I hope this helps. Note that only a small fraction of this involves anything that 3d can automate. I don't know what level of experience you are starting with. I can break any of these steps down or clarify on whatever step you are stuck at. Will expand more on flow rate when I get back. I will need to know more about your local conditions and requirements. Local regulations require different methods of calculation. Read local ordinance and DEP requirments.
Most of rational is based on local rainfall and soil conditions. Time of concentration is lay of land, and vegetative cover. Do you have SCS County Soil manuals or equivalent where you are?
1. Ensure water can get from point A to Point B. - Create alignment and profile along flow path to check that water flows downhill with hopefully at least a 1% slope. Swales can go down to 1/2% but start getting bigger rapidly and start to become swamps.
(Don't laugh or roll your eyes. I see a lot of designs where I swear the engineer doesn't understand the concept that water flows downhill.)
At least in PA, slopes under 6% can receive credit as BMP's for NPDES permits if you deepen them an extra 6 inches, have a vegetative lining, and place check dams along them. Space the check dams for the six inch puddles.
Slopes > 10% require additional shear stress checks and usually more expensive, stronger linings.
If you can, try to keep the alignment flowing along the contours to obtain these goals.
2. Determine area of other means of flow rate. - The catchment functions only work for sumps, so will not be helpful for swales. (The one time I tried, it didn't work very well for sumps either. I had a 20 foot deep hole & it only caught the 6" puddles within.) You will need to do this the old fashoined way. Start at the bottom of the swale and draw poly lines from the bottom perpendicular to the contour lines until you reach the top. The surface water drop function works well to check this. (I have only done in LDD. You will have to play around with the 3d version.)
Go to the top of the swale and do the same thing. At some point, the lines should cross or you will be able to see a ridge to follow on the top of a hill. If this starts to become a large area, break into smaller areas where the slopes change or other reasonable places to change geometry.
In LDD, I will often create closed poly lines and use the parcell function to label. It also provides a backup in case the lines or labels are accidently erased or lost. (I also got tired of drafters or me losing my redlines.)
3. PADEP allows swale design based on cfs / acre or use the outflow values from storm pipe if you have that. Otherwise, I can step you through the rational method for determining flow rate. Know what year design storm is required.
4. Determine Cross sectional geometry - PADEP E&S manual has several criteria.
Know what equipment contractor prefers if possible. Some contractors prefer V shaped swales. I normally use a 2 ft bottom minimal width as that is usually a standard backhoe bucket width.
PADEP has a 12:1 requirement for bottom width to depth. If the bottom width is greater than 12 times the flow depth, you don't get uniform flow and the flow splits and meanders. For very small flow rates, you often don't have any other choice.
Hydraulically, keep the depth at 2/3 of the bottom width or below if possible.
Side slopes should be 2 hor to 1 vertical min. 3:1 are prefered by DEP for NPDES permits.
Try to keep flow depths under 2 feet or 1 foot to minimize stress to the linings.
Swales are sized with the Manning's equation. I have this programming into spreadsheets and would need to look up. I don't have here at home. Shear stress is based on slope and flow depth.
5. Determine lining. Rip Rap stone sizes vary by area. Local regulatory agencies usually have charts of which lining can withstand what velocity and shear stress.
The TRM manufacturer's often provide software that will calculate all of this and provide the appropriate lining. They usually take a great deal of paper for the printouts however. You will need to check both the vegetated and none vegetated conditions. Vegetation runs from A-E depending on height - D = lawn - B = roadside swales that are not mowed.
Synthetic Industries has a good program that I think can now be downloaded from their web site. (Standard answer for contractors - I only specify Pyramat if nothing else works except grouted rip rap or concrete. Yes, it is expensive.)
North American Green has their own software as well.
All of them should have documentation of which linings are temporary, only until the grass grows. Permanent ling would be grass only.
Permanent are plastics that reinforce the grass.
6. I have standard swale assemblies that adjust for cut and fill that I started in 2009. I haven't updated since then, but they should still work.
Ann Wingert, P.E.
![Autodesk Design & Make Report](https://forums.autodesk.com/t5/image/serverpage/image-id/1350519iCB55845EBFC43016/image-size/large?v=v2&px=999" "Autodesk Design & Make Report")