Hydrology

geography

Drainage Basin

The area from which a river collects water or the area drained by a river.

Watershed

The highland area that separates two drainage basins.

Gauging Station

A location at the end of a drainage basin where river flow data is collected.

Input

Precipitation:Water received by the drainage basin from various forms of precipitation like rainfall, snowfall, sleet-fall, and hailfall.

Interception Storage

The amount of water retained by vegetation when precipitation falls on it before reaching the ground.

Overland Flow

Thin sheet of water flowing over the surface, often causing river floods after a rainstorm.

Runoff (Discharge)

The total water leaving the drainage basin through surface routes, also known as channel flow.

Hortonian Overland Flow Model

A model by Robert E. Horton that explains the formation of overland flow based on rainfall intensity and soil infiltration capacity.

Hortonian Overland Flow

A type of overland flow responsible for the formation of gullies and rills, occurring when rainfall intensity exceeds the soil's infiltration capacity.

Saturation Overland Flow Model (SOF)

An alternative model to Hortonian overland flow, where rainfall infiltrates initially but saturates the soil over time, leading to surface runoff once the soil is saturated.

Throughflow/Interflow

The horizontal movement of water after infiltration in the vadose zone, contributing to channel flow after a rainstorm.

Baseflow/Groundwater Flow

The horizontal movement of water in the phreatic zone below the water table, contributing to river discharge after overland flow and through flow.

Evaporation

The process of water changing from liquid to gas, influenced by factors like insolation, latitude, seasons, time of day, and sky conditions.

Transpiration

The process by which water is evaporated from the internal structure of plant leaves through stomata.

Evapotranspiration

The total water loss from a drainage basin to the atmosphere, including evaporation from soil, water surfaces, and transpiration from plants.

Potential Evapotranspiration

The total water loss that would occur from a fully vegetated drainage basin with soil moisture above field capacity.

Actual Evapotranspiration

The total water loss from a drainage basin that may not be fully vegetated and with soil moisture not necessarily at field capacity.

Discharge

The volume of water flowing through a river or stream at a specific point, typically measured in cubic meters per second (m3/s).

Stage-Discharge Relationship

A method used by hydrologists to quickly obtain discharge data by relating the water level (stage) to the discharge rate.

Storm Hydrograph

A graph showing the variation in discharge over time during a rainstorm, illustrating the response of a drainage basin to precipitation.

Discharge

The volume of water flowing through a river at a specific point, often measured in cubic meters per second.

Peak Discharge

The highest level of discharge in a river during a specific storm event, usually corresponding to the amount of rainfall received.

Bank Full Discharge

The discharge at which a river is at full capacity and any additional water will lead to flooding.

Overland Flow

The movement of water over the land surface towards a river after a rain event, considered the fastest way to deliver water to a river.

Hydrograph Separation

A method used by hydrologists to determine the contributions of different components (such as overland flow and base flow) to the total discharge of a river.

Lag Time

The time period between the maximum rainfall and the peak discharge in a river, influenced by factors like overland flow and infiltration.

Recurrence Interval/Return Periods

The time it takes for a flood of a specific magnitude to reoccur in a drainage basin, used in flood predictions based on historical discharge data.

Embankments

Structures built along rivers to prevent flooding by increasing the water channel's capacity to hold water.

Flood Action Plan (FAP)

A comprehensive plan consisting of 26 steps aimed at minimizing the extent of hazards and damage caused by floods in Bangladesh.

Drainage Basin

The area of land where all the water that falls in it, or drains off it, goes to a common outlet, such as a river.

Flood Shelters

Structures built in Bangladesh to provide shelter to citizens during floods, helping them survive and stay safe.

Deforestation

The clearing of trees and forests, which can contribute to flooding by reducing interception of rainwater and increasing soil erosion.

Urbanization

The rapid growth of cities and towns, leading to changes in land use that can impact drainage basins and increase flooding risks.

Tista River System

A river system in Bangladesh originating in the Sikkim Himalayas and entering the country at the Kharibari border.

Bengal Basin

The region where most of the river drainage basin of Bangladesh is located, affecting the physiography of the country.

Flood Warning Scheme

A system in Bangladesh involving signals and symbols to warn people about floods, allowing them to make preparations and seek safety.

Rainwater Erosion

The process of rainwater carrying eroded soil, increasing its speed and reaching the ground quickly.

Fresh Water Demand

The increased need for fresh water due to population growth in Bangladesh, leading to extensive use of the Ganges river for activities like irrigation, industrial water supply, and agriculture.

Stability of Flood Defenses

The quality and effectiveness of flood defenses in Bangladesh, impacted by insufficient funds for construction, ineffective current defenses, and deforestation leading to vulnerable protection schemes.

