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I am involved with a local group trying to better understand the hydrology of the October 2015 flood that impacted South Carolina from roughly Columbia to the coast. Different sources indicate the rain was on the order of a 1,000 event. During computer simulation studies to recreate the flooding and streamflow, modelers have used CN values as low as 65 and place high confidence in runoff volumes and peaks simulated by the models they are using. As FYI, these models use standard CN and compute Ia=0.2S. Said CN values were applied for the entire storm duration.
The following are snippets from one report about the event.
Hourly rainfall rates at the Forest Acres Richland County Emergency Services Gills Creek automated gage (Forest Drive and I-77) recorded 1.76 inches from 2-3:00 a.m., 3.76 inches from 3-4:00 a.m., 3.00 inches from 4-5:00 a.m. and 2.12 inches from 5-6:00 a.m. yielding an unprecedented 10.64 inches over four hours. At noon, the Forest Acres Gills Creek gage rainfall amount had risen to an incredible 15.51 inches for the past twelve hour period. (FYI, I live in the Forest Acres area and across the road from Gills Creek.
Please provide your thoughts/opinions about CN application to this event. Should the CN value be increased during the event as the ground became saturated?
Help me, please.
I have successfully replicated a recorded stage in a reservoir with a CN and a precipitation of about 12 inches as I remember. So I would assume that CN hydrology would work in you case. I used standard NRCS procedures.
Hi Mike and all,
I took a quick look at an 8000 acre region in Lexington/Richland County just below Columbia with Web Soil Survey. There appear to be many A/B soils with some significant C/D soils in the region. Slopes were zero to 2% for the most part. Available (0 to 60 inch) storage ran about 10 inches (the major block of Congaree loam of 2000 acres). Part of this storage was no doubt filled before the event began. The layer K value was roughly 22 inches per day with lots of other higher values. I don’t know the situation below 60 inches but it is probably much less permeable.
With the series of rainfalls you outlined, I don’t believe you experienced complacent/violent behavior but I would speculate that you were losing storage S as the event unfolded. This would in effect result in increasing CN values as you are thinking.
This could be at least a significant footnote for the ongoing revision. Thanks for sharing this.
I agree with Bill… even not knowing the details, like how much runoff (Q) was there?
May not have had the lead-in Complacent limb, but surely approaching Violent response. That is 1:1 with dQ/dP
At the VERY large event we don’t have much experience. Expectations are that it’s a different game. Well past the curvy part of the CN’s P:Q plot.
Plus, if an instrumented research installation encounters one, it often blows out the site and leaves no record.Assuming that it‘s a Curve Number world out there….The 65 CN doesn’t look impossible as a start-of-storm CN. depending on the site, but the rain surely way beyond experience limits.
Averaging the 2 rainfalls comes to about 13 inches. On a CN(20) of 65, that gives P/S=2.4 . which is well into the development of runoff processes.
Don’t know if it’s useful perspective, but in the 46 years of record at Coshocton ws 26020, [with cn=70] the biggest P/S they ever got was 1.1.
Continuing with the CN belief system, the CN 65 and 13 inches of rain gives a contributing area fraction (dQ/dP) of about 90%.
All above assumes Ia/S=0.20
This is also a problem that comes up in PMFs for spillways: P= mean + 15 standard deviations! Really big storms.
Also, there may be as much uncertainty with the hydrographing in the model as with the CNs? In my thoughts they’re as suspect as the CNs. What model was being used?
Hope this was of some help. Let me/us know how it comes out.
One question …
Are you more interested in Q (volume) or q (peak discharge)?
To follow on to Pete’s discussion. Q for CN=65 with 10.64” of P = 6.12” or about 57% of P.
Assuming an AMC III at the start of the storm, CN goes up to 82. Q for CN=82 with 10.64” of P = 8.78 inches or about 83% of P.
Runoff volume will certainly affect peak discharge, but more of the effect on peak is probably seen in the rainfall distribution, the unit hydrograph, and the Tc used in the model.
With the largest volume increments towards the middle to end of the storm, the system is essentially primed before the heavy rains arrive and we can expect larger qs than if the heavy rain occurred during the beginning of the storm.
All of these parts have to play together to get the whole picture.
Are there gages to do some comparison? I’m guessing not, but it would certainly be helpful to have something to calibrate to. High water marks might be a potential.
I hate to pour cold water on your desire to model extreme flood events with curve number (CN) hydrology, but I have to ask the question: Why are you using this method?
CN hydrology was invented when we were using slide rules to calculate flood events. I still own a slide rule (I have been doing flood studies since the early 1970s), but I don’t use it any more. This simplified method does not take into account the impact of changing soil moisture conditions on runoff timing, volume, and magnitude. A soil that has a curve number of 65 in average antecedent soil moisture conditions (whatever they are) is not going to act the same during a 100-year or 1000-year event. Trying to guess what the curve number should be and how it changes during large storm events is a pointless activity. Today with computers there are better ways to model hydrology. Specifically, I am talking about continuous simulation hydrology. Continuous simulation hydrology is not new and it is not too complicated. I describe the argument for using continuous simulation hydrology instead of the old, single-event hydrology methods in my paper “Dumbing Down Hydrology” published in the July/August 2016 issue of Stormwater magazine. The paper is also available on my LinkedIn page. Also, I am happy to send a copy of it to anyone who is interested. The bottom line is: today we have better hydrologic modeling tools than CN hydrology. As professionals we should be using them.