Stormwater runoff coefficient C for grassed landscaped concrete decks

[FinnB]

I am working on a building development in Ireland where I must provide a storm water atenuation tank in order to prevent flooding the Local Authorites drainage system. A flow restrictor at the exit of the tank will control discharge.I am using the Rational Method to access storm water runoff.My problem is I have landscaped roof decks and I am unsure what "C" value to use.The roof deck is concrete slab overlain with 4"(100mm)of free draining gravel which is covered with 8"(200)of soil. Do I really have to take this surface as impervious or can I allow some soakage into the gravel/soil? Taking a C value of 90% as I have read elsewhere seems very conservative. Given I am a structural engineer without specialist knowledge with a fast approaching deadline can anyone provide me with some guidance and possibly a reference. I do not have the time/resources to investigate alternative to the Rational Method). Thank you.

 

[gibfrog]

C for impervious areas is 0.95 not 0.90, especially if you are dealing with storms that require attenuation.
I would not reduce the C for the landscaped roofs, because the I would assume antecedant conditions of maximum soil moisture Clifford H Laubstein
FL Certified PE #58662

 

[KThomson]

I would have to agree with Gibfrog - It's like the idea of a pool. It is not impervious area, there is probably about 6" to a foot of storage. BUT - once that storage is filled, any more rain will run off as if it were concrete.

At the beginning of the storm there will be some storage within the layer of soil, but after saturation, the water has no place to go and will run off with a C about 0.95.

However, if you have outlet control, you can calculate your storage and use it as a detention basin.

I would love to hear if the landscaped roof works - I came across the idea on the web, and I would like to use them in future designs!

Kate

 

[proturf]

I would enjoy discussing some of my experience in working with on-structure (rooftop) gardens and landscapes (including athletic fields). Let me know if anyone is interested in this related topic.

 

[bltseattle]

Most of us have heard of "source controls" for pollutants in runoff, but green roofs and ball fields can also provide "source control" for storm runoff volumes, an idea that is growing under the umbrella of the current sustainability movement in urban civil engineering (e.g. LEED accreditation).

FinnB,
I've been analyzing hydrology of "eco-roofs" consisting of drained soil media on an impervious roof and here are some thoughts on the subject as it pertains to your question:

1. Surface runoff will be dictated by the 8-inch soil layer you mention. Does surface runoff occur? If the soil is permeable enough and the rainfall light enough in intensity then you might not have surface runoff for the storm you are considering. You can compare the peak rainfall intensity to the saturated infiltration rate as a first check. If it is a well drained soil such as sand you might not get any surface runoff, instead you might get seepage flow.

2. It sounds like you are using the Rational method to estimate the peak runoff rate. In this case, if step 1 showed that runoff occurs, use a C factor compatible with a similar surface - sandy grass might be C=0.3, for example - when computing peak flow rates.

3. If step 1 showed that runoff percolates through the rooftop soil horizon, then you have to consider the timing of peak flows on the overall catchment draining to your detention system. Chances are, runoff from conventional roof surfaces are going to reach your tank ahead of the peak seepage flow from the green roof. A time series analysis is most helpful here because the Rational Method is not robust enough to quantify the delay in peak from the green roof.

4. If you are trying to establish the runoff volume (instead of the peak rate), then using the C of 0.90 or 0.95 is appropriate for a saturated green roof. Knowlege of your climate patterns is important here. If you are analyzing a summer thunder storm you could expect that the rooftop soil is sort of dry and could absorb 10-20% (C=0.8)of it's volume in moisture content, depending on the media specifications (up to say 1.6 inches of rainfall for an 8-inch layer, starting dry). If it is winter back-to-back storms, you may not get any absorbtion but would still have the benefit of attenuated peak flows due to the time it takes for water to percolate through and flow laterally.

I've been working on a spreadsheet based hydrology model for just this kind of site, although it is based on a time series hydrograph analysis rather than the Rational method, that I will be presenting at the 2004 Green Roof Conference in Portland, Oregon, in June.

Given that background, here is my recommendation for your application and short timeframe:

Use the higher C value say 0.95 since the soil will likely be saturated for infrequent design storms. However, when you use the travel time to compute the peak rain intensity so you can apply the Rational Method, use the percolation time for the green roof areas. This could be from say 60 min to 240 min, corresponding to 8 inches depth at 8 inch/hr (sand) to 2 in/hr (silty loam) depending on the soil type. I also recommend the check from #1, above.

Good luck - I hope this helps!

Proturf -
I am very interested in your insights. I've found that there is no simple analysis to estimate run-"through" from eco-roofs and athletic fields consisting of freely drained media. I've worked out a method to address percolation time and storage within the media voids. The really challenging part is figuring out the delay and discharge relationship as flow collects laterally into underdrain. I have heard anecdotaly that ball fields produce very little runoff and attenuate flows greatly.

 

[proturf]

bltseattle:

Typically intake rates and storage account for most precip duration/intensity. Lateral flow is controlled by the same gradients as vertical flow.... dependent upon slope, head, total gradient..... and thus the discharge relationship is purely a function of drainage spacing. These relationships are also influenced by whether the pourous media (rootzone) is free-draining to drainage pipe or if there is a gravel under layer that impedes flow to drainage pipes (except under saturated flow of positive pressure gradients). These 2 system types are the basic diffences between media designs that we would do for on-sturcture systems as opposed to detention/retention structures.