O Level Notes : Geography - landforms and landscape processes - Disaster risk management

It involves reducing and managing conditions of hazards, exposure and vulnerability. The activities for disaster risk management include: 

• Prevention

These are activities and measures to avoid existing and new disaster risks, for example, g relocating people from affected areas.

• Mitigation

Involves lessening or limiting the adverse impacts of hazards and related disasters.

• Transfer

Is shifting financial consequences from one part to another, for example,  getting financial benefits from the government or another country after a disaster.

• Preparedness

Involves reducing negative impacts of disasters through acquiring knowledge and capabilities of governments and professionals to respond to the disasters.

The ways to respond to disasters differs according to the nature of the hazards. Some of these hazards and the ways they are managed are:

1. Floods

Causes of floods

The causes of floods are both physical and human. They include:

• Bursting of dams

When dams burst the water is released in large volumes. This water floods the surrounding areas , for example, Tokwe- Mukosi in Masvingo bursted in 2014.

• High rainfall intensity of long duration

When an area receives a lot of rainfall lasting for a long period, the soil's infiltration capacity is exceeded. The excess water flows down the slope causes floods in low lying areas.

• Melting of snow

Polar glaciers or areas covered with snow melt due to effects of global warming. This leads to the melting water flowing down slopes to low lying areas where flooding takes place.

• Deforestation

Cutting down trees reduces interception of water. The rain water quickly flows into the channel leading to flooding.

• Stream bank cultivation

This causes siltation of dams and lakes. The rivers become shallow hence are quickly filled with water causing flooding of surrounding areas.

Effects of floods

When floods occur, they bring adverse effects to human communities. These effects include:

• Death of people
• Destruction of property and homes
• Destruction of infrastructure, for example, roads, bridges and power lines.
• Displacement of people
• Destruction of crops
• Spread of water borne diseases

Disaster risk management

This disaster is managed through mitigation by:

• Evacuating people from affected areas using helicopters.
• Early warning systems
• Provision of valuable information
• Counselling of the affected people.
• Provision of clean water
• Provision of temporary shelter such as tents.
• Moving people to areas of higher elevation.
• Donations of clothing and other necessities.

Challenges likely to be faced

• Resistance from the affected people to be evacuated.
• Shortage of materials for use, for example, boats
• Lack of finance to provide necessities to the affected people.
• Lack of training and professional knowledge of risk disaster management.
• Poor communication

Case Study: Tokwe Mukosi Disaster

In February 2014, Zimbabwe's Masvingo Province experienced heavy torrential rainfall. The Tokwe Mukosi area is one of those areas which received these rains. The heavy rains led to rapid stream discharge in the streams feeding into Tokwe Mukosi Dam. This led to the dam experiencing great pressure. Due to the pressure Tokwe Mukosi Dam collapsed. The water from the collapsing dam flooded the areas downstream. The water rose to high levels burying and sweeping away homes. Over 1500 families were left homeless. Crops and homes were destroyed and most families lost their belongings. Disaster Risk Management measures were initiated and these focused on:

• Provision of aid.

The affected families received food and other important needs. A family was given 500g dried beans, 2kg flour, 2kg sugar, 2 litres cooking oil, 500g salt, a packet of candles, and 1kg kapenta per 2weeks.

• Evacuation

The affected villagers were relocated to higher ground.  About 4,500 villagers living along the flood basin of the dam were evacuated. Zimbabwe Air Force Helicopters were used to evacuate the affected families and their remaining belongings. They were evacuated from the area downstream to villages upstream of the dam which included Chingwizi, Chisase, and Masungula, where they were provided with tents.

• Donations

A musical concert was organized running under the theme, "Remembering Tokwe Mukosi One Environmental Family". Afrika Revenge, Cynthia Mare and Zahara are some of the musicians who performed at the concert. Songs were recorded, for example, Tokwe Mukosi by Ras Caleb to show solidarity with the affected families. Mitigation strategies implemented in the case of Tokwe Mukosi assisted in lessening the damage caused by the floods. Challenges faced in providing the assistance included lack of enough material.

 Tokwe Mukosi Disaster Mitigation

Volcanoes

Volcanoes are a real cause of concern because when they occur they cause several challenges.

Problems

• They lead to death of people.
• They destroy infrastructure and property.
• Destruction of ecosystems and biodiversity.
• They lead to acidic rain which corrodes monuments.
• Dust produced by volcanoes causes respiratory diseases.
• They cause tsunamis especially when they occur near Islands.
• triggers heavy rains by providing condensation nuclei

Disaster Risk Management

It is difficult and to be precise, impossible to stop volcanoes from erupting. However, human beings have to find ways of reducing the damage caused by these natural hazards. That reduction of damage caused is called mitigation. The following are helpful mitigation strategies in the occurrence of volcanoes.

• Putting in place early warning systems.
• Evacuating people from the areas affected or likely to be affected.
• Diverting lava flows so that they don't destroy infrastructure.
• Quick dissemination of information to avoid unnecessary loss of life.
• Putting in place disaster preparedness systems.
• Putting in place gas detectors to monitor eruptions.
• putting on protection clothing, for example, gas masks.

Earthquakes

Earthquakes are sudden Earth movements of vibration caused by the movement of plates. There is need for understanding that earthquakes occur in those areas where all the other tectonic activities are active. These occurrences trigger each other hazards, for example an earthquake triggers a tsunami.

Effects of Earthquakes

Negative effects

• Destruction of infrastructure.
• Death of people.
• Landslides and rock falls.
• Tsunamis, for example, Fukushima (Japan) in 2011.
• Blackouts due to disturbance of power supply.
• Bursting of water pipes and dams.
• Spread of diseases due to pollution of water.
• Loss of income and livelihoods.
• Loss of important landmarks and cultural landmarks.
• Aftershocks.

