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the 1 in 100 flow rate - for example this opening size until the 1 in 1 year event is
may be 350mm for a permeable attenuated to the 1 in 1 discharge rate and
pavement or up to 600mm for basins. make a note of the resulting maximum
storage depth.
4. Make a note of the calculated opening
size to achieve the 1 in 100 flow rate at 8. Re-run the calculations for the 1 in 100
this storage depth. year event based on the changed Detailed Design
opening. The maximum flow rate will now
5. Based on the same storage component be below the allowable discharge rate
design and flow control opening, calculate resulting in more storage than is
how a 1 in 1 year rainfall event will behave necessary. To overcome this, a second
– make a note of the maximum storage opening may be placed above the 1 in 1
depth and maximum flow rate. Note that storage depth noted in step 7. Add a
the volume and therefore driving head will second opening so that it’s lower most
be significantly smaller for the 1 in 1 year point (invert) is at or above the 1 in 1
rainfall event and therefore the flow rate storage depth and recalculate the storage
through the orifice will be significantly behavior in a 1 in 100 event. Adjust the
lower. opening size and height above the 1 in 100
6. If the calculated maximum flow is less storage depth until the 1 in 100 flow rate is
that the 1 in 1 year control rate then the achieved at the maximum storage depth
opening does not need changing. for the 1 in 100 event.
7. If the calculated maximum flow for the 1 in
1 event is larger than the 1 in 1 year control Design Notes:
rate then reduce the opening size and
recalculate based on the 1 in 1 event being Both the 1 in 1 and 1 in 100 discharge
mindful that the 1 in 100 year scenario will rates can be achieved by any
have to be reconsidered. Amend the combination of the following: 100
■ Adjusting the depth of each defined
storage tier by adjusting the area and
therefore volume of each tier
■ Incorporating one or more additional
required 1 in 100 flow rate
openings
Other options can be explored where
there is difficulty in matching outflow
flow rates derived by area drained rates for both the 1 in 1 year and 1 in 100
and respective growth curve year flows:
■ Try different types of openings such
as rectangular and v-notch weirs.
■ Store for a different return period – it
is not necessary to store for the 1 in
100 year return period in every sub-
required 1 in 1 flow rate catchment. The final discharge from
the site must meet requirements of
NSTS.
Flow
The Royal Borough of Kensington & Chelsea SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates
