Technical Scour Calculations Supplement 14B (210–VI–NEH, August 2007) Technical Supplement 14B Scour Calculations Part 654 National Engineering Handbook Issued August 2007 Cover photo: Where sediment transport capacity of a stream exceeds the sediment supply, scour may occur. Scour may be estimated using analytical tools and accommodated or re-mediated by the design. Advisory Note Techniques and approaches contained in this handbook are not all-inclusive, nor universally applicable. Designing stream restorations requires appropriate training and experience, especially to identify conditions where various approaches, tools, and techniques are most applicable, as well as their limitations for design. Note also that prod- uct names are included only to show type and availability and do not constitute endorsement for their specific use. (210–VI–NEH, August 2007) Technical Scour Calculations Supplement 14B Contents Purpose TS14B–1 Introduction TS14B–1 Processes TS14B–2 Effects of stream types TS14B–4 Scour computations for design TS14B–5 Long-term bed elevation change TS14B–6 Armoring .........................................................................................................TS14B–6 Equilibrium slope ...........................................................................................TS14B–7 Cohesive beds .................................................................................................TS14B–9 Sand and fine gravel—no bed-material sediment supplied from ...........TS14B–10 upstream Sand and fine gravel—reduced sediment supply from upstream ..........TS14B–10 Beds coarser than sand—no sediment supplied from upstream ...........TS14B–11 Sediment continuity analysis ......................................................................TS14B–12 More complex approaches for long-term aggradation or .......................TS14B–13 degradation General scour TS14B–13 Process description .....................................................................................TS14B–13 Contraction scour ........................................................................................TS14B–14 Live-bed contraction scour .........................................................................TS14B–15 Clear-water contraction scour ....................................................................TS14B–17 Bridge scour .................................................................................................TS14B–17 Bend scour ....................................................................................................TS14B–17 Bedform scour TS14B–19 Process description .....................................................................................TS14B–19 Bedform predictors .....................................................................................TS14B–19 Scour associated with structures TS14B–21 Structures that span the full width of the channel ..................................TS14B–21 Sills ................................................................................................................TS14B–22 Step-pool structures ....................................................................................TS14B–23 Grade control structures and weirs ...........................................................TS14B–23 Structures that partially span the channel ................................................TS14B–26 Example computations TS14B–29 Sand-bed reach .............................................................................................TS14B–29 Gravel-bed reach ..........................................................................................TS14B–32 Design features and measures to address scour TS14B–35 List of symbols TS14B–37 (210–VI–NEH, August 2007) TS14B–i Technical Supplement 14B Scour Calculations Part 654 National Engineering Handbook Tables Table TS14B–1 Examples of bridge failures associated with TS14B–2 scour Table TS14B–2 Streambed erosion and deposition processes TS14B–3 Table TS14B–3 Types of scour analyses TS14B–5 Table TS14B–4 Constants for computation of minimum armor TS14B–7 particle size Table TS14B–5 Approaches for determining equilibrium slope TS14B–8 Table TS14B–6 Ranges for data set underlying the Yang TS14B–11 sediment transport relation Table TS14B–7 Constants for equilibrium slope formulas for TS14B–12 coarse-bed channels with little or no sediment load input Table TS14B–8 Constants for Lacey and Blench relations, TS14B–14 U.S. units (D50 in mm) Table TS14B–9 Constant K for Lacey and Blench relations, TS14B–14 SI units (D50 in mm) Table TS14B–10 Exponent a for contraction scour relation TS14B–15 Table TS14B–11 Summary of scour analyses and applicability TS14B–28 to various bed types Figures Figure TS14B–1 