Experiments in the recirculating flume sapping box have modeled valley formation by groundwater sapping processes in a number of settings. The effects of the following parameters on sapping channel morphology were examined: surface slope; stratigraphic variations in permeability cohesion and dip; and structure of joints and dikes. These kinds of modeling experiments are particularly good for: testing concepts; developing a suite of distinctive morphologies and morphometries indicative of sapping; helping to relate process to morphology; and providing data necessary to assess the relative importance of runoff, sapping, and mass wasting processes on channel development. The observations from the flume systems can be used to help interpret features observed in terrestrial and Martian settings where sapping processes are thought to have played an important role in the development of valley networks

Kochel, R. Craig

Simmons, David W.

English

NASA Technical Reports Server

GROUNDWATER SAPPING CHANNELS: SUMMARY OF EFFECTS OF EXPERIMENTS WITH VARIED 
STRATIGRAPHY 
Kochel, Re Craig and Simmons, David W., Department of Geology, 
Southern I l l i n o i s  Un ive r s i t y ,  Carbondale, IL 62901 
Experiments i n  our r e c i r c u l a t i n g  flume sapping box have modelled v a l l e y  
f o r m a t i o n  by groundwater sapping processes  i n  a number of s e t t i n g s .  W e  have 
examined t h e  e f f e c t s  of t h e  fol lowing parameters on sapping channel 
morphology: 1 )  s u r f a c e  s lope ;  2) s t r a t i g r a p h i c  v a r i a t i o n s  i n  permeabi l i ty  
cohesion and d i p ;  and 3) s t r u c t u r e  - j o i n t s  and dikes .  
F i g u r e  1 i l l u s t r a t e s  t h e  v a r i e t y  of  d e s i g n s  u s e d  t o  s i m u l a t e  t h e  
gently-dipping s t r a t a  and j o i n t s  c h a r a c t e r i s t i c  of t h e  Colorado Plateau.  
Run 3 can be viewed a s  a c o n t r o l  because i t  used uncemented, homogeneous 
sediment. Runs 8 - 11 were designed t o  observe t h e  e f f e c t  of j o i n t s  i n  a 
v a r i e t y  of s t r a t i g r a p h i c  s e t t i n g s .  J o i n t s  were cons t ruc ted  by excavat ing 
t h e  f i n e  sand i n  l i n e a r  t roughs and b a c k f i l l i n g  with c o a r s e r ,  more permeable 
s a n d .  Runs 27 and 28 i n v e s t i g a t e d  t h e  e f f e c t s  of  v a r y i n g  c o h e s i o n .  
Cohes ion  was va r i ed  by mixing d i f f e r e n t  amounts of cement (between 0.5 and 
5% cement) o r  l o e s s  i n  t h e  f i n e  sand and by us ing  sediments of vary ing  g r a i n  
s i z e .  
Slope of t h e  sediment s u r f a c e  and t h e  s l o p e  of t h e  i n t e r n a l  
s t r a t i g r a p h y  were v a r i e d  between runs  t o  determine t h e  e f f e c t  of s l o p e  upon 
t h e  r a t e  of sapping. I n  p a r t i c u l a r ,  we were i n t e r e s t e d  i n  how s l o p e  a f f e c t s  
t h e  r a t e  of sapping channel development and sapping channel morphology. 
I n i t i a l  experiments using homogeneous sediment (Run 3 )  i nd i ca t ed  t h a t  t h e r e  
e x i s t e d  a t h r e s h o l d  s l o p e  of  a b o u t  9' be low which no s a p p i n g  c h a n n e l s  
formed. Below t h i s  c r i t i c a l  s l ope ,  a seepage f a c e  formed, but channel 
i n c i s i o n  f a i l e d  t o  occur  because sediments were no t  en t ra ined .  This  s l o p e  
v a l u e  i s  probably d i a g n o s t i c  of t h e  f i n e  sand used i n  t h e s e  experiments. 
0 Experiments w i th  i n i t i a l  s l o p e s  above 11 experienced s i g n i f i c a n t  slumping 
a t  t h e  expense of channel formation. 
The e f f e c t s  of s l o p e  i n  experiments w i th  layered  s t r a t i g r a p h y  appear t o  
b e  more complex. Va r i a t i ons  i n  t h e  d i p  of s t r a t a  seem t o  be more important  
i n  channel development t han  s u r f a c e  s lope .  Runs 14 and 27 contained layered  
s t r a t a  wi th  markedly d i f f e r e n t  s u r f a c e  and d i p  s lopes .  The 9' s u r f a c e  s l o p e  
