The relatively young mountains of the Hindu Kush Himalayan (HKH) region are vulnerable to erosion and sedimentation, from both natural (e.g. precipitation) and man-made (e.g. road construction, agriculture/habitation on steep slopes) factors. Transport processes in rivers move this sediment to downstream locations such as floodplains. Sedimentation in the primary river basins of HKH is therefore very high. For example, sediment in the Ganges and Brahmaputra basins, shared by Bhutan, India, Nepal, and China, have been estimated to range from roughly 270 to 700 million tons/year for the Brahmaputra River and from 400 to 730 million tons/year for the Ganges River. To put these sediment figures in perspective, sediment loads normalized by river basin drainage area (km 2) are estimated at over 650 million tons/year for the Ganga-Brahmaputra basin, compared to 195 million tons/year for the Amazon basin and 44 million tons/year for the Nile Basin (estimated from Table 6.1 in this paper ).
Addressing high levels of sedimentation in HKH is critical considering the high economic cost of erosion and sedimentation across multiple sectors and to the livelihoods of the millions of people. In the agriculture sector, soil erosion and sedimentation lead to soil nutrient loss and reductions in crop yields and increase the need for farming inputs such as fertilizers. These processes also reduce the amount of soil available to hold water for crops and increase the need to acquire and transport water from other places. Erosion and sedimentation also affect water supply through siltation of waterways that reduces water quality, and erosion can diminish vegetated areas that impact water flow and recycling processes. Hydropower, a major source for strategic energy development in the HKH region, is hindered by excessive sedimentation that reduces the lifespan of reservoirs by decreasing storage capacity, while increasing short-term operational costs and reducing generation efficiency. Erosion, landslides and flooding impact the road sector, causing blockages and road washouts that cut off the flow of goods and people, in addition to requiring significant costs for repair. At the same time, road construction that does not incorporate best practices for erosion and sediment control contributes to erosion and landslide events that increase sedimentation in waterways. Disaster management is also related to sediment management, both in terms of landslides and flooding. The HKH region is very prone to landslides due to intense rainfall during the monsoon season coupled with fragile topography. Landslides are both a major source of sediment in mountainous catchments and a major risk to life, property, and other assets that are located on unstable slopes. Sediment management, by reducing the risk of landslides, therefore also contributes to disaster management. Excessive sedimentation in waterways similarly can increase the risk of flooding. Climate change is projected to further exacerbating existing erosion and sedimentation challenges, with the increase in intense weather patterns.
Erosion control and sediment management therefore bring economic benefits across multiple sectors and also contribute to climate resilience, making livelihoods less vulnerable to climate change, making infrastructure resilient, and reducing the risk of natural disasters. The interconnected nature of erosion and sedimentation processes and impacts within a watershed, locally and regionally, upstream and downstream, make watersheds an appropriate and effective unit for erosion and sediment management. Watershed management – referring to a wide variety of practices that fall under the umbrella of “investment in green infrastructure”, such as slope correction using terracing, planting hedgerows and cover crops – is a Nature-based solution (NBS) that can significantly minimize the loss of soil and downstream sedimentation, with positive secondary impacts to the many sectors affected by sediment. By reducing erosion and sedimentation, these practices also regulate water flows, stabilize soils, maintain soil fertility, improve soil water holding capacity, regulate water quality in downstream rivers, and mitigate shallow to medium depth landslides. Watershed management practices also sequester carbon. The multiple benefits of watershed management therefore accrue not only to the agriculture, energy, and water sectors, but also have implications for disaster risk reduction, transportation, and climate change mitigation.
Given the potential benefits associated with watershed-scale sediment and landslide risk management, this e-book presents guidance notes on each subject. The first, Sediment Modeling and Management , has been developed for both World Bank teams and external clients and development partners. The second, NBS for Landslide Risk Assessment , has been developed for an internal World Bank audience but has broad application to clients and development partners as well. These guidance notes are described below, and links to the full online notes are provided. These guidance notes have been developed to inform planning and prioritization of sediment management and landslide risk management at the watershed scale.
Deep Dive 1: Planning & Prioritization for Sediment Management
Guidance Tool on Sediment Modeling and Management
Measuring and modeling sediment can help manage the problem of accelerated erosion and sediment. The approach of managing land-use practices and water management practices to manage erosion and sediment falls under the umbrella of Watershed Management. Planned at a basin level, watershed management interventions can have several benefits such as regulating water flows, stabilizing soils, maintaining soil fertility, improving soil water holding capacity, regulating water quality in downstream rivers, mitigating shallow to medium depth landslides, and sequestering carbon.