Economic Debts

The economic crisis faced by Bangladesh due to annual flooding during the monsoon season, resulting in financial strain, inability to recover fully, and deteriorating economy affecting flood defense maintenance.

Monsoon Climate

The climate in Bangladesh characterized by heavy rain and snow, causing rivers to overflow during monsoon seasons, leading to soil erosion, increased runoff, and significant flooding.

Tectonic Uplift

The uplift of the Himalayas causing increased sediment erosion rates, depositing residue in Bangladesh rivers, choking river channels, and reducing their efficiency in managing water flow.

Relief Runoff

The melting snow from the Himalayas flowing into Bangladesh during spring and hot seasons, increasing river discharge and contributing to flooding, exacerbated by deforestation and climate change.

Sea Levels

The rising global sea levels affecting land elevation, causing inundation when rivers overflow, and reducing river flow gradient, impacting flooding in coastal areas like Bangladesh.

River Processes

Natural processes like erosion in river systems contributing to Bangladesh's vulnerability to flooding, with river erosion and submerging of coastal islands being significant phenomena.

1998 Flooding Incident

A catastrophic event in Bangladesh caused by melting ice caps in the Himalayan Mountains, heavy monsoon rains, and poor flood defenses, resulting in significant damages to the country.

Bangladesh

A country in South Asia that experiences frequent flooding due to its geographical location, being a flood plain with over 80% of its land susceptible to floods.

Flood Control Systems

Measures and infrastructures implemented to reduce flood risks and minimize damage caused by flooding incidents, such as embankments, dykes, and flood warning systems.

LEDC (Less Economically Developed Country)

A country with a low standard of living, underdeveloped industrial base, and low Human Development Index (HDI), often facing challenges in dealing with natural disasters like floods.

Human Causes

Factors contributed by human activities that exacerbate flooding incidents, such as deforestation, urbanization, and lack of maintenance of flood infrastructures.

2004 Flooding Incident

A devastating flood in Bangladesh caused by heavy monsoons, melting snow in the Himalayan Mountains, and rapid urbanization, leading to widespread destruction and loss of lives.

United Nations

An international organization that provides resources and aid to countries facing natural disasters, like the flooding incidents in Bangladesh, to support relief efforts and help affected populations.

2012 Flooding Incident

A flood in Bangladesh triggered by the heavy monsoon season, poor flood warning systems, and decreased international aid, causing significant challenges for the country in coping with the disaster.

Monsoon season

A seasonal wind pattern that brings heavy rainfall to certain regions, often associated with the Indian subcontinent.

Poverty line

The minimum level of income deemed adequate in a particular country, below which individuals or families are considered to be in poverty.

Malnutrition

A condition resulting from an inadequate or unbalanced diet, leading to health issues due to lack of essential nutrients.

Fluvial processes

Processes related to rivers, including erosion, transportation, and deposition of sediment by flowing water.

Hydraulic radius

A measure of the efficiency of a river channel in transporting water and sediment, calculated as the cross-sectional area divided by the wetted perimeter.

Gradient

The steepness of a slope or the rate at which a river descends along its course.

Laminar flow

A type of streamflow characterized by water moving in parallel layers in an orderly manner, often with low velocity and minimal turbulence.

Turbulent streamflow

Water flow characterized by irregular movement, random directions, high velocities, and collisions with physical barriers, leading to mixing between layers.

Helicoidal flow

Cork-screw-like water flow in a meander, contributing to slip-off slopes and river cliffs formation.

Corrasion

River erosion process where the load carried by the river wears out the bed and banks by rubbing and dislodging sediments.

Hydraulic action

Erosion process involving the sheer force of flowing water, pushing water into cracks, and cavitation weakening banks over time.

Corrosion or solution

Dissolution of soluble rocks on riverbanks and beds due to water with high acid concentration.

Attrition

Erosion process where load particles strike each other, break down, become rounded, and smaller.

Vertical erosion

Erosional process concentrating on the riverbed, leading to the formation of deep narrow V-shaped gorges.

Headward erosion

River lengthening towards its source, involving plunge pool formation and retreat of the river's source.

Suspension

Movement of very fine particles like clay and silt within the water, influenced by river velocity and turbulence.

Saltation

Movement of sand and pebbles lifted from the riverbed into the water current and later bouncing back.

Competence

Maximum weight of the largest fragment a stream can transport based on its velocity.

Capacity

Maximum amount of load a river can move, influenced by velocity and river competence.

Cobbles and Boulders

Very large particles that require high stream velocities of more than 500 centimeters per second for erosion due to their weight.

Potholes

Cylindrical holes drilled into the bed of a river in the upper course, formed by the swirling motion of pebbles trapped in depressions.

