Diversion Dams

Part of Irrigation

Diversion dams raise the water level in a stream just enough to redirect flow into an irrigation channel. Unlike storage dams, they do not attempt to hold back large volumes — they simply steer water where you need it.

A diversion dam is not meant to store water. It is a low barrier across a stream that raises the water level upstream by 30 centimeters to 2 meters, creating enough head to push water into a side channel. The stream continues flowing over or around the dam — you are borrowing water, not blocking it. This distinction matters enormously: a storage dam failure can be catastrophic, while a diversion dam failure is merely inconvenient.

Choosing a Dam Site

The ideal location for a diversion dam meets several criteria:

• Narrow channel: Less material needed, stronger structure per unit of effort
• Stable banks: Rock or firm clay banks that will not erode around the dam ends
• Bedrock or gravel bottom: Provides a solid foundation (soft mud bottoms require additional engineering)
• Upstream pool area: Space for water to back up without flooding anything important
• Irrigation channel takeoff: The point where your channel begins should be on the upstream side of the dam, at a height that allows water to flow into the channel when the dam raises the stream level

Flood Considerations

Every stream floods. Before building, find the high-water marks (debris lines on banks, discolored rocks, scour patterns) and ensure your dam design can survive or be quickly rebuilt after flooding. For streams with violent floods, plan for annual dam rebuilding — many traditional irrigation systems rebuilt brush dams every year as a community event.

Types of Diversion Dams

Brush Dams

The simplest and most ancient type. A brush dam is essentially a woven fence of branches and poles placed across a stream and weighted down with rocks.

Construction method:

1. Drive two rows of sturdy stakes (8-10 cm diameter, sharpened) into the streambed, spaced 50-80 cm apart, across the full width of the stream. Stakes should extend at least 60 cm above the desired water level.
2. Weave flexible branches (willow is ideal) horizontally between the stakes, creating a dense wall.
3. Pack the upstream face with leaves, grass, straw, or sod to reduce water flow through the dam.
4. Place heavy rocks along the base on both sides to anchor the dam against current.
5. As water backs up, fine sediment naturally seals remaining gaps.

Specification	Brush Dam
Maximum height	0.5-1.0 m
Stream width	Up to 5 m
Construction time	1-3 days (2-4 people)
Lifespan	6-18 months
Materials	Poles, branches, rocks, grass
Flood survival	Low — expect annual rebuilding

Willow Stakes Come Alive

If you use green willow stakes, many will root and sprout, creating a living dam that grows stronger over time. Drive stakes deep (at least 30 cm into the streambed) and leave them tall enough that leaves can reach above the water surface. Within one growing season, the living willows will reinforce the structure significantly.

Rock Dams

More durable than brush dams, rock dams use the stream’s own materials. They work best where the streambed provides abundant cobbles and boulders.

Construction method:

1. During low water, clear the dam site down to firm bottom material.
2. Place the largest available boulders as a foundation row across the stream. These should be heavy enough that the current cannot move them — typically 50+ kg each.
3. Build up layers of progressively smaller rock behind the foundation row, creating a wedge shape (thicker at the base, tapering to the crest). The upstream face should slope at about 45 degrees; the downstream face can be steeper.
4. Fill gaps between rocks with gravel and small stones.
5. If available, pack clay into the upstream face to reduce seepage through the dam.

Target dam dimensions: base width should be at least 2-3 times the height. A 1-meter-tall rock dam needs a 2-3 meter thick base.

Specification	Rock Dam
Maximum height	1.0-2.0 m
Stream width	Up to 10 m
Construction time	3-10 days (4-8 people)
Lifespan	5-20 years
Materials	Boulders, cobbles, gravel, clay
Flood survival	Moderate — may need repair, rarely destroyed

Check Dams (Gabion Style)

Check dams combine rock with a containing structure. Traditional gabions use woven wire baskets filled with rock, but pre-industrial versions use log cribs or woven branch baskets.

Log crib construction:

1. Lay two parallel logs across the stream, extending into both banks by at least 1 meter.
2. Lay two perpendicular logs on top, creating a square frame.
3. Continue alternating layers (like building a log cabin), notching the intersections for stability.
4. Fill the interior of the crib with rocks and gravel.
5. Pack the upstream face with clay.
6. Anchor the crib by extending the bottom logs into trenches cut into the banks.

Bank Anchoring Is Critical

The most common dam failure mode is not overtopping — it is water flowing around the dam ends, eroding the banks until the dam is bypassed entirely. Every dam type must be anchored into both banks. Extend the dam structure at least 1-2 meters into solid bank material on each side. This buried section is called the “wing wall” or “key.”

Wing Walls

Wing walls are extensions of the dam structure that angle upstream into the banks. They serve two purposes: they prevent water from eroding around the dam ends, and they funnel water toward the center of the dam (and toward your irrigation intake).

