In a previous post, we discussed the potential for vegetation as a solution to reducing landslide risk in hilly terrains. While this measure has been implemented widely in many places to reduce landslides along isolated unstable slopes, a large-scale application of forestry to protect urban centers have been few. Developing an integrated urban forestry plan with a specific focus to reduce landslide risk can be a strong and cheap alternative to build resilience in landslide-prone cities of the world. By stabilizing loose soil and reducing surface runoff, vegetative cover can protect cities from losing their hard-earned economic growth to landslides. Such a solution falls under the umbrella of Nature-based Solutions for disaster resilience. In this post, we’ll discuss the advantages of such an approach and take a look at how Seattle, Washington used vegetation to limit landslide hazards.
The advantage of vegetation
Landslides are commonly triggered due to four reasons; seismic activity, heavy rainfall/groundwater flow, human activities and deforestation. In each of the four triggers, the land is loosened/already loose, leading to the flow of debris along slopes. For example, seismic activity leading to earthquakes can very easily trigger landslides in areas consisting of loosely consolidated materials and high slopes. Heavy rainfall can cause landslides when water levels in the soil because so high that no more infiltration can be supported, leading to surface runoff with the mass movement of debris.
In both cases, the response to the trigger can be improved in two ways to prevent landslides: (1) increasing soil binding capacity and (2) decreasing surface runoff by either adding barriers or increasing the infiltration capacity of the soil.
Vegetation performs an important role in both regards. Tree roots and plant roots have a powerful effect on the soil by binding it together. As explained in the previous post, trees and shrubs that coppice or have strong roots have the best binding effect, preventing soil from moving and causing shallow landslides. In addition, vegetation is a physical barrier that slows down the movement of debris and water. Vegetation intercepts direct rainfall before raindrops hit the soil, reducing or even eliminating rainspash erosion. When forest litter gets collected, it acts as a spongy layer that can hold water, improve the water holding capacity of the soil, reduce the intensity of water flow and lessen erosion.
Increasing landslide risk in cities of India
As cities expand, more and more land area is incorporated into the urban landscape. Particularly, the peri-urban areas see the greatest conversion of land. In cities located in valleys, like Kohima, the city expansion is taking place on the slopes of the surrounding hills. In cities located on hilltops, like Shimla, extensive infrastructure is being developed to improve connectivity to the city. In urban agglomerations like Dehradun-Mussoorie, Dehradun is expanding upwards towards Mussoorie to link the two urban centres for better economic growth. In all three cases, the land is being stripped of tree cover in favor of infrastructure development. However, the sloping land on which expansion is occurring is highly prone to disasters as most Indian cities in the Himalayas are prone to earthquakes. The heavy rainfall during the monsoon season is also being exacerbated due to climate change. Thus, deforestation for urban expansions on vulnerable slopes is increasing the risk of landslides in such cities.
Case Study: Regulatory efforts to promote urban forestry for landslide risk reduction in Seattle, Washington
Seattle frequently experiences landslides and incurs huge economic losses because of it. The occurrence of landslides is expected to increase due to climate change, which brings high temperatures that can leave the soil loose and dry, along with heavier rainfall. After the disastrous landslides in 1995-96, and 1996-97, the City of Seattle took important regulatory steps to curb landslide risk in collaboration with the US Geological Survey and the State of Washington.
Strong government support, implementation of regulation and public outreach provided the key to Seattle’s landslide risk reduction program.
The regulatory steps were incorporated into the Seattle Municipal Code on the back of rigorous scientific evidence that delineated landslide-prone areas and buffer zones. Two regulations, in particular, were important for risk reduction. Under “Development of Standards for Steep Slope Areas” the regulation specifies that development in steep slopes should keep disruption of existing topography and vegetation minimum. These areas have come to be called “Environmentally Critical Areas”. Removal of trees is prohibited unless specified on a case-to-case basis. The “Trees and Vegetation” regulation specifies maintenance of the urban vegetation to ensure continued resilience. The regulatory measures also highlight the need to remove invasive species and plant vegetation that is native to the area, to ensure fast growth and harmony with the existing climate and soil type.
The initiative was supported by the State Government through the Growth Management Act (GMA) that requires all jurisdictions in Washington to identify and regulate geologically hazardous areas. This policy lead to constant monitoring and development of a database that ensured that development in risk-prone locations is strictly regulated. The regulation incorporated support from the public, who were encouraged to stabilize slopes by protecting vegetation and undertaking activities like mulching. Vegetation restoration activities were also supported under the regulation to improve the health and functioning of landslide-prone slopes. A list of recommended native trees, shrubs and groundcover plants and supported conditions are provided under the regulation. These include trees like madrones, sword ferns and douglas fir, and shrubs/plants like red flowering currant, serviceberry, Indian plum and native roses.
Lessons and opportunities
Strong government support, implementation of regulation and public outreach provided the key to Seattle’s landslide risk reduction program. Such measures can be very powerful in India, given that there is growing momentum in disaster management and disaster risk reduction in the country. Urban growth is increasing and is being supported under the Smart Cities Mission and Atal Mission for Rejuvenation and Urban Transformation (AMRUT) of the Government of India.
Many of the cities focused under these schemes, like Kohima, Nagaland and Dharmshala, Himachal Pradesh, are prone to landslide. This has been recognized in their Smart Cities Proposals. These cities have identified structural regulations to mitigate risk, like enforcement of building codes. Slope stability techniques through structural reinforcements are also very popular.
Urban forestry measures can greatly supplement these measures and also prove to be cost-effective. The government in Uttarakhand, India has recently taken regulatory measures to open up forests between Dehradun and Mussoorie for development activities. While this will improve investments, care should be taken that there is a strong urban forestry policy in place to protect natural vegetation and reduce the risk of landslides. Similarly, under the National Disaster Management Plan 2016, landslide mitigation measures highlight structural measures. There is an opportunity to explore options of nature-based solutions like urban forestry.