HEALTH AND SANITATION
23
Rainwater harvesting:
overflow provisions
On occasion, the volume of rainwater collected from the
roof catchment will exceed the storage capacity of the
rainwater storage tank, causing the tank to overflow.
By Water Research Commission
If overflow handling provisions are not in place,
excess rainwater will back up rainwater conveyance
and top-up drainage piping, until it reaches a point
from which it can most easily discharge/overflow. APPLICABLE CODES, STANDARDS
AND GUIDELINES SELECTED PROVISIONS AND DESIGN
AND INSTALLATION IMPLICATIONS
SANS 10400-R: 1990
The application of the National Building
Regulations Part R: Stormwater disposal RR4 Access to stormwater drains
RR5 Connection to stormwater sewer
RR6 Use of street surface drainage system
This may be at the downspout-to-conveyance drainage
pipe transition, or at less ideal locations like the access
opening of the tank, or at the air gap of a top-up system.
Overflows at these points may damage the rainwater
tank itself, or do water damage to a building’s exterior
or interior. SANS 10400-P: 2010
The application of the National Building
Regulations Part P: Drainage 4.10 Discharges from swimming baths,
swimming pools, fountains or reservoirs
4.14 Sizing of discharge pipes
4.15 Sizing of drains
SANS 10252-2: 1993
Water supply and drainage for buildings
Part 2: Drainage installations for buildings 6.3.1 Hydraulic load
6.3.2 Sizing of drains
All tanks, whether underground or above ground require
some form of overflow. For underground tanks, the
overflow is typically piped underground, carrying overflow
water to a lower elevation. Above ground tanks can
have the overflow above or below ground. Both overflow
systems should be designed using only gravity to carry
the water away from the tank location.
Table 1 Standards and codes applicable to catchment and
conveyance network
OVERFLOW DISCHARGE LOCATIONS
The purpose of the overflow system is to handle
excessive rainwater flows, directing them away from the
rainwater storage tank to a suitable location. Overflow
volumes can be directed to grade, a storm sewer, or even
an onsite soakaway pit. In each case, rainwater can be
conveyed via gravity flow or by pumping.
The size of an overflow pipe is determined by the size of
the inlet pipe. The overflow pipe size must be at least as
large as the inlet to ensure maximum flow discharge. The
water carried out through the overflow typically will not
require any type of filtration unless discharge
regulations mandate.
APPLICABLE STANDARDS AND GUIDELINES FOR
OVERFLOW DISCHARGE
The applicable standards found in the South African
national standards are listed in Table 1.
www.plumbingafrica.co.za
Due to the consequences of not properly handling
excessive volumes of rainwater, it is important that the
RWH system includes sufficient overflow provisions.
The design of overflow systems involves deciding where
excess volumes of rain can be appropriately discharged,
and how to convey these overflow volumes from the
storage tank to the point of discharge.
A cutaway of a typical swale when completed.
July 2019 Volume 25 I Number 5