Introduction to Check Dams Kabul, Afghanistan February 2011 3 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
Understand the processes at work in the system What happens in if all the vegetation is removed from a watershed? 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.
Section The channel will get deeper
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 Assessment Tool: Channel Stages Good for collection and transport zone Stage 1 Stage 2 Stage 3 Section
Test Time STAGE 1
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 Stage 2 Profile
A check dam will keep the channel from getting too deep Profile
Check Dams Lots of different types Lots of different materials
Components of a check dam 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 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 1 m 60 cm section
May need a filter with log check dams Energy Dissipation Section: Absorbs energy as water flows over top. Key into Channel Bottom: Keeps water from eroding under check dam Same rules for all types of check dams Profile
The check dams work as a team Downstream erosion might undermine the upstream check dam Profile
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
Answer: nothing major Question: What is wrong with this check dam?
Question: What is wrong with these check dams? Same site 1+ years later Answer: nothing major
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
Stone Sizing For S < 0.1 For So less than 10% ( ) 1. 1.5 0. 529 D = 12 923qS 50 0.10<S<0.40 ( ) 0. 0.58 0. 529 D = 12 233qS 50 For So between 10% and 40% Robinson et. al, 1998 D 50 in inches
Oklahoma
Oklahoma
Chevron Weir
Traditional or Rigid grade control and drop structures Figure 12.12 St. Anthony Falls (SAF) Type Drop Structure (Blaisdell, 1948)
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?