Source: https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-18-river-management-hard-engineering [Revision notes](https://senecalearning.com/en-gb/revision-notes.md) › [IGCSE](https://senecalearning.com/en-gb/revision-notes/igcse.md) › [Geography](https://senecalearning.com/en-gb/revision-notes/igcse/geography.md) › [Edexcel IGCSE](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse.md) › River Management: Hard Engineering # River Management: Hard Engineering Geography: Edexcel IGCSE, topic 1.1.18 ## Flood Defences - Hard Engineering Hard engineering strategies aim to reduce the effects of flooding by building man-made constructions that control a river's flow. Examples of hard engineering strategies include: ### Dams and reservoirs ![Dams and reservoirs](https://image-v2.cdn.app.senecalearning.com/2018-04/7dc57d4f-edc5-4c86-aa0d-6d5261305e07/shutterstock_533558680.jpg) - A dam is a large earth, stone or concrete wall built across a river in the upper course. Behind the dam, a reservoir (man-made lake) forms. - Reservoirs can reduce flooding risk by filling up with water when there is a lot of rainfall. This water can either be drunk or used to produce energy (hydroelectric power). - But, dams are expensive and habitats and towns can be destroyed by the flooding needed to create a reservoir. - The soil downstream from a reservoir usually loses some of its fertility because minerals are deposited in the reservoir instead of downstream. ### Channel straightening ![Channel straightening](https://image-v2.cdn.app.senecalearning.com/2018-04/1b22e8ea-2edb-4807-9c45-79ece78ff167/shutterstock_711414538.jpg) - Bends and meanders are removed by constructing man-made, straight channels connecting the meanders. - Straight channels reduce flood risk by keeping the water flowing through an area quickly. - But, downstream areas are more at risk of flooding and erosion because more water flows there more quickly. ### Embankments ![Embankments](https://image-v2.cdn.app.senecalearning.com/2018-04/dac644fa-5bbb-4dc8-ba3b-971431e86d4f/shutterstock_1007081728.jpg) - Embankments are walls built either side of the river channel to raise the height of the banks. - Embankments reduce flooding risk by increasing the capacity of the river. - But, embankments are expensive to build and if the banks do burst, then flooding can still happen. ### Flood relief channels ![Flood relief channels](https://image-v2.cdn.app.senecalearning.com/2018-04/dcd73809-d00d-478d-b06c-a664b0038405/shutterstock_325379636.jpg) - Flood relief channels are secondary channels built alongside rivers that divert water away from particular areas or take it somewhere else if there is too much rainfall. - The amount of water flowing through a given point in a river is reduced (river discharge falls). - But, the river discharge will increase at the junction between the relief channel and the river it links up with. ## Test yourself on River Management: Hard Engineering - What do we call a large wall built across a river in the upper course? - With channel straightening, which areas are more at risk of flooding? [Practise this topic for free](https://app.senecalearning.com/classroom/course/ef6d2f62-5eb8-4168-aaaf-48efdb77968e/section/ee8680dd-0fcb-4ce7-be24-898cb389a3ae/session) - Previous topic: [1.1.17 Human Activities on Rivers](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-17-human-activities-on-rivers.md) - Next topic: [1.1.19 River Management: Soft Engineering](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-19-river-management-soft-engineering.md) ## Geography: Edexcel IGCSE contents ### 1 Physical Geography - 1.1 River Environments - [1.1.1 Weathering](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-1-weathering.md) - [1.1.2 Mass Movement](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-2-mass-movement.md) - [1.1.3 Erosion](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-3-erosion.md) - [1.1.4 Transportation](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-4-transportation.md) - [1.1.5 Deposition](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-5-deposition.md) - [1.1.6 River Landforms: Waterfalls & Gorges](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-6-river-landforms-waterfalls-and-gorges.md) - [1.1.7 River Landforms: Interlocking Spurs](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-7-river-landforms-interlocking-spurs.md) - [1.1.8 River Landforms: Flood Plains](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-8-river-landforms-flood-plains.md) - [1.1.9 River Landforms: Point Bars](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-9-river-landforms-point-bars.md) - [1.1.10 River Landforms: Levees](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-10-river-landforms-levees.md) - [1.1.11 River Landforms: Meanders](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-11-river-landforms-meanders.md) - [1.1.12 River Landforms: Oxbow Lakes](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-12-river-landforms-oxbow-lakes.md) - [1.1.13 River Landscape Changes: Long Profile](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-13-river-landscape-changes-long-profile.md) - [1.1.14 River Landscape Changes: Cross Profile](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-14-river-landscape-changes-cross-profile.md) - [1.1.15 Vertical & Lateral Erosion](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-15-vertical-and-lateral-erosion.md) - [1.1.16 Flood Risk Factors](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-16-flood-risk-factors.md) - [1.1.17 Human Activities on Rivers](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-17-human-activities-on-rivers.md) - [1.1.18 River Management: Hard Engineering](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-18-river-management-hard-engineering.md) - [1.1.19 River Management: Soft Engineering](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-19-river-management-soft-engineering.md) - [1.1.20 Case Study: The River Eden - Landforms](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-20-case-study-the-river-eden-landforms.md) - [1.1.21 