Hi Everybody! I’ve been away since easter and was swamped on my return. Saying that, I should point out that I’m actually swamped right now but I figured this post was too easy to pass over.
I completed a preliminary drainage impact study for a new school recently. I was excited to see the school wanted to install a green roof and I made promises that we could reduce runoff volume and rates enough to satisfy BREEAM criteria through the use of the roof and roof alone on the site.
As a short background into this, I recently looked into the effect of green roofs on runoff using Micro Drainage in two parts: Part 1 and Part 2.
Essentially, BREEAM (BRE Environmental Assessment Method) is a way for scoring how green a new development is. In most cases it will ask that the post development surface water discharge is reduced by 50%. The standards used to be painfully vague but are now much simpler to understand and design for. Enough of the background, let’s get to the school in question.
The school is proposing a ~500m2 extension on an area of hard-standing. Effectively, the development will not be increasing the discharge from the site as it will remain hard-standing post development. This means that any reduction to flows we gain from the green roof will act to reduce the runoff from the site.
The existing drainage system is comprised of small diameter (100-150mm) pipes which drain to the north. The drainage was constructed to fall by gravity to an outfall manhole near the boundary with a depth near 1.3 meters.
In order to simulate the existing and proposed networks a storm drainage model was created using the Micro Drainage WinDes software package. The network was input into the software with a theoretical pipe (2.000) that drains the existing hard play area where the new build is sited. In order to compare the pre to post change due to the green roof this pipe was analyzed as existing hard-standing and as an area of green roof in two different models.
The results from the analysis can be seen below:
As is apparent from the above results, the green roof has a significant on the existing network. It removes approximately 3 cubic meters of discharge from the area and reduces flow from the existing hard-standing zone to a 1/3 in the small return periods and by 1/2 in the extreme event.
The above table demonstrates that the proposed development can achieve a sustainable approach to drainage without the need for additional buried attenuation structures. The calculations showed that the system does suffer from some flooding in extreme events.
I am not certain of what the cost implications of green roofs over buried attenuation to achieve the same drainage results but I think it is safe to say that current precedent does not factor costs into green development! However, this proves that for small extensions placed over existing hard-standing developers can achieve sustainable drainage strategies (and satisfy BREEAM criteria) without the need for buried attenuation.
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