214
13 Tunnel Flow and Erosion Processes …
13.4 Results and Discussion
13.4.1 Hydrological Processes of Tunnel Flows
During 1989 and 1990, tunnel flow occurred in twelve of the fifteen runoff-generation
storms. Monitoring the responses to natural storms in arid and semiarid regions was
always problematic due to the low frequency of events. In addition, most of the storms
occurred at night. This was not an unusual characteristic of storms in these regions,
but it did pose greater difficulties for all the assistants to arrive at the monitoring
stations at the beginning of the storms. The consequence of all these was that a
complete record of each storm at each tunnel outlet was virtually impossible to
obtain. In spite of all the logistical difficulties, reasonable data were available from
four tunnels (1, 3, 4 and 6). In particular, the largest systems, Tunnels 3 and 6, as
well as the catchment outlet had completed records of runoff and sediment data in
most of the storms. Thus, it was possible to estimate the contribution of tunnel flows
to catchment sediment yield.
(1) Hydrological response of tunnel flows
Tunnel flow start time is here defined as the lag time between the onset of rainfall
and the emergence of flow at the tunnel outlet. During 1989 and 1990, tunnel flows
occurred in twelve of the fifteen monitored storms. Overall, their response to rainfall
was very fast, with the start lag times ranging from 1 to 67 min (Table 13.1). No
distinctive difference exists between tunnel flow start time and overland flow start
time, which implies that tunnel flow velocity is very fast. Although no measurements
were made, we could roughly estimate it. The shortest start time for Tunnel 6 was
5 min. Even if we assume that overland flow was initiated immediately and entered
the last tunnel inlet after the rainfall start, the tunnel flow still had an average velocity
of 0.26 m/s. It is noted that the tunnel length of this section was actually measured
through crawling through by me.
A total of 42 discharge peaks were observed from 35 complete process records
at four tunnels (1, 3, 4, 6). Most tunnel hydrographs only had a single peak, which
occurred within half an hour after the start of tunnel flow. The number of peaks seems
to be determined by rainfall characteristics and is not affected by tunnel complexity.
However, exceptions do exist. For example, the storm on July 11, 1990 was characterized by a single rainfall peak (Fig. 13.5). Likewise, one discharge peak occurred
in Tunnel 1, 4 and 6. However, two tunnel flow peaks were produced in Tunnel 3.
The reason is still unclear. One possible explanation is that this is due to the runoff
generation zonation. In light or medium storms, runoff in the tunnel catchment is
mainly generated from the cultivated slopes and the steeper valley side slopes and
could be quickly directed into the tunnel system. However, in heavy storms such as
the above-mentioned one, a large amount of runoff could also be generated from
the terrace lands and took a considerably longer time to reach the tunnel inlets than
the former owing to the relatively remote locations and gentle slopes. Therefore, it
13 Tunnel Flow and Erosion Processes …
13.4 Results and Discussion
13.4.1 Hydrological Processes of Tunnel Flows
During 1989 and 1990, tunnel flow occurred in twelve of the fifteen runoff-generation
storms. Monitoring the responses to natural storms in arid and semiarid regions was
always problematic due to the low frequency of events. In addition, most of the storms
occurred at night. This was not an unusual characteristic of storms in these regions,
but it did pose greater difficulties for all the assistants to arrive at the monitoring
stations at the beginning of the storms. The consequence of all these was that a
complete record of each storm at each tunnel outlet was virtually impossible to
obtain. In spite of all the logistical difficulties, reasonable data were available from
four tunnels (1, 3, 4 and 6). In particular, the largest systems, Tunnels 3 and 6, as
well as the catchment outlet had completed records of runoff and sediment data in
most of the storms. Thus, it was possible to estimate the contribution of tunnel flows
to catchment sediment yield.
(1) Hydrological response of tunnel flows
Tunnel flow start time is here defined as the lag time between the onset of rainfall
and the emergence of flow at the tunnel outlet. During 1989 and 1990, tunnel flows
occurred in twelve of the fifteen monitored storms. Overall, their response to rainfall
was very fast, with the start lag times ranging from 1 to 67 min (Table 13.1). No
distinctive difference exists between tunnel flow start time and overland flow start
time, which implies that tunnel flow velocity is very fast. Although no measurements
were made, we could roughly estimate it. The shortest start time for Tunnel 6 was
5 min. Even if we assume that overland flow was initiated immediately and entered
the last tunnel inlet after the rainfall start, the tunnel flow still had an average velocity
of 0.26 m/s. It is noted that the tunnel length of this section was actually measured
through crawling through by me.
A total of 42 discharge peaks were observed from 35 complete process records
at four tunnels (1, 3, 4, 6). Most tunnel hydrographs only had a single peak, which
occurred within half an hour after the start of tunnel flow. The number of peaks seems
to be determined by rainfall characteristics and is not affected by tunnel complexity.
However, exceptions do exist. For example, the storm on July 11, 1990 was characterized by a single rainfall peak (Fig. 13.5). Likewise, one discharge peak occurred
in Tunnel 1, 4 and 6. However, two tunnel flow peaks were produced in Tunnel 3.
The reason is still unclear. One possible explanation is that this is due to the runoff
generation zonation. In light or medium storms, runoff in the tunnel catchment is
mainly generated from the cultivated slopes and the steeper valley side slopes and
could be quickly directed into the tunnel system. However, in heavy storms such as
the above-mentioned one, a large amount of runoff could also be generated from
the terrace lands and took a considerably longer time to reach the tunnel inlets than
the former owing to the relatively remote locations and gentle slopes. Therefore, it