This Guidance Tool covers the applications of sediment modelling and management as they would be applied at a basin level. The Tool is designed to guide practitioners and teams working on watershed management projects and is also relevant to sectors of agriculture, hydropower, water supply, transport and disaster management who could benefit from incorporating nature-based solutions and green infrastructure to address the environmental impacts of their interventions. Guidance is based on lessons and applications from actual projects which are covered as case studies in the note.
The tool explains how to prepare a sediment budget and provides options, based on data quality and budget consideration, for prioritizing locations and types of watershed management interventions. It highlights that analytical planning can be considered even in case of data and budgetary constraints.
This guidance tool highlights the key elements for decision making to prioritize investments for sediment management at a landscape level . The introduction provides an overview of the challenge that accelerated erosion and sediment pose. The next section highlights the applications of sediment modelling and its value-add to specific sectors such as Agriculture, Hydropower and Water Supply. A section on Sediment modelling explains the various tools that can be used individually or together to gather the best data and model the process of sediment loss and erosion at a landscape level. The next section of the guidance note presents a phase wise approach to prioritizing investments to manage sediment at a landscape level. It provides guidance for practitioners on undertaking a thorough planning process, preparing a sediment budget and finally undertaking a prioritization exercise to plan investments in the landscape based on the findings of the sediment budget.
An overview of the key contents of the Guidance Note on Sediment Modeling & Management is provided in the PPT presentation below. The full Guidance Note is available at this link.
It is often the case that the challenge of sediment loss, the land area the problem is spread across and the associated environmental and economic costs are large while the resource available to address the challenge are limited. Sediment modeling and prioritizing interventions, based on the results of the modeling, can help identify the most effective locations and nature-based solutions to address the problem. These positive impacts have measurable economic benefits for sectors such as agriculture, hydropower, and water supply. Some global examples of how this has been done in practice are highlighted below:
Watershed Management for Agricultural Productivity
Accelerated soil erosion is a threat to agricultural productivity because it results in the loss of fertile topsoil and in pollution in downstream wetlands and water bodies. The issue is aggravated in certain contexts such as semi-arid areas, areas with steep slopes, deforested areas, and in regions with intense rainfall. Depending on the conditions in a basin, erosion can be caused by water, wind, and tillage. Soil erosion also causes huge losses to the economy.
Modelling erosion in a watershed can be used to design effective measures such as reduced tillage and no-till agriculture practices, improve vegetation cover as well and plan structural measures such as terraces.
Case Study: Soil Loss Assessment for the country of Malawi
Recognizing soil loss as a major threat to the agricultural development and overall economic development in Malawi, the Government of Malawi (GoM) together with UNEP, UNDP and FAO undertook a study to assess the current rates and trends of soil loss in Malawi as a baseline for future monitoring of soil loss in the country. Through the application of a sediment model called SLEMSA, the State was able to identify the hotspots for soil erosion and the main causes of soil loss rates. Source: http://www.fao.org/3/I6387EN/i6387en.pdf
Watershed Management to Enhance Hydropower Efficiency
Sediment is a huge challenge for hydropower projects in South Asia, especially in the Himalayan region. Himalayan rivers across Pakistan, India and Nepal carry enormous amounts of sediment that lead to an erosion of turbine runners, a rise in the riverbed level that can increase flooding risk as well as sediment settling in reservoirs, leading to reduced storage capacity and sediment flow downstream. Apart from measuring and modelling the sources of, in this case, the constitution of the sediment is also important. A high concentration of harder and courser sediment such as quartz particles can be more detrimental to turbines than softer soils.
Case Study: Green Infrastructure Solutions for Sediment Management in Kali Gandaki Watershed, Nepal
Kali Gandaki A is Nepal's largest hydropower plant. Since it became operational in 2002, the plant has experienced multiple issues caused by sedimentation, including turbine erosion due to the abrasion from inflowing sediment combined with cavitation, leading to frequent repairs. In addition, dead storage capacity in the reservoir was already filled by the time the plant was operational due to the small reservoir volume and large monsoon sediments. Sediment Modelling was undertaken as part of a World Bank project in collaboration with Stanford University’s Natural Capital Project; The study not only identify the sub-watersheds that were contributing the most sediment and the processes contributing the sediment such as landslides, roads, glaciers and hill and gully erosion, but also modelled the activities that could be the most effective to mitigate erosion as well as the most cost effective in having an impact. Source: https://openknowledge.worldbank.org/bitstream/handle/10986/32757/Case-Study-of-Kali-Gandaki-Watershed-Nepal.pdf?sequence=1&isAllowed=y
Watershed Management to Enhance Drinking Water Quality
Watershed interventions can also be designed to manage sediment to enhance the quality of municipal and urban water supply schemes. Investments in green infrastructure using natural systems to trap sediment and regulate water often provide a more cost-effective approach than relying solely on grey infrastructure such as reservoirs and treatment systems. Designing such projects are based on the principle that it is cheaper to prevent water problems at the source than to address it further downstream. One of the world’s most famous examples of this is the Water Supply of New York that funds a watershed protection Programme in the Catskill mountains for USD 1.5 Billion annually, but this saved over USD 10 Billion, the cost of a massive filtration plant and its operation whilst contributing to multiple environmental co-benefits.
