Taupo District Flood Hazard Study - Whareroa Stream June 2012

Taupo District Flood Hazard Study - Whareroa Stream
Table 3.2: Summary statistics of the synthetic flow record for Whareroa Stream (1978-
2011) derived as a function of the ratio of A 0.8 (m³/s).
Minimum Median Maximum Mean
Lower
quartile
Upper
quartile
Whareroa Stream 0.615 1.915 34.193 2.432 1.367 2.881 0.706
COV
Table 3.3:
Annual maximum flows recorded for the synthetic Whareroa record scaled
using the ratio of A 0.8 (1979-2010).
Rank Year
Flow
Flow
Flow
(m 3 Rank Year
/s)
(m 3 Rank Year
/s)
(m 3 /s)
1 2001 34.2 12 2003 15.1 23 1990 10.7
2 2004 28.7 13 2007 14.5 24 2005 10.5
3 2002 21.9 14 1982 13.7 25 2010 10.2
4 1989 21.6 15 1979 13.6 26 1983 9.5
5 1992 17.6 16 2008 13.0 27 1980 8.7
6 1996 17.4 17 2010 12.8 28 1997 8.3
7 1991 16.3 18 2006 12.0 29 2009 7.7
8 1998 16.2 19 1985 11.6 30 1981 7.7
9 1988 16.2 20 1994 11.5 31 1984 6.7
10 2000 15.9 21 1993 11.2 32 1987 5.6
11 1995 15.8 22 1999 11.2
3.5 Characteristics of flood events
Figure 3.11 shows the flood hydrograph for the largest flow in the synthetic record i.e., 9
December 2001. This hydrograph highlights a number of features of flood events in the
Whareroa catchment. In general, it takes rainfall events of from 6-12 hours duration to
generate a significant flood.
Longer duration rainfall events produce more sustained flows, but usually with a lower peak
discharge. High intensity rainfall events tend to produce sharp, short duration flood peaks.
Also, once the catchment has been ‘wetted up’ i.e., all the storage is full, the river responds
rapidly and sharply to any additional rainfall.
Despite the variability of specific storms, there is a high degree of similarity in flood response.
Figure 3.12 compares the recorded 2004 flood hydrograph with the 2001 flood hydrograph
from the synthetic record. The peak discharges are only approximately 5m³/s different. The
2001 flood contained two smaller peaks on the rising limb, while the 2004 event had a much
higher initial base flow. The shapes of the flood hydrograph for each of these events,
however, are remarkably similar despite them being measured in two different catchments.
This indicates that using the 2004 flood measured in the Whareroa Stream as a basis to
scale a flood hydrograph for the 100-year event is suitable for hydraulic modelling.
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