CHAPTER 7 FLASH FLOOD MODELLING
7. Introduction
7.2. Results
Design rainfall data for the U20H quaternary catchment for 1 hour, 2 hour (storm) and 3 day (regional) rainfall durations are presented for various return periods as extracted from Hydrorisk (Smithers & Schulze 2003). These are shown in Tables 7.1, 7.2 and 7.3 and list the variable values and formulae used to calculate the rainfall intensity (i) (a from Table 7.1 and d, c from
Table 7.2) and the Rational Formula (RF) peak discharges (Qt) (C, A from Table 7.3). An important point to note is the magnitude difference in rainfall depths, where a one hour storm can produce approximately 40% of the predicted three day rainfall for the equivalent return period (Table 7.1). Three U20H sub-catchments marked in Figure 7.2 are used to show the intensity calculations (Table 7.2) and discharges (Table 7.3). Table 7.4 compares discharges for both the regional and flash flood return periods.
The lumped parameters used to determine the run-off coefficient for the semi-distributed flash flood model (Chapter 4), show very little variation. Catchment slopes for the area ranged between 10% and 26%, with a predominant grassland land cover. Overall soil infiltration is poor, with thin soils (< 100 mm deep) across the whole quaternary catchment. Based on the value ranges from Mark and Marek (2011), the run-off coefficient was determined to be 0.725 for all the sub-catchments within this quaternary catchment. Most sub-catchments have a time of concentration (Tc ) between 1 – 2 hours (Figure 7.2).
Figure 7.1. Detailed map of two U20 H sub-catchments showing the QM1 0 0 and QMRM F in comparison to the community reported flooded homesteads.
A stepped series of calibration factors (CF) for the various reach slope categories were used to model flood elevation surfaces until the closest fit to the field data was found. Calibration factors were then refined to achieve a bit fit across all reach slope categories. The calibration factors for the Flash Flood Model had to be significantly adjusted, particularly in the higher slope reaches to produce a best fit (Table 7.5). The results of the flash flood elevation modelling show that the surface with the closest fit to the control data (homestead data and GPS marked flood deposits) is the one derived from the FFM200 estimated return period (Fig. 7.3). While the Flash Flood Model calibration factors produced consistent results within this catchment, the study needs to be extended to other quaternary catchments to verify if these can be also applied universally in the same manner as the Flood Zone Model calibration factors.
Table 7.2. The calculated values for three U20H sub -catchment variables are listed together with the rainfall intensity calculation results. See Figure 7.2.
Sub- catchment
Time of concentration
Power factor35
Rainfall Intensity, i=ad-c (mm/hr) Return Period (Years)
d c 10 20 50 100 200
u20h_10b,1 1.4 -0.762 50.1 60 74.7 87.3 101.4
u20h_11,1 1.4 -0.762 51.5 61.7 76.8 89.7 104.3
u20h_10,1 2.2 -0.762 35.5 42.5 52.9 61.9 71.9
Table 7.1. Comparison for hourly and three day design rainfall depths for certain return periods (Extracted from Hydrorisk (Smithers & Schulze 2003). Rainfall
Intensity
Return Period (Years) a
2 5 10 20 50 100 200
1 Hour
mm/hour 31 44 53 64 80 94 109
2 Hour
mm/hour 38 53 65 78 96 113 131
3 Day
(mm/day) 79 110 135 161 201 234 273
Spatially the estimated flash flood 200 year return period (FFM200) flood risk area can be considerably larger than the QM100 and QMRMF flood risk areas (Fig. 7.3). A relatively small flash flood in the U20H quaternary catchment can affect upward of 700 homesteads whilst a regional flood may only affect 100 homesteads (Table 7.6). From Figure 7.3, the modelled FFM200 surface matches the survey respondent control quite closely.
Table 7.4. Comparison of regional and flash flood discharges for various return periods.
Sub catchment
Regional Flood Return Period (Years) Flash Flood Return Period (Years)
Area QM100 QM200 QMRMF 20 50 100 200
km2 m3/s m3/s m3/s m3/s m3/s m3/s m3/s
U20H_10b,1 2.01 9 11 18 24 30 35 41
U20H_11,1 5.19 24 30 46 65 80 94 109
U20H_10,1 10.12 47 58 89 109 135 158 184
Table 7.3. Variables and calculations used for the RF calculations for the three selected U20H sub-catchments. See Figure 7.2.
Sub- catchment
Area km2 Run-off coefficient
Rational Formula Qt = CiA/3.6 (m3/s) Return Period (Years)
A C 10 20 50 100 200
u20h_10b,1 2.01 0.725 20 24 30 35 41
u20h_11,1 5.19 0.725 54 65 80 94 109
u20h_10,1 10.12 0.725 72 86 108 126 146
Table 7.5. Characteristics of floodplain slope categories and comparison of defined slopes categories for the Flood Zone (FZM) and Flash Flood Models (FFM).
Slope category
FZM Calibration factor applied
FFM Calibration
factor applied Floodplain characteristics
< 0.001 0.001 ?
Reaches with very little or no elevation change. Valleys tend to be broad. Rivers tend to meander within the flood plain. Primarily major rivers. Common on coastal flood plains
0.01 –
0.001 0.05 0.3
Low relief. Valleys tend to be broad. Rivers tend to meander within the flood plain. Primarily major rivers.
0.01 -
0.025 0.2 1.2
Intermediate phase between the low relief drainage and the steep drainage.
> 0.025 0.4 6 High relief with steep sided valleys incised into the topography. Narrow floodplains.
Figure 7.2. Map of the U20H quaternary catch ment showing the sub-catchment time of concentration (Tc). The sub-catchments used in the tables as examples
are also marked.
The comparison between the data measurement sites and the modelled elevation surfaces for U20H is presented in Figures 7.4 and 7.5.
Figure 7.3. Detailed map of two U20H sub -catchments showing the QM1 0 0 and QM R MF and FFM2 0 0 flood risk areas in comparison to the community reported
flooded homesteads.
Table 7.6 List of community reported flooded homesteads and estimated current homesteads at risk in quaternary catchment U20H for selected flash flood and
regional flood return periods .
Estimated flood return
period
Flooded homesteads
Current homesteads at
risk
FFM20 328 753
FFM 50 343 870
FFM 100 357 969
FFM 200 371 1069
QM100 31 74
QMRMF 44 104
Comparison of the cross-sections in Figure 7.5 shows that the difference in elevation between that of the QMRMF and the FFM200 can be at least nine metres. Spatially this has a significant impact on areas demarcated as flood risk areas (Figs. 7.3 and 7.4).
Figure 7.4 Map of the U20H Quaternary catchment showing positions of cross - sections in relation to the various modelled flood elevations .