2013 – Application of modern dam safety management practices to older farm dams

John Duder, David Bouma and Paul McCallum

The authors have been involved in the safety inspection and remediation of many older (pre-dating the 2004 Building Act) farm dams over the past decade coupled with considerable corporate knowledge from dams inspected by Tonkin & Taylor Ltd in its 50+ year history. This paper presents a summary of the varied benefits and risks of these older dams and the difficulties encountered in bringing them into alignment with current practice.
The many farm dams around New Zealand provide considerable benefit to the owners and often to the environment and wider community including the obvious stock water and irrigation, but also micro hydro, recreation, flood detention, release of environmental flows and flows for downstream users, and wetland habitat.
However, when applying current dam safety practice, and looking forward to the implementation of the Dam Safety Regulations, some of the older farm dams have significant dam safety issues that are often challenging to address. Although there is a high degree of variability, typical issues include:
 Little or no documentation of geotechnical investigations, design or construction,
 Design standards, particularly for spillway capacity have generally increased,
 Little or no formal surveillance or maintenance carried out or recorded since commissioning,
 Many farm dam owners have a poor understanding of their obligations under the Building Act and the Conditions of their Resource consents,
 Consent conditions may not require dam safety related monitoring and maintenance, and/or the conditions may not have been historically enforced.
Many of these farm dams have been constructed by small contractors at the request of the farmers, often with only “standardised” engineering design and little specific geotechnical investigation. Typically there are no as-built records and the dam owners have been left with a general lack of understanding of owner’s responsibilities to monitor and maintain the dam.
Given that there are often very limited funds available for upgrade work, it has proved important to apply sound engineering judgement and a high degree of pragmatism to realise the greatest possible reduction in dam safety related risk for the available funds. Good cooperation between the Regional Authority, the Building Consent Authority for dams (often they are different organisations), the dam owner, and the dam engineer, together with a pragmatic approach is vital in moving toward current best practice for management of these dams.
Case studies are presented for the Northland Region, where the farm dams are typically homogenous earth fill dams in the order of 8 to 12 m high, fulfilling functions as irrigation, stock water supply, recreation and flood detention structures. The findings are considered relevant to earth fill farm dams across the country.