Run-off from tall/ residential development 3

[Johnny001]

I am trying to find some guidance with regard to calculating surface water run off from a collection of mult storey buildings. Building regs give guidance on roof pitch but not in relation to the catchment created by the building elevation.

Any guidance would be appreciated.
Many thanks

 

[civilperson]

The run off coefficient is close to 0.98 or 1.0 since the permeability is nil. The intensity should be for a minimum time storm, probably 10 minutes for most cities. Are you planning to store this volume or just retain it to match previous undeveloped state?

 

[gbam]

Wouldn't the land use for this type of situation equate to commercial? Your percent impervious should represent the landuse. If I have the full picture, I typically treat this as a commercial development.

 

[RWF7437]

What are you trying to do ? Design a storm drain collection system ? Size a detention/retention pond ? Size roof drains ? How large in land area is your project ? Who will review and approve your design ? Why is the height of the buildings apparently considered to be a factor of importance ?

Try restating your question and providing more detail. Answers given by others seem to assume too many of these unknowns.

good luck

 

[Johnny001]

Apologies, I have had permission to directly connect all of our drainage, unattenuated to the public sewer (which makes a change!). I am simply sizing and providing flow rates for the development and may have to spec basement pumps where required. My question is should an allowance for the elevation of the buildings be considered - as the elevation which is also exposed to the elements, is greater than the plan area (eg sky scraper). I was once given a formula by an old colleague but with no explanation of its application or parameters. Imagine a square court surrounded by blocks of flats on 3 sides:
Ae=0.5 Sq.root (Aa.Sqaured + Ab.squared-2Aa-Ab.cos.theta)
Aa and Ab being the area of the vertical walls.
I can't find any refrence or guidance in this respect.

Many thanks for your assistance

 

[TerryScan]

Johnny,

I see no relevance of building elevation with respect to total runnoff.

If it rains straight down, your footprint area is the roof area.

If you had rain blowing sideways or...lets say 45 degrees...the roof gets the same...
...and one side of the building gets some rain that reaches the street and is collected by normal street drainage.
...which is offset by the otherside which blocks rain getting to the street.

I'd say "it all comes out in the wash"...it is over complication to consider the building height with respect to storm runoff.

 

[ptmoss]

Actually, I understand the question now (I've never worked in an area with skyscrapers, so it took a while..). If you have no tall buildings, just ground and houses, it really won't matter if the wind is blowing, you'll still effectively get the same volume of rain hitting any particular area on the ground.

However, put a tall building in the way and it will certainly intercept a volume of rainfall based on the angle of rainfall and height of building.

Downstream I can see it all evening out, but those drains immediately adjacent to the buildings, I can see where they could be impacted. Depending on how tall, prevailing winds, and how many other buildings are situated around it, it might be inconsequential or it might be something you should reasonably consider.

And I think it's one more reason I'll just stay out in the country..

 

[cvg]

I think you are overthinking this a bit. It seems that the area of the side of the building catching the rain is fairly small. Probably less than an acre unless we are talking about a really large building. The building also shadows part of the site which would then be dry, so it partly evens out. In the end, you might see a few cfs more peak runoff - if you can believe that the rain is indeed coming at 45 degrees for the entire duration of the storm.

This can easily be accounted for in the C factor - bump it up by a few percent or just add a safety factor to your design.

 

[bltseattle]

Standard civil practice is that building height is not considered in runoff, due to factors mentioned by TerryScan above. I understand that mechanicals take vertical surfaces into account when sizing on-structure drains and building plumbing, but it is typically the civil computation used when discussing net runoff leaving a site.

 

[Johnny001]

many thanks for your advice and comments. As you suggest I may be over complicating it!

Kind Regards

johnny

 

[BRIS]

Interesting question - at the extreme if rain was horizontal none would hit the ground and 100% would hit the building and convert to run off: so the shadow effect theory does not hold up. With a tall building you can capture much more than than estimated from the plan area, but the overall area run off remains the same. The draiange network design is not effected but you may have flooded basements if the local building drainage cannot cope.