Coastal Sediments
4/1/2016
Coastal Sediments
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D.
Professor Emeritus
Shoreline sediments
•Sandy coasts are predominant worldwide
•Pebble and cobble (shingle) beaches occur in areas of glacial and fluvio‐glacial deposits
•Silt and clay coastal sediments occur in areas of low wave energy, like lagoons and estuaries, or near the mouths of large rivers
•Low tide elevations and below in macro‐tidal coasts like Cook Inlet and Bristol Bay, Alaska
•Sediments may be sorted by size and shape by wave energy and current changes across a beach profile
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
Module 9a ‐
Coastal Sediments
2
1
Coastal Sediments
4/1/2016
Classification by sediment origin
▪ Clastic or lithogenous sediments are composed of detrital grains eroded from rocks (e.g., most sand)
Portland Bill, at end of Chesil Beach, UK
▪ Terrigenous sediments were eroded from consolidated deposits on the continents
▪ Biogenic sediments are composed of calcium carbonate grains from shells, skeletons, and invertebrates (e.g., coral sand)
▪ Hydrogenous sediments are formed from chemical reactions with seawater (e.g.,
manganese nodules on deep ocean floor)
Module 9a ‐
Coastal Sediments
CE A676 Coastal Engineering
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Classification by mineral composition
▪ The mineralogy (chemistry) of sediments is associated with the source from which they were originally eroded
▪ River drainage
▪ Adjacent coastal deposits
▪ Coastal bluffs or cliffs
▪ Density, , and specific gravity, γ
, are critical sediment transport parameters
▪ Quartz is the most common beach mineral, mixed with Feldspar, and Calcite (e.g., degraded limestone)
▪ Heavier minerals ( > 2.87, usually < 1%) useful as tracers to eroded source, e.g., Aragonite, Garnet, Hornblende,…
▪ Lighter materials, like shell fragments, coral, and other carbonates, may dominate beaches of tropical islands
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
Mineral found in beach sand
Density, 
(kg/m3)
Aragonite
2930
Augite
3400
Calcite
2710
Foraminifera shells
1500
Garnet
3950
Hornblende
3200
Magnetite
5200
Muscovite
2850
Quartz
2650
Feldspar
2550‐2760
Zircon
4600
Module 9a ‐ Coastal Sediments
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Coastal Sediments
4/1/2016
Grain Size Analysis
▪ Sieves 2.0 mm – 0.062 mm (‐1.0 – 4.0 ) usually good for beach sand, but consider gravel and cobbles found on many Alaskan beaches
▪ Coarser fractions, with gravel, pebbles, or cobbles, call for a larger sample volume to test
▪ Consider wet sieving for high content of fine material
▪ Vacuum filtering, settling tube, or other specialized analysis is necessary for silt and clay size distribution
▪ Otherwise, D < 0.062 is all classified as “fines”
CE A676 Coastal Engineering
Module 9a ‐ Coastal Sediments
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Module 9a ‐
Coastal Sediments
6
Size Classification Scales
Wentworth advantages
• Finer divisions for sand
• Follows  scale
Wentworth disadvantages
• Finer sand divisions hard to distinguish in well‐graded beach material
• Departs from conventional gravel definitions for construction CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
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Coastal Sediments
4/1/2016
Phi Units
“Phi” units () = ‐log2D

(D = diameter in mm)
 = 0, D = 1 mm (USC medium sand)
  > 0, D < 1 mm :
  < 0, D > 1 mm
 Wentworth size classes are whole numbers in  units
  = 0, D = 1 mm, limit coarse/very coarse, Wentworth
  = 1.0, D = 0.5 mm, limit medium/coarse
  = 2.0, D = 0.25 mm, limit fine/medium
CE A676 Coastal Engineering
Module 9a ‐
Coastal Sediments
7
Gradation Curves
“uniform”
D50
“graded”
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
Module 9a ‐
Coastal Sediments
8
4
Coastal Sediments
4/1/2016
Phi Size Parameters
Mean phi size: in terms of phi sizes for 16, 50, and 84% passing
M 
 16   50   84
3
Standard deviation phi size:
 
