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RESULTS AND DISCUSSION
Landscape Structures and Flow Velocity Variation
Diversified artificial and natural buffer/detention landscape structures, included stone
dams (SDs), roadside grassed ditch (RGD), grassed filter strip (GFS), dry ponds (DPs)
and riparian buffer zone (RBZ), were found distributed along the ephemeral stream
channel upslope to downslope in the experimental watershed. These structures had
many different characteristics (Table 1). All the landscape structures constituted the
whole buffer/detention structure system to control the transport process of diffuse
polluted run-off.
Table 1:
Characteristics of the buffer/detention structures along the
ephemeral stream channel
Type*
114
Constituting
Parts
Elevation
(m)
Height
or
depth
(m)
Length†
(m)
Width
(m)
Area
(m 2 )
Adjacent
land use
SDs Dam 1 48.3 1.2 19.0 16.0 304.0 Hilly land
Dam 2 47.2 0.9 39.0 14.0 546.0 Hilly land,
village
Dam 3 46.0 1.1 59.0 21.0 1.24 × 103 Hilly land,
village
Dam 4 40.6 0.8 86.0 11.0 946.0 Village,
orchard
RGD – 26.2–27.1 0.4 260.0 2.0 520.0 Road,
cropland
GFS – 24.3–25.4 – 182.0 7.0–7.5 1.27 × 103 Cropland
DPs Pond 1 24.1 0.9–1.0 6.3 – 31.5 Cropland
Pond 2 22.6 1.2 23.5 – 212.0 Cropland
RBZ – 16.8–17.3 – 302.0 – 2.87 × 105 Cropland,
reservoir
*SDs, stone dams; RGD, roadside grassed ditch; GFS, grassed filter strip; DPs, dry
ponds; RBZ, riparian buffer zone.
†Length refers to distance from inlet to outlet of the structure.
The change of flow velocity in the water pathway plays an important role in detention
time and export load of pollutants. From the view of the whole system, the flow
velocity presented a fluctuant pattern with many instances of increase and decrease
(Figure 2). During a continuous run-off event, the mean flow velocity was 36.9 cm/s at
the inlet of the system and decreased to 13.6 cm/s at the outlet. Due to the junction
of large tributary flow from the village area (resulting from the low infiltration rate of
the ground), there was a sharp elevation of flow velocity from 10.1 to 26.2 cm/s. But
soon it decreased to 12.0 cm/s when the flow encountered a stone dam. Because of
transporting in the sloping stream channel, the flow velocity was gradually increased
and reached 32.3 cm/s at the inlet of roadside grassed ditch. Due to the enhancement
of surface roughness by vegetation, the flow velocity gradually decreased. But it
increased again at the inlet of the grassed filter strip before entering the tributary