Case Study: The River Eden - Climate](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-21-case-study-the-river-eden-climate.md) - [1.1.22 Case Study: The River Eden - Geology](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-22-case-study-the-river-eden-geology.md) - [1.1.23 Case Study: The River Eden - Management](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-23-case-study-the-river-eden-management.md) - [1.1.24 Case Study: The River Eden - Human Activity](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-24-case-study-the-river-eden-human-activity.md) - [1.1.25 End of Topic Test - River Environments](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-1-25-end-of-topic-test-river-environments.md) - 1.2 Coastal Environments - [1.2.1 Weathering](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-1-weathering.md) - [1.2.2 Mass Movement](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-2-mass-movement.md) - [1.2.3 Erosion](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-3-erosion.md) - [1.2.4 Deposition](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-4-deposition.md) - [1.2.5 Longshore Drift](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-5-longshore-drift.md) - [1.2.6 Sediment Transportation](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-6-sediment-transportation.md) - [1.2.7 Wave Action on Coasts](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-7-wave-action-on-coasts.md) - [1.2.8 Other Factors Influencing Coasts](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-8-other-factors-influencing-coasts.md) - [1.2.9 Coastal Landforms: Wave-cut Platforms & Cliffs](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-9-coastal-landforms-wave-cut-platforms-and-cliffs.md) - [1.2.10 Coastal Landforms: Headlands & Bays](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-10-coastal-landforms-headlands-and-bays.md) - [1.2.11 Coastal Landforms: Caves, Arches & Stacks](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-11-coastal-landforms-caves-arches-and-stacks.md) - [1.2.12 Coastal Deposition](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-12-coastal-deposition.md) - [1.2.13 Spits, Bars & Sand Dunes](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-13-spits-bars-and-sand-dunes.md) - [1.2.14 Human Activity on Coasts](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-14-human-activity-on-coasts.md) - [1.2.15 Coastal Defences: Hard Engineering](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-15-coastal-defences-hard-engineering.md) - [1.2.16 Coastal Defences: Soft Engineering](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-16-coastal-defences-soft-engineering.md) - [1.2.17 Coastal Defences: Managed Retreat](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-17-coastal-defences-managed-retreat.md) - [1.2.18 Case Study: Holderness Coast Overview](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-18-case-study-holderness-coast-overview.md) - [1.2.19 Case Study: Holderness Coast Management](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-19-case-study-holderness-coast-management.md) - [1.2.20 Case Study: Holderness Coast Defences](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-20-case-study-holderness-coast-defences.md) - [1.2.21 End of Topic Test - Coastal Environments](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-2-21-end-of-topic-test-coastal-environments.md) - 1.3 Hazardous Environments - Tropical Cyclones - [1.3.1 Characteristics of a Tropical Cyclone](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-1-characteristics-of-a-tropical-cyclone.md) - [1.3.2 Structure of a Tropical Cyclone](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-2-structure-of-a-tropical-cyclone.md) - [1.3.3 Physical Hazards of Tropical Cyclones](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-3-physical-hazards-of-tropical-cyclones.md) - [1.3.4 The Impacts of Tropical Cyclones](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-4-the-impacts-of-tropical-cyclones.md) - [1.3.5 Responses to Tropical Cyclones](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-5-responses-to-tropical-cyclones.md) - [1.3.6 Vulnerability to Tropical Cyclones](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-6-vulnerability-to-tropical-cyclones.md) - [1.3.7 Preparing for Tropical Cyclones](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-7-preparing-for-tropical-cyclones.md) - [1.3.8 Case Study: Katrina](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-8-case-study-katrina.md) - [1.3.9 Case Study: Nargis](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-3-9-case-study-nargis.md) - 1.4 Hazardous Environments - Earthquakes & Volcanoes - [1.4.1 Earthquakes](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-1-earthquakes.md) - [1.4.2 Features of an Earthquake](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-2-features-of-an-earthquake.md) - [1.4.3 Types of Earthquake](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-3-types-of-earthquake.md) - [1.4.4 Case Study: L'Aquila](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-4-case-study-laquila.md) - [1.4.5 Case Study: Chile](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-5-case-study-chile.md) - [1.4.6 Case Study: Japan & Pakistan - Planning](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-6-case-study-japan-and-pakistan-planning.md) - [1.4.7 Volcanoes](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-7-volcanoes.md) - [1.4.8 Types of Volcanoes](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-8-types-of-volcanoes.md) - [1.4.9 Volcanic Eruptions](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/1-4-9-volcanic-eruptions.md) ### 2 Human Geography - 2.1 Economic Activity & Energy - [2.1.1 Increasing Energy Consumption](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-1-1-increasing-energy-consumption.md) - [2.1.2 Non-Renewable Energy - Nuclear](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-1-2-non-renewable-energy-nuclear.md) - [2.1.3 