Build wing walls at approximately 30-45 degree angles from the dam face, extending into the bank. Construct them from the same material as the dam (rock, timber, or a combination). Dig into the bank material so the wing wall is partially buried — at least half its height should be below the natural bank surface.

Spillway Design

Every diversion dam needs a spillway — a controlled overflow point that allows excess water to pass safely during high flows. Without a spillway, rising water will find its own path over or around the dam, usually causing damage.

Simple Spillway

Cut a notch in the dam crest at the point farthest from your irrigation intake. The notch should be:

• Wide enough to pass the stream’s normal high-water flow
• Lined with rock or timber to prevent erosion
• At the same height as your desired upstream water level

The spillway crest sets the maximum water level upstream of the dam. Water above this level flows through the spillway rather than over the entire dam.

Armored Overflow

For larger streams, armor the entire downstream face of the dam with tightly fitted rock. This allows water to flow over the full length of the dam during floods without eroding the structure. Place the largest, flattest stones on the downstream face with their flat sides up, creating a rough but continuous surface.

Spillway Type	Best For	Capacity	Construction Effort
Simple notch	Small streams, low floods	Limited	Low
Wide notch	Moderate streams	Moderate	Moderate
Full armored overflow	Large streams, severe floods	High	High

Irrigation Intake Design

The intake is where water leaves the stream and enters your irrigation channel. Its position and design determine how much water you can divert and how much sediment enters your system.

Intake Placement

Position the intake on the inside of a stream bend if possible — sediment deposits on the inside of bends (the point bar) while cleaner water flows along the outside. If the stream is straight, place the intake at least 2 meters upstream from the dam face.

The intake opening should be:

• Above the streambed (at least 15-20 cm) to avoid drawing in bed sediment
• Below the dam crest to ensure water enters the channel before overflowing the spillway
• Fitted with a gate to control or shut off flow

Sediment Exclusion

Even with careful intake placement, sediment will enter your channel. Install a sluice gate on the dam near the intake — a gate at streambed level that can be opened periodically to flush accumulated sediment downstream. Open the sluice for 15-30 minutes weekly during irrigation season.

Trash Rack

Place a grid of sticks or poles (spaced 3-5 cm apart) across the intake opening to catch floating debris. Clean the rack daily during leaf fall and after storms. A clogged trash rack reduces your water supply; a missing trash rack lets debris clog your channels.

Construction Sequence

For any dam type, follow this general sequence:

1. Build during low water: Late summer or dry season is ideal. Some communities temporarily divert the stream around the construction site using a small side channel.

2. Prepare the foundation: Clear loose material from the streambed where the dam will sit. You need to reach firm material — gravel, clay, or bedrock.

3. Key into the banks: Dig trenches into both banks for the wing walls before starting the main dam structure. If you build the dam first and add wing walls later, the connection will be weak.

4. Build from the banks inward: Start construction at both banks simultaneously, working toward the center. The center section — where you close the dam — is the most difficult because the full stream flow concentrates there. Close quickly with pre-positioned materials.

5. Install the intake and gates: While the dam is under construction, build the intake structure and first section of your irrigation channel. Test them before the dam is complete.

6. Seal and fine-tune: Once the dam is closed, pack clay or sod on the upstream face. Adjust the spillway height to achieve your desired upstream water level.

Seasonal Maintenance

Task	When	Purpose
Inspect dam structure	Weekly	Catch erosion or displacement early
Clean trash rack	Daily to weekly	Maintain intake flow
Flush sluice gate	Weekly	Remove accumulated sediment
Repair minor damage	As needed	Prevent small problems from growing
Full inspection after floods	After every significant flood	Assess and repair damage
Annual rehabilitation	Before irrigation season	Rebuild or reinforce as needed
Remove seasonal debris	Spring	Clear branches, leaves, ice damage

Safety Considerations

Dam Failures and Downstream Risk

Even small diversion dams hold water that, if released suddenly, can damage downstream property or endanger people. Never build a dam that impounds more than 2-3 meters of water depth without professional engineering. Ensure people downstream know about your dam. If you notice cracking, unusual seepage through the dam body (as opposed to over the spillway), or bank erosion around the wing walls, lower the water level immediately by opening all gates and investigate before the dam fails uncontrollably.

Avoid working in flowing water whenever possible — even shallow, slow-moving streams can knock people down, and cold water causes rapid fatigue. Work in pairs, wear sturdy footwear, and never work in a stream during rising water.

Summary

Diversion dams raise stream levels to redirect water into irrigation channels. Choose from brush dams (simple, annual rebuild), rock dams (durable, 5-20 years), or log crib check dams depending on available materials and stream size. Every dam requires wing walls keyed into both banks, a spillway for flood flows, and a properly positioned intake with sediment control. Build during low water, key into banks first, and close the center last. Maintain weekly and inspect after every flood. Keep dam heights modest (under 2 meters) for safety.