Interlocking Spurs

Resistant mountain ends that the river avoids in the upper course, taking the easiest path down the slope.

V-Shaped Valleys

Deep river valleys with steep sides resulting from vertical erosion in the upper course, maintaining a narrow channel due to limited lateral erosion.

Waterfalls & Gorges

Formed by a change in rock type along the river's course, leading to a sudden drop in gradient and erosion of less resistant rock, creating a plunge pool and eventually a gorge of recession.

Rapids

Sections of a river with steep gradients and hard rock beds, causing increased turbulence and velocity.

Meanders

Bends in a river formed by continuous erosion on the outer bank and deposition on the inner bank, increasing sinuosity over time.

Braided Channel

A network of river channels separated by islands, characterized by a braided appearance due to sediment deposition.

Floodplain

An area of land next to a river that experiences flooding during periods of high discharge, consisting of fertile soils deposited during floods.

Meander

A winding curve or bend in a river caused by erosion and deposition, leading to the formation of oxbow lakes.this occurs due to the helicodial movement of water in the river wherby the river flows faster on the outside of bends and slower on the inside. This difference in speed causes erosion on the outer bank (cut bank) and deposition on the inner bank (point bar)

Over time, the erosion on the outer bank and deposition on the inner bank accentuate the bends, making them more pronounced. This process continues, creating a series of sinuous curves known as meanders.

As the river continues to erode the outer banks and deposit sediments on the inner banks, the meanders migrate downstream. This migration can lead to the formation of oxbow lakes if the meanders become too exaggerated and the river cuts through the narrow neck of a bend^.

Levees

Natural embankments formed along riverbanks due to sediment deposition during floods, helping to contain the river within its channel.

Oxbow Lake

A U-shaped body of water formed when a meander is cut off from the main river channel, eventually drying up over time.

Infiltration

The process by which precipitation or water soaks into subsurface soils and moves into rocks through cracks and pore spaces. This process occurs because the rainfall intensity <infiltration capacity

Deflocculation

the vertical erosion of clay minerals via a porous regolith. Clay minerals are removed from the upper soil layers and may be deposited in a specific area within the vadose zone to form a clay pla

Bangladesh Case study (human causes)

Rapid Urbanization

• Population Growth: Bangladesh’s population increased from 1.81 million to 30 million in 40 years since 1951, reaching 163 million by 2013.

• Impact on Drainage and Flooding: Urbanization increases water entry and drainage basin capacity due to construction, leading to higher flood susceptibility.

• Land Use Changes: Agricultural lands are replaced by buildings and roads, reducing flood prevention capacity and increasing peak flow.

Deforestation in the Upstream Region

• Increased Deforestation: Driven by food and fuel wood demands, especially on steep slopes.

• Flood Intensity: Deforestation accelerates water flow, increasing flood intensity, particularly in the Himalayan foothills affecting the Ganges and Brahmaputra rivers.

• Soil Erosion: Lack of trees leads to faster rainwater flow and soil erosion, exacerbating flooding.

Fresh Water Demand

• Increased Demand: Population growth escalates fresh water needs, heavily utilizing the Ganges for irrigation, industry, and agriculture.

• Impact on Floodplain: Human activities remove silt from the river, disrupting natural floodplain construction.

• Well Construction: 100,000 wells built, lowering the water table and land level, increasing flood susceptibility.

Stability of Flood Defenses

• Poor Flood Defenses: Bangladesh’s debt limits effective flood protection measures.

• Ineffective Defenses: Current defenses are minimal and often fail, leading to increased debt and economic issues.

• Deforestation Impact: Deforested floodplains increase water flow speed, compromising flood defenses.

natural causes

Monsoon Climate

• Heavy Rain and Snow: Monsoon climate causes main rivers to overflow between July and mid-August, leading to soil erosion and leaching.

• Relief Rainfall: Convectional rainfall from monsoons is supplemented by relief rainfall from the Himalayas, contributing to annual floods.

• 1988 Flood: A significant monsoon downpour in the Ganges, Brahmaputra-Meghna system caused massive flooding, inundating two-thirds of Bangladesh.

Tectonic Uplift

• Increased Erosion: Tectonic uplift of the Himalayas increases sediment erosion, which clogs river channels in Bangladesh, reducing their efficiency.

Relief Runoff

• Melting Snow: In spring and hot seasons, melting snow from the Himalayas increases river discharge, contributing to flooding.

• Deforestation Impact: Deforestation causes soil erosion, reducing the land’s ability to absorb water, exacerbated by global warming.

Sea Levels

• Rising Sea Levels: Global sea level rise decreases land elevation, increasing flood risk. The local sea level rise is about 7mm/year around Bangladesh’s coast.

