Dynamics of Moving Water

Dynamics of Moving Water


1. Hydrologic Cycle- Infiltration- Runoff


When it rains and the water hits the ground, two things can happen:

1.The water begins to seep (percolate) into the soil, this is called Infiltration.

2.If the ground is made up of material that will not allow water to seep in or if the ground is saturated then the rain water begins to runoff.

1. Hydrologic Cycle

Condensation

Precipitation

Percolation or Infiltration

Surface Runoff

Ground Flow/Stream Flow

Evaporation

Hydrologic Cycle

Infiltration: precipitation that seeps into the ground.

Factors affecting infiltration: Interparticle size – the size and

connectedness of the particles in the ground, eg., sand vs clay

Vegetation (associated organisms) – loosen the soil.

Rainfall intensity Seasonal factors, eg., freezing of the ground Permafrost – is there any, its depth Urban landscape - paving, buildings, etc.

Hydrologic Cycle

Infiltration con’t: When rain falls, the first type of erosion

is splash erosion. Raindrop impacts reduce infiltration rate

because the soil particles become more compacted

movement of material downhill.

Hydrologic Cycle

Hydrologic Cycle

Runoff: flow of water from the land as both surface and subsurface discharge.

Occurs when rate of precipitation is greater than rate of infiltration.

Usefulness: recreational, sewage disposal, domestic, irrigation, food source, hydroelectric power, etc.

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Hydrologic Cycle

Runoff Categories: Surface Runoff/Overland flow:

excessive overland flow can lead to sheet erosion forming small rills or tiny rivers.

can include Channel Flow (water traveling in a river or stream).

Subsurface Runoff/Ground Water: travels beneath the surface towards a

body of water.


2. Stream Flow- Velocity- Volume (Discharge)- Load


When the ground becomes saturated and the water begins to travel from a high point to a low point in a depression or valley – we have a river and we have Stream Flow.

With stream flow we have Erosion, with erosion comes Transportation and finally with transportation we eventually get Deposition.

The ability for a river to do the above three things we need to study the Velocity of the river, the Volume of the river and the material (Load) the river is carrying.

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Stream Flow

The velocity (speed) of flow in a river is controlled by:

Gradient (slope) Friction due to resistance of water on the

bed of the channel. Volume

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Stream Flow

Volume (Discharge): volume of water that flows past a given point can be determined by the following factors: Q = A x V

Q is discharge A is cross-sectional area of stream V is mean velocity (in m3/sec usually)

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Stream Flow

Load: ability of a stream to carry load of solid material varies with volume, velocity and particle size.

Stream Transport: Load

The ability of a stream to carry load of solid material varies with discharge, velocity and particle size.

load

discharge

load

velocity

load

particle size

Question: Using these graphs, explain in words the relationship between discharge, velocity, particle size and load (the amount of material carried within the stream).

Solid matter carried by a river is called its STREAM LOAD.

A river’s ability to do work (pick and move material) depends upon its energy.

Stream Load can be carried in three different forms:

1. Dissolved load

2. Bed load

3. Suspended load

Stream Transport: Load

Physical Geography, Strahler and Strahler

Dissolved Load

The dissolved load is not usually visible; it is carried in solution. This is material carried in the form of dissolved ions resulting from mineral alteration.

3.9 million tonnes annually are removed from the land in dissolved load.

Amount depends on rock type (bedrock) and climate.

This is the reason an Ocean is salty!

Bed load is material that rolls, bounces, slides along the bottom of the river and is not supported by the water.

Sand, gravel and larger particles move along as bed load.

Moves slowly and it is hard to measure

A stream can move larger particles at times of flood

Depends on the stream gradient, discharge, and size of the load.

Bed Load

Finer particles (clay and silt) are carried in suspension. Held within the water by turbulence and flow of the stream.

Once these particles are picked up, very little energy is needed to transport them, less than the energy needed to pick them up.

Larger particles are near the bottom; smaller ones evenly distributed

Greater the stream discharge the greater the suspended load.

Suspended Load

This image is of Wilket Creek in Toronto after a flood. Below is the channel that the river created when it flooded. Notice the heavier material on the sides and the lighter material on the inside. The river just could not carry the heavy material so it deposited the rocks to the side.

Floods increase the capacity of a stream to carry load (both suspended and bedload) because the volume of water flowing downstream increases.

Floods are usually associated with increased velocity and this also increases a river’s capacity to move material.

Eg., capacity to move bedload goes up about 3rd to 4th power of the velocity

If a stream’s velocity is doubled in a flood, its capacity to carry bedload goes up 8 to 16 times, and the overall load will increase even more due to increased volume.

Rivers, therefore, can change dramatically in floods.

Importance of all of this Knowing the load of a river is important when predicting the changes in river dynamics due to FLOODS.

The Power of Wilket Creek in Toronto during a flash flood in 2005

Sometimes a river cuts into a bank that it can cause large amounts of land to fall off so to speak (landslide). Image A is a picture of a section of the Don River that was being eroded away. The Don was actually re-routed (next slide) and the land was saved – Picture B

A B

How the Don was re-routed to save some land and property from falling into the river.

Once a river undercuts its bank and the land above is not held together by the roots of trees, the land, with the help of gravity falls down creating a landslide. This is Wilket Creek in Toronto.


A river reaches what is called Stream equilibrium when the supply of load it gets is equal to the amount that it can carry. In other words there is no need for the river to erode, it’s hunger so to speak is satisfied. Usually a river is at it’s mature stage.

The river cannot cut downwards without upsetting its equilibrium and therefore it tends to meander, cutting sideways at the banks.

As it erodes material from one bank (on the outside - Pool) it deposits material on the other bank (the inside - Riffle) and the load remains constant and the river remains at equilibrium.

Stream Equilibrium


The picture on the left is the Don River – note the Pool and the Riffle and the meander.

Rejuvenation: The river’s equilibrium changes due to a change in velocity or volume.

The river becomes young again and begins to erode vertically.

Can occur in three ways:

Eustatic rejuvenation – caused by the lowering of the sea level during an ice age

Dynamic rejuvenation – caused by the uplift of the land during tectonic activity

Static rejuvenation – caused when either the load is reduced therefore increasing the capacity or when rainfall increases thereby increasing capacity.

Stream Equilibrium: Rejuvenation

During a flood, Wilket Creek grew in volume and velocity. It began to erode vertically and it cut right underneath its sides. This is called undercutting. Once the flood was over it stopped eroding and went back to it’s equilibrium.

This is the Humber River in Toronto and a human attempt at Rejuvenation. This was done to increase the speed of the river so that its flow became straighter. How can these ‘steps’ also help in flood control?


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Dynamics of Moving Water