3.1. Selecting Monitoring Nodes in Each Drainage Area and Stream
The selection of monitoring nodes is required for the integrated operation of drainage facilities, such as a CR and a DR. This is because a CR’s operation, or a cooperative operation between a CR and a DR, is based on the level of monitoring nodes. In this study, monitoring nodes were selected using two methods, based on the first flooding node and the maximum flooding node.
The first flooding node generally occurs between branch conduits, rather than between main conduits. This makes it difficult to use for the integrated operation of drainage facilities. A section is categorized as the main conduit based on the product of the runoff coefficient (C) and drainage area (A). If this is greater than 0.12 km2
(CA ≥ 0.12 km2
), then it is the main conduit. If it is smaller than 0.12 km2
(CA < 0.12 km2
), then it is a branch [16
]. The first flooding node is selected using the results of rainfall runoff simulations, with synthetic rainfall events generated by the Huff distribution. The amount of synthetic rainfall is increased from 1 mm in 1 mm increments, and this is applied to the runoff model until the first flooding event occurs. The first flooding nodes in each drainage area are shown in Table 6
The rainfall amount required to cause the first flood of the drainage system in the Daerim3 pump station is higher than that in the others. This means that the drainage system in the Daerim3 pump station is relatively strong against initial flooding. Conversely, the drainage systems in the Sinlim1, Sinlim2, and Sinlim5 pump stations are relatively weak against initial flooding. Some drainage areas have different first flooding nodes for different rainfall durations. The drainage areas in Guro1, Dorim2, and Daerim3 show different first flooding nodes at 90 min because some conduits in these areas have reverse gradients, causing different initial flooding patterns. However, the Guro2, Guro3, Guro4, Guro4, Sinlim1, Sinlim2, Sinlim5, Mullae, and Daerim2 pump stations all demonstrated the same first flooding node for all durations. When a drainage area had different first flooding nodes at 90 min, the node that appeared in the largest number of results was selected.
The maximum and first flooding nodes were selected as the monitoring nodes for the integrated operation in each drainage area. To select the maximum flooding nodes, historical rainfall events were used, rather than synthetic ones, as simulations using synthetic rainfall events produce various maximum flooding nodes, making the selection of monitoring nodes difficult. Rainfall data from 23 September 2010 and 27 July 2011, when historical flooding occurred in the target watershed, were used to identify the maximum flooding nodes in each drainage area. The maximum flooding nodes in each drainage area are shown in Table 7
Several maximum flooding nodes demonstrated a greater flooding volume than those in other drainage areas, namely Sinlim1, Sinlim2, Sinlim5, Mullae, and Dorim2. This is because of the capacity shortage of conduits and backwater effects produced by the level of the CR. A single node is selected as a monitoring node if the first flooding node is the same as the maximum flooding node. Two nodes are selected as monitoring nodes if the two are different. In the real-time integrated operation, Guro1, Guro3, Guro4, Sinlim1, Sinlim5, Mullae, Daerim2, and Daerim3 each have two monitoring nodes, while Guro2, Sinlim2, and Dorim2 have one. If the depths of the two monitoring nodes differ, they are converted into a dimensionless parameter. The depth of the monitoring node is converted to 1.0D if it is 1.5 m, and the level is converted to 0.6D if it is 0.9 m.
In Korea, monitoring nodes in urban streams are constructed under bridges. There are six bridges across the Dorim stream: the Dorim, Guro1, Sindaebang, Gwanakdorim, Sinlim3, and Seoul National University Bridges. All streams in Korea have a designed freeboard, and this is 0.6 m for the Dorim stream. The monitoring candidates in the Dorim stream are shown in Figure 10
Rainfall events with various frequencies (30, 50, 80, 100, and 200 years) and with a duration based on the time of concentration in the Dorim stream (360 min) were applied to select the monitoring nodes. The water level in the Dorim stream, height of the bank, and freeboard for events with 30-, 50-, 80-, 100-, and 200-year frequencies are shown in Table 8
For the 30-year frequency, all monitoring candidates satisfy the designed freeboard of the Dorim stream. For the 80- and 100-year frequencies, the right bank at Sindaebang Bridge lacks a freeboard. Overflow occurs here with a 100-year frequency. The other monitoring candidates satisfy the designed freeboard of the Dorim stream. However, the banks of various monitoring candidates, such as the left bank at the Dorim Bridge, both banks at the Sindaebang Bridge, and the right bank at the Sinlim3 Bridge, also lack a freeboard. Overflow occurs at the right bank at the Sindaebang Bridge, and the overflow from a 100-year frequency event is the same as that for an 80-year frequency event. Both banks at the Dorim, Sindaebang, and Gwanakdorim Bridges, and the right bank at the Sinlim3 Bridge, also lack a freeboard. Overflow occurs at the Dorim and Sindaebang Bridges. The results in Table 8
shows that overflows at Sindaebang Bridge occur with 80-, 100-, and 200-year frequencies. This section of the Dorim stream is most vulnerable to overflow.
