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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 [email protected] or Otto Gonzalez at [email protected] for more information on this workshop.
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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
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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
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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
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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.
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The stream or gulley will start by attacking the bottom of the channel first.
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The channel will get deeper
Section
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Groundwater can drop
What else happens when a channel degrades?
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The channel will keep getting deeper until the sides start to fall in
Section
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Then it starts to get wider
Now it is really starting to get bad
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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
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STAGE 1
Test Time
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Headcut
STAGE 2
Channel downcutting creates straighter, steeper gully.
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widening STAGE 3
Channel widening, cut banks on both sides.
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It is best to stop this process early in Stage 2
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To stop it: use a check dam
Profile
Stage 2
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Profile
A check dam will keep the channel from getting too deep
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Check Dams Lots of different types
Lots of different materials
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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
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Control Section: Keeps the water in the channel
Bank Key: Keeps water from eroding around check dam
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•Control Section should have the same area as a stable section of the stream or gully.
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Energy dissipation section should be two times the structure height Maximum height should be less than 1.5 m shorter is better!
H
2H
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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
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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
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Profile
The check dams work as a team Downstream erosion might undermine the upstream check dam
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Space so that the slope can be level between the check dams and the upstream one will not be undermined.
Profile
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Test Time
Question: What is wrong with this check dam
Answer: bank key and adjacent protection
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Question: What is wrong with this check dam?
Answer: no control section.
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Question: What is wrong with this check dam?
Answer: no control section. Made worse by bad bank key
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Answer: everything, bad spacing, no control section, no bed or bank key in, no energy dissipation
Question: What is wrong with this check dam?
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Question: What is wrong with this check dam? Answer: nothing major
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Question: What is wrong with this check dam?
Answer: nothing major
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Question: What is wrong with these check dams?
Answer: nothing major
Same site 1+ years later
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Rock and Brush Grade Stabilization
Arid areas
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Excellent design guidance is
available
Note the basics remain the same!
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Check Dams •Can Also be used in Transport Zone
But be careful!
•Prevents serious stream/river erosion •Irrigation diversions •Water supply
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Rock Ramp Largest stones are placed at crest and on downstream face of structure
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Rock Chute
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( ) 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%
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Oklahoma
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Oklahoma
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Chevron Weir
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Figure 12.12 St. Anthony Falls (SAF) Type Drop Structure (Blaisdell, 1948)
Traditional or Rigid grade control and drop structures
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Excellent design guidance is available for the design of rigid drop structures
NEH 654 14G
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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
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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
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Questions?