Published Papers


Proposed Kotuku flood detention dam - geology and geotechnical design features
Author:  Daniel Scott
Presented/Published:  10th ANZ Young Geotechnical Engineering Conference - 10YGPC, 2015
In the North Island of New Zealand, volcanic soil and rock pose seepage issues for water retaining structures, and very careful investigation and design is necessary. The proposed Kotuku Flood Detention Dam has been designed to reduce flooding within Whangarei City, and is expected to commence construction in the 2014/2015 earthworks season. The proposed dam is classified a high potential impact classification (PIC) dam, and hence the geotechnical issues have been rigorously analysed. The proposed dam and catchment are located within a complex geological environment, where a basalt flow and flow margin, firm alluvial soils, and massive sandstone rock are present in the proposed dam foundation. This paper describes the geotechnical issues posed by site geology in preparing a detailed design of the 18m earth embankment dam. Investigation and testing has been undertaken to assess the site geology, including founding conditions, seepage potential and materials for dam construction. Permeability testing, including packer, falling and constant head tests, provided input to forming a geological model of the subsurface conditions. Transient groundwater analysis on the geological model was undertaken for a 36 hour, 1% annual exceedance probability (AEP) flood event. This analysis led to the development of a detailed design for the proposed dam, which comprises a clay liner, undercuts of alluvial and selected residual and flow margin materials, toe drains and internal drainage.

Upgrade of Mangamahoe dam spillway
Author:  D Tate, G Euinton, W Robbins
Presented/Published:  2005
Keywords:  Dam, spillway, mangamahoe, New Zealand Dam Safety Guidelines
The Mangamahoe Dam is located on the Mangamahoe stream, a tributary of the Waiwhakaiho River, eight kilometres southeast of New Plymouth. The Waiwhakaiho flows through New Plymouth in a fairly deeply incised valley until it reaches the industrial section of Fitzroy, on the eastern extremity of the New Plymouth. Here valley widens out and there are some light industrial buildings within the flood plain.  The consent permits up to 7 cubic metres per second to be diverted from the Waiwhakaiho into the reservoir, the 4.4MW power station is located fifteen hundred metres north of the dam where the discharge is returned to the Waiwhakaiho. The dam was constructed as a central concrete core wall supported by earthfill shoulders, 25 metres high and 155 metres long in 1930 and 1931. The dam is considered high potential impact by the criteria in the New Zealand Dam Safety Guidelines.

Cobb dam sinkholes
Author:  Bruce Walpole, Craig Scott
Presented/Published:  NZSOLD/ANCOLD Conference, Queenstown 2007
Keywords:  Dam safety, embankment, sinkholes, foundations, dam drainage, geophysical investigations
Monitoring and surveillance is crucial to managing the ongoing performance of dam structures.  The true value of appropriate monitoring, surveillance and review processes is only realised when potential dam safety issues arise. Trustpower’s civil safety monitoring and surveillance program includes nineteen hydro schemes throughout New Zealand and incorporates structures with Potential Impact Classifications (PIC) ranging from Low to High.  TrustPower promotes a continual improvement policy on its management of safety issues and conducts inspections on a regular basis. Routine and periodic independent inspections of the key components within a scheme are paramount to the viability of the safety management system. The importance and purpose of these inspections has recently been highlighted by the discovery of two sinkholes on the face of the earth dam associated with the Cobb hydro electric power scheme.  This paper provides an example of the need for continual monitoring and surveillance, vigilance of observations, good archiving systems and documentation. It discusses the broader issues surrounding the subsequent response processes to potential dam safety deficiencies, and the success (or otherwise) of investigative methods. It also highlights that an adequate dam safety compliance system has commercial value as there is a measurable reduction in dam performance uncertainty and hence greater efficiency in the speed at which accurate resolutions can be drawn.

