GCA5Z6C Water Flow & River Sediment - Hope River (Earthcache) in Prince Edward Island, Canada created by El Nimrod

GCA5Z6C ▼

A cache by El Nimrod Message this owner

Hidden : 3/14/2023

Difficulty:

Terrain:

Size: Size: other (other)

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Geocache Description:

Welcome to a popular local Island fishing stream. There is a pool that contains eroded sediment and rocks that have been transported down the stream. The purpose of this earthcache is to learn about the flow of water as it relates to the transportation and deposition of river sediment.

Erosion, Transportation & Deposition

Water plays a significant part in the weathering and erosion of the earth's surface. This earthcache looks at one aspect of the transport of eroded rock by water streams and rivers. Flowing water picks up sediment, transports it (click) as it flows, and deposits it when the flow slows down. How fast must water flow to pick up the sediment, and when will the sediment be deposited as the water slows? The Hjulström curve (click), named after Filip Hjulström (1902–1982), is a graph used by hydrologists and geologists to determine whether a waterway will erode, transport, or deposit sediment. Originally published in 1935, the graph takes sediment particle size and water velocity into account.

The upper curve shows the critical erosion velocity in cm/sec as a function of particle size in mm, while the lower curve shows the deposition velocity as a function of particle size. Note that the axes are logarithmic. The plot shows several key concepts about the relationships between erosion, transportation, and deposition. For particle sizes where friction is the dominating force preventing erosion, the curves follow each other closely and the required velocity increases with particle size. However, for cohesive sediment, mostly clay but also silt, the erosion velocity increases with decreasing grain size, as the cohesive forces are relatively more important when the particles get smaller. The critical velocity for deposition, on the other hand, depends on the settling velocity, and that decreases with decreasing grain size. The Hjulström curve shows that sand particles of a size around 0.1 mm require the lowest stream velocity to erode.

The curve is not perfect as there are other factors that come into play, such as the depth of the water and characteristics of the sediment. The curve has been expanded on by other geologists to take these factors into account. In plain English, the graph shows the relationship between the size of sediment and the velocity (speed) required to lift it, carry it along, and deposit it. The velocities for transport are lower than that for erosion, because it takes much more energy to lift sediment than to maintain it in transport (think about carrying a heavy load;, it takes more energy to initially lift it than to carry it). The other unusual pattern is that it takes more energy to erode some of the smallest particles. This is because they are clay particles which stick together, so they need a lot more energy to be eroded.

Factors Affecting Water Flow Velocity

Water flow velocity is decreased by friction along the stream bed, so it is slowest at the bottom and edges and fastest near the surface and in the middle. In fact, the velocity just below the surface is typically a little higher than right at the surface because of friction between the water and the air. On a curved section of a stream, flow is fastest on the outside and slowest on the inside.

Other factors that affect stream-water velocity are the size of sediments on the stream bed — because large particles tend to slow the flow more than small ones — and the discharge, or volume of water passing a point in a unit of time.

Smaller particles may rest on the bottom some of the time, where they can be moved by saltation and traction, but they can also be held in suspension in the flowing water, especially at higher velocities. Turbulent (chaotic flow and rough water surface) flow is more effective than laminar flow (straight-line flow and a smooth water surface) at keeping sediments in suspension.

Stream water also has a dissolved load, which represents (on average) about 15% of the mass of material transported, and includes ions such as calcium (Ca+2) and chloride (Cl-) in solution. The solubility of these ions is not affected by flow velocity.

It is important to be aware that a stream can both erode and deposit sediments at the same time. At 100 cm/s, for example, silt, sand, and medium gravel will be eroded from the stream bed and transported in suspension, coarse gravel will be held in suspension, pebbles will be both transported and deposited, and cobbles and boulders will remain stationary on the stream bed.

Examples: 

• At 20 cm per second fine sand is eroded while pebbles would be deposited.

• The smallest clay particles require velocities of 200 cm per second to be eroded. For the smallest silts it is approximately 120 cm per second.

• Cobbles are eroded upwards of 170 cm per second.

• Deposition starts to occur at 0.1 cm per second for particles of approximately 0.01mm in size.

• Boulders require the smallest velocity for deposition, at only 150 cm per second for the smallest boulders 250 mm in size.

• Course sand of 0.5 mm in size is transported between 3 cm per second and 16 cm per second.

Logging Requirements:

Please send the answers to the following required questions to me through my geocaching profile before logging as found. DO NOT post any answers on your log, encrypted or otherwise.

List the name GCA5Z6C Water Flow & River Sediment - Hope River in the first line of your email. Also list the number and name(s) of people in your group.

(1) Estimate the rate of water flow in the stream on the west side and east side of the road during your visit. Describe your method of measurement and the prevailing weather conditions at the time of your measurement (e.g., rain, snow-melt, clarity of water, etc). Are the rates of water flow the same? If not, what is one factor that could account for the difference?

(2) Examine and compare the river sediment on the east & west sides of the road. Are they the same or different? On the west side of the road (at the posted coordinates, north side of the stream), there is almost entirely sand & silt along the riverbank -- no pebbles or gravel — why might this be?

(3) Now examine the stream on the west side of the road, south side of the stream, and describe the type of sediment seen. Why might this be? (4) Per current www.geocaching.com guidelines, post a picture of yourself and your GPSr with your log that shows the stream in the background.