Flashboard Riser Inlet Structures

Our Flashboard Riser Inlet Structures are perfect for larger ponds and impoundment drains where significant water flow must be accommodated.   These are also known as dropboard risers, weirs or monks. 

Designed For HDPE Pipe   These flashboard inlet boxes are designed to work with double-wall corrugated high density polyethylene drain tiles from 8” to 30” in diameter.   

Corrosion Resistant   Our standard product line is divided into two offerings. 

One-piece heavy-gauge aluminum structures for smaller diameter pipes and limited height.    We offer optional powder coating when additional corrosion protection or visibility are needed. 

Heavy-duty steel structures for larger diameter pipes and tall structures.   These steel structures feature a two-step powder coating process with a corrosion resistant zinc primer and tough top coat that is resistant to abrasion, impact and UV light.  

Standard Lumber Flashboards   Our units are designed to accept standard “2 by” dimensional lumber as flashboards.    

Modular Design  Our heavy-duty steel flashboard risers are modular.  A 36-inch tall bottom section attaches to the pipe and holds up to 36 inches of flash boards.   18-inch tall top boxes can be added to achieve any height desired up to 10 feet.

Trash Guards   We offer optional trash guards to keep debris out of the inlet box.  Floating debris will clog your inlet structure.  Beavers are smart and will immediately recognize the front of your inlet as prime real estate for their dam.  They will push sticks and mud over the flashboards into the inlet box and drain pipe.   Debris is very difficult to remove once it is in the box and pipe.

Important Considerations 

Sealing    Wooden flashboards made from ordinary lumber do not provide a perfect seal.  They will leak a little bit when first installed.  They will naturally swell and seal up with mud, algae and debris in a short time.   The small amount of leakage is not a problem when ample water flow is available.   If necessary, an immediate watertight  seal can be achieved by using the widest possible flashboards and placing sealing material between the flashboards.   

Water pressure can generate surprisingly large forces on the lower flashboards and the structure in general.    These forces multiply with the width and height of the structure.   Lower flashboards in a tall structure will experience higher pressure and demonstrate a greater propensity to leak.  The lower flashboards can be difficult to remove on wide or tall structures.  

We offer optional flashboards that resist swelling and have integral seals and handles. 

We offer optional weighted top boards to prevent wooden flashboards from floating.   These are called "sinker" boards.   Sinker boards are made of steel tubing filled with concrete. A sinker board can be installed on top of regular wooden boards to prevent them from floating.   A permanent sinker board is often used as the bottom board to maintain the bottom seal.  

Floating and Leaning are the design factors that led us to split the standard product line between one-piece aluminum and heavy-duty modular structures.

Leaning   The weight of soil behind tall structures can push on the back causing the structure to lean forward.  

Floating     Single-sided box-type flashboard inlet structures want to “float” when the water is blocked and the inlet box is dry.    Water weighs 62.4 pounds per cubic foot so the floatation force is equal to the cubic feet of water displaced by the box times 62.4.

All structures will resist floating to some degree through a combination of their weight and soil trapped against the sides and the pipe flange. Floating becomes a significant problem on large and tall boxes where the floatation force can reach thousands of pounds.   

Additional ballast can be added to counteract the floating force.  The density of these materials varies with moisture content and the degree of compaction.   Conservative estimates of does it for some likely ballast materials are:

Gravel                              89   lb/cuft

Sand                                90   lb/cuft

Compacted clay soil     95   lb/cuft

Aluminum                       172  lb/cuft

Concrete                        140  lb/cuft

Steel                               490 lb/cuft

Multiply the Width x Depth x Height of the inlet box to get the cubic feet of water displaced.  Multiply this by 62.4 to get the floating force.   Divide the floating force by the density of the ballast to get the minimum number of cubic feet required.

There are optional methods to secure ballast to  the inlet box:

  • The bottom section can be fastened to a user-supplied concrete footing pad with lag bolts.   The volume of the footing pad should be roughly equal to the volume of the inlet box. 

  • We offer optional frames that can be added to the inlet structure that utilize soil behind the structure to secure it.  These frames prevent both floating and leaning.   The frames also provide structural support for the pipe and coupling.  These frames are fitted with anchor points that align with pre-cut holes in the inlet structure.   Backfilled soil or gravel serves as the ballast.  We offer optional skirts for the frames to stabilize the backfilled soil.  

Safety    Large pipes and taller inlet structures can present safety risks.  High head pressure combined with large water flow can create significant suction forces that could pull people or animals into the drain.   Use appropriate fencing and safety grates to prevent accidental access.  

Installation is Straightforward

Our standard units are designed to be installed with simple tools.   No concrete or welding are required.

Prepare the area in front of the drain tile to receive the bottom section. The bottom surface will sit four inches below the bottom of the inside surface of the drain tile.   If the bottom section will sit on soil or a user-supplied pad, ensure it is flat and level, 4 inches below the bottom of the inner surface of the pipe.     If the optional footing form is used, prepare the area in front of the pipe following the instructions provided for that unit.

Slip the skirt on the bottom section into the inside of the plastic drain tile.   Check the fitment to the base and adjust as required.  

Drive the supplied rebar earth anchors through the base to secure the unit.   If an optional user supplied pad is used, lag the base to the pad using concrete lag bolts.  If the optional footing unit is used, bolt the unit to the corresponding anchor points.

Place the supplied self-drilling screws into the supplied brackets and drive them through the pipe into the inner steel skirt using a power driver.   For reference, the screws are Elco Dril-Flex ® 1/4-inch diameter with a 1/2-inch hex washer head.   

If top boxes are used to increase the height, install the box with the supplied hardware following the instructions for that unit.

Place the flashboards into the box

Backfill as desired ensuring that large rocks or clumps of earth do not damage the pipe or box.

Ditch Dam Flashboard Structure

Our "ditch dam" flashboard risers are flow-through channels designed to be installed in a ditch.   They are perfect for providing a couple feet of adjustment for pond levels or diverting water between ditches.  They are also useful as a "check dam" to slow water flow in a ditch and catch sediments.  Check dams are commonly used in desert areas to increase the rate of water absorption into the soil by slowing the water flow and allowing it to soak in.   Our ditch dams do not attach to a pipe.   The units are made from heavy-gage powder coated steel and come in a range of widths and heights.  Installation is simple.  Just place them in the ditch, tamp them down and backfill along the sides.