Hydro: Floods

Question 1

describe excessive rainfall as a cause of flood

Answer 1

Monsoons (eg. Pakistan: floodplain of Indus River)
Mid-latitude depressions (eg. Winter flooding in the UK)
- prolonged rainfall causes water to fall on already saturated ground, leading to rapid Horton overland flow which results in floods when storm flow exceeds bank full discharge.

Question 2

Describe rapid snowmelt as a cause of flooding

Answer 2

Occurs in later spring and early summer, may bring vast amounts of water downstream, leading to flooding (eg. Bangladesh - monsoon rains and snowmelt from the Himalayas channels large amounts of water down)

Question 3

Describe landslides as a cause of flooding

Answer 3

Huge slide of rock into a body of water can result in a huge wave of water that can overtop a dam. Can cause flooding of settlements downstream. Landslides can also create a flood upstream behind the dam due to build up of water. (Eg. 337 deaths in Gansu Province in China when torrential rains in 2010 caused landslides which dammed the Bailong river)

Question 4

What are the causes of floods?

Answer 4

1) excessive rainfall
2) rapid snowmelt
3) volcanic action (*lahar and mudflow is NOT flooding)
4) landslides
5) dam failures (but not that common)
6) breakage of artificial levees too

Question 5

What are the 3 flood intensifying conditions?

Answer 5

1) urbanisation
2) deforestation
3) overgrazing

Question 6

How does urbanisation and channelisation of rivers cause floods?

Answer 6

asphalt and concrete used to build roads for urban pavements and roads have low infiltration capacity, more overland flow (storm flow) and less baseflow contrast natural areas with vegetation. Water is forced to runoff rapidly into nearby streams. (Eg. 2010 SG flood 300mm in orchard road which affected many businesses and roads in the CBD due to drainage inefficiencies ie. Debris-choked Stamford canal)

Question 7

How does deforestation cause floods?

Answer 7

Vegetation loss leads to loss of interception, soil moisture and groundwater storages due to:

1) rainsplash action - in vegetated areas, rain droplets are intercepted by leaves, reducing rainsplash action. Without vegetation, rain droplets strike the ground directly to compact the soil
2) decreases infiltration by affecting soil structure
3) resin from fires sealing the soil by sticking soil grains tgt, reduces water infiltration
4) logging skids logs on the ground, removing vegetation and leaves ground vulnerable to erosion. Gullies can form, carrying more sediment to streams.

Question 8

How does overgrazing lead to floods?

Answer 8

Cattle and sheep grazing on open slopes removes surface vegetation. Rainfall running off poorly protected soil erodes gullies, carrying more sediment to streams. (Eg. Deforestation of hillsides near mandano in Jan 2014 resulted in torrential rains that killed 13 ppl and displaced over 40,000 ppl)

Question 9

How do we construct a flood frequency graph for flood prediction?

Answer 9

Use recurrence intervals = (n+1)/R (return periods of a flood) based on floods recorded in the past. Maximum daily flows are identified for each year. Peak discharge for the respective years are then ranked according to discharge volume (highest ranked 1). Construct a flood frequency graph and use it to predict that a flood with a particular discharge is likely to recur within a _ year period.

Question 10

What are the limitations of recurrence intervals?

Answer 10

Any new larger flood can reduce the rank of the flood in question and can dramatically reduce its recurrence interval.

Question 11

What are the 3 methods for flood prediction?

Answer 11

1) recurrence intervals
2) rational runoff method
3) Hazard mapping

Question 12

Describe the rational runoff method in predicting floods.

Answer 12

The rational runoff method predicts peak run off rates by assuming that stormflow discharge is a fixed proportion of the rainfall intensity. It’s calculated by Qpk=0.278CIA. Works best for urban areas with high run off rates and areas with little channel storage. Assumes Horton overland flow. (Qpk = peak rate of runoff, C=rational runoff coefficient. I=rainfall intensity, A= drainage area)

Question 13

Describe Singapore as a floods case study.

Answer 13

Excess rainfall: high rainfall of 2550mm per year. Intense rainfall causes huge amounts of stormflow to concentrate in river channels in short times. Usually occurs during monsoon at beginning/end of year.
Topography: conducive for flooding - Bukit Timah Granite & Jurong formation are particularly flood prone due to steep sided valleys concentrating flood waters to valley floors
Urbanisation: built up areas increased from 30% in 1960 to 60% in 2012.

Question 14

Describe Bangladesh as a floods case study.

Answer 14

Bangladesh is a low lying flood plain
Convergence of 3 major rivers that swell yearly due to snowmelt from the Himalayas
Coincidence of monsoon season often exacerbates the effects

Question 15

Describe hazard mapping as a prediction measure of floods.

Answer 15

Flood Hazard mapping is used to determine the areas susceptible to flooding when discharge of a stream exceeds bankfull discharge. Using historical data and discharge trends, maps can be constructed to show areas expected to be covered with floodwaters for various discharges. Limitation: such models only work if they are kept up to date with drainage modifications

Question 16

Describe Artificial levees as a mitigation measure of floods.

Answer 16

Natural levees are broad, low ridges of fine alluvium built along the sides of a channel. Artificial levees are heightened natural levees by earth dykes designated to hold in ordinary floods. (eg. Mississppi River)

Question 17

Explain a limitation of artificial levee

Answer 17

1) material will be deposited in the channel during high discharge rather than spill into the flood plain. With the build up of the bed, less water is required to overtop the original levee. Height of the levee must be raised periodically.
2) Artificial levees reduce the width of the flood-flow part of the river and raise the water level during flooding. Can lead to higher and faster water flow and higher floor levels downstream.
3) false sense of security

Question 18

Describe dams as a mitigation measure of floods

Answer 18

Flood control dams are built up to stop downstream flooding since they store flood water and let them out slowly. Dams are built high enough to contain a certain magnitude of expected flood.

Question 19

Describe the limitations of dams as a mitigation measure of floods

Answer 19

1) Flood protection capacity decreases over time as reservoir fills with sediment with deforestation and urbanisation
2) barriers to fish migration
3) dam failure (eg. Baldwin Hills 1963
4) Thermal stratification behind the dam which may affect salmons

Question 20

State 4 non structural mitigation approaches of floods

Answer 20

1) Floodways (eg. Mississippi river - spillway is opened so water drains into Lake Ponchartrain)
2) Mortgage limitations
3) Building codes
4) Flood insurance