WSUD On-site Detention in xprafts 2013
Content
1. Introduction of ODS
2. General
a. Impervious Area
b. Pervious Area Capture
c. Average Allotment Density
d. Developed Area/Total Area
3. On-site Detention Unit
a. Type of ODS system
b. Extended Detention Storage
c. Flood detention Storage
d. Primary orifice outlet
e. Secondary orifice outlet
f. Site Storage Requirements (SSR)
g. Site Reference Discharge (SRD)
h. Permissible Site Discharge (PSD)
i. Secondary Permissible Site Discharge
j. High Early Discharge (HED)
k. Base of Storage to Discharge Control Pit Invert (H1)
l. Height of Extended Detention Storage (H2)
m. Height to Spillway (H3)
n. Volume of Initial Water / Total Water Volume of Storage at start
o. Infiltration
4. Rainwater tank
a. Available Air Space Volume (Dedicated Airspace)
b. Available Water Space Volume
c. Roof Capture
d. Volume of Initial Water/Total Volume of Water Storage at start
e. Height of Outlet to Spill
f. Spill Width/Developed Area
1. Introduction of On-site Detention/Retention
On-site Detention (OSD) involves the temporary storage and controlled release of stormwater
generated within a site. Without adversely affecting the property, it relies on thoughtful design and
passive engineering during site development to achieve significant reductions in downstream
flooding. OSD is required to ensure that the change in stormwater runoff from a site due to
development does not increase flooding problems downstream except severe events. OSD
systems must be properly maintained to make sure that stormwater flows from the site are
regulated for the life of the development. More details on OSD can be found in the handbook
called On-site Stormwater Detention Handbook, the 4th Edition, published by Upper Parramatta
River Catchment Trust, 12/2005.
OSD can be provided most efficiently and effectively when it is considered at early stage of the
development process. The system is easy to maintain when owners have a clear idea of the
location and function of each component in the system.
Aims: Detention on development sites has been seen as the solution to problems of established
areas where additional development or redevelopment is required. Generally, it is not possible,
either physically or financially, to progressively enlarge drainage systems to accommodate
increases in impervious areas, runoff rates and volumes occurred due to redevelopment. Detention
and/or retention facilities are normally planned for inclusion in urban stormwater management
systems for the following purposes:
WSUD On-site Detention - xprafts 2013
Page 1
 to remedy a situation where some part of the downstream drainage system is undersized, and
cannot be enlarged conveniently or inexpensively;
 to reduce flows from a developing area, so that the flows from the fully urbanised catchment
are no greater than those which would occur under present conditions, or for the catchment
in its natural state; or
 to develop the most cost-effective drainage system possible, by reducing the sizes and cost
of downstream pipes and channels, or as long as this reduces the overall net cost of the
total drainage works
At first the OSD policy was designed to prevent increased flooding during very large storms (e.g.
100 year ARI), and had no impact on smaller but more frequent storms (e.g. 1 year ARI). In
environmental terms, these smaller storms may cause more damage to watercourses and
disturbance to aquatic habitats. Furthermore, from a sustainability viewpoint, it would be desirable
to have the stormwater runoff from developed sites more closely mimic pre-development
conditions. Consequently, using a two-stage outlet to control site runoff in both the 1.5 year ARI
storm and the 100 year ARI storm, and by implication all intervening storm magnitudes, was
preferred.
Several significant changes to the OSD policy were assessed including:
• an on-line OSD storage;
• dual outlets, i.e. primary and secondary outlets;
• an uncontrolled primary outlet, i.e. outlet without High Early Discharge (HED) and
• a discharge control pit for the secondary outlet only i.e. outlet with HED.
Under the alternative OSD arrangement all site runoff is directed to the OSD storage. The water
level in the OSD storage rises gradually. In this way, the discharge through the orifice also
increases gradually as the depth of water (the „head‟) above the orifice increases. In small storms
the discharge leaving the site through the primary outlet (low level orifice) will be much less than
occurred previously due to the adoption of a reduced Permissible Site Discharge (PSD) for the
primary outlet. In major storms a secondary outlet with a higher PSD would control outflows from
the OSD storage. In combination these two outlets achieve overall aims of reducing peak flows in
frequent storms as well as in major storms.
OSD Solution: The Trust's catchment OSD policy, developed in conjunction with the four local
councils in the catchment – Baulkham Hills, Blacktown, Holroyd and Parramatta – is the result of
an extensive series of computer simulations using a detailed hydrologic model of the upper
Parramatta River catchment. Since late 1991, all four councils have been applying the same OSD
policy to their portion of the catchment.
The catchment OSD policy aims to ensure that subsequent developments will not increase flooding
or stormwater flows at any downstream locations, in all flood events up to and including 100 year
ARI events.
Rainfall/runoff modelling of the Upper Parramatta River catchment has been undertaken on a semi
continuous basis for over 25 year. The xprafts rainfall/runoff program has been adopted for the
hydrologic inputs to more advanced hydraulic analysis of the major tributaries throughout the
catchment. Initially the catchment was divided into only a few sub-catchments to represent the
inflows from the various tributaries. Subsequent modelling, carried out by the Trust, has
progressively expanded this model to 778 sub-catchments with 18 public retarding basins, 13
private basins and 22 natural storages that act as de facto basins. xprafts has been used to analyse
OSD solutions for all four editions of the handbook mentioned above.
2. On-site detention policy
For a complete guidance and example of OSD systems users are referred to the handbook called
th
“On-site Stormwater Detention Handbook” the 4 Edition, published by Upper Parramatta River
Catchment Trust in December 2005.
WSUD On-site Detention - xprafts 2013
Page 2
2.1 Statement
The catchment OSD policy aims two main points:


