3

Introduction to Check Dams

Kabul, Afghanistan February 2011

This watershed rehabilitation and restoration training was prepared by the U.S. Department of Agriculture (USDA) team of Jon Fripp (Civil Engineer – USDA/NRCS), Melvin Westbrook (Director USDA-NRCS/IPD), Otto Gonzalez (International Agricultural Development Specialist - USDA Foreign Agricultural Service), Clark Fleege, (Nursery Manager, USDA Forest

Service, and George Hernandez (Forester - USDA Forest Service), in consultation with Lief Christenson, (USA CJTF101 Water Resources Coordinator, Afghanistan). Contact Jon Fripp at jon.fripp@ftw.usda.gov or Otto Gonzalez at Otto.Gonzalez@fas.usda.gov for more information on this workshop.

Module Topics: • Purpose of Check Dams • Design basics • Design basics of a check dam in

collection zone • Design overview of a check dam in

transport zone

Check Dams •Typically used in Collection Zone •Can also be used in Transport Zone (use caution!)

•Stop erosion •Traps sediments and promotes infiltration •Watershed rehabilitation

These are structures • Use caution • Understand the processes at work in the

system • Know the components of a check dam

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

Groundwater can drop

What else happens when a channel degrades?

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 Stages Good for collection and transport zone

STAGE 1

Test Time

Headcut

STAGE 2

Channel downcutting creates straighter, steeper gully.

widening STAGE 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 Dams Lots 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 basics You 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 shorter is better!

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.

Question: What is wrong with this check dam?

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

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

Same site 1+ years later

Rock and Brush Grade Stabilization

Arid areas

Excellent design guidance is

available

Note the basics remain the same!

Check Dams •Can Also be used in Transport Zone

But be careful!

•Prevents serious stream/river erosion •Irrigation diversions •Water supply

Rock Ramp Largest stones are placed at crest and on downstream face of structure

Rock Chute

( ) 529.05.150 923.112 qSD =

For S < 0.1

For So less than 10%

Robinson et. al, 1998

Stone Sizing

0.10<S<0.40

( ) 529.058.050 233.012 qSD =

D50 in inches

For So between 10% and 40%

Oklahoma

Oklahoma

Chevron Weir

Figure 12.12 St. Anthony Falls (SAF) Type Drop Structure (Blaisdell, 1948)

Traditional or Rigid grade control and drop structures

Excellent design guidance is available for the design of rigid drop structures

NEH 654 14G

Be Careful!! • Especially in transport zone • Interruption of sediment flow • Upstream effects • Fish Passage • Flood Control • Dam Safety Issues • Bank Stability • Infrastructure

Example Problem: •Water backed up behind irrigation diversion •Water overtopped banks •Flanked structure •Damaged pasture land

You need an engineer to help with the design if:

• If project is in the transport zone • 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

Questions?