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Johnson), and River Mile 1589 (Nohly). These sites represent a range of typical conditions along the Missouri River in eastern Montana.

For the purpose of this study we assumed a hypothetical spring release every three years on the Missouri River downstream of Fort Peck Dam, to trigger migration and spawning by Pallid Sturgeon (Scaphirhynchus albus). The simulated regime involved raising discharge from a base flow level, taken to be 216 m3/sec (8000 ft3/sec) to a maximum level of 675 m3/sec (25000 ft3/sec) in stages over 12 days, maintaining flow for between 6 and 36 days (depending on water temperature) and then returning flow to the original base level over 12 days. These two extremes were taken as the ‘best-’ (6 days) and ‘worst-’ (36 days) case scenarios from a bank stability perspective, and an interim scenario with a peak of 18 days was also tested.

A two dimensional hydrology model, GeoSlope SEEP/WTM (GeoSlope International Ltd 1998) was used to evaluate the effect of the simulated flow regime on streambank pore-water pressures. These pore-water pressures were combined with geotechnical field data to perform bank stability assessment using a bank- stability model (Simon et al. 2000). The Bank Stability and Toe-Erosion Model was used to investigate the effects of high flows on bank-toe scour, and resulting bank geometry. The eroded bank profiles were then re- analyzed in the Bank-Stability Model to differentiate changes in bank stability due to hydrologic effects and those due to erosion.

Table 1. Geotechnical and hydrologic parameters input to

Pore-water pressure modeling

The SEEP/WTM software package was employed to model pore-water pressures created under the imposed hydrologic conditions. SEEP/W is a two-dimensional finite element hydrology model that simulates the movement of water and the resulting pore-water pressures for both saturated and unsaturated conditions using Richard’s equation.

A finite-element mesh was created for each site based on profiles measured in the field (Simon et al. 1999a) to provide a framework to model pore-water pressures created under the simulated flow regime. Saturated hydraulic conductivity (Table 1) required for the SEEP/W modeling was measured in the field during the summer of 2001. Initial soil moisture conditions were simulated running a steady state analysis on each mesh with average spring groundwater level and slight surface evaporation, to create a realistic soil moisture distribution prior to imposition of the flow release. Local stage vs. time functions were developed from rating curves and used as boundary conditions for the transient analysis, simulating flow as a series of time- dependent heads on nodes along the bank toe and face.

Bank-stability analysis

The Bank-Stability Model calculates the ratio [Factor o f S a f e t y ( F s ) ] b e t w e e n t h e f o r c e s t h a t d r i v e a n d r e s i s mass-bank failure. The model accounts for the geotechnical properties of the bank material including soil shear strength (cohesion, angle of internal friction, and unit weight), positive and negative pore-water SEEP/W, Bank Stability Model and Bank and Toe t

Erosion Model.

Friction angle,

Cohesion, c’

USCS

φ

ML CH-CL CL CL SM SM CL SP SM CL-CH SM CH

. 32.9 26.9 0 35.0 37.7 13.4 37.9 9.9

(degrees) 30.1 29.1 26.9 55

(kPa) 13.2 7.34 9.4 31.5 1.85 0.36 78.9 0 0 22.3 0 27.7

Milk River 1762

Site Name

River Mile

Nohly

1589

Tveit- Johnson

1624

Woods Peninsula

1676

Pipal

1716

Saturated unit weight

φb

(kN/m3) 21.4 22.2 20.6 20.8 23.0 21.4 21.6 21.6 21.0 21.4 20.9 20.2

(degrees) 17 17 17 17 17 17 17 17 17 17 17 17

Saturated conductivity (ms-1)

3.2e

-7

9.9e

-7

3.5e

-7

3.5e

-7

5.0e

-6

2.0e

-6

2.0e

-6

1.3e

-6

8.5e

-6

2.3e

-8

8.5e

-6

4.3e

-6

Critical shear stress (Pa)

3.94 10 7.06 7.06 1.34 1.34 7.06 0.31 1.34 10 1.34 13.4

Erodibility coefficient (k) (cm3/N-s)

0.5 0.32 0.38 0.38 0.86 0.86 0.38 1.8 0.86 0.32 0.86 0.27

71

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