to ensure that new developments and redevelopments do not increase peak stormwater
flows in any downstream area during major storms up to and including 100 year ARI (1%
AEP) event;
to reduce post development peaks throughout the catchment in the 1.5 year ARI event to
be as close to natural levels and to encourage the integration of OSD with other water
quality measures.
The OSD solution should create a sustainable solution for peak stormwater flow management,
which complements any WSUD aspects of the development.
2.2 Objectives


To limit flow peaks throughout the catchment, in a 100 year ARI event, to estimated peak
flows under 1999 conditions, even if the further development is equivalent to full
medium/high density redevelopment throughout the catchment, thereby preventing any
increase in downstream peak flows by temporarily storing on-site the additional and
quicker runoff generated;
To prevent increases in downstream flooding and drainage problems that could:
- increase flood losses
- damage public assets
- reduce property values
- require additional expenditure on flood mitigation or drainage works.




To reduce post development peaks, throughout the catchment, in the 1.5 year ARI event to
as close to natural levels as practical;
To encourage integration of OSD systems into the architectural design and layout of the
development so that adequate storage areas are included in the initial stages of the site
design;
To encourage integration of the OSD facilities into a sustainable overall water management
plan for the site; and
To require construction supervision of OSD systems by the OSD designer to improve
construction standards.
2.3 Policy Application
OSD systems temporarily detain stormwater on a site, in order to limit the discharge leaving
the property to a pre-determined rate ensuring that the development does not increase
downstream flood discharges for storms up to the 100 year ARI event.
OSD is applied as a condition of development consent by Council under the Environmental
Planning and Assessment Act 1979. Any existing obligation to provide and maintain an OSD
system will be found in the development consent applicable to the property or on the property
title itself. In the case of a proposed development, the guidelines are given whether OSD will
be necessary.
OSD is not generally required on residential lots created by subdivision prior to 1991 when a
common OSD policy was adopted by the four councils in the upper Parramatta River
catchment. All lots created after 1991 have a requirement for OSD applied as a condition of
development consent for the subdivision.
The stormwater drainage system (including surface gradings, gutters, pipes, surface drains
and overland flowpaths) for the property must:
o
be able to collectively convey all runoff to the OSD system in a 100-year ARI (1%
AEP) event with a duration equal to the time of concentration of the site; and
WSUD On-site Detention - xprafts 2013
Page 3
o
ensure that the OSD storage is by-passed by all runoff from neighbouring properties
and any part of the site not being directed to the OSD storage, for storms up to and
including the 100-year ARI event.
3. xprafts General tab

Impervious Area
Sub-catchment Land Use and OSD Capture

i. Land Use (in %) is classified into three groups: Roof, Road and Paved
Areas. These three components percentage should be added up to 100%.
ii. ODS Capture (%) is percentage of impervious areas that enter the OSD.
Pervious Area Capture (%)
Percentage of pervious areas that enter the OSD.

Average Allotment Density (lots/ha)
This is required mainly for the Rainwater Tank component of any OSD systems.

Developed Area / Total Area
Developed Area/Total Area is a ratio describing the area of controllable private land by
OSD over the total sub-catchment area. For example the total sub-catchment has a large
an urban area component, that requires the OSD, and also has 20% parkland and public
road, that is not controlled by the OSD, then the ratio of Developed Area/Total Area would
be 0.8.
WSUD On-site Detention - xprafts 2013
Page 4
4. xprafts On-site Detention Unit

Type of ODS system
Two different types of On-Site Detention units are considered in xprafts as shown in the
diagram in a typical side view below:
(i)
(ii)
type either rectangular or circular, and
type triangular which is more naturally ground based sloped ponds with the deepest
part at the outlet.
In addition the outlet orifices can be controlled by a High Early Discharge chamber (HED) or
discharge proportionally to the driving head with direct outlets in the outlet wall.
WSUD On-site Detention - xprafts 2013
Page 5

Extended Detention Storage
The lower portion of the OSD storage, which detains stormwater in smaller, frequent storms
up to the 1.5 year ARI event in order to reduce stormwater runoff closer to the rates under
natural, pre-development conditions. This helps to minimise damage and disturbance to
downstream watercourses and aquatic ecosystems.

