AGEN 2363 Chpt. 10 notes: Terraces and Diversions

 

This chapter deals with conservation structures designed to reduce the energy of runoff water and depositional loads. The design of these structures depends on:

- topography

- soil

- objectives

- costs

- labor, maintenance, availability of resources

 

I. Three categories of terraces:

A. Bench.

1. Where used?

2. Advantages?

3. Disadvantages?

4. How are they designed?

 

 

B. Graded (broadbase) terraces.

1. Where used?

2. Advantages?

3. Disadvantages?

4. How are they designed?

 

VI = xS + y VI =

x =

S =

y =

 

HI = (VI/S) x 100 HI =

 

Other factors to consider in HI determination:

 

 

C. Grass-back terraces:

 

 

D. Grass-back, back slope terraces:

 

 

 

 

 

 

 

 

II. Diversion ditches grassed waterways.

A. Basic design: trapezoid b = bottom width

z = side slope ratio (EX. 8:1)

T t = wetted top width, 10 year return storm

t d = wetted depth

T = top width = b + [2 X z(d + 0.2)]

d

b

B. Suggested grass species: bermudagrass, tall fescue, bahiagrass

 

C. Underground inlet/outlet systems:

 

 

III. Gully control/renovation.

A. How does a gully form?

 

B. Three Rs of renovation:

Reduce or Re-route Runoff

Re-shape if possible

Re-vegetate

 

C. Other ideas:

1. Start at the top! Consider renovation in Phases for very large gullys.

It is essential to greatly lower or eliminate runoff energy!

 

2. DO NOT throw cars, refrigerators, etc into the gully!

 

3. Ideas for runoff control and sedimentation collection:

- broken rock

- BARRIERS: rock, brush, logs, fences, timber, hay bales, etc

- drop structures/weirs

- graded and concrete chutes

- small earthen dams with pipe drains

 

4. Revegetation.

a. Cover crops: annual ryegrass, millets, wheat, oats

b. Proven species:

- weeping lovegrass - sericea lespedeza - bermudagrass

- bahiagrass - switchgrass - annual lespedeza

- tall fescue - perennial ryegrass - crabgrass

 

 

 

 

IV. Stream bank protection.

A. Current deflectors, know the following:

1. regular dikes

 

2. vane dikes

 

3. groynes

 

4. sills

 


B. Bank Protectors

1. toe protectors

 

2. hardpoints

 

3. revetments

 

4. windrow revetment

 

 

V. Flood control.

1. sand bags

 

2. dikes

 

3. levees

 

4. floodways

 

5. reservoirs

 

 

VI. Wind erosion control structures.

1. woven mat barriers-

 

2. snow fences/fences

 

3. vegetative wind breaks: trees, hedges, grass