Design, construction, and commissioning of the Deep Stream hydro-electric scheme
Author:  D R Tate, V M Martin, J Pearson
The Deep Stream hydro-electric scheme is the first green field scheme developed by Trustpower, which has a diverse range of existing hydro electric assets, most of which have been purchased since 1999. Development of the scheme in a cost-effective manner provided a number of challenges that have been successfully overcome. The paper outlines the key features of the scheme, with particular reference to risk-based decisions made during design and construction. The paper focuses on the civil aspects of the scheme and, in particular, major water retaining elements. It concludes with a discussion of the commissioning process and the value of close monitoring of performance.

Successful performance of detention dams in the February 2004 Manawatu floods
Author:  Don Tate
Presented/Published:  IPENZ Conference 2006
Keywords:  Detention dam, Porewa, Tutaenui, Matarawa, Manawatu floods
The Manawatu floods of February 2004 were an extreme event, causing widespread damage over a wide area.  The townships of Scotts ferry and Tangimoana, located adjacent to the Rangitikei River, were inundated, as well as parts of Fielding.  Large areas of the Manawatu River flood plain were also flooded.  A number of stopbank breaches associated with this flooding have been widely publicised.  However, the successful performance of a large number of detention dams subjected to the same extreme event has not been recognised.  These dams total 53 in number and are owned  by Horizons Regional Council.  These dams fulfilled their design function of limiting flooding and, in particular, limited flooding to the towns of Hunterville and Marton and in Wanganui East.  Most dams were subjected to the greatest flood event since their construction, and in most cases the spillways operated for the first time.  This paper describes the background of the dams, the flood event, and observations made of their performance, and describes the damage to a few dams.  Risk areas for typical detention dam designs are then discussed, followed by historic management aspects of the Manawatu dams in the context of the pending dam safety legislation and possible impacts on their ongoing management.

Management of the Trustpower Ltd dam portfolio
Author:  Peter Lilley, Kelly Deighton, Don Tate, Craig Scott
Presented/Published:  NZSOLD/ANCOLD 2001 Conference on Dams
Keywords:  Dam safety, risk surveillance systems
In recent years Trustpower has undergone a rapid transition from a part owner of three dams in the Kaimai ranges south of Tauranga and the Hinemaiaia and Wheao schemes near Taupo in 1998, to the present ownership situation.  Today Trustpower owns 22 dams comprising a range of structure types, including arch, earthfill, rockfill, concrete gravity and a number of embankment canal systems.  The dam classifications for the dam portfolio vary from small to large and the NZSOLD potential impact ratings vary from very low to high.  The portfolio includes some of the largest dams in New Zealand, for example Matahina Dam a 70m high central core rock fill, Patea Dam an 80m high earthfill dam and Mahinerangi Dam a 40m high concrete arch dam with concrete gravity abutments.  The dam structures vary significantly in terms of age, potential impact and risk to Trustpower.  The Dam Safety Management Procedures (including monitoring and surveillance systems, inspections, and reviews) that existed for each dam also showed considerable variation in comparison.  The approach adopted for dam safety management is described, and the interrelationship with commercial objectives and commonly accepted standard practices.

Piping failure of the Poihipi reservoir
Author:  Don Tate
Presented/Published:  NZ Geotechnical Society, Tauranga, 2003
Keywords:  Piping failure, active fault, volcanic terrain
The Pohipi Reservoir formed water storage for firefighting purposes at the Poihipi Geothermal Scheme located near Taupo.  Soon after initial filling of the HDPE lined reservoir, the reservoir breached by a piping mechanism.  The nature of the failure was remarkable in several respects.  The outflow from the breach occurred at two separate locations, located 500m and 300m, respectively from the reservoir in opposite directions.  The failure seepage path followed a linear active fault which was present beneath the footprint of the reservoir.  This paper describes the mechanism of the piping process, the investigations which were carried out for an alternate reservoir site and the design solutions adopted.  A discussion of recent research by Australian researchers on piping failures in the context of this failure is also presented.  This event highlighted many risk factors associated with seepage from water retaining structures in general, and specific risks associated with water retaining structures in volcanic terrain.

