mikeflood model predictions for 28 october 1998 flood event · 2013. 3. 17. · peak flood...

NZ Transport Agency

Peka Peka to North Ōtaki Expressway

Hydraulic Investigations for Expressway

Crossing of Mangaone Stream and Floodplain

Status Issue 2 Page 99 January 2013

Project Number 5C1814.00 PP2O_vol3_TR9D Mangaone

Appendix C

MIKEFLOOD Model Predictions for 28 October 1998 Flood

Event

NZ Transport Agency






The predictions of the MIKEFLOOD model were checked against anecdotal evidence of flood inundation

patterns following the aftermath of the 28 October 1998 flood (Figures 4-2 and 4-3).

Figure C-1 shows the measured flood discharge hydrograph from the Ratanui gauging station for the 28

October 1998 flood event while Figure C-2 shows the inflow hydrographs used as upstream boundary

conditions for the same event in the MIKEFLOOD model scaled from the measured hydrograph in Figure C-1.8

Figure C-3 shows predicted stage hydrographs upstream of the NIMT railway and SH1 culverts on the main

stream channel for the MIKEFLOOD model simulation of the 28 October 1998 flood while Figure C-5 shows

corresponding discharge hydrographs at the same locations. Similarly Figure C-5 shows predicted stage

hydrographs upstream of the NIMT railway and SH1 culverts on the Mangaone Overflow for the model

simulation of the flood event and while Figure C-5 shows corresponding discharge hydrographs at the same

locations.

Figure C-7 shows the predicted flood inundation pattern across the alluvial fan surface for the 28 October

1998 flood.

Figure C-4 shows a slight reduction in the peak flood discharge through the NIMT railway and SH1 culverts

from about 23.0m3

/s to 21.5m3

/s. On the other hand Figure C-6 shows an increase in peak flood discharge

through the NIMT railway and SH1 culverts on the Mangaone Overflow from about 17.4m3

/s to 18.2m3

/s. The

increase is due to augmentation of the SH1 culvert flow by flows in the roadside drain spreading southwards

towards the Mangaone Overflow from surcharging of floodwaters upstream of the SH1 culvert on the main

stream channel9

. (the NIMT railway culvert at the time of the 28 October

Figure C-7 indicates that significant flood ponding would have occurred upstream of the NIMT railway line

which matches what can be seen in Figures 4-2 and 4-3 after the peak of the flood. It also indicates

overtopping of the railway line and SH1 by the northern overland flow path at the Te Waka Road intersection

with SH1 and by the School Road drain flow at the Gear Road intersection with SH1. Between the NIMT railway

line and SH1, railway overtopping flows spread along the roadside drain and flowed over SH1 at several

locations through Te Horo Village (as well as augmenting the flow through the SH1 culvert on the Mangaone

Overflow). It is not known whether this reflects the actual observed locations of flood inundation experienced

except that the predictions of shallow overflow across SH1 match anecdotal evidence of peak flood depths. In

reality there would probably have been seepage of floodwaters through the track ballast on the railway line as

well as overtopping.

Figure C-7 also indicates shallow inundation (< 0.2m) through parts of Te Horo Village to the west of SH1.

Again it is not known how accurate these predictions are of actual inundation patterns although presumably, if

there was overflow of SH1 at several locations, there would also have been some minor inundation of

properties through the village.

8

In Figure C-2, the leading edges of the “Mangaone Stream” and “North-1” component inflow hydrographs have been

modified to incorporate a dummy base flow to prevent the Mangaone Overflow from running dry. This was done to

overcome problems of model instability. This departure from reality does not affect the results of the model verification as it

is the predictions of the model at the flood peak that are of primary interest.

9

At the time of the 28 October 1998 flood, the NIMT railway culvert on the Mangaone Overflow had a smaller discharge

capacity than the downstream SH1 culvert – the railway culvert has since been replaced.

NZ Transport Agency






Figure C-7 shows that the Lucinsky Overflow would have operated which again matches the photographic

evidence in Figures 4-2 and 4-3. However Figure 4-2 does suggest that the source of flow for the Overflow

may in fact have been the main channel just upstream of the access drive to the Lucinsky property through the

line of trees in the centre of the photograph – the part of the access drive across the direct line of the Overflow

appears not to have been inundated. This suggests some uncertainty about the topographic detail in the

MIKEFLOOD model about this location.

Despite this uncertainty over the topographic detail of one small area of the DTM in the MIKEFLOOD model

there is at least some evidence to suggest that the predicted flood inundation pattern for the 28 October 1998

flood broadly reflects the pattern of inundation actually experienced. If the pattern of flood inundation is over-

predicted, this could be more a consequence of the uncertainty over the actual magnitude of the fled event

where the NIMT railway line and SH1 cross the Mangaone Stream and Overflow.