• River Gradient: Rising sea levels reduce river flow gradient, affecting flood dynamics.

River Processes

• Erosion: River erosion and submerging of coastal islands are major natural disasters, making land more susceptible to flooding.

• Homelessness: River erosion displaces approximately 0.1 million people annually in Bangladesh.

• Historical Data: Significant land loss due to erosion in rivers Jamuna, Ganges, and Padma over the past decades.

Causes of Erosion

• Tidal Pressure: Downward tides and wave intensity cause coastal erosion.

• Greenhouse Gas Emissions: Warming atmosphere due to greenhouse gases accelerates glacier melting, increasing water flow into Bangladesh’s rivers.

bangladesh case study

Case Study: Bangladesh Flooding Incident of 2024Overview

In August 2024, Bangladesh experienced severe flooding, primarily affecting the northern regions. This disaster was triggered by heavy monsoon rains, which caused rivers to overflow and inundate vast areas.

Engineering Techniques Used

The primary engineering structures involved were embankments and flood control dams. However, many of these structures were either breached or overwhelmed by the sheer volume of water. Efforts included:

• Embankments: Designed to contain river water, but many were breached due to the intensity of the flood.

• Flood Control Dams: Used to regulate water flow, but some failed under the pressure of the excessive rainfall.

Positive Impacts

• Awareness and Preparedness: The disaster has increased awareness about the importance of flood preparedness and the need for resilient infrastructure.

• International Aid: The flooding prompted a significant international response, bringing in aid and resources to help with recovery efforts.

Negative Impacts

• Loss of Life: The floods resulted in the deaths of at least 32 people.

• Displacement: Over 5.7 million people were affected, with many displaced from their homes.

• Infrastructure Damage: Extensive damage to homes, roads, and public facilities, disrupting daily life and economic activities.

Size and Scale of the Flood

The flooding was extensive, affecting 11 districts and displacing millions of people. The floodwaters inundated large parts of the northern regions, causing widespread devastation.

Effects on People

• Displacement: Thousands of residents were forced to leave their homes and seek refuge in shelters.

• Health Risks: Increased risk of waterborne diseases due to contaminated water supplies and inadequate sanitation.

• Economic Hardship: Many people lost their livelihoods as agricultural lands were submerged and businesses were destroyed.

Other Effects of Flooding

• Environmental Damage: Significant soil erosion and destruction of natural habitats.

• Disruption of Services: Essential services such as electricity, water supply, and healthcare were severely disrupted, complicating rescue and relief efforts.

Flood Control Systems

Flood Action Plan: In response to the disaster, there is a pressing need for a comprehensive flood action plan. This plan should include:

• Regular Maintenance: Ensuring that existing embankments and flood control structures are regularly inspected and maintained.

• Early Warning Systems: Implementing advanced early warning systems to alert residents of impending floods.

• Infrastructure Upgrades: Investing in more resilient infrastructure to withstand extreme weather events.

• Community Preparedness: Educating and preparing communities for flood risks through drills and awareness programs.

The Bangladesh flooding incident underscores the critical importance of robust infrastructure, effective disaster preparedness, and proactive flood management to mitigate the impacts of such natural disasters in the future.

: Forbes : BBC

hard and soft engineering

1. Embankments: These are raised barriers along riverbanks designed to contain river water and prevent it from flooding adjacent land. However, during the 2024 floods, many embankments were breached due to the intensity of the floodwaters¹.

2. Flood Control Dams: These structures are used to regulate water flow and store excess water during heavy rains. Some of these dams failed under the pressure of the excessive rainfall¹.

Soft Engineering Techniques

1. Flood Forecasting and Warning Systems: Bangladesh employs advanced hydrological and hydrodynamic models, such as MIKE11-GIS, and satellite imagery processing systems to predict and warn about potential floods².

2. Community Preparedness Programs: These include educating and preparing communities for flood risks through drills and awareness programs¹.

3. Afforestation and Reforestation: Planting trees to increase water absorption and reduce runoff, which helps in mitigating the impact of floods¹.

Annual Regime

An annual hydrograph is a graphical representation of river discharge (flow) over the course of a year. It's a river's flow regime.

Discharge: This is the volume of water flowing through a river channel per unit time, typically measured in cumecs (cubic meters per second).

Base flow: The normal, day-to-day discharge of a river, primarily supplied by groundwater seepage into the channel.

Peak flow: The maximum discharge recorded on the hydrograph, often associated with periods of heavy rainfall or snowmelt.

Lag time: The time between peak rainfall and peak discharge, influenced by factors such as basin size, slope, and land use.

Seasonal variations: Changes in discharge throughout the year due to factors like precipitation patterns, temperature, and evapotranspiration rates.