2.2. Experimental design and methods
According to the change of riparian slope in topographic structure, 4
field natural slopes with few human disturbances (5°, 10°, 15°, 20°)
were selected in the riparian zone (Fig. 1c). Meanwhile, the soil
properties and vegetation types were similar among the 4 slopes (Table
1). Four experimental plots were set up on the slope of the riparian
zone and each experimental plot was surrounded by steel flumes (300 cm
length × 100 cm width × 30 cm height) so as to separate the plots from
the surrounding environment to prevent runoff from overflowing.
[Insert Table 1]
Vegetation patches are severely fragmented according to field survey in
the study area of riparian zone, therefore, 3 vegetation cover in this
study were preliminarily designed as 0%, 15% and 30% respectively in
the experimental plots. The extraction method of vegetation cover in the
experimental plots is that: Firstly, simply weed the grass and repair
the ground surface; Secondly, the vertical photography of the
experimental plots is taken with a camera (Canon SX60 HS), Finally,
Photoshop CS4 software (Adobe Systems Inc., San Jose, California, USA)
was used to process the photos to obtain vegetation cover. In order to
ensure the accuracy of vegetation cover, the process is repeated many
times until the vegetation cover obtained is within 0.5% of the error
compared with the actually designed vegetation cover.
The experimental system included a storage water tank, a water scouring
supply system (composed of a constant head scouring setup, water pipes,
water pump, and water valve), steel catchment collector, and plastic
container (Fig. 1e). The water tank was filled with water to ensure
consistent hydraulic pressure throughout the experiment. The constant
head scouring setup was placed at the top of the steel flume to keep a
uniform water flow into the experimental plots, and a steel catchment
collector was placed at the foot of the steel flume to collect the
runoff and sediments. Then, a plastic container (20 L) was used to catch
the runoff and sediments at the outlet of the steel catchment collector.
According to the discharge per unit width produced by the heavy rain in
the study area, the scouring intensity was set to 15 L/min. In order to
avoid any effects of river sediment on the experiment, the water in the
tank was obtained from underground through a water pump.
The runoff scouring experiments were conducted with 4 slope gradients
(5°, 10°, 15°, 20°) and 3 vegetation cover levels (0%, 15%, 30%),
respectively. A total of 12 treatments were conducted, and each
treatment lasted 30 min and repeated twice. Considering the
heterogeneity in the initial soil moisture of the different slopes of
the experimental plot, a sprinkle water process was applied to soil
moisture uniform. The volume of the sprinkling water depended on the
saturation of the soil surface, which should be fully saturated with no
runoff. The water discharge was adjusted close to 15 L/min and repeated
3 times to ensure the error was within 2%. Runoff-initiating time was
measured by a stopwatch at the beginning of each experiment. During each
experiment, runoff and sediment samples were collected in a marked
plastic container at 1 min (0–5 min) and then at 5 min (5–30 min)
intervals for a total of 10 samples. The container was changed several
times in each interval, and the total runoff volume of each interval was
recorded.