Field Methodologies: Detailed Investigation Andrew Simon USDA-ARS National Sedimentation Laboratory, Oxford, MS

Aims of this Section To describe the methodologies and instruments used to collect the necessary data for bank-stability modeling.

Fundamental Processes Behind Bank Stability If we want to predict bank stability we need to quantify the underlying processes controlled by force and resistance to mass failure and hydraulic shear: Bank shear strength (resistance to mass failure) vs. Gravitational forces Bank-toe erodibility (resistance to hydraulic erosion) vs. Boundary shear stress National Sedimentation Laboratory

Bank Profile and Stratigraphy Select critical bank geometry and survey profile to thalweg; Notate bank stratigraphy (including bank toe dimensions and slope), dominant size class and layer thickness from bank face or during augering; sample each layer for particle-size distribution; Determine what techniques will be required to determine critical shear stress and erodibility of the bank toe and other layers;

Bank Shear Strength

Measuring Soil Strength In-situ tests – Borehole shear test (BST) Torvane – cohesion and friction combined Shear vane (undrained clays only) Laboratory test – shear box and triaxial cell

Iowa Borehole Shear Tester

Soil Strength Testing

Some “Ball Park” Figures (based on more than 800 tests) *

Measuring Pore-Water Pressure Measure directly using tensiometers and piezometers Infer from water table height  w = h.  w. where  w = pore water pressure (kPa); h = head of water (m);  w = unit weight water (kN/m 3 )

Measuring Matric Suction in the Field Auger to desired depth for BST testing Take undisturbed core with hammer sampler (take second core sample for bulk unit weight) Insert digital tensionmeter Record readings every 15 sec for 6 – 10 minutes

Unsaturated Shear Test, Goodwin Creek Bend, MS Incorporating Suction in a Strength Test C a = 22.7 kPa  ’ = 0.37 = 20.3 o r 2 = 0.99 Matric suction = 17kPa At  b of 14° this gives; 17 x (tan 14 ° ) = 4.2 kPa added cohesion Therefore; c’ = 22.7 – 4.2 = 18.5 kPa

Hydraulic Erosion Processes (Bank Toe)

Hydraulic Erosion Processes Terms used in this section Hydraulic shear stress – the force exerted by water flowing over material, Pascals (1Pa=1N/m 2 ) Boundary shear stress  o, critical shear stress  c, excess shear stress  e Erodibility – amount of erosion per unit excess shear stress, per unit time, m 3 /Pa/sec (m/sec) Erosion rate – rate of bank retreat, m/sec

Shields Diagram by Particle Diameter (For Non-Cohesive Materials) Excludes cohesives Rule of Thumb for Uniform Sediments:  c (in Pa) = diameter (in mm)

Erosion Rate and Excess Shear Stress: Cohesives  = k (  o -  c )  = erosion rate (m/s) k = erodibility coefficient (m 3 /N-s)  o = boundary shear stress (Pa)  c = critical shear stress (Pa) (  o -  c ) = excess shear stress Critical shear stress is the stress required to initiate erosion. Obtained from jet-test device

Measuring Bank and Toe Erosion and Erodibility (Cohesives) Jet test device scours a hole in the bank or toe and measures the shear stress and erosion rate From this we calculate critical (threshold) shear stress and erodibility coefficient, k Measuring bank erodibility with the ARS non-vertical jet test device

From Relation between Shear Stress and Erosion We Calculate  c and k Shear Stress, Pa Erosion Rate, cm s -1 cc k (cm 3 N -1 s -1 ) An Example: Test 2, Hungerford Brook, Rowell property, VT.  c = 2.46 Pa

Original Relation for Erodibility (k) Erodibility, m 3 /N-s k = x  c y = 0.2  c -0.5 Where;  c = critical shear stress (Pa), x, y = empirical constants Hanson and Simon (2001)

Distributions: Critical Shear Stress

Distributions: Erodibility Coefficient

Erodibility Relation: Yalobusha River System, MS

Erodibility Relation, Kalamazoo River, MI

Erodibility Relation: Shades Creek, AL

Erodibility Relation: James Creek, MS

Revised Erodibility Relation

Cohesive Strength Meter (CSM) The CSM consists of a water-filled chamber 30 mm in diameter that is pushed into the sediment. The jet of water comes from a downward directed nozzle in the chamber. The velocity of the jet is increased systematically through each experiment. Bed erosion is inferred from the drop in the transmission of infrared light across the chamber caused by the suspension of sediment.

Example CSM Results  c = 11 Pa

Comparison of Methods:  c

Comparison of Methods: k