New Zealand Dam Safety Guidelines 2000 - summary of changes
Author:  Peter Riley
Keywords:  Dams, safety, guidelines, legislation, environment, potential impact
The New Zealand Dam Safety Guidelines were released in 1995.  A full review has been completed for release in November 2000.  The update conforms with current international practise in Dam Safety, and New Zealand legislation relating to dams.  Responsibilities under the Resource Management Act are described.  Potential impact of loss of the dams including environmental effects has been used to define guideline requirements.

Patea hydro scheme case history - environmental predictions and the outcome
Author:  Peter Riley
Presented/Published:  Commission Internationale Des Grandes Barrages, Dix-septieme Congres des Grands Barrages Vienne, 1991

The Patea Hydro Electric Scheme was the first of the local authority hydro electric schemes to be subjected to full environmental scrutiny. Many environmental planning techniques which are now standard practice were developed during the approval processes for the Patea Scheme. This paper examines the procedures which were followed, the predictions which were made on scheme behaviour, and compares these with the present situation 6 years after the lake has been filled.  In the 1970s the world economy was severely affected by disruption in oil supply from middle east states. A potential world energy crisis was seen which would affect New Zealand badly. A Government policy was formulated encouraging local authority Electric Power Boards to develop hydro resources within their area. Favourable Government finance was offered. Local energy resources were examined, including environmentally sensitive rivers. The Egmont Electric Power Board, a local power supply authority, responded by investigating hydro resources within its own region. Consultants were appointed, with Beca Carter Hollings & Ferner Ltd as lead consultant, and the author as Project Manager. The Patea River was chosen as having the potential to supply a large proportion of the Board's electricity demand.  The chosen scheme had a dam in a remote section of the Patea River 42 km upstream from the mouth. The dam-would raise the water level by 60 m creating a narrow sinuous lake which extended 47 km back through remote bush covered gorges. The power station output of 30 MW would generate directly into the Egmont Electric Power Board supply network. Fig. 1 shows the dam location and catchment area.

Rosedale dam - issues associated with a high-hazard dam in an urban area
Author:  D Tate, P Riley, S Singleton
Presented/Published:  IPENZ Proceedings of Technical Groups 26/1
Keywords:  High-hazard, dams, remedial works, urban development, safety evaluation, Rosedale, North Shore
The Rosedale Dam retains the No 1 oxidation pond at the Rosedale Wastewater Treatment Plant facility. The pond has a surface area of 36 hectares and an impounded volume of over 1 million cubic metres. The dam owner, the North Shore City Council, is presently applying to the Auckland Regional Council and the North Shore City Council for renewal resource consents for the Rosedale Wastewater Treatment Plant facilities. The treatment plant handles all of the sewage from the North Shore area, which has undergone rapid growth since the plant was commissioned in the early 1960s. Stormwater flows from the surrounding catchments are diverted around the reservoir by a ring drain.  Growth of the North Shore has enveloped the dam, transforming rural landscape into residential and industrial suburbia. Consequently, the hazard rating of the dam has escalated from Medium to High, requiring increasing dam safety standards. public awareness of the dam and its functions has also increased with aesthetic awareness.  Various upgrade works to the dam were completed in the period 1992 to 1994 to alleviate concerns with various aspects of the dam's performance, principally with respect to seepage and internal erosion. In 1996 a further safety evaluation highlighted uncertainties with respect to the capability of the emergency spillway to safely discharge extreme storm events. Remedial measures have been implemented in 1999/2000 to raise the crest level, and improve the emergency spillway.  The consent process has highlighted discharges from the emergency spillway and potential downstream impacts as a significant issue of concern. The is paper describes the issues associated with a high-hazard dam in an urban area which has been subject to rapid downstream development and some of the complicating factors when the retained reservoir is a key component to the city's sewerage infrastructure.


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