The Waikato Regional Flood Event of 9-20 July 1998

The Australasian Journal of Disaster
and Trauma Studies
Volume : 1998-2

The Waikato Regional Flood
Event of 9-20 July 1998

A J Munro

Flood Duty Officer
Environment Waikato
PO Box 4010
Hamilton East
New Zealand

Abstract

This report gives a brief synopsis of the recent flood event on the Lower Waikato River, located approximately 50 km south of Auckland, New Zealand. Extremely high river levels were experienced on the Waikato and Waipa Rivers between 9-20 July due to widespread, and in places record rainfall. As a result, urban centres including Hamilton, Ngaruawahia, Huntly, and Mercer were inundated. Farms adjacent to the Waipa River, Whangamarino Wetland, and properties fringing Lakes' Taupo and Waikare were also affected. This event came off the back of a previous deluge that occurred only a week before. These back to back deluges (along with already saturated ground conditions) and successive frontal bands meant that rivers were unable to cope with the copious amounts of water entering the catchment. Flood warnings and management systems generally worked very well, and the Lower Waikato Waipa Control Scheme performed as per design.

The Waikato Regional Flood
Event of 9-20 July 1998

Background

Introduction

Rainfall

The design rainfall frequency for the Lower Waikato Waipa Control Scheme is 200-250mm over three days.

The 7-day total rainfall from 8-15 July for the relevant rainfall recorders are presented in Table 1. Most of the rain fell 9-11 July, with Ngaroma (near Te Kuiti) receiving its July normal in less than two days. The 7-day figures are compared with rainfall in a 'normal' July.

River Flows

Recorded peak flows are presented in Table 2 below.

Lake Taupo Levels and Gate Settings

The Taupo control gates were again set to minimum on 15 July to reduce lower Waikato River levels. Between 16-17 July, river levels generally declined, with Lake Taupo swelling to its highest level in 40 years (357.49 m) due to continued high tributary input. In fact, it has been calculated that the total inflow for Lake Taupo for July 98 is the biggest monthly inflow since records began in 1905.

The Tongariro Power Development scheme (diversion of foreign waters) was shut down when Lake Taupo reached its winter maximum control level of 357.25 metres.

On 17 July, Lake Taupo control gates and Karapiro Dam discharges were increased to reduce Lake Taupo levels. The control gates were maintained at their maximum setting until the level of lake fell significantly below the winter maximum control level of 357.25 m (this was reached on Monday 3 August, more than two weeks after the gates were fully opened).

The main effects from these high lake levels were:

• foreshore flooding and erosion at lakeside settlements due to wave/wind action. This was particularly evident on the foreshore adjacent to Taupo township;
• increased pressure on sewerage and stormwater systems in some areas (the main event occurred on the night of 15 July).

  Erosion:
  Many lakeside reserves and public footpaths suffered damage from erosion (Figure 1), undermining a section of sewer pipes and a carpark on the northern shore. The Kuratau Boat Ramp also lost some of it's foundations.
  

  Stormwater:
  Number of outfalls damaged, and capacity of system reduced (fortunate that there was no heavy rain event during this time).
  

  Sewerage:
  Five Mile Bay and Waitahanui worst affected. Infiltration into system was four times the normal flow. Septic tanks still exist in the area, many overflowed into main system due to the high lake levels (not designed for those levels). Not too many problems experienced with this increase.
  

• many jetties/boat ramps became unusable.
  

Note: maximum lake levels for planned storage are 357.14 m and 357.25 m, for summer (January-March) and winter (April-December) respectively. The maximum level is lower in summer because cyclonic rainfall is more likely then.

Karapiro Flows

It has become apparent that the tributary inflows into the Waikato system between Karapiro and Mercer were significant (0.4 cumecs / km² between Karapiro and Hamilton, as opposed to 0.12 cumecs / km² between the dams).

It is apparent that the volume of water entering the Waikato system from minor tributaries between Karapiro and Mercer played a relatively major role in contributing to the flood levels experienced on the lower Waikato.

