2. INTRODUCTION
The Geomorphology class met to investigate the morphology and
discharge of Tomahawk Creek in Leawood, KS, to observe and collect data
relating to the fluvial geomorphic processes taking place. Measurements
obtained included determining the discharge and bankfull discharge regarding
cross-sections of the wetted channel, estimating the bedload using the belt line
transect method, and mapping the natural meander characteristics and mitigation
measures.
METHODS
To obtain the cross-section, members of the team measured the bankfull
width, and the vertical depths. With the tape measure stretched horizontally from
bank to bank (bankfull width), and another from the stretched tape down to the
surface of the ground or water (vertical depths), the vertical measurements were
taken in 1 meter intervals throughout the bankfull widths from the 3 different
cross sections. To obtain the wetted cross-section members of the team
measured the wetted width and reproduced a vertical measurement method as
described above only the depths were from the surface of the water down
vertically to the bottom of the streambed. The horizontal intervals were changed
from 1 meter as done on the bankfull widths to 1/10 of the total wetted width.
Velocity was also measured at each site, and using the float down method
a ball and a stopwatch were used and timed down the section. Members
measured out 10 meters and allowed the ball to float downstream. In Calculating
3. velocity, the average surface velocity (distance floated / average float time) is
found to get the m/s ratio.
Bedload was measured at the site as well. To obtain bedload data, group
members selected meander bends to designate a transect at the widest point of
the point bar towards the cutbank, and sampled pebbles in 1-meter by 1-meter
square and selected approximately 15 pebbles from the wetted and the dry point
bar. The pebbles were later measured for axis length and sorted by shape
ranging from very angular to angular to sub angular to sub rounded to rounded to
very rounded, which helps define the streams competence.
RESULTS AND DISCUSSION
The morphology of the stream suggests that it has typical characteristics
of a meandering stream, and the point bars and cutbanks are clearly visible and
directly adjacent to each other, and are followed by riffles and pools. The
elevation difference, although not measured, is obviously minor and attempts to
mitigate the erosional effects of the stream that have been put in place near C1
(refer to sketch map), and if not appropriately improved, will allow for further
direct erosion and also erosion downstream. Heavy rocks much larger than the
competence of the stream under normal conditions were positioned to reduce
any further erosion at C1, although these attempts have been unsuccessful as
most likely during the heavy rainfall events have destroyed the attempted
purpose as a new scarp is preceding it and further eroding the park. It also
appears that more rock will need to be placed directly across from the already
4. altered location to stop the opposite bank from being cut from underneath by the
stream as evidence by the riffles.
The discharge values calculated from the data collected are not equal at
each site along the stream: C1=1.65, C2=5.55, C3=1.94. Since there were no
tributaries along the section chosen for evaluation, the only possible explanations
are either infiltration or runoff somehow leading back into the stream, or the
incorrect measurement of either cross-sectional area or velocity. The discharge
calculations mentioned earlier were listed descending downstream. If the
proposed hypothesis of infiltration or runoff leading back to the stream were valid,
discharge values would be expected to increase downstream, this is not the case
as the C3 section decreased significantly over the first two sections. The second
hypothesis involving incorrect measurement is the most likely source of error. It is
also possible that the surface velocity is not an accurate measurement due to the
fact that it does not account for the differences that could occur in streambed
surface texture and shape.
Site C3 had the largest cross-sectional area but in contrast had the lowest
calculated discharge. Site C2 is most likely to flood due to its discharge being
nearly triple that of the largest section C3 but with only nearly 1/2 the bankfull
width, and compounded by a high velocity in C1 as well and a only slightly higher
channel width.
The competence of the stream during discharge conditions was similar for
C1 and C2, but much different for C3. The average size particle that could be
eroded for the sampled portion of the stream is .4mm, with the C1 and C2 cross
5. sections having .3mm and .1mm particle size contained in the transportation
threshold of the diagram, however the C3 section had a .8mm particle size
located in the deposition threshold of the diagram, most likely minimizing it
transfer ability. The maximum size particle that could be eroded for each of the
average velocities we measured would be .02 mm and the minimum size particle
would be .009mm, meaning these are rather small and possible testing error
comes into play.
CONCLUSION
Tomahawk Creek’s low velocity, discharge and mostly sub-angular to
angular bedload suggest that it is of low competence, and does not carry larger
particles very often or carry them from great distances. There is however a need
for mitigation as the park situated right next to the banks is continually being
eroded and danger for the loss of more park in the future. For future
investigations, knowledge of stream order and the reduction of errors by students
could prove to be more beneficial as groups would each have one task, and
compare notes. Then, on another visit sample the river using the same methods
with the groups changing task, and then compare and contrast each day to see if
human error can be minimized and depict a truer steam velocity, bedload, and
competence.
