Flood modelling in a high country mountain catchment, New Zealand: comparing statistical and deterministic model estimates for ecological flows

Figures & data

Table 1 Summary flow statistics for the Upper Ahuriri River at South Diadem gauge (period of record 1963–2009)

Flow statistic	Flow (m^3/s)
Mean annual flow	23.3
Mean annual maximum flow (flood)	175
Mean annual minimum flow	8.5
Mean annual 7-day low flow	8.9
Maximum instantaneous flow	570
Minimum instantaneous flow	6.6

Fig. 1 Location map of (a) the Upper Waitaki River Basin major hydrological features and (b) HEC-GeoHMS catchment map showing the extent of the Ahuriri River, major tributaries, and 31 sub-basins used in the HEC-HMS model.

Fig. 2 Schematic flow chart for runoff curve number mapping for input to HEC-HMS.

Fig. 3 Calibration storm (14–18 May 2009) used for the Ahuriri River catchment HEC-HMS model.

Fig. 4 Mean monthly flows for the Ahuriri River at the South Diadem gauge (1963–2009).

Table 2 Parameters for the two best-fitting distributions, flood peak estimates (m³/s), and the average values of the two distributions

Distribution	Parameters			Peak flow
	Location	Scale	Shape	ARI
				2	5	10	20	50	100
				AEP
				0.5	0.2	0.1	0.05	0.02	0.01
Lognormal	−47.3	5.6	0.42	217	327	401	476	574	649
GEV	181.6	92.4	0.056	215	326	403	480	584	666
Average				216	327	402	478	579	658

Fig. 5 Plot of comparison of estimated flood magnitudes and return periods for the five best fitting probability distributions for the Ahuriri River flood data.

Fig. 6 HEC-HMS model calibration results showing the Ahuriri River observed and modelled flow for the calibration event.

Table 3 Summary of comparison of HEC-HMS model results to statistical and other estimates for all flood peaks evaluated in this study (m³/s). PEPF (Per cent Error in Peak Flow) compares HEC-HMS peak to statistical mean peak. ARI is annual recurrence interval (years) and AEP is annual exceedence probability

ARI	AEP	HEC-HMS	Statistical mean	PEPF	Regional	WRENZ
2	0.5	210	216	3	126	254*
5	0.2	283	327	13	229
10	0.1	357	402	11	329
20	0.05	455	478	5	451
50	0.02	641	579	11	665
100	0.01	844	658	28	882	690
Note: *Value is mean annual flood for comparison.

Fig. 7 HEC-HMS model validation results showing the Ahuriri River observed and modelled flow.

Fig. 8 Comparison of the Gringorton plotting position of observed Ahuriri River flood data, GEV and lognormal statistical probability distributions mean, calibrated HEC-HMS model results based on the best curve numbers (CN), model results with both +5% CN and –5% CN, and results from Pearson (1995) based on regional L-moments and quantiles.

Table 4 Sensitivity analysis results of per cent change in curve numbers with resulting proportion change in modelled flood peaks. ARI is annual recurrence interval and AEP is annual exceedence probability

ARI	AEP	Per cent curve number change
		−10%	−5%	+5%	+10%
2	0.5	−0.30	−0.15	0.18	0.37
5	0.2	−0.28	−0.14	0.16	0.33
10	0.1	−0.26	−0.13	0.15	0.31
20	0.05	−0.24	−0.12	0.14	0.29
50	0.02	−0.21	−0.11	0.12	0.25
100	0.01	−0.19	−0.09	0.11	0.21

Fig. 9 Results of the curve number sensitivity analysis showing the significant increase in flows observed as the curve number increased.

Table 5 Comparison of Ahuriri River flood information with other selected nearby rivers with similar drainage areas. Data for all other rivers derived from WRENZ estimates. Rivers drain the east slope of the Southern Alps from north to south with the exception of the Landsborough River above the Clark River on the West Coast and the Orari River at the gorge in the eastern foothills, for comparison purposes

River	Catchment area (km^2)	Mean annual flow (MAF)	Mean annual flood	Mean annual flood/km^2	Mean annual flood/MAF	1% AEP Flood	1% AEP Flood/km^2	1% AEP Flood/MAF
Upper Rakaia below Lake Stream	638	51.7	619	0.97	12.0	1535	2.41	29.7
Ashburton South Branch above Stour River	534	13.7	58	0.11	4.2	172	0.32	12.6
Rangitata above Potts River	578	85.9	653	1.13	7.6	1424	2.5	16.6
Godley	337	66.4	550	1.63	8.3	1198	3.6	18.0
Tasman	829	167	1577	1.90	9.4	3548	4.3	21.2
Dobson	356	24.1	220	0.62	9.1	558	1.6	23.2
Hopkins	334	44.9	431	1.29	9.6	1061	3.2	23.6
Upper Ahuriri
HEC-HMS 1% AEP	560	23.3	175	0.31	7.5	844	1.5	36.2
Statistical 1% AEP	−	−	−	−	−	6.58	1.2	28.2
Matukituki	506	55.3	522	1.03	9.4	1227	2.4	22.2
Dart	612	72.5	1128	1.84	15.6	2436	4.0	33.6
Landsborough River above Clark River (West Coast)	612	155.3	2488	4.07	16.0	5474	8.9	35.2
Orari River at Gorge (eastern foothills)	553	9.7	254	0.46	26.2	1031	1.9	106.3

Pearson , C.P. 1995 . Regional frequency analysis of low flows in New Zealand rivers . Journal of Hydrology (NZ) , 33 ( 2 ) : 94 – 122 .

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