The peak discharge from Karapiro was 550 cumecs on 12/7/98. At Hamilton, the flow had increased to 750 cumecs due to another 200 cumecs entering the system between the two locations.

Without the effect of the hydro dams and the option to use Lake Taupo as a flood storage area, peak flood levels at Hamilton, Huntly, and Mercer would have been 1 metre, 0.5 metres, and 0.1 metres higher respectively.

Lake Waikare/Whangamarino Wetland Gate Settings

Many farms that fringe the lake were inevitably flooded as the local stopbanks (i.e. private) were overtopped. Likewise, some farms fringing the Whangamarino wetland were also inundated due to the swollen Maramarua and Whangamarino Rivers which continued to infill the wetland above private stopbank levels while the control gate remained closed.

Since the establishment of the Lake Waikare gate, the lake level can now be artificially controlled between 5.50-5.65 m. In pre-gate days, the average level of the lake hovered around 6.5 m (0.21 m higher than the flood peak in July 1998). The highest lake level on record is 8.38 m (1958). The design flood level of the scheme (when lake floodwaters actually flow over farmland and into the wetland) is 7.37 m. In this event, approximately 1.7 km² of land was flooded by Lake Waikare.

The Whangamarino Wetland area swelled from its normal 17 km² to 67 km² in this event as opposed to 126 km² in the 1958 flood. Without the presence of the gates, the wetland level would have been equal to the Waikato River level of 6.11 m or an extra 73 km² of land underwater, potentially totaling 140 km²).

Flood Warnings and Alerts

The flood alert levels for the Hauraki/Coromandel Rivers reached 4 in some areas.

The Flood Operations Centre was activated on Thursday 9 July at 11:00 pm. From that point on (until Monday 20 July), river levels and rainfall patterns were monitored by staff 24 hours per day. The telemetry system switched from normal day/night mode into alert mode meaning that sites were being polled (updated) every hour, instead of the usual 3 hours.

Environment Waikato flood duty staff kept in constant contact with district councils, ECNZ and residents living on or near the Waikato/Waipa Rivers, advising them of expected peak levels and times. Predicting the arrival of the peaks became a difficult exercise with the onset of more heavy rain, and the already high river levels from the previous event.

Regular contact was also made with the Paeroa Flood Response Officers in regard to the Piako River flood situation.

Flood warning recipients were notified by phone, fax, email, and/or paging. The 0832 Infolines were also operational during the event, and staff encouraged customers (where relevant) to use it as much as possible. Telecom has since advised that the Waipa/Waikato Infoline had a 335% increase in the number of calls between 1-20 July, taking pressure off key staff in the flood control centre.

Media Coverage

Flood Management Procedures

• Flood Management Rules (ECNZ and Environment Waikato)
• Flood Warning Procedures Manual (both 1994 and 1998 Versions)
• Huntly Flood Management Plan
• Lower Waikato/Waipa Control Scheme Asset Management Plan
• Various relevant Technical Publications (i.e. Rangiriri Spillway, Lake Waikare Operational Procedures etc).

Due to the complexities of this event, decisions at times were made with limited information available. Circumstances also changed very quickly, and were unpredictable.

The guidelines outlined in the above reports have been tailored specifically for the whole Waikato River system (i.e. including Lake Taupo, Waipa River, and other tributaries and designated flood storage areas). These are derived from many years of historical data, knowledge, and experience by the various scheme managers.

Flood Effects:

The peak flow in the Awakino River Taupo (estimated to have a probability of occurrence in any one year of 20% (5 year flow) inundated SH 3, and the road was closed due to flooding and slips.

The peak flows in the upper Waipa were the highest recorded since 1958. The peak flows at Otewa and TeKuiti were estimated to have a probability of occurrence in any one year of 1% (100 year flow), and a peak flow at Otorohanga of about 2% (50 year flow), and at Whatawhata approximately a 1.5% probability of occurrence (80 year flow).

