Taming the Little Miami River - Earthcache EarthCache

From the attached satellite photo you can see the Little Miami River splits into two. On the side where you are standing are a large number of residential properties. The dam is there to protect them from erosion. The dam helps to protect them by redirecting some the flow to the other branch and by dissipating some of the energy in the flow of the water. This dissipation also protects the dam structure itself.

Erosion

The erosion of the riverbank would be a threat to the homes nearby.

Rivers do most of the erosional work of landscape shaping. Rivers have been credited with 85 to 90 percent of the total sediment transported to the sea. (A grain of sand may require 1,000 years to be carried through a large river network to the sea.) This contrasts with about 7 percent transported by glaciers, 1 to 2 percent by groundwater and ocean waves, and less than 1 percent each by wind and volcanoes. Presumably, these volumes of sediment transport are proportional to the relative volumes of landscape eroded by the various agents. Therefore, any general consideration of landscape development must deal primarily with the work of rivers.

Most erosion by rivers is accomplished during the brief intervals of high discharge and flooding. At these times, rivers flow not only faster, deeper, and wider, but also much muddier. Because of the great increase in turbulence in a deep, fast-flowing stream, it can carry one hundred to one thousand times more sediment than it can at low-water stages. Thus, if a river at flood has one thousand times as much water, and that water is one thousand times muddier than at low flow, then one million times more sediment is being moved. Such numbers are typical of actual measured values.

Rivers erode their channels by:

* Grinding the rocky channel with abrasive particles already being carried;

* Plucking, or tearing out large blocks along preexisting fractures; and

* Dissolving rocks (such as limestone).

As with sediment transport, most riverbed erosion occurs during brief times of high discharge. By far, however, the most sediment in a river comes from the hill slopes on the valley sides rather than by direct river action. In that sense, rivers act more like passive gutters or storm drains that are forced to carry water and sediment that are delivered to them from their drainage basins rather than actively producing their own.

The dam before you is an attempt to minimize this erosion.

Hydraulic Jump

What this dam is an excellent example of the use of a hydraulic jump to dissipate the kinetic energy (the energy of the water’s motion) to protect the dam structure and prevent erosion of the streambed behind the dam. The idea to use a hydraulic jump and other features to protect dams and rivers was pioneered near here. Using a swimming pool at Colonel Edwards Deed’s farm in Moraine Ohio as a hydraulic laboratory - Miami Valley Conservancy District engineers pioneered the “jump” and other devices to dissipate the water’s energy. Some dams have a series of baffles, dips, pools, and finally a wall, which forces the water to “jump” back on itself, substantially reduces the water’s kinetic energy. Calmed, the water flow downstream at a much-reduced threat to people and property. It might be obvious where the jump is but here are some indicators: Before the jump the water flow is smooth and low in height – on our case the smooth flow over the cement of the dam itself. At the jump the water height is higher and rough and choppy.

Definitions:

Kinetic Energy = energy of an object which it possesses due to its motion

Potential Energy = is the energy stored in a body or in a system due to its position in a force field or due to its configuration. In this case it is the height of the water in the hydraulic jump and gravity is the “force field”.

Physics of the Hydraulic Jump

This dam allows the physics of the hydraulic jump to be easily observed. When the river water moving at a high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the river surface. The rapid velocity decrease of the water and increases in height converts some of the flow’s kinetic energy in potential energy, with some energy being irreversibly lost to heat due to the turbulence of the water.

We can estimate the speed decrease of the water based on the continuity principle. Basically, this means for the water (an incompressible liquid) the amount of water flowing over the dam is the same as the amount of water flowing down stream. Since our dam is a constant width it the relation is

(Velocity after the jump / velocity before the jump) = (water height (depth) before the jump / water height after the jump)

To log the earthcache do the following:

1) (Optional) Take a picture of yourself with the dam in the background showing the dam spillway and jump. (Optional) 2) Estimate the height of the dam from the top where the water spills over to the base of the hydraulic jump.

3) Based on the depth of the water flowing over the height of the jump estimate the velocity ratio decrease of the water at he jump ( Vj / Vo ) = ( Ho / Hj ) Would you expect the decrease in velocity to increase or decrease the erosion of the river bank downstream?

References: http://en.wikipedia.org/wiki/Hydraulic_jump#Height_of_the_jump

http://www.miamiconservancy.org/flood/index.asp

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