A Recommended Procedure

Generally when designing multi-stage outlets, you’ll need a device for each frequency you wish to model. For example, a 2, 10 and 100 year model will most likely need a culvert, orifice and either a secondary weir or a restrictor plate on the culvert. While it is possible to satisfy all frequencies with fewer devices, it’s not very probable.

Step-by-Step Approach

  1. Always start a pond design from the downstream end, working upstream. Do not work by the frequency. Instead add structures starting downstream, working upstream. Add the Culvert first. Ideally it is this device that will control the final outflow. Plus, all other structures (Except an emergency spillway) route through this anyway.
  2. Size the Culvert to meet or exceed the highest of the Target Qs. If the closest culvert size far exceeds the target Q, then you should plan on using secondary structures such as a weir or an orifice to satisfy this upper end. A restrictor plate over the Culvert is also a popular method. Perform a trial route to confirm. Use standard, commercially available sizes.

    Design from downstream to upstream, not by return period
  3. Next, add devices such as an orifice to satisfy the lowest Target Q. Perform trial routes to determine the maximum elevation reached. Use this elevation as the invert for the next structure. You can add a Riser at this point to satisfy the visuals. Just be sure to set its crest elevation above the maximum elevation reached during the Trial Routing in the previous step where you added the Culvert. You can adjust this later.
  4. Next, add a secondary weir or another orifice to satisfy the intermediate frequency(s). Perform trial routes to determine the maximum elevation reached. Use this maximum elevation as the invert elevation for the next structure. Repeat this step for each intermediate frequency. Experiment with v-notch as well as rectangular weirs. We have found more success with rectangular weirs however.
  5. Finally, if you haven’t already done so, add a Riser structure to contain the multi-stage devices. Note that it is not recommended to use a Riser as a controlling device but in some cases this will be the most economical design. First try to use it simply as a container for other devices such as orifices and weirs. Set the Riser crest elevation to just above the maximum elevation reached in Step 4. You could have added the Riser directly after adding the Culvert, but we did not know where to set its Crest elevation. In this case we could have guessed but set it high so that it would not affect outflows.
  6. Add any necessary emergency spillway weirs. These are not multi-stage.
  7. Read this article. How to Design a Detention Pond Design A Step-by-Step Approach

There’s More

Hydrology Studio has developed two sophisticated procedures for designing your outlet structures. They both use the Trial Route functionality. The First method utilizes the Schematic Chart while the second uses primarily the Stage-Discharge Curve.

  1. Design Using the Schematic – This approach allows you to setup the outlet structure right on the Schematic chart. This is a step-by-step example problem using this procedure applied to the Modified Rational method. Our preferred method.
  2. Design Using the Stage-Discharge Curve – This approach, while not as real-life visual as the Schematic method, is just as effective and fast.

 

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