The peak flows in the lower Waikato system from Ngaruawahia to Mercer hovered between a 2% (50 year flow) and a 1% (100 year flow) probability of occurrence, which were in places higher than the 1958 peak flow. The peak flow at Mercer reached a level of 6.11 metres, inundating unprotected areas of the floodplain in this locality, including the Morrison Road basin and the Mercer riverbanks which are located outside the scheme area. The Rangiriri Spillway operated as designed for the first time in 28 years (Figure 3).

The nature of the second deluge was to produce high flows in all catchment areas (i.e. upper and lower) due to record rainfall that was fairly widespread. The flood wave increased in severity as it moved downstream due to the heavy input from the middle and lower Waikato tributaries. This was paramount in the lower Waikato as the peak flow from the Waikato Hydro system passed through Ngaruawahia before the Waipa peak flow arrived.

Over time, controlling the timing of the peaks proved extremely difficult due to continued rainfall, which either re-elevated river levels or attenuated the peak flow.

This meant that the Waikato peak flows through Hamilton, and the Waipa peak flows through Whatawhata were sustained at a very high level for some time.

The normal time of travel for flows between Otorohanga and Whatawhata is about 16 hours, but this peak took about 90 hours to reach Whatawhata.

Hauraki/Coromandel Rivers
The peak flow in the upper Waitoa river system equated to a 5% (20 year flow) probability of occurrence in any one year.

The Piako Ponding Zones were used for the first time as per the Piako River Scheme design, with significant flooding occurring in these zones.

Provisional Flood Damage - Rural
It is estimated that 1000-1500 hectares of farmland were flooded, predominantly in the lower Waikato area. Information provided from Federated Farmers indicate that about 9 farms were completely under water, with another 30-40 partially affected.

Provisional Flood Damage - Residential
The following residential areas were reported to have suffered some form of flood damage:

• in Te Kuiti, four homes were evacuated (Mangaokewa River)
• Huntly West Rd (Ngaruawahia) closed (Waikato River)
• Huntly West homes, Huntly College and roads partially flooded (Waikato River)

It has been estimated that the costs for the Waikato Region in regard to state highway damage will lie somewhere between $7-9 million. This includes costs of about $5 million for the large slip on SH 3 near Mohoenui. The total cost is expected to top $17 million, including damage to the protection scheme, infrastructure, farms, and property.

Review Programme (Where to from here)

Full Review

Flood Event
Flood Management
Scheme Management

Debriefings

Internal
Key Agencies
Community

Reporting

Auditing

Scheme Repairs

Longer Term (Improving our Performance)

Addressing identified issues

Review of the the Flood Rules and Flood Warning Manual

Scheme Protection Standards

Hazard Management

land-use planning
mapping of floodplain areas

Establishing Emergency Management Groups

Summary and Conclusions

• Above normal rainfall - the highest on record for July. Note: June was also above normal in most areas
• Saturated ground conditions affecting the whole river catchment
• Back-to-back storms which meant river levels could not return to normal
• Two major rivers both in flood joining together at Ngaruawahia
• A succession of frontal bands that continued to pass over the region
• Heavy and sustained (in fact record-breaking) inflows into Lake Taupo.

Flood flows in the Tauranga-Taupo, Tongariro, Waipa, and Waikato River systems in this event have been regarded (at some sites) as being larger than the 1958 flood, but not quite rivalling the 1907 event. Hence, the Alert Level assigned to these systems reached 5 (major flooding, minor roads flooded, and major flows in river system).

Flood warnings and management systems generally worked very well, and the Lower Waikato Waipa Control Scheme operated better than design in many places. The general reaction from the community, both from the urban and rural sector, has been extremely positive.

It should be noted that the majority of the previous major flood events that are commonly referred to, occurred during the summer period (i.e. cyclone originated), whereas this event has been out of the norm occurring during the winter months.

MetService have since indicated that a very warm north Tasman Sea provided the energy for the successive depressions that formed during July, bringing with them the active rainfall bands that resulted in this flood.

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