3.2. Results of the Rainfall Runoff Simulation
Historical rainfall events in 2010 and 2011, when flooding occurred in the Dorim stream, were selected for the rainfall runoff simulation, in which the current and integrated approaches for operating drainage facilities, including CR and DR operations, were applied to the target watershed. The integrated operation, which includes the use of the CR, was applied to each pump station in the Dorim stream. The preparation time was between 5 and 30 min. The calculation was used to determine the initial operating level in the CR. This was applied to the Daerim3 pump station as follows. The product of the initial pump discharge (233 m3
/min) and the preparation time (30 min) were divided by 4. The required volume in the CR was 1711 m3
and the average area at each elevation in the CR was 11,400 m2
. The required depth was calculated by dividing the required volume in the CR by the average area at each elevation [16
]. Table 9
shows the operating levels of the drainage pumps in each drainage area of the Dorim stream.
These data are shown for both the integrated and current operations. The results, shown in Figure 11
a, indicate that the integrated operation produces a lower flooding volume than the current operation (which produces 2,905,874 m3
Overall, the integrated operation demonstrated good results, although they varied slightly according to the operating levels. When the level of the monitoring node was 0.2D, the maximum flooding volume was 2,743,103 m3
, and the minimum flooding volume was 2,741,478 m3
with a level of 0.3D. Figure 11
b shows the results of the current and integrated operations for 2011. The results in Figure 11
b also show that the integrated operation produced a lower flooding volume than the current operation (3,312,733 m3
) for all levels of the monitoring node. The integrated operation once more showed good results, although they still slightly differed from one other according to the operating levels. The maximum flooding volume was 2,951,973 m3
when the level of the monitoring node was 0.1D, and the minimum flooding volume was 2,944,196 m3
when the level was 0.9D. The results of Figure 11
show that the integrated operation was steadily better than the current operation at all levels of the monitoring nodes.
In Figure 12
, the results of flooding volume over time using the integrated operation and current operation are compared. The results in 2010 and 2011 are shown in Figure 12
a,b, respectively. The results in 2010 and 2011 show that the integrated operation had less flood volume per minute than the current operation.
Moreover, this study applied the system resilience to verify the ability to prepare for and recover from the malfunction (failure) of drainage facilities and inundation (system degradation) of drainage systems. The proposed resilience index was applied to the current and integrated operations for the 2010 and 2011 events. The results of the two operations were compared for both years, when the level of the monitoring node at the beginning of the integrated operation (including the CR operation) was 0.8D. The results of system resilience for the current and integrated operations are shown in Table 10
For the 2010 event, the system resilience of the current operation was 0.199, whereas that of the integrated operation was 0.238. This means that the integrated operation increases the system resilience of the urban drainage system in the target watershed by 0.039. The value of system resilience ranges from 0 to 1. A high value of system resilience in urban drainage systems means a more resilient drainage system. The integrated operation makes the urban drainage system in the target watershed resilient to failure (flooding).
For the 2011 event, the system resilience of the current operation was 0.064, whereas that of the integrated operation was 0.235. The system resilience increment between the current and integrated operations was thus 0.171. The system resilience of the current operation for the 2011 event was lower than that for 2010 because the total flooding volume of the 2011 event was larger than that in 2010 and system failure occurred frequently in 2011, as the flooding volume was widely distributed. The system resilience of the target watershed in Table 9
was calculated as low because the value of performance was calculated as zero if flooding occurred when there was no rainfall.