Measuring Surface Infiltration/Runoff?

[cfj104]

Does anyone have any experience trying to estimate surface infiltration? Ultimately, I would like to be able to look at the infiltration rate and relate that number to runoff, but wanted to see what luck people have had doing this?

The main problem that I think I will run into is preventing the water from escaping out the sides of the infiltration test area since it may be difficult to get a good seal.

Any advice/suggestions?

 

[gbam]

Perk test? Double Ring Infiltometer (SP?) has there been studies in your area that you can fall back on?

 

[brandoncdg]

Very curious as the kind of information you find out from here.

Civil Development Group, LLC
Los Angeles Civil Engineering specializing in Hillside Grading

http://www.civildevelopmentgroup.com

http://www.civildevelopmentgroup.com/blog

 

[cfj104]

I am actually trying to figure out what runoff does on a gravel lot. Typically, I model the surface with a CN of 85 (depending on soil type) but we are trying to figure out if these numbers are too high. Due to the gravel, we are concerned about getting a good seal with a double ring infiltrometer, but that would be my preferred method.

I will post back with what we find out.

 

[gbam]

How thick is the gravel? Can it be ignored? If so, move the gravel and evaluate the soil below. Otherwise you will have a storage affect from the porosity of the gravel. You can evaluate the average velocity through the gravel by doiing some simple tests like a constant or falling head column test. Its been a while since I was in school, but I remember doing similar tests in the lab.

 

[cvg]

if you have a compacted, well graded aggregate base material - you may get very little storage or percolation. For an open or gap graded material, you could get quite a lot of storage, but then it depends on how thick the gravel layer is and how well the underlying soil percolates. A layer of geotextile underneath will also affect the percolation rate. With heavy traffic loads and frequent or infrequent re-grading of the road or parking lot, the percolation conditions can change, usually getting more impermeable with time and traffic loads. In general, for hydrologic studies in support of drainage design, a conservative estimate is used and that is where the 85 comes from. Good luck trying to come up with a better number.

 

[gibfrog]

1) How thick is the gravel? Depending on the grading/Local Authority Having Jurisdiction, I assume a 30% - 40% voids volume. Once thes voids fill, the additional rainfall either percolates or runs off, depending on the underlaying soil & ammount of rainfall.

2) As CGV writes, compaction kills percolation. (Also as times goes on, fines will collect in the gravel and start to plug the underlaying soil.)

3) Consider adjusting your initial abstraction volume (inches of rainfall) based upon your gravel depth + your underlaying soil. While 4" of gravel & 20" of gravel, both present a gravel surface for your CN, the initial abstraction is very different.

4) You may adjust you underlaying soil CN based upon available soil storage above the seasonal high ground water. There is a correlation between depth to the SHGW and soil type. You can always get a NRCS soil technician to verify the on-site soils.

5) Infiltration rate is not directly related to soils type. We have many B/D soils with nice, >6"/hr DRI percolation rates but when the water table rises or mounding occurs, the percolation rate is dramatically reduced. I recently had a project with a 12"/hour DRI percolation test and a <0.12"/hr infiltration rate when groundwater mounding occurs.

6) Consider modeling the gravel as a shallow pond. Run a groundwater mounding analysis to see how it will percolate.

http://onlinemanuals.txdot.gov/txdotmanuals/hyd/nrcs_runoff_curve_number_methods.htm

Streets and roads:
A B C D
Paved; curbs and storm drains (excluding right-of-way)
98 98 98 98
Paved; open ditches (including right-of-way)
83 89 92 93
Gravel (including right-of-way)
76 85 89 91
Dirt (including right-of-way)
72 82 87 89



Clifford H Laubstein
FL PE 58662

 

[beej67]

What region of the country are you in, cfj104, and what are the soils like there?

Since you appear to be working with TR-55, Mr. Laubstein's approach of adjusting the initial abstraction in your model to account for the voids is probably the cleanest, and easiest to justify to other engineers or reviewers. To some degree, it was put in the model to handle just such a situation.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com