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Andrew Simon, Eddy J. Langendoen, Robert Thomas - page 4 / 7

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Simon et al. (1999a). The model was run using the simulated flow conditions as a driving input. The predicted bank profile was calculated on a daily basis and imported into the Bank Stability Model so that the stability of both the initial and the predicted bank profile could be assessed.

Boundary and critical shear stress used

Critical shear stresses for the bank materials measured are shown in Table 1, with values ranging from 0.3 to 13.4 Pa. The most resistant materials were clay layers (τc = 7.1 – 10.0 Pa) while the least resistant were sand layers (τc = 0.3 Pa). For any given flow, boundary shear stress at the five sites varies due to local channel gradient and channel geometry with narrow channels confining flow, resulting in higher shear stresses. Peak, local boundary shear stress at the break of slope between the bank and the toe for the five sites is as follows (from downstream to upstream); Nohly: 3.9 Pa, Tveit-Johnson: 4.9 Pa, Woods Peninsula: 2.0 Pa, Pipal: 3.2 Pa, Milk River: 4.5 Pa.

Bank Stability and Erosion Results

Results from the stability analyses are expressed in t e r m s o f a F a c t o r o f S a f e t y ( F s ) . A v a l u e o f 1 . indicates the critical case and imminent failure; values above one are theoretically viewed as stable. However, the uncertainty and variability of soil properties and failure geometries results are such that we consider values between 1.0 and 1.3 conditionally stable. 0

River mile 1624 (Tveit-Johnson)

The streambank is 9.6 m high and composed of a basal layer of sandy silt approximately 4.5 m thick with an upper layer of clay. Initial results show the s t r e a m b a n k t o b e s t a b l e ( F s = 1 . 6 9 ) d u r i n g b a s e f l o conditions, and that negative pore-water pressure in the streambank decreased during the initial 12-day rise in stage. Stability increased very slightly with the w r i s e i n f l o w d u e t o c o n f i n i n g p r e s s u r e ( F s = 1 . 7 1 During drawdown the streambank drained rapidly, and experienced a slight decline in stability as ) . c o n f i n i n g p r e s s u r e w a s r e l e a s e d ( R e g i m e 1 : F s = 1 . 5 4 , R e g i m e 2 : F s = 1 . 4 6 , R e g i m e 3 : F s = 1 . 4 0 ) drawdown assuming no bank-toe erosion. The results a f t e r f o r R e g i m e 1 s h o w e d t h a t F s h a d n o t s t a recover at the end of the flow release so this r t e d t o s i m u l a t i o n w a s e x t e n d e d t o e n s u r e t h a t reached critical levels. The value after 70 days was F s n e v e r

1.49, and after 140 days 1.48 where after it recovered slowly.

Although the non-eroded bank was quite stable, results indicate that streambank failure is possible when bank-toe erosion is accounted for. Flow at this site is somewhat confined, generating relatively high boundary shear stresses. Critical shear stress for the bank base material is 1.3 Pa, compared with a local boundary shear stress of 4.9 Pa during peak flow. End-of-simulation stability values accounting for e r o s i o n w e r e a s f o l l o w s ; R e g i m e 1 : F s = 1 . 3 1 , R e g i m e 2 : F s = 1 . 1 3 a n d R e g i m e 3 : F s = 0 . 9 8 with the non-eroded simulation, under Regime 1 the . A s F s v a l u e h a d n o t r e c o v e r e d a t t h e e n d o f t h e i n i period and an extended simulation was performed, resulting in a minimum value of 1.28 after 70 days. Bank-toe erosion produced increasingly large failures with each successive flow regime. t i a l

Results highlight the vulnerability of this site to bank-toe erosion. Even the shortest regime results in approximately 1.5 m of bank-toe erosion, with approximately 2 m of erosion under the worst-case flow release.

River mile 1589 (Nohly)

The streambank is 6.5 m high and is composed of a basal layer of clay approximately 2.5 m thick with an upper layer of silt. Initial results show the streambank t o b e s t a b l e ( F s = 1 . 4 5 ) d u r i n g b a s e f l o w c o n d i t i o n s Negative pore-water pressure in the streambank decreased during the initial 12-day rise in flow and continued to decline during the period of maintained . h i g h f l o w . S t a b i l i t y i n c r e a s e d ( F s = 1 . 5 8 ) d u r i n g t initial 12-day rise in flow as confining pressure h e i n c r e a s e d F s m o r e r a p i d l y t h a n r i s i n g p o r e - w a t e r p r e s s u r e s c o u l d d e c r e a s e i t . F s d e c r e a s e d d u r i n maintained high flow as pore-water pressure continued to increase due to water infiltration from the channel into the bank. During drawdown stability declined but the streambank remained stable under g t h e t h e t w o s h o r t e r r e g i m e s ( R e g i m e 1 : F s = 1 . 3 4 , R e g i m e : 2 F s = 1 . 3 1 ) a n d c o n d i t i o n a l l y s t a b l e u n d e r t h e l o n g e s t r e g i m e ( R e g i m e 3 : F s = 1 . 2 5 ) a s c o pressure was removed faster than drainage allowed the pore-water pressures to equilibrate. n f i n i n g

Critical shear stresses for the materials at this site (10 Pa at the bank toe) exceeded the boundary shear stresses (3.9 Pa at the base), resulting in no erosion during the flow release. The results suggest that the simulated flow regime incorporates sufficiently slow

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