Drains software- Australia-General Hydrology

[Allen1362]

Hi,
I have 2 questions about drains software used in AUS for designing storm water drainage system. My first question is more general and the second one is related to software.
1- which intensity duration do we have to use for our design? Is it time of concentration? If so, how should I calculate it?
As I remember, in uni we added together property drainage time, over land flow time and gutter flow time for different components of each inlet flow and then chose the longest one. But the problem is, I need intensity first to calculate the over land flow time (T= 6.94*(L*n)^0.6/(I^0.4*S^0.3). How can I fix it?

2- This question is also related to the time of concentration. On sub catchment data sheet (DRAINS software) what we have to choose for ADDITIONAL TIME section? Is it time of concentration? The drains manual does not explain this parts so clear or i can not understand it!!!

Cheers.

 

[pinkpig]

not familiar with Drains software but here are my 2 cents.

1. time of concentration is the water drop travel time of the longest hydraulic path in your watershed. Note it is longest hydraulic path, not necessarily the longest physical travel path, though most of the time, they are the same;

2. for calculating T which requires I, you can assume a I ,then to see if the calculated T conforms with I. Do the iterations then. Or you can use SCS method, which requires a 2-yr 24hr rainfall depth. google TR-55 manual you will get the formula.

 

[Allen1362]

Thanks a lot pinkpig!

 

[CivilEngAus]

DRAINS is a good software to know and I generally find the PDF manuals to be quite useful.

1. It depends on the site and the size of the catchment. If you are modelling small urban catchments (subdivision work etc) then you can simply assume Tc. Typical values of Tc are 5 minutes for impervious and 10 minutes for pervious. Yes, the actual values won't be exactly these although these are a decent approximation and you will find that a lot of Council design manuals recommended to use these [some won't accept a Tc of less than 5 minutes].

2. Do not use this. Additional time/lag time as it is called in newer versions of the software is exactly that; a lag time applied to the catchment. E.g. if you put in an additional time of 60 minutes then the time for the storm runoff to reach the node will be the Tc + the additional time; generally this would not be desired. Additional time should be set to 0 for general use.

One thing to be wary of with DRAINS is that the model is quite sensitive. If you make a small change to the model then you can sometimes get quite fluctuating results.

 

[NicGreen]

Two things i have found really helpful in regards to this type of thing is the Australian Rainfall & Runoff Guide on these matters.

In regards to your Intensity question. It is depended on a number of things, most importantly what scenario you are trying to model. If its a small subdivision in an urban area. The Rational Formula and the intensity method illustrated by others may be applicable.

In rural area where water flow over a longer reach becomes concentrated, sheet flow equations or methods such as those involving those presumably taught to you at uni may need to switched out for other methods. Neither the less the ARR book highlights which circumstances require which analysis method and some formulas and examples for each.

In terms of drainage software. EPA SWMM is also a good program. I find it a little more intuitive then drains and easier to use. Although it uses percentage impervious over Coefficient of runoff values (which essentially is just your imperviousness, infiltration, evapotranspiration, depression storage, etc all rolled into one estimate)