Drainage Basins, River Morphology, and Flooding

Flashcards covering key concepts related to drainage basins, river systems, erosion, and flooding.

What is a drainage basin?

The area from where a river obtains its water from, or the area drained by the river.

What is a watershed?

The area of highland that separates two drainage basins.

What is a gauging station?

A place located at the end of the drainage basin where river flow data is collected.

What are the major components of a drainage basin as a hydrological system?

Input (precipitation), Transfers (infiltration, overland flow), Stores, and Output (evapotranspiration, river flow into the sea)

What are the different forms of precipitation?

Rainfall, Snowfall, Sleet-fall, Hailfall

What is Interception Storage?

The percentage of precipitation that is retained on or intercepted by vegetation.

What is Throughfall?

Rainfall that is not intercepted and passes through the foliage of leaves to the surface.

What is Stemflow?

Water released from interception storage by flowing towards the surface along the stems of trees or grass.

What is River Runoff (Discharge)

The total amount of water that leaves the drainage basin via surface routes, also called channel flow.

What are the components of runoff/sources of water in the river?

Overland flow, Throughflow/Interflow, Baseflow, Direct precipitation

What is Overland Flow?

A thin sheet of water flowing over the surface, responsible for causing river floods.

According to Robert E Horton, what two variables determine whether overland flow is produced?

Rainfall Intensity and Infiltration Capacity of the soil

In the Hortonian Overland Flow Model, under what conditions does overland flow occur?

When Rainfall Intensity becomes greater than the Infiltration Capacity.

Where is the Hortonian Overland Flow Model said to be more active?

Arid and semi-arid regions where sun baking has reduced the surface’s infiltration capacity to low levels.

What are the effects of vegetation in the humid regions?

Increasing the soils’ infiltration capacities because the roots of vegetation opens the soils, and the humus produced by decaying vegetation tends to soften the soil and so increases the soils’ infiltration capacity beyond any intense rain rainstorm.

What is the Saturation Overland Flow model (SOF)?

Most of the rains in a drainage basin should infiltrate, since in these vegetated ecosystems, Rainfall intensity < Infiltration Capacity. The infiltrating water in the long term begins to saturate the soils, from below. Once saturated, any additional precipitation fails to infiltrate but becomes surface runoff/overland flow.

What areas of the Drainage Basin become easily saturated?

Areas that are close to the river channel with a higher water table, areas with thin soils, and areas supplied with a large amount of water by irrigation.

What is Throughflow/Interflow?

The horizontal movement of water after infiltration in the vadose or aeration zone, eventually reaching a channel.

What are the factors that influence through flow?

Presence of clay pans, percolines, impermeable sub soil, Presence of horizontal joints, bedding planes or a platy soil structure

What is Baseflow/Groundwater Flow?

The horizontal movement of water in the phreatic zone below the water table.

According to Darcy’s Law, what are the two factors that determine the total amount of water (Q) contributed by baseflow to a river?

The hydraulic gradient of the water table and the hydraulic conductivity of the soil material

What is Evaporation?

The change of state of water from liquid to gas.

What factors influence the process of evaporation?

Insolation, Wind Speed, Human Actions

How does insolation affects evaporation rates?

Latitude, seasons and time of the day alters evaporation rates

What is Transpiration?

The evaporation of water from the internal structure of the leaf through the stomata.

What factors influence transpiration?

Vegetation type, meteorological conditions (latitude, seasons, time of day, sky conditions, wind speed), influence of human activities

What is Evapotranspiration?

The total water loss from the soil, snow, vegetation interception, free water surfaces, and transpiration to the atmosphere.

What is Potential evapotranspiration?

This is the total water loss that would occur from the drainage basin that is fully covered with vegetation and whose soil moisture content is above field capacity.

What is Actual evapotranspiration?

This the total water loss from the drainage basin whose surface is not necessarily fully covered by vegetation and whose soil moisture content is not necessarily at field capacity.

What is the equation of discharge?

Q= Velocity X Cross -sectional Area of the river

What does the falling limb of the storm hydrograph represent?

From the beginning of the rainstorm up to its end, precipitation falling on the drainage basin is being held elsewhere by Processes such as interception, surface storage, and the initial formation of Overland flow

What does the rising limb of the storm hydrograph represent?

Sharp increases in discharge because all the stores that were holding onto the rainwater have finally released the water, and it has finally joined the 1st order tributaries, 2nd order tributaries and eventually into the mainstream.

When does the river go into flooding?

If the peak discharge of the rainstorm is above the bank full discharge of the river

Hydrologists came up with a method of showing relative importance of the three components that deliver water to a river. What is this method and what components are taken under consideration?

Hydrograph separation and Overland flow, Throughflow and Base flow are taken under considertion.

What is Lag time?

The time period between Maximum rainfall and Peak discharge

What is flood predictions?

These are attempts by hydrologists to foretell when a flood of a specific magnitude is likely to occur in a drainage basin.

What is Recurrence interval/ return periods?

This is the time it takes for a flood of a specific magnitude to reoccur in a drainage basin.

What is flood forecasting??

flood forecasting is a short-term way of telling whether a rainstorm that has been recently received in a drainage basin is going to cause flooding or not.

What are some Flood Control Technologies used in Bangladesh?

Boats, food storage, aids from other countries, embankments flood shelters and Flood Warning Scheme

What are the main human causes for the river floods?

Urbanization, riverbed aggradation, ploughing and deforestation

List some Natural Causes of Flooding in Bangladesh .

Monsoon Climate, Tectonic Uplift, Relief Runoff, Sea Levels

What are the three primary shapes of delta?

cuspate, arcuate and bird’s foot.

Define the process and contribution of Vertical Erosion

The channel's erosional processes are concentrated onto the riverbanks. Lateral erosion is quite common in the middle and the lower course of a river where the increases in volume provides the river with a full load of sediments that allows it to attack its banks

Define the process and contribution of Headward Erosion

River extendending its length towards the source or headwaters of the river. In this case the rivers erosional processes concentrated onto the riverbed

Define the process and contribution of Lateral Erosion

Vertical erosion leads to headwater extensions where Overland flow entering the river at its source produces a plunge pool that develops below the waterfall.

What is competence in River Load?

The maximum weight of the largest fragment that a stream of a given velocity can transport.

What is capacity in River Load?

This is the maximum amount of load that a River moves.

Give an example of where the river deposits course sediments into the riverbed and the deeper sections that are in between them.

Riffles vs Pools.

Relate Helicoidal Movement in River Morphology

The corkscrew-like flow of water called Helicoidal Flow moves material from the outside of one meander bend and deposits it on the inside of the next bend.

Describe braided channels and the impact they have on River Morphology

A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, aits or eyot

Define Floodplains

is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.

Define River Levees

Levees are natural embankments produced, ironically, when a river flood. When a river floods, it deposits its load over the flood plain due to a dramatic drop in the river’s velocity

Describe the importance of the Bottom set beds in River Deltas

The bottom most bed, the bottomset bed, is composed primarily of clay and some other fine-grained sediments.

Describe the importance of the Fore set beds in River Deltas

The fore-set bed lies on top of the bottomset bed. The fore-set bed is composed of coarser sediments that are deposited due to a fall in the river’s velocity and are not transported very far into the stationary body of water that the river flows into.

Describe the importance of the Top set beds in River Deltas

The topset bed is, as the name suggests, the topmost bed of the delta. It too is composed of coarse sediment but, unlike the fore-set bed, the topset bed does not dip, it is horizontally bedded.