of Run 14 should have r e s u l t e d  i n  r ap id  channel formation. However, on ly  
s m a l l  channe l s  formed d i r e c t l y  above t h e  t o e  of t h e  coa r se  l a y e r .  Slumping 
occurred downslope from t h i s  po in t .  The s t r a t i g r a p h y  i n  Run 14 was p a r a l l e l  
t o  t h e  s u r f a c e  s lope  and no l a y e r i n g  was exposed on t h e  seepage face .  Most 
of t h e  groundwater d i s cha rge  through t h e  coa r se  l a y e r  e i t h e r  flowed along 
t h e  f l u m e  f l o o r  t o  induce slumping o r  emerged d i r e c t l y  above t h e  t o e  of t h e  
coa r se  l a y e r ,  t ak ing  t h e  s h o r t e s t  r o u t e  through t h e  f i n e  l aye r .  The s u r f a c e  
0 
s l o p e  of Run 27 was only 3 , w e l l  below t h e  t h r e sho ld  of t r a n s p o r t  seen  i n  
homogeneous f i n e  sand,  but experienced s i g n i f i c a n t  channel development. 
Channels developed because t h e  coarse ,  permeable l a y e r  was exposed on t h e  
f a c e  of a low sca rp  a t  t h e  toes lope ,  
https://ntrs.nasa.gov/search.jsp?R=19870014028 2020-03-20T10:24:13+00:00Z
The d e p t h  of t h e  sapping canyons a l s o  appeared t o  have been d i r e c t l y  
r e l a t e d  t o  t h e  t h i cknes s  of t h e  sediment i n  t h e  upper s t r a t a ,  I n  s i t u a t i o n s  
where t h e r e  was a  more permeable upper l a y e r  (weakly cemented) over a  less 
p e r m e a b l e  base ( s t r o n g  cement o r  l o e s s  mix ture) ,  t h e  b a s a l  l a y e r  ac t ed  a s  a 
b a s e  l e v e l  c o n t r o l  on i n c i s i o n .  Width of  t h e  s a p p i n g  c h a n n e l s  v a r i e d  
c o n s i d e r a b l y  w i t h  t h e  t h i c k n e s s  of  t h e  s t r a t a  and c o h e s i o n .  Cohes ion  
l i m i t e d  t h e  r a t e  of l a t e r a l  c u t t i n g  by r e t a r d i n g  t h e  r a t e  of channel wal l  
slumping, r e s u l t i n g  i n  narrower v a l l e y s .  
L a i t y  and Malin (1985) drew a t t e n t i o n  t o  t h e  r o l e  of s t r u c t u r e  i n  
c o n t r o l l i n g  t h e  p a t t e r n  of channel networks developed by sapping i n  t h e  
Navajo Sandstone of t h e  Colorado P la teau ,  They noted t h a t  on a  r eg iona l  
s c a l e  t r i b u t a r i e s  t o  t h e  Esca lan te  River  were asymmetrically d i s t r i b u t e d  on 
o p p o s i t e  s i d e s  of t h e  channel.  S t r u c t u r a l  f e a t u r e s  such a s  j o i n t s  and 
f a u l t s  c r e a t e  zones of increased  pe rmeab i l i t y  i n  consol ida ted  rocks which 
a r e  p r e f e r r e d  pa ths  f o r  groundwater flow. Runs 8 - 11 were designed t o  
o b s e r v e  t h e  e f f e c t  of j o i n t s  upon t h e  development of sapping channels.  I n  
gene ra l ,  ' main channel t r e n d s  followed j o i n t  p a t t e r n s  and t r i b u t a r i e s  
d e v e l o p e d  p a r a l l e l  t o  j o i n t s .  Our e x p e r i m e n t s  w i t h  l i n e a r  zones  o f  
i n c r e a s e d  permeabi l i ty  suggest  t h a t  i f  s i g n i f i c a n t  j o i n t s  a r e  p re sen t ,  
sapping v a l l e y s  w i l l  p r e f e r e n t i a l l y  extend along t h e s e  avenues of increased  
g r o u n d w a t e r  d i s c h a r g e .  The d e g r e e  of i n f l u e n c e  j o i n t s  w i l l  h ave  upon 
channel l o c a t i o n  probably depends upon t h e  r e l a t i v e  d i f f e r e n c e s  between t h e  
p e r m e a b i l i t i e s  of t h e  j o i n t s  and t h e  hos t  rock. A s  t h i s  d i f f e r e n c e  becomes 
g r e a t e r ,  t h e  i n f luence  of j o i n t i n g  should become more pronounced. 
We are c u r r e n t l y  experimenting wi th  t h e  development of channels  by t h e  
combination of groundwater sapping and r a i n f a l l  runoff  processes .  Channels 
formed i n  Run 28 were e s t a b l i s h e d  f i r s t  by sapping and then  supplementd by 
p e r i o d i c  r a i n f a l l .  The r e s u l t i n g  channels  exh ib i t ed  a  t ape r ing  head a r e a  
