HEALTH AND SANITATION
• Infiltration trench
• Infiltration basin
For RWH systems with an aboveground tank, two methods apply:
• Discharge to grade via gravity flow – this method applies when
rainwater overflows can be directed to grade via gravity flow.
• Discharge to storm sewer via gravity flow – this method applies
when rainwater overflows can be discharged into a storm sewer
via gravity flow, although the tank cannot be directly connected to
the sewer.
OVERFLOW DISCHARGE
LOCATIONS/METHODS
25
Infiltration practices use temporary surface or underground storage
to allow incoming stormwater runoff to exfiltrate into underlying soils.
Runoff first passes through multiple pre-treatment mechanisms to trap
sediment and organic matter before it reaches the practice.
ADVANTAGES DISADVANTAGES
Discharge to grade via
gravity flow • Simplest method to design, install and
operate.
• Low probability of rainwater backing
up the overflow drainage piping. • If discharge location not prepared properly, may cause soil erosion at site.
• May pose a nuisance/safety issue if discharging large volumes from big catchment
surfaces.
• Overflow drainage piping may freeze if large sections are above the frost penetration
depth; ice may build up at the point of discharge if not designed properly.
Discharge to storm sewer
via gravity flow • Ideal for below-ground tanks as storm
sewers are also located below grade.
• Storm sewers are specifically designed
to collect roof runoff and direct it to an
appropriate location off-site. • Design must prevent backflow from storm sewer into rainwater tank.
• Stormwater discharges can have negative environmental impacts on receiving water
bodies.
Table 2 Comparison of the advantages and disadvantages associated
with overflow discharge locations/methods
OVERFLOW TO BIORETENTION
Bioretention systems store stormwater runoff and pass it through a
filter bed of engineered soil media composed of sand, soil, and organic
matter. Filtered runoff may be collected and returned to the conveyance
system, or allowed to infiltrate into the soil.
Design variants include: traditional bioretention, streetscape bioretention,
engineered tree pits, stormwater planters and residential rain gardens.
Bioretention systems are typically not designed to provide stormwater
detention of larger storms (e.g., two-year, 15-year), but they may be
in some circumstances. Bioretention practices are generally combined
with a separate facility to provide those controls.
To prevent possible groundwater contamination, infiltration must not be
used at sites designated as stormwater hotspots.
OVERFLOW TO GRASS CHANNELS OR DRY SWALES
Grass channels
Grass channels can provide a modest amount of runoff filtering and
volume attenuation within the stormwater conveyance system resulting
in the delivery of less runoff and pollutants than a traditional system of
curb and gutter, storm drain inlets, and pipes. The performance of grass
channels will vary depending on the underlying soil permeability. Grass
channels, however, are not capable of providing the same stormwater
functions as dry swales as they lack the storage volume associated with
the engineered soil media. Their retention performance can be boosted
when compost amendments are added to the bottom of the swale.
Grass channels are a preferable alternative to both kerb and gutter and
OVERFLOW TO INFILTRATION PRACTICES
Infiltration practices capture and temporarily store the design's storm
volume before allowing it to infiltrate into the soil over a two-day period.
Design variants include:
As the stormwater penetrates the underlying soil, chemical and
physical adsorption processes remove pollutants. Infiltration practices
are suitable for use in residential and other urban areas where field
measured soil infiltration rates are sufficient.
A typical design of a swale.
www.plumbingafrica.co.za
July 2019 Volume 25 I Number 5