| dc.description.abstract | A study on two gullies (A and B) on a volcanic ash soil in a
semi-arid area at Olkaria, in Hell's Gate National Park, was
done to determine the main gully development processes,
characteristics that contribute to gullying and to test low
cost measures of control. Gullying was common in this study
area where geothermal power development was taking place. The
study on gully A included its morphometry, development
processes, catchment, soil, and rainfall characteristics, and
tests on various control measures. Similar investigations
were done on gully B except that they were less detailed and
did not include rainfall characteristics and tests on control
measures.
The main development processes observed were bed scouring,
undercutting and sidewall collapse. Observations revealed
that a rainfall event that caused the most bed scouring of
68.0 em had a 130 of 24.4 mm/hr which yielded runoff of 0.21
~/s into the gully. This bed scouring was more than that
caused by a later rainfall event, with a 130 of 28.8 mm/frr,
that produced runoff estimated at 1.40 m3/s which scoured the
bed for only 20.·Q cm . It was deduced that erosion of the
gully bed depended, to a large extent, on the erodibility of
the bed which was in contact with the runoff and the moisture
condition or rainfall amount 5 days prior to a runoff event.
The erodibility of the volcanic ash soil at Olkaria was rated
high and it was established that land use changes, like
overgraz ing and unprotected disposal of geothermal development
waste water downhill, contributed to gUllying.
Results of control measures showed that brushwood and loose
stone check dams had an average deposition in their upstream
of 51.0 cm and 57.0 cm respectively but the former check dams
were more stable than the latter since brushwoods were secured
between posts unlike the loose stones. The main problem
encountered on brushwood check dams was key failure.
Conversion of the channel of gully A to a grassed parabolic
waterway did not prevent bed scouring but minimized
undercutting and sidewall collapse. A reshaped and grassed
gully sidewall got rilled and its loose soil at the toe region
was undercut by runoff making grass establishment difficult.
Tests done on four grass species, on an adjacent land which
had been bulldozed in an unsuccessful attempt to fill the
gully, showed that star grass (Cynodon dactylon) had the
highest survival rate.
Due to the high erodibility of the volcanic ash soils,
disturbed or overgrazed lands should immediately be
revegetated to avoid gully erosion. Once gullies have started
minimization of the main erosion processes can be done through
the installation of loose stone and brushwood check dams (to
prevent bed scouring) and a grassed parabolic waterway (to
minimize undercutting and sidewall collapse). Shaping of the
gully wall needs further investigation since the | en |
