Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 94


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 93
93 CHAPTER 12 SCOUR EXAMPLE PROBLEMS 12.1 EXAMPLE 1: SINGLE CIRCULAR PIER 0.094 1000 3.36 2 - 1 1 WITH APPROACHING CONSTANT VELOCITY log(8 .4 10 6 ) 10 = 52.28 N Given: m2 Pier geometry: Pier diameter B = 2.5 m, circular pier is read on the EFA curve Channel geometry: Channel upstream width B1 = 50 m (4) The initial rate of scour Z Flow parameters: Water depth H = 3.12 m, (Layer 1) at = max Approaching constant velocity V = = 8.6 mm/hr Z 3.36 m/sec Angle of attack: 0 degrees (5) The maximum depth of scour Zmax is EFA result: Layer 1: Thickness 10 m; critical shear stress 2 N/m2 Zmax (Pier) = 0.18Kw Ksp Ksh R e 0.635 = 0.18 0.916 Layer 2: Thickness 20 m; critical shear (8.4 10 ) 6 0.635 = 4112.3mm stress 4 N/m2 Flood period: 2 days for hand calculation (6) The equation for z (t) is 2 years for computer calculation t t ( hrs) Determine: The magnitude of maximum pier z= = 1 + t 1 t ( hrs) scour depth + Z Z max 8 . 6 4112 .3 (7) The flood lasts 2 days (48 hours), therefore 12.1.1 SRICOS-EFA Method: Hand Calculation Z = 375 mm or 9.1% of Zmax (1) Calculate the K factors for max and Zmax: 12.1.2 SRICOS-EFA Method: H 3.12 Computer Calculation -4 -4 kw = 1 + 16 e B = 1 + 16 e 2.5 = 1.109 Use SRICOS-EFA program Option 1: Complex Pier Scour ( ) ( ) 0.34 0.34 3.12 K w = 0.85 H B = 0.85 = 0.916 2.5 Results: After a 2-year period of the flood having 3.36 m/s velocity, Since the pier in this case is a circular pier, ksh = 1 and the final pier scour is Ksh = 1. It is a single pier, so ksp = 1 and Ksp = 1. Z=4m There is no attack angle of the flow, so k = 1. Table 12.1 and Figures 12.1 through 12.3 provide further (2) Calculate Reynolds Number as information. Figure 12.4 illustrates the scour depth develop- ment with time. Vd 3.36 2.5 Re = = = 8.4 10 6 v 10 -6 12.2 EXAMPLE 2: SINGLE RECTANGULAR PIER WITH ATTACK ANGLE AND APPROACHING HYDROGRAPH (3) Maximum hydraulic shear stress around the pier is Given: Pier geometry: Pier width B = 1.22 m, pier length max = kw ksh ksp k 0.094 V 2 - = 1.109 1 1 log R e 10 Lpier = 18 m, rectangular pier

OCR for page 93
94 TABLE 12.1 Summary of data input (Example 1) Input Unit SI 1 Output Unit SI 1 First Date of Analysis 01-01-2003 Last Date of Analysis 01-01-2005 No. Of Input Data 730 Upstream Channel Width 50 Type of Pier Circular Pier 1 Pier Diameter 2.5 Time Step Hours 24 Type of Hydrologic Input Velocity 2 Number of Regression Points Velocity vs. Water Depth 1 Values of Regression Points Velocity, Water Depth 3.36, 3.12 No. Of Layers 2 Properties of 1st Layer Thickness 10 Critical Shear Stress 2 Number of Regression Points Shear Stress vs. Scour Rate 8 1, 0 Estimate Initial 2, 0.1 Scour Rate Value of Regression Shear Stress, Scour Rate 4,1 Points 6,2 9, 3 20, 6 40, 8 60, 8.9 Properties of 2nd Layer Thickness 20 Critical Shear Stress 4 Number of Regression Points Shear Stress vs. Scour Rate 8 3, 0 Estimate Initial 4, 0.1 Scour Rate Value of Regression Shear Stress, Scour Rate 6,1 Points 9,2 18.5, 4 27, 5 40, 6 60, 6.9 EFA Result (Layer 1) 10 9 8 Scour rate Shear stress 7 (mm/hr) (N/m2) Scour Rate (mm/hr) 0 1 6 0.1 2 5 1 4 4 2 6 3 9 3 6 20 2 8 40 8.9 60 1 0 0 10 20 30 40 50 60 70 Figure 12.1. EFA Results for Soil Layer 1 (Example 1).