armoring. In debris, trees overhanging the channel, and excessive these cases, evaluation of small tributaries will bank erosion without resultant downstream growth of indicate how deep the stream may have degraded. Be point bars. Streambed armoring is indicative of cause tributaries are higher-slope streams that potential degradation. If the armor layer is carry less perennial flow, they maintain irregulari breached during a flood, degradation may be rapid. ties longer. Their banks and beds may also be com Streams with high sediment yields have been shown to posed of more resistant material than the main be more prone to degradation. stream, and the influence of vegetative controls is greater for small streams. If a tributary has an abrupt change in slope at its junction with a main stream or if it contains headcuts, this indicates Figure 4. Degradation on Homochitto River at US-61 crossing near Doloroso, Mississippi, resulting in severe impact on lateral stability (indicated by change recent degradation. While the floodplain is being in stream form at right of photograph). evaluated, the elevation of abandoned meanders and overflow channels should be documented. The age of these features can be estimated by the growth of vegetation and sedimentation characteristics. If these features are considerably higher than the present streambed (taking into consideration infill ing due to silting and flood stages), the difference in elevation is indicative of degradation. Aggradation is easier to evaluate than degrada tion because depositional features are usually ob vious and can develop longer and more fully before they pose a danger to crossings. Aggradation can occur on any type of stream and is common in bedrock or mountainous streams. The first sign of aggrada tion is often accelerated growth of point bars as well as deposition elsewhere. A shift from a mean dering to a braided pattern is indicative of aggra dation Ill . Aggradation is most active wherever there is a change of slope or in areas of back water. Therefore, the first signs of aggradation are often at confluences, behind culverts, and at Figure 5. Severe bank erosion caused by degradation on Perry Creek at 1-55 bridges (see Figure 6). The growth of natural near Grenada, Mississippi. levees and the occurrence of more frequent bank overtopping during moderate floods indicate stream aggradation.
PREDICTION OF GRADE CHANGES BASED ON WATERSHED ACTIVITIES
Dramatic examples of degradation and aggradation have been documented by Lane (8). These cases were caused by mining in the river - channel, landslides, dams, and water diversion. Dams, channelization projects, gravel mines, and flow diversion projects are easy to spot in aerial photographs. However, establishing the connection between impacts and ob served geomorphic characteristics is more diffi cult. The occurrence of an activity within a water shed does not necessarily mean that grade changes will occur. The extent of the activity and the stability and flow history of the stream are signif icant factors that determine final effects (10). Case histories illustrating the types of grade prob Figure 6. Aggradation on Yellowstone River in Yellowstone National Park, lems that result from different watershed activities illustrated by deposition at confluences and culverts. have been documented, as have methods for calculat ing the extent of grade change <1.1!>·
MEASUREMENT OF STREAM PROPERTIES
The most direct method for evaluating a grade change is measurement from a fixed object (i.e. , a bridge deck) to the streambed. Progressive and continual trends in bed elevations over an extended time period indicate a grade change. Original bridge design plans often take note of the distance from the bridge deck to the streambed at the time of con struction. Comparison of the results of a quick field measurement with these existing data is useful to establish whether a change has occurred. Engineers should be careful to avoid the measure ment of local effects. Streams, as part of their natural character, have streambeds that fluctuate up 10 Transportation Research Record 896 and down. Engineers should also avoid comparing bed port Technology. Water Resources Publications, elevations that result from high flows with those of Fort Collins, co, 1977, 807 pp. a dry period. Repeated measurements from different 2. L.B. Leopold, M.G. Wolman, and J.P. Miller. positions across the stream will yield the best re Fluvial Processes in Geomorphology. W.H. Free sults for comparative