Non-Renewable Energy - Fossil Fuels](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-1-3-non-renewable-energy-fossil-fuels.md) - [2.1.4 Renewable Energy - Solar](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-1-4-renewable-energy-solar.md) - [2.1.5 Renewable Energy - Water & Bio](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-1-5-renewable-energy-water-and-bio.md) - [2.1.6 Renewable Energy - Hydro](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-1-6-renewable-energy-hydro.md) - [2.1.7 Energy - Statistical Skills](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-1-7-energy-statistical-skills.md) - 2.2 Rural Environments - [2.2.1 Rainforests](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-2-1-rainforests.md) - [2.2.2 Temperate Forests](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-2-2-temperate-forests.md) - [2.2.3 Characteristics of a Temperate Forest](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-2-3-characteristics-of-a-temperate-forest.md) - [2.2.4 Goods & Services in Temperate Forests](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-2-4-goods-and-services-in-temperate-forests.md) - [2.2.5 Deserts](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-2-5-deserts.md) - [2.2.6 Tundra](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-2-6-tundra.md) - 2.3 Urban Environments - [2.3.1 Urbanisation in Different Countries](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-3-1-urbanisation-in-different-countries.md) - [2.3.2 Causes of Urbanisation](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-3-2-causes-of-urbanisation.md) - [2.3.3 Consequences of Urbanisation](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-3-3-consequences-of-urbanisation.md) - [2.3.4 Sustainable Urbanisation](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-3-4-sustainable-urbanisation.md) - [2.3.5 Case Study: Rapid Urbanisation in Mexico City](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-3-5-case-study-rapid-urbanisation-in-mexico-city.md) - [2.3.6 End of Topic Test - Urbanisation](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/2-3-6-end-of-topic-test-urbanisation.md) ### 3 Global Issues - 3.1 Fragile Environments & Climate Change - [3.1.1 Natural Causes of Climate Change](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-1-1-natural-causes-of-climate-change.md) - [3.1.2 Effects of Climate Change on the Environment](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-1-2-effects-of-climate-change-on-the-environment.md) - [3.1.3 Effects of Climate Change on People](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-1-3-effects-of-climate-change-on-people.md) - [3.1.4 Impacts of Deforestation](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-1-4-impacts-of-deforestation.md) - [3.1.5 End of Topic Test - Climate Change](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-1-5-end-of-topic-test-climate-change.md) - [3.1.6 Increasing Water Supply](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-1-6-increasing-water-supply.md) - [3.1.7 Sustainable Rainforest Management](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-1-7-sustainable-rainforest-management.md) - 3.2 Globalisation & Migration - [3.2.1 Globalisation - Transport](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-1-globalisation-transport.md) - [3.2.2 Globalisation - Communication Networks & Banking](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-2-globalisation-communication-networks-and-banking.md) - [3.2.3 Push & Pull Factors](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-3-push-and-pull-factors.md) - [3.2.4 TNCs & Globalisation](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-4-tncs-and-globalisation.md) - [3.2.5 Tourism Landscapes](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-5-tourism-landscapes.md) - [3.2.6 The Growth of Tourism](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-6-the-growth-of-tourism.md) - [3.2.7 The Economic Benefits of Tourism](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-7-the-economic-benefits-of-tourism.md) - [3.2.8 The Social & Cultural Benefits of Tourism](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-8-the-social-and-cultural-benefits-of-tourism.md) - [3.2.9 The Economic Costs of Tourism](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-9-the-economic-costs-of-tourism.md) - [3.2.10 The Social & Cultural Costs of Tourism](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-10-the-social-and-cultural-costs-of-tourism.md) - [3.2.11 Managing Tourism - Sustainability](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-11-managing-tourism-sustainability.md) - [3.2.12 Managing Tourism - Quotas & Tourism Hubs](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-2-12-managing-tourism-quotas-and-tourism-hubs.md) - 3.3 Development & Human Welfare - [3.3.1 Ways of Defining Development](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-3-1-ways-of-defining-development.md) - [3.3.2 Different Ways of Measuring Development](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-3-2-different-ways-of-measuring-development.md) - [3.3.3 Environmental Factors Causing Uneven Development](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-3-3-environmental-factors-causing-uneven-development.md) - [3.3.4 Historical Factors Causing Uneven Development](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-3-4-historical-factors-causing-uneven-development.md) - [3.3.5 Economic Factors Causing Uneven Development](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-3-5-economic-factors-causing-uneven-development.md) - [3.3.6 Patterns of Development](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-3-6-patterns-of-development.md) - [3.3.7 How Can we Reduce the Global Development Gap?](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-3-7-how-can-we-reduce-the-global-development-gap.md) - [3.3.8 Top-Down & Bottom-Up Development](https://senecalearning.com/en-gb/revision-notes/igcse/geography/edexcel-igcse/3-3-8-top-down-and-bottom-up-development.md)