Disaster Risk Management

• Early warning systems.
• Training people in disaster preparedness.
• Having first aid kits in homes.
• The provision of clean water to avoid water borne diseases.
• Air dropping of food in affected inaccessible areas.
• Provision of tents for shelter.
• Rescue operations.
• Funding, for example, through donations.
• Building more earthquakes resistant buildings.

 Mass Wasting

The term mass wasting refers to the downslope movement of materials due to the pull of gravity. These movements can be rapid or slow. They include rock falls, mudflows and landslides.   When these occur they are disastrous. They cause damage to people, property and infrastructure. Mass wasting have same effects as most disasters and to mitigate against their effects there is need for taking measures such as early warning systems and relocation to areas where there are stable.

 Measuring Stream Discharge

Stream discharge is the volume of water passing through a given point in a channel. It is measured in cubic meters per second. Discharge is measured or expressed as:

Q =Ax V     where: Q is stream discharge

A is the cross sectional area

V is velocity

This means that for one to measure stream discharge, there is need to find the cross sectional area and then the velocity of the water flowing from one point to another. Measuring discharge or any aspect in the field calls for proper planning and this planning involves acquiring proper equipment for the survey. To measure stream discharge the following equipment is needed:

• Stop watch
• Tape measure
• Floats
• sounding pole
• Datum line
• Pegs

According to the formula for calculating discharge, Velocity is multiplied by the cross sectional area. It means there is need for us to start by measuring the two in a channel.

MEASURING RIVER VELOCITY

River velocity refers to the speed at which water flows in a river channel measured in meters per second (m/s). River velocity is measured using two methods thus the traditional method and the modern method.

Traditional method of measuring river velocity

The traditional method of measuring river velocity requires the following items:

• Floats

These are light materials which float on top of the water, for example,  certain rubbers and orange peels.

 The Stop watch

To measure stream velocity, follow the steps below:

1. Select points along a river channel from which the river velocity will be measured.
2. Allocate team members areas to cover. One member should be upstream and another downstream to record the time taken by floats.
3. Let floats flow from a point upstream and record the time the floats take to reach a marked point downstream.
4. Repeat the above procedure several times.
5. The data collected at different points and intervals should be averaged to generate an accurate mean velocity.

MEASURING VELOCITY USING CURRENT METER

Measuring a river velocity using a current meter requires that for data to be collected there should be the following equipment:

• Mounting pegs
• Athick supporting cable
• Current meters
• Rigid vertical cables

Procedure

1. Select an accessible part of a river channel.
2. Mount a supporting cable across the selected accessible part.
3. Subdivide the channel into segments using rigid vertical cables.
4. At each of these points place a current meter at different depths.
5. Record the velocities on the note pad or notebook.
6. Current meters are placed at 0,2 and 0.8 of water depth to cater for variations in velocities caused by friction drag when water in transit rubs against river bank and river beds.

After collecting the data, the average velocity is then calculated by adding the 0,2 velocities and 0,8 velocities then divide them by two.  The value is obtained at different levels, for example, level 1`to 4 should be added and divided by 4 to get the final average velocity. Divide the distance between the two points by the average time to get the velocity.

Measuring stream cross sectional area

Stream depth refers to the vertical distance between the river bed and the upper most part of the water in the channel. The following instruments are required:

• Measuring tape
• Sounding rod
• Metre rule
• Recording sheet/ note book/lap top

Procedure

When measuring stream depth, a tape is stretched across the channel. The sounding rod is placed in an upright position to take the readings.

Measure depth at equidistance intervals

The rod is used to measure depth at 9 points which are separated by an equal distance. The number of points from where depth is measured depends on the width of the channel and the distance between the points is the surveyor's choice guided again by channel width.

These measurements of depth at equidistance intervals across the channel are then plotted on a graph paper using a scale. The number of the boxes covered by what has been plotted is the cross sectional area of the channel at a given point. By counting the boxes on a graph paper one is able to get the area which is then converted to the actual area of the channel using a scale.

The cross sectional area is multiplied by the velocity to give stream discharge.

Measuring stream discharge

(a)Acquire the equipment needed to measure stream discharge

(b) measure stream discharge at 2 points in the stream near your school.

Here is what we have discussed on this topic

• River experience vertical, lateral and head ward erosion.
• Abrasion, solution, hydraulic action, cavitation, and attrition are the processes of river erosion.
• Stream erosion form features such as rapids, waterfalls, potholes and plunge pools.

• Depositional features by rivers include braids, levee's and floodplains.
• Deltas are formed by deposition when rivers enter the sea.
• Through  river   capture,   stronger   consequent   streams   capture   the headwaters of the weaker consequent streams flowing parallel to it.
• Wind erodes through deflation, attrition and abrasion.
• Wind deposition forms sand dunes.
• Disaster risk management involves lessening the risks presented by natural hazards.

Definitions of terms used in this topic

River Source: it is the point in the upper course where rivers start.

Velocity: it is the speed of the flowing water at a given point.

Stream Discharge: it is the amount of water passing through a given point at a given time.

Rejuvenation: it is the revival of the river's erosive power.

Alluvial fans:   it is a mound of deposits deposited by water emerging from a canyon onto a Pedi plain.

River Long Profile: it is the stretch of the river from the upper course to the lower

course.

Consequent streams: these are streams which follow the dip of the original slope, they are consequent upon the slope.

Subsequent streams: these are streams joining the consequent stream at right angles, they develop as a result of headward erosion.

Watershed :it is an area dividing two adjacentdrainage basins