purposes. man and Co., San Francisco and London, 1964, Analysis of stream cross sections and longitudi 522 pp. nal profiles can be used to determine the effects of 3. T.N. Keefer, R.S. McQuivey, and D.B. Simons. grade changes. Comparisons of cross sections mea Stream Channel Degradation and Aggradation: sured in the past with those taken later can be used Causes and Consequences to Highways. FHWA; to show whether the river has incised or risen (11). Rept. FHWA-RD-80-038, 1980, 91 pp. If it is feasible to measure the profile of the 4. J.C. Brice and J.C. Blodgett. Countermeasures stream along its length, these data are very useful for Hydraulic Problems at Bridges: Volumes l (7). Measurements will identify hard points or and 2. FHWA, Repts. FHWA-RD-78-162 and P'HWA base-level controls within the channel. Stream pro RD-78-163, Sept. 1978, 184 and 554 pp. files drawn for stable streams are often concave 5. S.A. Gilje. Debris Problems in the River Envi upward. A stream profile concave downward is in ronment. Public Roads, Vol. 42, No. 4, March dicative of a degrading channel river engineering is not occurring 8. E.W. Lane. The Importance of Fluvial Morphol on every stream nationwide. However, approximately e<:'IY in Hydraulic Engineering. ASCE. Vol. 81. one-fifth of the hydraulic hazards that are severe No. 745, 1955, 17 pp. in nature are due to grade changes. Most of these 9. P.C. Klingeman. Hydrological Evaluations in incidences can be traced directly to an obvious Bridgepier Scour Design. Journal of Hydraulics cause: channelization, streambed mining, or a dam. Division, Proc., ASCE, Vol. 99, 1973, pp. 2175- Hydraulic engineers should be aware of the potential 2184. consequences of a significant change in the use or 10. J.C. Brice. Stability of Relocated Stream control of a stream. When such an impact is made on Channels. FHWA, Rept. FHWA-RD-80-158, 1980, a stream with crossings in place, highway engineers 184 pp. should inspect the stream and crossings annually. 11. H.W. Shen, S.A. Schuman, J.D. Nelson, D.O. Degradation problems are far more common than Doehring, M.M. Skinner, and G.L. Smith. Meth aggradation problems. Streams carrying large sedi ods for Assessment of Stream-Related Hazards to ment loads are more prone to problems. Methods of evaluating and calculating degradation or aggrada Highways and Bridges. FHWA, Rept. FHWA-RD-80- 160, 1980, 252 pp. tion, as well as an evaluation of countermeasures 12. J. Gessler. Aggradation and Degradation. In for hydraulic problems caused by grade changes. are River Mechanics (H.W. Shen, ed.), Colorado presented in the literature
Assessment of Commonly Used Methods of Estimating Flood Frequency
DONALD W. NEWTON AND JANET C. HERRIN
A study made to determine what are likely to be the most accurate and consis determinations at gaged locations by using statistical estimating procedures. tent procedures for determining peak flood flow frequencies for ungaged water The most obvious reasons for this superior performance are the definition of sheds in the Tennessee Valley region is described. The study was based on in the parameters and the formulation of the prediction equation. The procedures formation developed during a pilot test, which is currently the most compre that performed best (a) used parameters that were well-defined and could be hensive and objective method available of examining the performance of com consistently determined, (b) were formulated so that the flood-frequency esti monly used procedures for estimating peak flow frequencies at ungaged loca mates were not sensitive to parameter variations, and (c) were calibrated to a tions. Test results showed significant differences in performance when proce large number of gage records in a relatively small, well-defined hydrologic dures were evaluated by using the criteria of accuracy, reproducibility, and region. practicality. Although the pilot test evaluation was limited to the Midwest and Northwest, it is concluded that the observed differences in performance result A common problem for hydrologists is estimation of from fundamental differences in procedure formulation that can be expected to occur in the Tennessee Valley and in other regions as well. The most ac peak flow frequencies at locations for which there curate and reproducible procedures evaluated were found to be regression-based are few or no flood records--ungaged locations. The proced11res in which prediction equations are calibrated to flood-frequency different procedures in common use often provide