by Myles Lind, CPEng, President, IPWEA NZ
This article was originally published in the APWA Reporter, July 2020 issue
Introduction
Around 40% of New Zealand’s road network is gravel (unsealed) roads. During dry periods, particularly in the summer months, dust is generated as vehicles drive on the gravel roads. Dust from gravel roads creates safety and health hazards for road users and those living or working nearby. There are also the economic costs from reduced productivity of land, crops and livestock located close to gravel roads.
Emerging issues
In general, gravel roads service sparsely populated rural areas. However, the recent increase in lifestyle interest in rural areas has meant that a higher number of people are now being exposed to dust effects from gravel roads. For economic reasons, new rural houses tend to be built closer to the public road than traditional farmhouses. As a result, health professionals and residents have expressed concerns about the health impacts of exposure to dust from gravel roads which has encouraged road managers to consider their options.
New government requirements
The impacts of land transport planning and investment decisions often continue for decades, or centuries, as transport infrastructure is long-lived and shapes how our cities and regions develop.
As the national planner, funder and regulator for land transport, the New Zealand Government has reviewed how its decision-making processes influence national well-being. This review has looked beyond how people and products travel, by including broader well-being outcomes, such as human health, social development, economic prosperity, global connectivity, the liveability of towns and cities, and the quality of the environment.
In 2019, this work resulted in the New Zealand government publishing a new outcomes framework for land transport. This framework established five connected well-being outcomes the New Zealand government expects road managers to achieve across the country. Road managers are each expected to establish how they are impacting, and intending to contribute to, delivery of these national outcomes. One of the specific outcomes from the framework is to protect people from exposure to harmful pollution from the transport system.
The dust problem
The amount of dust created from a gravel road is a factor of the speed and size of the vehicles, with faster speeds and larger vehicles producing more dust. Often locally sourced road materials used in the construction and maintenance of gravel roads are a big part of the problem. Getting the right mix of materials for a gravel road can be a challenge in some regions.
Airborne dust from gravel roads can deposit on roofs contaminating household drinking water supplies, as well as a deposit on gardens and houses causing nuisance and reducing amenity value. Nuisance dust is typically made up of more substantial size particles, referred to as total suspended particulate (TSP). TSP are those particles with an aerodynamic diameter of up to 100 microns. The finer dust particles, with an aerodynamic diameter of fewer than 10 microns (known as PM10) are the ones that increase risks with regards to public health.
Dust is also known to interfere with the photosynthesis process in plants, with ramifications for horticulture and forestry sectors as well as the environment.
Public health effects
Historically, airborne dust from gravel roads was primarily considered a nuisance issue. It is only since the turn of this century that the actual health impacts of road dust in the air began to be recognized. There is now a global scientific consensus that exposure to particulate pollution causes respiratory and cardiovascular effects than can include coughs, bronchitis, and exacerbation of asthma. These effects are known to impair people’s ability to attend a school or go to work. In New Zealand, it has been reported that dust from gravel roads results in increased visits to doctors for some people and hospital admission for others.
Changing investment
In New Zealand, road managers are working together, across regions, to share learnings on good-practice gravel road management. Road managers are faced with the challenge of how to best to allocate scarce funds across all of their road networks.
Road dust represents the loss of the fine material essential to the integrity of a gravel road, hastening the need for maintenance grading and replenishment of loosened and lost gravel. Allowing this vital component of the road to be lost in this way is increasingly environmentally and economically unsustainable. Effective minimization of road dust is integral to efficient asset management of gravel roads. This is especially important as climate change is predicted to bring more extended periods between rainfall days, exacerbating road dust events.
The most effective method of dust suppression is to seal the road. The cheapest suppressant is spraying water, but the effectiveness of water can last from half a day to half an hour, depending on the traffic and the weather. Proprietary dust suppressants are being introduced to support gravel road maintenance and reduce the loss of fine materials.
Cost management
The cost-benefit of any mitigation needs to be considered carefully. A risk-based methodology was developed by the New Zealand government to help assist road managers assess and prioritize dust mitigation-related activities. This approach considers risk factors, including:
- Traffic characteristics (nature, number and speed of vehicles)
- Site characteristics (local weather patterns and topography)
- The number and proximity of sensitive receptors (residential dwellings, sensitive ecological or horticultural areas)
Conclusions
Using these new approaches, dust levels reduce, and material replacement intervals can be extended. However, it remains essential that gravel roads are correctly shaped and have proper drainage channels.
Myles Lind can be contacted at (+64) 9448-7298 or myles.lind@at.govt.nz.