Case Study: Establishing the Upper-Tana Water fund to address upstream erosion impacting Nairobi’s water supply
Nairobi in Kenya receives 95% of its water supply from the Tana River, originating in the highlands above Nairobi. Due to agricultural activities in these upper reaches, siltation of water has impacted downstream communities and reduced water supply and water quality to Nairobi. A few erosion and reservoir sedimentation studies to document the extent of sedimentation existed for the watershed. To address the challenge, the Upper Tana Nairobi Water Fund (UTNWF) was set up by The Nature Conservancy (TNC). To plan the fund, the economic impact of different land conservation interventions was modelled for three key basin stakeholders: small-holder farmers; Nairobi City Water & Sewerage Company (NCWSC) and Kenya Electricity Generating Company (KenGen). Land conservation measures further upstream in collaboration with small-holder farmers were deemed to hold mutual benefits - For example, reduced soil erosion was projected to increase agricultural yields and to increase water quality further downstream.
Deep Dive 2: Landslide Risk Management
Guidance Tool on Nature Based Solutions for Landslide Risk Management
Landslides are a leading cause of loss of life and damage globally. A recent study on the global patterns of loss of life due to non-seismically induced landslides estimated that 2,620 fatal landslides occurred globally in the period 2004-2010, causing 32,322 fatalities with a concentration of human loss in the Himalayan Arc and in China. (Petley 2012) A later study analyzing a global dataset of fatal non-seismic landsides from 2004-2016 found 4,862 landslide events that killed 55,997 people, with Asia having the largest concentration of events. (Froude and Petley 2018) Global landslide hazard data recently released by the World Bank estimates that roughly 400,000 significant rainfalltriggered landslides and 130,000 earthquake-triggered landslides occur per year globally. UNISDR estimates that between 1998 and 2017, over 5% of all disasters globally were landslides, affecting nearly 5 million people. Further, landslides, volcanic activity and mass movements led to US$ 8 billion in damages in this period. While landslides are distributed across the globe, South Asia is a hotspot for landslide activity and for loss of life and damages due to landslide events.
This Guidance Tool seeks to increase knowledge among World Bank teams of nature-based solutions (NBS) for landslide risk management, particularly at the landscape scale. A recent study found that in some areas landslides are nearly 6x more likely to occur on non-forested lands than on forested lands and estimated that it was 16 times more cost-effective to promote forest corridors (conservation or reforestation along roads) than to pay for the expected landslides damages. While one study cannot be generalized globally, research suggests that NBS have an important role to play in reducing landslide risk and protecting people and assets. Further, NBS confers multiple co-benefits for people, biodiversity, and the environment through leveraging forests and vegetation to reduce risk. The Sendai Framework for Disaster Risk Reduction (2015-2030) has recognized NBS as a strategy that can reduce disaster risk across various disasters and at the same time increase climate adaptation and community resilience. Understanding NBS as a potential solution to manage disaster risk is growing, particularly for mitigating floods and coastal erosion, but application to landslide risk is more limited. This Guidance Note seeks to fill this knowledge gap by collating information about landslide risk assessment and providing examples of how NBS can be integrated into the landslide risk assessment and management process to prioritize its use.
The goal of this Guidance Note is to increase awareness among World Bank teams to methodologies that exist for landslide risk assessment, and NBS to manage identified risks. World Bank activities are both affected by landslides and other types of geohazards and, if not properly implemented, can contribute to increased landslide and geohazard risk. Specifically, landslides interface with forestry, agriculture, water, transportation, housing and urban sectors, among others, both increasing risks of loss of life and economic damages to these sectors as well as being impacted (potentially triggered) by activity within these sectors. Identifying and managing landslide risk is critical to the success of development projects across multiple sectors, and to the lives and livelihoods of the people who are meant to benefit from development activities.
Specifically, the NBS Solutions for Landslide Risk Assessment guidance note seeks to help World Bank teams to:
This guidance tool seeks to answer the following question, raised through numerous consultations with Bank staff, development partners, scientists, and experts working in the field of landslide risk assessment and NBS:
This guidance tool provides insights to these questions through answering the following sub-questions:
An overview of the key contents of the Guidance Note on Nature Based Solutions for Landslide Risk Management is provided in the PPT presentation below. The full Guidance Note is available at this link.