13 Check Dams 2006

This training was prepared by the U.S. Department of Agriculture (USDA) team of Otto Gonzalez-USDA Foreign Agricultural Service (Team Leader), Jon Fripp (Civil Engineer) and Chris Hoag (Wetland Plant Ecologist)-USDA Natural Resources Conservation Service (Civil Engineers). Fripp and Hoag were the primary authors of this material. The U.S. AID provided funding support for the USDA team.

8

Introduction to Check Dams

Check Dams•Usually Used in Collection Zone•Can Also be used in Transport Zone

These are structures

• Use caution

• Understand the processes at work in the system

• Know the components of a check dam

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What happens in if all the vegetation is removed from a watershed?

Understand the processes at work in the system

Answer:More erosion of surface

So what happens if you have more water running down the watershed collection zone?

Answer: Erosion. Gullies will form and grow. The channel will get hungry and eat its boundary.

The stream or gulley will start by attacking the bottom of the channel first.

The channel will get deeper

Section

The channel will keep getting deeper until the sides start to fall in

Section

Then it starts to get wider

Now it is really starting to get bad

This is process is called the channel stages

Stage 1

Section

Stage 3

Stage 2

Assessment Tool: Channel StagesGood for collection and transport zone

STAGE 1

Test Time

Headcut

STAGE 2

Channel downcutting creates straighter, steeper gully.

wideningSTAGE 3

Channel widening, cut banks on both sides.

It is best to stop this process early in Stage 2

To stop it: use a check dam

Profile

Stage 2

Profile

A check dam will keep the channel from getting too deep

Check DamsLots of different types

Lots of different materials

A Check Dam must• Have a Control Section

• Have a Energy Dissipation Section

• Be Keyed into Channel Bottom

• Be Keyed into Channel Bank

• Be appropriately spaced

Components of a check dam

This introduction is going to cover the basicsYou may want to get more detailed training

Control Section: Keeps the water in the channel

Bank Key: Keeps water from eroding around check dam

•Control Section should have the same area as a stable section of the stream or gully.

Energy dissipation section should be two times the structure height

Maximum height should be less than 1.5 m

H

2H

Key into channel bottom for a minimum of 60 cm

Key into channel bank for a minimum of 1 m

Top of check dam should be at least 30 cm below top of channel

section

1 m

60 cm

Energy Dissipation Section: Absorbs energy as water flows over top.

Key into Channel Bottom: Keeps water from eroding under check dam

May need a filter with log check dams

Profile

Same rules for all types of check dams

Profile

The check dams work as a team

Downstream erosion might undermine the upstream check dam

Space so that the slope can be level between the check dams and the upstream one will not be undermined.

Profile

Test Time

Question: What is wrong with this check dam

Answer: bank key and adjacent protection

Question: What is wrong with this check dam?

Answer: no control section. Made worse by bad bank key

Question: What is wrong with this check dam?

Answer: bank key in

Answer: everything, bad spacing, no control section, no bed or bank key in, no energy dissipation

Question: What is wrong with this check dam?

Question: What is wrong with this check dam? Answer: nothing major

Question: What is wrong with this check dam?

Answer: nothing major

Question: What is wrong with these check dams?

Answer: nothing major

You need an engineer to help with the design if:

• The river has high velocities

• The river is large

• The erosion is significant

• The river system is unstable

• There is something very important on the bank

• The project will cost a lot of money

• Laws state you must have an engineer

The End