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Check Dams

Modeling Check Dams

What are Check-Dams?

Check-dams are partial blockages constructed across a channel, commonly constructed of broken rock.  They can be used to reduce the flow velocity (in order to reduce erosion) or to create pooling (to enhance water quality.)

How are check-dams modeled?

The best approach to modeling check dams will depend on the prevailing flow regime within the channel:

bulletIf the flow depth is large in relation to the height of the check-dams, you may be able to use a reach routing with an appropriate Manning's value.  For details read about reach routing.
 
bulletFor low-flow conditions, in which a level pool is formed behind each check-dam, you can use a series of ponds to represent the check-dams.  Details on this option are given below.

Modeling check-dams as ponds

Like any pond, you need to specify storage and discharge characteristics for each check-dam:

bulletStorage above the dam can be defined with whatever storage option is most applicable to the check-dam.  In many cases, custom stage-data will be the best solution, allowing you to specify a surface area at each contour elevation.  This elevation range should start at the inside base of the check-dam (zero area), and extend up to some elevation above the top of the check-dam.  It's important that the data extend above the dam, so that the exact storage can be calculated when water is flowing over the dam, otherwise a storage exceeded warning will occur.  When defining surface areas above the top of the check-dam, include only the area upstream of the dam, representing the storage volume that is controlled by the check-dam.
 
bulletFlow over the check-dam can be defined with one of the standard outlet devices, such as a sharp-crested rectangular or trapezoidal weir.  (Although a broad-crested weir may provide greater accuracy in some applications, the difference in routing results may not justify the additional complexity.)
 
bulletIf there is significant seepage through the check-dam, this can be modeled as an additional outlet device.  Depending on the expected behavior of the seepage, you may be able to use an exfiltration outlet, or perhaps an orifice array.  In any case, be sure the seepage device is routed to Primary (not discarded!) so that its discharge will be recombined with the weir flow for further routing.  For a detailed discussion of flow through rock fill see Design Hydrology and Sedimentology for Small Catchments pages 151-155.
 

Setting-up multiple check-dams

Before creating the ponds for additional check-dams, check the routing results for a single check-dam to make sure you're satisfied with the results.  In most cases, you should start with the first (upstream) check-dam.  You can then create clones of the first pond without having to re-enter all the data!

To create the first clone, drag the existing pond while holding down the Ctrl key.  This sequence will create a second check-dam that is an exact duplicate of the selected node, but with a new node number.  With HydroCAD 8.0 (and later) you can also also Copy the node to the clipboard and use Paste to create each clone.

To set the correct (lower) elevation for the cloned check-dam, right-click the new pond and select Raise/Lower from the menu.  Enter a negative value indicating the elevation difference from the first to second check-dam.  This will lower all the elevation values within the pond by the specified amount.

Finally, drag the primary outflow handle from the first pond to the clone, so that the flow will be routed to the second check-dam.

Repeat this process for each subsequent check-dam as required.

Handling tailwater effects

HydroCAD will issue a warning message if a potential tailwater dependency is detected between the ponds.  If this occurs, you may need to select a tailwater-sensitive routing procedure, such as the Dynamic Storage-Indication method.  In the absence of a tailwater warning, you should be able to use the default Storage-Indication procedure for pond routing.

Assessing the results

As always, remember to use the simplest model that will address your specific objectives.  Although check-dams can be modeled in great detail, this may or may not have a significant effect on the overall results of your model.  If in doubt, do a trial study to determine the effects and plan your model accordingly.  Although properly constructed check-dams can be useful in controlling erosion, their effect on hydrograph routing may be minimal for all but the smallest rainfall events.  For further details please see the general modeling tips.

 

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