 84   16
2
Skewness (departure from symmetry):   
“Phi” units () = ‐log2D
M    50

(D = diameter in mm)
CE A676 Coastal Engineering
Module 9a ‐
Coastal Sediments
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Module 9a ‐
Coastal Sediments
10
Sphericity and Roundness
• Sphericity, X, of sediment grains is defined as:
• where V is the volume of the grain and • L is the length (principle dimension). X
 
6V
1
3
L
• A sphere has a sphericity of 1. • Most grains have sphericity significantly less than 1.
• Roundness, R, or angularity is defined as:
• where Rc is the average radius of corners and edges and
R
Rc
Rsphere
• Rsphere is the radius of the maximum inscribed sphere
• Roundness is a measure of the sharpness of the corners.
•
Roundness is more commonly classified as angular, subangular, subrounded, rounded, or well‐rounded.
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
5
Coastal Sediments
4/1/2016
Grain size and density effects
▪ Sediment transport is
▪ proportional to current velocity and wave height
▪ inversely proportional to sediment grain size and density
▪ Distribution of size and shapes of sediment on beach profiles is often sorted
▪ Size sorting relates to wave exposure
▪ Larger disk‐shaped smooth cobbles (shingles) found at high tide level
▪ Silt and clay at low tide level
Chesil Beach, southern England
▪ Sediments are rounded by extended exposure to high wave energy
CE A676 Coastal Engineering
Module 9a ‐ Coastal Sediments
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Module 9a ‐ Coastal Sediments
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Fall Velocity
▪
Commonly used parameter in sediment transport analyses (ref. CEM III‐1‐4)
▪
Stokes Fall Velocity assumes laminar flow (Re < 400) around a sphere
; V = fluid speed, D = grain size,  = kinematic fluid viscosity
▪
▪
 = 1.007 x 10‐6 m2/s (fresh water at 20C)
▪
 = 1.05 x 10‐6 m2/s (S = 35 ppt at 20C)
▪
laminar drag coefficient for a sphere: ▪
Balance forces for steady state:
▪
▪
▪
This form used often, without further consideration of flow regime
Transitional flow range (400 < Re < 200,000), ▪
▪
1
s = sediment density;  = ambient water density
1.6
1
Fully turbulent range (Re > 200,000), ▪
2.6
1
0.5
⁄
0.2
⁄
CEM Figure III‐1‐5
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
6
Coastal Sediments
4/1/2016
Fall Velocity
Fall
Quartz spheres falling in water and air
CEM Figure III‐1‐6
CE A676 Coastal Engineering
Module 9a ‐ Coastal Sediments
13
Beach slope, grain size, and wave energy
High wave energy
• Larger grain size
• Steeper beach face slope
From text (Sorensen)
Low wave energy
• Smaller grain size
• Shallower beach face slope
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
Module 9a ‐
Coastal Sediments
14
7
Coastal Sediments
4/1/2016
Sampling Littoral Materials
Sampling strategy
▪ Grain size distribution
▪ Composition (mineralogy)
▪ Variation of above with ▪ horizontal and ▪ vertical position
▪ Variations with time
Sampling method
▪ Grab samples
▪ Stratigraphic samples (pits, Note‐keeping
•
•
•
•
•
•
•
•
•
cores, geophysical)
▪ Suspended sediments Date and time
Serial sample number
Position (latitude, longitude, elevation)
Horizontal location of samples
Spacing between samples
Volume of sample
Vertical location (surface, depth, etc.)
Sampling technique
Notes: field classification, context information (near waterline, berm, scarp, etc.)
(samples, acoustic, optical)
CE A676 Coastal Engineering
Module 9a ‐ Coastal Sediments
15
Submerged sediment sampler
Sediment Grab Sampler
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
Module 9a ‐
Coastal Sediments
16
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Coastal Sediments
4/1/2016
Stratification
Pit exposing beach strata
Module 9a ‐
Coastal Sediments
CE A676 Coastal Engineering
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Acoustic Measurements
3.5 kHz Sub‐bottom profile west of Knik Arm Shoal
400 Hz continuous seismic reflection – Knik Arm Shoal
Towed “fish” for geophysical (acoustic) measurements
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
Module 9a ‐
Coastal Sediments
18
9
Coastal Sediments
4/1/2016
CE A676 Coastal Engineering
CE A676 Coastal Engineering
Orson P. Smith, PE, Ph.D., Instructor
Module 9a ‐
Coastal Sediments
19
10