Scour observations from typical reaches of TS14B–1 alluvial rivers Figure TS14B–2 Examples of local scour TS14B–3 Figure TS14B–3 Examples of general scour TS14B–3 Figure TS14B–4 Conceptual representation of the relationship TS14B–6 between long-term average vertical stability and sediment transport Figure TS14B–5 Definition of terms for armor limited scour TS14B–6 Figure TS14B–6 Definition of equilibrium slope, S TS14B–7 eq Figure TS14B–7 Headcut migrating upstream through cohesive TS14B–9 streambed toward bridge in north central MS TS14B–ii (210–VI–NEH, August 2007) Technical Supplement 14B Scour Calculations Part 654 National Engineering Handbook Figure TS14B–8 Empirical equilibrium slope—drainage area TS14B–9 relationship for Yalobusha River watershed in northern MS Figure TS14B–9 Critical shear stress for channels with TS14B–10 boundaries of noncohesive material Figure TS14B–10 Fall velocity for sand-sized particles with a TS14B–16 specific gravity of 2.65 Figure TS14B–11 Downstream face of Horse Island Chute TS14B–17 bridge near Chester, IL, as viewed from left (north) embankment Figure TS14B–12 Definition of recommended length for TS14B–19 protection downstream from a bend apex, Lp Figure TS14B–13 Recommended length of protection divided TS14B–19 by hydraulic radius, Lp/R, as a function of Manning’s roughness coefficient for the bend, n, and hydraulic radius, R Figure TS14B–14 Relative relationships between progression of TS14B–20 alluvial bedforms and flow intensity Figure TS14B–15 Scour associated with low stone weir TS14B–21 Figure TS14B–16 Definition sketch for computing scour TS14B–22 associated with sills Figure TS14B–17 Definition sketch for computing scour TS14B–23 associated with step-pool structures Figure TS14B–18 (a) Low- and (b) high-drop grade control TS14B–24 structures Figure TS14B–19 Definition sketch for computing scour TS14B–25 associated with grade control structures Figure TS14B–20 Scour associated with stone spur dike TS14B–26 Figure TS14B–21 Definition sketch showing crest length, Lc, TS14B–26 and side slope angle, θ, for spur dikes Figure TS14B–22 Four methods for designing stone structures TS14B–35 to resist failure due to bed scour (210–VI–NEH, August 2007) TS14B–iii Technical Scour Calculations Supplement 14B Purpose average condition. Blodgett (1986) presented informa- tion regarding bed elevation fluctuations from 21 sites on streams with a range of bed material sizes. Monthly Scour is one of the major causes of failure for stream or annual measurements were made at the same loca- and river projects. It is important to adequately as- tion within generally straight reaches, free of features sess and predict scour for any stream or river design. like bedrock, bridge piers, or large boulders that might Designers of treatments such as barbs, revetments, or cause local scour. Mean and maximum scour depths weirs (that are placed on or adjacent to streambeds) are plotted in figure TS14B–1 (Blodgett 1986) as a must estimate the probable maximum scour during the function of median bed-material size. In this figure, the design life of the structure to ensure that it can adjust scour depth is defined as the depth of scour below a for this potential change. This technical supplement reference plane, which was set at the highest thalweg provides guidance useful in performing scour depth computations. elevation measured during the period of observation. Clearly, scour depths can be quite significant. Scour is perhaps the primary cause of failure of riv- Introduction erine hydraulic structures, and failure to adequately assess and predict scour hazard represents a major Streams continually mold and remold their streambeds design flaw. For example, most failures of continuous by eroding and depositing sediments. Scour and fill of bank protection projects like revetments are due to alluvial channels not undergoing long-term aggrada- toe scour. The most spectacular examples of structural tion or degradation occur as fluctuations about some failure due to scour involve bridges, such as those Figure TS14B–1 Scour observations from typical reaches of alluvial rivers Medium diameter of bed material, D50 (mm) 0.1 1.0 10 100 100 Sand Gravel Cobble Boulders -0.115 10 Zt(max)=6.5 D50 ) 10 (m) t Z (ft t , Z , 1.0 1.0 Depth of scour Depth of scour -0.115 Z (mean)=1.42 D50 t Mean depth of scour 0.10 Maximum depth of scour 21 sites 0.1 0.0001 0.001 0.01 0.1 1.0 Medium diameter of bed material, D50 (ft) (210–VI–NEH, August 2007) TS14B–1 Technical Supplement 14B Scour Calculations Part 654 National Engineering Handbook summarized in table TS14B–1 (Lagasse and Richard- the structure extends below maximum scour depth. son 2001). Less well known, but also important, scour This technical supplement provides guidance