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Andrew Simon, Eddy J. Langendoen, Robert Thomas - page 5 / 7





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changes in stage to maintain bank stability by allowing pore-water pressure to equilibrate.

River mile 1676 (Woods Peninsula)

The streambank is 6.4 m high and is composed of a 2.4 m thick basal layer of sand, a middle 0.6m thick layer of clay and an upper 3.4m thick layer of sandy silt. Initial results show the streambank to be barely s t a b l e ( F s = 1 . 0 6 ) 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 was reduced during the initial 12-day rise in flow, with a lag effect due to the low permeability of the bank materials. Stability r e m a i n e d c o n s t a n t ( F s = 1 . 0 6 a f t e r 1 2 d a y s ) a s t h resisting force of confining pressure matched the driving force caused by the loss of negative pore- water pressure. Negative pore-water pressure continued to fall during the period of maintained high e f l o w , a n d F s f e l l a c c o r d i n g l y . T h e b a n k b e c a unstable under all three regimes, with minimum m e v a l u e s o f F s = 0 . 6 1 , 0 . 5 9 a n d 0 . 5 7 r e s p e c t i v e l y f o r three flow scenarios. t h e

Although the bank toe material at this site has a low critical shear stress (0.3 Pa) the applied boundary shear stresses are also quite low (peak stress at bank toe of 2.0 Pa) due to the wide channel and low gradient. The relatively small amount of bank toe erosion that occurred was not sufficient to displace the optimum location for the failure surface significantly, and the minimum stability values are almost unchanged for the eroded banks. The results suggest that this site is already very vulnerable to instability, and that the simulated flow regime is likely to trigger streambank failure due to detrimental hydrologic effects.

River mile 1716 (Pipal)

t The streambank is 6.7 m high and is composed of a 2.8 m thick basal layer of sandy silt, a middle 0.9m thick layer of brown clay and an upper 3 m-thick layer of sandy silt. Initial results show the streambank o b e c o n d i t i o n a l l y s t a b l e ( F s = 1 . 2 8 ) d u r i n g b a s e f l o w conditions. Negative pore-water pressure decreased rapidly during the initial 12-day rise in stage. Stability declined sharply as the resisting confining pressure was less than the driving force caused by the loss of negative pore-water pressure. Negative pore- water pressure continued to decline during the maintained high flow as water continued to infiltrate the streambank. Under all three regimes the bank failed before drawdown began, with minimum values o f F s = 0 . 9 4 , 0 . 9 1 a n d 0 . 8 r e a s s u m i n g n o b a n 74 k

erosion. A small amount of toe erosion was predicted b y t h e m o d e l , r e d u c i n g b a n k s t a b i l i t y t o F s = 0 . 8 8 0.81 and 0.75 under the three regimes. Failure was due to a combination of loss of matric suction and bank-toe erosion. Both bank saturation and bank undercutting are critical issues at this site. ,

River mile 1762 (Milk River)

The streambank is 6.7 m high and is composed of homogenous dark brown silty clay. Initial results s h o w t h e s t r e a m b a n k t o b e e x t r e m e l y s t a b l e ( F s 3.71) during baseflow conditions. A high level of stability is maintained throughout the simulated flow regime due to the relatively low bank angle compared to other sites, and the cohesive nature of the bank material. Negative pore-water pressure declined rapidly during the initial 12-day rise in flow level. = S t a b i l i t y o v e r t h i s p e r i o d i n c r e a s e d s l i g h t l y ( F 3.80) suggesting that the confining pressure of the flow was able to offset the rapid loss of suction caused by the infiltration of water into the streambank. Pore-water pressure remained fairly constant during the maintained high flow indicating rapid equilibration between channel and banks during this period and resulting in a fairly constant factor of safety. During the 12-day drawdown period the s = s t r e a m b a n k r e m a i n e d v e r y s t a b l e a l t h o u g h F s d e c l i n e d ( F s = 3 . 3 2 , 3 . 3 1 a n d 3 . 3 1 u n d e r t h e t regimes) as confining pressure was removed faster than drainage allowed the pore-water pressure to equilibrate. Due to the high critical shear stress of the bank material (13.4 Pa compared with a peak local boundary shear stress of 4.5 Pa) no bank or toe erosion occurred. The combination of non-vertical banks, high cohesion and high critical shear stress resulted in a very stable bank. h r e e

Discussion and Conclusions

A combination of hydrology, erosion and bank stability modeling has been used to predict the impact of flood release on five riverbanks typical of conditions along the Missouri River in eastern Montana. The simulations and field data collection undertaken show a range of responses illustrative of different processes controlling bank-stability. Two sites (Milk River and Nohly) appear to be relatively stable and are unaffected by the simulated flood release. One site (Tveit-Johnson) resists the hydrologic (infiltration) effects of the flood, but is sensitive to basal undercutting of the banks. This site would require bank-toe protection to maintain stability. Woods Peninsula is close to the failure

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