more  i n d i c a t i v e  of r u n o f f  v a l l e y s  and was a l s o  c h a r a c t e r i z e d  by more 
b i f u r c a t i o n  than  v a l l e y s  produced only  by sapping. However, c l o s e  
i n s p e c t i o n  revealed a  d i s t i n c t i v e  sca l loped  morphology of a lcoves  developed 
along t h e  channel wal l s .  
We not iced  t h e  importance of groundwater p i r acy  i n  t h e  evo lu t ion  of 
channel networks during most of t h e  sapping experiments. Subsurface p i r acy  
was commonplace i n  a l l  types  of s t r a t i g r a p h i c  s e t t i n g s  and r e g a r d l e s s  of t h e  
presence of j o i n t s .  Between 3 and 6 channels  t y p i c a l l y  formed a t  r egu la r ly -  
s p a c e d  p o s i t i o n s  ac ros s  t h e  seepage f a c e  during t h e  i n i t i a l  few hours of 
s a p p i n g  runs.  During t h e  course  of a  run ,  one o r  two of t h e s e  channels  
ex tended  headward more r a p i d l y  u n t i l  i t  captured groundwater from 
surrounding a r e a s  t h a t  would have flowed i n t o  neighboring channels.  Once 
dominance of a  g iven  channel began, t h e  process  became self-enhancing and 
t h e  d i s p a r i t y  between development of neighboring channels became even more 
appa ren t .  Eventua l ly ,  t h e  p i r a t e d  channels  became i n a c t i v e  and channel 
evo lu t ion  was terminated.  
Groundwater p i r acy  was b e s t  developed during s e v e r a l  runs 7 and 18 
where cond i t i ons  were e s t a b l i s h e d  t o  mimic t h e  i n t e r s e c t i o n  of channels  with 
s u b s u r f a c e  high-level  a q u i f e r s  i n  experiments designed t o  s imu la t e  channel 
development on Hawaii (Kochel and P ipe r  1985),  Head reg ions  of t h e  f i r s t  
channels  t h a t  reach  t h e  high-level  a q u i f e r s  widened d rama t i ca l l y  and grew a t  
t h e  expense of neighboring channels  t h a t  were p i r a t e d ,  This process  i s  
probably  r e spons ib l e  f o r  t h e  l igh t -bu lb  shaped v a l l e y s  t y p i c a l  of sapping 
v a l l e y s  on t h e  Hawaiian I s l ands ,  
These k inds  of modelling experiments a r e  p a r t i c u l a r l y  good f o r :  1 )  
t e s t i n g  concepts ;  2) developing a s u i t e  of d i s t i n c t i v e  morphologies and 
m o r p h o m e t r i e s  i n d i c a t i v e  of  s a p p i n g ;  3 )  h e l p i n g  t o  r e l a t e  p r o c e s s  t o  
morphology; and 4) providing d a t a  necessary  t o  a s s e s s  t h e  r e l a t i v e  
i m p o r t a n c e  of  r u n o f f ,  s a p p i n g ,  and mass w a s t i n g  p r o c e s s e s  on c h a n n e l  
development. The observa t ions  from t h e  flume systems can be used t o  he lp  
i n t e r p r e t  f e a t u r e s  o b s e r v e d  i n  t e r r e s t r i a l  and M a r t i a n  s e t t i n g s  where 
s a p p i n g  p r o c e s s e s  a r e  t h r o u g h  t o  have  p l a y e d  a n  i m p o r t a n t  r o l e  i n  t h e  
development of v a l l e y  networks. 
References 
Kochel, R.C., Howard, A.D., and McLane, C , ,  1985, Channel networks developed 
by g r o u n d w a t e r  s a p p i n g  i n  f i n e - g r a i n e d  s e d i m e n t s :  Analogs  t o  some 
Martian v a l l e y s :  - i n  Woldenberg, M., ed., Models i n  Geomorphology; Al len  
& Unwin, Boston, p. 313-341. 
Koche l ,  R.C., and P i p e r ,  J.F., 1985, Morphology of l a r g e  v a l l e y s  i n  Hawaii: 
Evidence f o r  groundwater sapping and comparisons t o  Mars: 17 th  Lunar 
P l a n e t ,  Sc i .  Conf., Houston, p. 424-425. 
L a i t y ,  J.E., and Malin, M.C., 1985, Sapping processes  and t h e  development of 
theater-headed v a l l e y  networks on t h e  Colorado P la teau :  Geol. Soc. Amer .  
v. 96, p. 203-217. 
Fig. 1. Schematic of s t r a t i g r a p h i c  s t y l e s  used i n  sapping experiments,  


Groundwater sapping channels: Summary of effects of experiments with varied stratigraphy

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