The MIKEFLOOD model is used in a comparative sense in this investigation to assess the relative effects of the

Expressway compared to the existing situation when the same boundary conditions are applied. Therefore any

errors in the base model will be common to both the existing and proposed situations and will affect the

results of the model simulations similarly.

The EPA’s Technical Reviewer has queried the validity of using an approximately 15% AEP flood (estimated

peak discharge 41.5m3

/s at the SH1 stream crossing) as validation for the effects of much larger floods (peak

discharges up to about twice the peak discharge of the 28 October 1998 validation event). The MIKEFLOOD

model is effectively a floodplain storage model simulating the ponding effects of the transport links crossing

the Mangaone Stream alluvial fan and floodplain system. Despite the two-fold increase in the magnitude of the

peak flood discharge at the SH1 stream crossing between the 28 October 1998 validation flood and the 1% AEP

flood adjusted for possible climate change effects to 2090 (refer Tables 2-5 and 2-6 for flood discharge

values), the model predicts peak flood levels in the existing situation at the NIMT railway culvert on the main

stream channel of 18.81m (Figure C-3) and 18.89m (Table 4-2) respectively, an increase of only 0.08m. Any

uncertainty in the estimated magnitudes of the 1% and 0.5% AEP floods adjusted for possible future climate

effects is therefore not significant in terms of the extent of predicted flood inundation.

The MIKEFLOOD model as configured for the 28 October 1998 flood simulation is therefore suitable to use as a

tool for assessing the effect of the Expressway on existing flood patterns across the Mangaone Stream alluvial

fan and floodplain system.

NZ Transport Agency






Figure C-1 Measured flood discharge hydrograph at Mangaone at Ratanui gauging station for 28 October

1998 flood

Figure C-2 Assumed inflow hydrographs for 28 October 1998 flood for input to MIKEFLOOD model

0

5

10

15

20

25

Dis

char

ge (

m3 /

s)

Time (dd/mm/yyy hh:mm)

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40 45

Dis

char

ge (

m3 /

s)

Time (hours)

Mangaone Stream

North_1

South_1

North_2

School Road

Combined Discharge

NZ Transport Agency






Figure C-3 Predicted stage hydrographs at NIMT railway and SH1 culverts on main stream channel for 28

October 1998 flood

Figure C-4 Predicted discharge hydrographs at NIMT railway and SH1 culverts on main stream channel for

28 October 1998 flood

17.6

17.8

18

18.2

18.4

18.6

18.8

19

00

:00

:00

01

:00

:00

02

:00

:00

03

:00

:00

04

:00

:00

05

:00

:00

06

:00

:00

07

:00

:00

08

:00

:00

09

:00

:00

10

:00

:00

11

:00

:00

Leve

l (m

MSL

We

llin

gto

n)

Time (hh:mm:ss)

US of NIMT Railway - Chainage 6450

US of SH1 - Chainage 6465

0

5

10

15

20

25

00

:00

:00

01

:00

:00

02

:00

:00

03

:00

:00

04

:00

:00

05

:00

:00

06

:00

:00

07

:00

:00

08

:00

:00

09

:00

:00

10

:00

:00

11

:00

:00

Leve

l (m

MSL

We

llin

gto

n)

Time (hh:mm:ss)

NIMT Railway Culvert

SH1 Culvert

NZ Transport Agency






Figure C-5 Predicted stage hydrographs at NIMT railway and SH1 culverts on Mangaone Overflow for 28

October 1998 flood

Figure C-6 Predicted discharge hydrographs at NIMT railway and SH1 culverts on Mangaone Overflow for

28 October 1998 flood (note that NIMT railway culvert was a smaller structure than the present

culvert at the time of the 28 October 1998 flood)

17.6

17.8

18

18.2

18.4

18.6

18.8

19

00

:00

:00

01

:00

:00

02

:00

:00

03

:00

:00

04

:00

:00

05

:00

:00

06

:00

:00

07

:00

:00

08

:00

:00

09

:00

:00

10

:00

:00

11

:00

:00

Leve

l (m

MSL

We

llin

gto

n)

Time (hh:mm:ss)

US of NIMT Railway - Chainage 6450

US of SH1 - Chainage 6465

0

2

4

6

8

10

12

14

16

18

20

00

:00

:00

01

:00

:00

02

:00

:00

03

:00

:00

04

:00

:00

05

:00

:00

06

:00

:00

07

:00

:00

08

:00

:00

09

:00

:00

10

:00

:00

11

:00

:00

Leve

l (m

MSL

We

llin

gto

n)

Time (hh:mm:ss)

NIMT Railway Culvert

SH1 Culvert

NZ Transport Agency






Figure C-7 Predicted peak inundation map for 28 October 1998 flood

NZ Transport Agency






mikeflood model predictions for 28 october 1998 flood event · 2013. 3. 17. · peak flood...

Documents