Manning's N - Farm Fields

[ilopran]

I'm working on a HEC-RAS study for a new subdivision. On either side of a good portion of the stream there are farm fields as soon as you get into the floodplain. Trying to determine an appropriate Manning's n to use for the floodplains in the area; it appears I need to determine if the crops would be considered "row crops" or "field crops" (using Manning's values from Chow, 1959). Based on some quick searching it seems these would be considered "row crops" considering you can see rows in aerial images over the last few years (also, based on site visits the fields this year were all either corn or soybeans).

Anybody have some knowledge on these two classifications? I can't really find anything defining them or referring to "field crops" outside of the agricultural finance world, which I believe is a different definition than I would be looking for relative to hydraulics.

 

[Steve S.]

My first question if a subdivision was in the watershed area for row crops would be how are the fields irrigated and drained? (especially is there a drainage ditch and what is the capacity?) You are probably already looking at that, but I think that is far and away a more influential detail than mannings n of the field itself. If the row crop field really does drain into/through whatever storm system you design, that could really complicate any pond design because of nitrogen.

Maybe choose both mannings "n" and run your model twice. Depending on field size and steepness, you may find the difference between (.025-0.045) (mature row crop) and (.030-.050) (Mature field crop) isn't really much of a difference.

According to Montana hydraulics manual chapter 9 (free online) row crops are things like sugar beets and corn. Field crops would be things like wheat and barley. It makes sense that row crops would have a lower n as the area between rows are shallow channels- indeed most sugar beets and corn in my area are flood irrigated. Chow references Woody L Cowan: Estimating hydraulic roughness coefficients, Agricultural Engineering, vol 37, no7 pp473-475, maybe you can find it. I think most of the agricultural roughness information came from SCS (NRCS) and they have a lot of information.

For me the struggle is not so much picking a number now, but choosing a number that would likely coincide with the design storm over the design period. Chow's range of numbers are for mature field and row crops, so that may not be the most realistic n for the field, unless you have a good idea your design storm is likely to coincide with mature crops

For the limited hydrology work I have done, it made sense to follow the procedure of department of transportation which provides guidance developing a model since you won't simply be able to choose values from a table and circle the answer (MDT requires at least 3 different models- ideally the "best" model would be calibrated against a known storm, if information is available). Based on my limited experience, the field roughness did not have a huge effect on time of concentration because of watershed topography. Your case may be different, though.

EI

 

[PEinc]

Use the worst case n for row or field crops. You never know what will be planted in the future.

http://www.PeirceEngineering.com

 

[Steve S.]

I'm not sure if there is such a thing as "worst case n." In my limited experience, the biggest impact of roughness in overland flow is in estimating time of concentration, which influences the design storm and timing of peak flow.

choosing a .025 or .05 (the ends of the spectrum) based on one being "conservative" may end up selecting the wrong storm.

detention volumes and hydrologic soil conditions are good opportunities for being conservative, though


EI