Sedimentation Basins

Question 1

What are the benefits of decreasing the velocity (and turbulence) of the moving water in sediment control?

Answer 1

Decreasing velocity will reduce the ability of the water to transport sediment and reduce the ability of the run-off to detach soil particles.

Question 2

What are “Sediment Traps”

Answer 2

They are small temporary structures used at various points of distrubed areas to detain run-off for a short period of time and trap heavier sediment particles.

Question 3

What are the two types of sediment ponds (basins)?

Answer 3

Dry basin and wet basin.

Question 4

What is a dry basin as it pertains to sediment control?

Answer 4

A basin used to trap sediment in an off-stream location that is dry most of the time.

Question 5

What is a wet sediment basin as it pertains to sediment control?

Answer 5

It’s a structure created to impound water for the purpose of settling sediment in the run-off.

Question 6

What is meant by “Overflow Rate”?

Answer 6

The ratio of pond discharge to the surface area of the pond.

Question 7

What will happen to the overflow rate if the flow rate of water to a basin is unchanged, but the area of the basin is increased?

Answer 7

The overflow will decrease and smaller particles will be settled out.

Question 8

What factors could disturb efficient settling of particles in a basin?

Answer 8

Short circuiting, bottom scouring, turbulence, non-uniform deposition, entrance and exit effects, irregular shape of the suspended material.

Question 9

What is the function of baffles in a pond?

Answer 9

To increase uniformity of detention time and to provide full utilization of the pond (or reduce short circuiting).

Question 10

What will the effect of partitioning a pond into a number of chambers and allowing water to overflow from each chamber along the entire width be?

Answer 10

This can improve pond performance in retaining suspended solids, because it will reduce short circuiting.

Question 11

Maximum efficiency of a rectangular sediment pond can be expected at a certain length-to-width ratio. What is the optimum ratio?

Answer 11

Length to width= 5:1.

Question 12

What can construction of an energy dissipator in the sediment pond entrance reduce?

Answer 12

Influent velocity.

Question 13

Modifying the inflow to the sediment basin so that the flow enters along the entire _______ of the basin, as much as possible, can increase the suspended solids removal efficiency of the basin.

Answer 13

Width.

Question 14

Wrapping a plastic filter cloth around a standard PERFORATED riser can increase the retention of what kind of material?

Answer 14

Fine-grained.

Question 15

Why is it preferred to have a very wide overflow weir instead of a standard riser pipe for the solids removal effeciency of a sediment pond?

Answer 15

It will reduce the outflow velocity of the water.

Question 16

Two or more ponds used in series, instead of one large basin covering the same area, have been shown the increase the removal efficiency. One reason for this improvement is _________________________.

Answer 16

Reduction of short circuiting.

Question 17

Short circuiting is undesirable in a sediment basin. What is short circuiting?

Answer 17

A hydraulic condition occuring in part of a basin where the time of travel is less than the theoretical flow-through time.

Question 18

All sediment containment structures require an inspection after major __________?

Answer 18

Rainstorms.

Question 19

What is the most important maintanence need for a sediment containment basin?

Answer 19

The removal of accumulated sediment.

Question 20

A general rule that can be used the determine when to clean out a sediment basin is:

Answer 20

When the sediment basin has reached 50% of its sediment storage capacity or six months after the mining operation was started.

Question 21

Small quantities of sediment from a sediment basin may be disposed of by:

Answer 21

Burial in the active pit or disposal by dewatering and covering with soil and vegetation.

Question 22

Large quantities of sediment from a sediment basin may be disposed of by:

Answer 22

Special provisions should be made to either bury it in an area designated for that purpose, or to stockpile, dewater, spread, adequately cover, soil stabilized, and vegetate it properly.

Question 23

Accumulated sediment that will be left in place after mining is completed, should be?

Answer 23

Spread, covered with topsoil, and vegetated, or stabilized by mechanical means, to prevent it from eroding back into the stream.

Question 24

Once surface mining is complete, what should be done with an earth bank built across a natural drainage way?

Answer 24

The embankment and all accumulated sediment should be removed to a predetermined area within the mine property and the area where the embankment was soiled, stabilized, and vegetated properly.

Question 25

What is a “Check Dam”?

Answer 25

It is a small dam constructed in a gully or other small water course to decrease the stream flow velocity, minimize channel scour and promote deposition of sediment.