5.4.1 Rainfall/Runoff Data Collection
One of the major purposes of the data collection program conducted by VU was to develop and calibrate urban drainage models for the monitored drainage systems. Therefore, it was necessary to collect complete rainfall hyetographs and runoff hydrographs for storm events. Therefore, the catchments were continuously monitored for rainfall and runoff storm events. The details of the monitoring program are found in Maheepala et al. (1998), Maheepala et al. (1999), Maheepala and Perera (1999), Maheepala (1999). Some details are given below. The rainfall/runoff data collection program was conducted as a separate project, not as part of this PhD project.
Automatic electronic tipping bucket type pluviometers with 0.2 mm accuracy (i.e. one tip of the bucket is equivalent to 0.2 mm of rainfall over the catchment) were used to monitor the temporal pattern and the magnitude of storm events. These pluviometers were installed close to the centroid of each catchment. The rainfall data records stored in pluviometers were downloaded into a notebook computer at two-month intervals. The magnitude and the temporal variation of storms were then obtained from these records.
For each of the 22 catchments, the stormwater runoff was monitored at the catchment outlet. Ultrasonic Doppler type flowmeters were installed on the inverts of drainage conduits to measure the flow depth and velocity, continuously at two-minute intervals. The
measured data were stored in the flowmeters and downloaded into a computer at monthly intervals.
5.4.2 Rainfall/Runoff Data Analysis
Raw hydrologic data (i.e. rainfall/runoff data) acquired from the data acquisition program were carefully checked for accuracy and consistency as part of the program. Whenever possible, the collected rainfall data were checked against independent rainfall data obtained from other nearby measuring stations operated by Melbourne Water and Bureau of Meteorology, for selected ‘large’ storm events. After the data were downloaded from pluviometers and flowmeters, they were manually checked for apparent malfunctioning of instruments, and errors and inconsistencies in the raw data.
The storm events were selected from the runoff data series by removing the data recorded during dry periods. The rainfall data for storm events and the corresponding runoff data were then checked for consistency in terms of temporal trends. Graphical time series plots of recorded flow depth and velocity were used in this preliminary data checking process. Figure 5.2 shows a time series plot showing temporal trends of velocity and depth of flow for a selected storm event together with rainfall of the event. The flow velocity measured at a given monitoring point should increase as the flow depth increases and vice versa, as shown in Figure 5.2.
The quality of rainfall/runoff data can be best observed by plotting rainfall and runoff in the same chart. When a catchment has several monitoring stations as the case in this data acquisition program (i.e. subcatchments within the major catchment), the data obtained at the outlet and at other upstream monitoring points can be plotted as shown in Figures 5.3, together with the rainfall of the event. These plots can be used to detect timing errors of rainfall/runoff data of storm events. As shown in Figure 5.3, the runoff peak should occur some time after the rainfall peak. Furthermore, the time of concentration (i.e. approximately estimated as the time between peak rainfall and peak runoff) of internal subcatchments should be smaller than that of the whole catchment.
112 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 0 11 22 33 44 55 66 77 88 99 110 T im e (m in ) Rainfall Intensity (mm/h) 0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0
Flow Depth (mm), Velocity (mm/s
)
R a in f a ll I n t e n sit y F lo w D e p t h
F lo w Ve lo c it y
Figure 5.2: Time Series Plots of Measured Flow Depth and Velocity
0 20 40 60 80 100 120 140 160 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 Time (min) Rainfall Intensity (mm/h) 0 50 100 150 200 250 300 350 400 450 500 Stormwater Runoff (l/s) Rainfall Intensity
Runoff - Catchment Outlet Runoff - Sub Catchment 1 Runoff - Sub Catchment 2
These plots are good indicators of the accuracy and consistency of rainfall and runoff data. They were prepared for all storm events of the study catchments, as part of the monitoring program.
After each data download, the rainfall data for storm events and the corresponding runoff data were checked for consistency in terms of rainfall and runoff volumes. For each storm event, rainfall and stormwater runoff depths of each catchment were computed and compared. The runoff depth should be always less than the rainfall depth.
Further data analysis was conducted under this PhD project to select the storm events for calibration and verification of urban drainage models of the study catchments. These data analyses are explained in Section 7.2.3.2.
5.4.3 Catchment and Drainage System Data Collection
The physical data of the catchments and their stormwater drainage systems were also collected for modelling of study catchments. This was conducted as part of this PhD project. These data include pipe system and land-use layout, catchment areas, percentage impervious and pervious areas, soil type or infiltration characteristics of catchment soils, topography of the catchments, lengths of overland flow paths, and dimensions, slopes and roughness parameters of drainage conduits, etc. These data were compiled from several sources such as drainage, contour, land-use and soil maps, areal photographs, drainage design and asset management information, reports on previous studies, VicRoad road directory and site visits.