November 21, 2010

The recent floods in Canada, Europ, China and elsewhere in recent months have again brought to the fore the ability of extreme rainfall to take lives, destroy homes and displace communities. Evidence suggests climate change is causing increases in extreme precipitation, leading to a greater risk of flooding in urban areas. As a result, agencies around the world are responding to this threat by incorporating climate change into their decision-making. But often the focus around rainfall – in the data being collected and the projections for the future – is on daily totals. Yet the increasing severity of “sub-daily” rainfall – such as hourly – accumulations can be overlooked.

This article looks at why sub-daily rainfall is crucial for flood risk, how it is becoming more severe as the climate warms, and the implications for planning our cities. In urban settings, drainage systems can typically cope with rain from a long-duration storm with a relatively low intensity. But a short storm of high intensity can bring rain that falls faster than the system can drain it away, resulting in a flash flood. Flash flooding is the result of short, intense bursts of rainfall that is followed within minutes or hours by an increase in surface water flow. Due to their rapid onset and difficulty in providing early emergency warning, flash floods can be particularly devastating.

This type of flooding is generally caused by convective storms. These occur when warm air at the Earth’s surface rises quickly on a hot day. This air cools as it ascends and the moisture it contains condenses to form clouds. In the right conditions, huge cumulonimbus clouds can form, which are commonly associated with thunder, lightning, strong winds and sudden changes in temperature. A warming climate means that the risk of these short-duration rainfall extremes is increasing. There are three lines of evidence for understanding how sub-daily rainfall extremes are changing

1. Physics: as the Earth warms the atmosphere can store more moisture.
2. Historical changes: extreme rainfall has been increasing – and the more extreme the event, the greater the increase. .......
3. Model projections: convection-permitting models, detailed enough to resolve convective rainfall processes, consistently show increases in the intensity of the most extreme precipitation events of 10-14% per degree of warming.

While the world’s eyes are on Glasgow — and the important COP26 negotiations — Paris officials announced last month a new bike plan that could make the city “100 percent bikeable,” according to David Belliard, the deputy mayor in charge of urban transformation. According to the plan, from 2021 to 2026, Paris will invest some $291 million in improvements to the city’s bikeability, adding 81 miles of new bike lanes and installing more than 130,000 new places for cyclists to park their bikes. “I’m really impressed with the speed and the scale with which Paris is thinking about this,” Kyle Wagenschutz, vice president of local innovation for the Colorado-based nonprofit People for Bikes, told me. Paris’ new plan to expand biking infrastructure builds on progress made previously in the city’s 2015 to 2020 cycling plan, which invested $174 million to double bike-lane mileage in the City of Light. That effort got a big boost during the early days of the COVID pandemic, when Paris accelerated the rollout of bike lanes for critical workers.Now, the city is going far beyond infrastructure toward supporting the “cycling ecosystem” — for example, by funding the creation of workshops to help people to repair their own bicycles, and by starting bike learning programs in elementary schools. As of 2019, nearly one-third of France’s greenhouse gas emissionscame from transportation— mostly from cars. But Paris’ bike plan may help bring that share down as it disincentivizes driving. In 2020, for example, Paris decided to do away with 72 percent of its on-street car parking spaces.                                    https://grist.org/beacon/

Not surprisingly, the report found that urban green spaces — from trees and parkettes to green roofs and large natural spaces — generally provide significant health benefits for residents and the community. It also found that these ecological benefits are directly related to the size, quality and density of the green space. Why is it important to reduce urban heat effects and air pollution? It is estimated that tens of thousands of Canadians die prematurely each year due to acute air pollution and that high summer temperatures lead to increased illnesses, hospitalizations and deaths, especially among older adults. As the Canadian population ages and extreme heat waves become more common across the country, urban green spaces can provide essential, natural protection. This report examined various types and scales of green space, and generally found that urban green space can provide cooler, cleaner air at the site, neighbourhood and city level. Emerging evidence also suggests that closely spaced and connected smaller green spaces can provide greater cooling effects to adjacent urban areas than large individual parks with open grass areas. It found that the density and spatial configuration of an urban forest — the sum of all urban trees, shrubs, lawns and pervious soils located in an urban setting — clearly affect land surface temperatures in the city and that these elements are critical for improving urban air quality. In general, the research suggests that balancing urban forest density, particularly in areas with low green space density, would greatly improve both local and city-wide urban air quality. Various plant species provide heat and pollution-mitigating capacities, and compact multi-layering of diverse plant species can help improve overall resiliency to drought, heat and pollution. Among plant types, trees have an exceptional ability to capture and filter multiple air pollutants, including ground-level ozone, sulphur dioxide, nitrogen oxides and particulate matter. Trees are also significantly associated with improved thermal comfort and relief from heat stress at the street level and neighbourhood scale, particularly during hot seasons and times of day. The report also highlighted growing evidence of disproportionate heat- and air-pollution-related health burdens associated with unequal distribution of green space in urban neighbourhoods. Further investigation is needed regarding the prevalence of green space-related health inequalities, considering evidence in Canada that dense, low-income inner-city neighbourhoods are generally more vulnerable. The report concludes with recommendations that include improving the quantity, quality and connectivity of green spaces; prioritizing green strategies for vulnerable urban areas; and integrating greening policies with broader health and land-use planning policies.

 https://davidsuzuki.org/wp-content/uploads/2017/09/impact-green-space-heat-air-pollution-urban-communities.pdf