Flood Detention Storage
The upper portion of the OSD storage that detains stormwater to prevent any increase in
downstream flooding in moderate to major storms, e.g. 100 year ARI events. Water held in the
Flood Detention storage drains away through both the primary and secondary orifice outlets.

Site Storage Requirements (SSR)
SSRT is total volume (in m3/ha or in m3 as applied to a specific site) required for overall
storage. The SSR for the OSD storage is partitioned into Extended Detention Storage (lower)
and Flood Detention Storage (upper).
The total SSRT for an OSD storage is 455 m 3/ha in which the maximum SSR for the extended
3
detention storage is 300 m /ha. The SSR is only adjusted if a rainwater tank is included in the
development/redevelopment and an airspace “credit” is claimed to partially offset the SSR.
It is common for an Authority to nominate a minimum SSR requirement for an on-site detention
unit within an individual allotment development. xprafts can, therefore, simulate an individual
allotment with a node at the outlet point of every allotment or simulate the accumulative effects
of a number of allotments within a single xprafts sub-catchment. The developed portion of the
sub-catchment is the area enclosing all allotments that contain individual on-site units. For
example if the sub-catchment contains parkland that will not include an on-site detention unit
then the developed portion of the sub-catchment will be less than the total sub-catchment.

Site Reference Discharge (SRD)
The SRD for the primary (lower) orifice outlet (SRDL) is 40 l/s/ha. The SRD for the
secondary (upper) orifice outlet (SRDU) in the DCP is 150 l/s/ha. This will need to be adjusted
in accordance with the procedures when the entire site cannot be drained to the storage.

Permissible Site Discharge (PSD)
WSUD On-site Detention - xprafts 2013
Page 6
PSD is the maximum allowable discharge leaving the site in litres/sec/hectare (l/s/ha) or in
litres/sec (l/s) when applied to a specific site. Refer also to Site Reference Discharge.

Secondary Permissible Site Discharge
This is the maximum permissible discharge (in l/s/ha) for an optional second discharge point
from an On-Site Detention Unit at a higher elevation within the unit‟s outlet orifices. The reason
for an optional second higher outlet point with different diameter is to allow the optimizing of
units to meet downstream maximum peak flow requirements for two different flow frequencies.
For example for a 5 year return period event only the lower outlet is required operating while
both primary and secondary outlets operating is required for a 100 year flood flow.
Note that by running simulations to limit the 5 year flow peak downstream to the pre-developed
level it is possible to determine appropriate SSR and primary PSD. These runs will also provide
the maximum water level in the detention unit. This level will then be used to set the height of
the secondary outlet whereby its size can be adjusted to meet downstream maximum peak
requirements in the 100 year return period event.
It is also necessary to run a range of ARI storm durations to locate the one that produces the
maximum water level in the detention unit.

High Early Discharge (HED)
HED is the method for ensuring that the discharge from a Discharge Control Pit (DCP)
approaches the PSD soon after flow spills into the DCP.
There are two options included in xprafts: HED on Primary and HED on Secondary

Primary orifice outlet
This is the lower orifice located at the base of the OSD storage. It controls the rate of release of
runoff from the site in smaller, frequent storms when water only fills part of the (lower)
Extended Detention portion of the OSD storage.

Secondary orifice outlet
Larger orifice located at the base of the Discharge Control Pit. Because it is much larger the
secondary orifice controls the rate of release of runoff from the site in larger, rarer storms as
water has filled the lower Extended Detention storage and starts to fill part of the upper Flood
Detention storage.

Base of Storage to Discharge Control Pit Invert (H1)
It is a vertical measure (in meter) between the invert of Discharge Control Pit and the base of
the OSD (see figure below).

Height of Extended Detention Storage (EDS) (H2)
This height is a vertical measure (in meter) between the invert of the OSD‟s lower outlet and
the invert of Flood Detention Storage.

Height to Spillway (H3)
This height is a vertical measure (in meter) between the lower outlet invert and the spill level of
the OSD spillway. This is a flag to indicate if a High Early Discharge (HED) pit is in operation.
If it is then the maximum discharge rate is reached almost immediately after a relatively small
volume inlet pit is filled prior to water discharging into the main OSD. If no HED is utilized the
discharge rate for the outlet/s is progressively increased to the Permissible Site Discharge
(PSD) at a stage equal to the Primary Height to Spill (LWHT). This will be equal to the invert of
the secondary upper outlet if there is one.
WSUD On-site Detention - xprafts 2013
Page 7

Infiltration
Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate
is a measure of the rate at which the soil is able to absorb rainfall or irrigation in inches per
hour or millimeters per hour. The rate at which the flow occurs is dependent on the properties
of the material and the relative volume of voids (air spaces) it contains. The rate decreases as
the soil becomes saturated. If the precipitation rate exceeds the infiltration rate, runoff will
usually occur.

Pond Infiltration
xprafts provides for „leakage‟ from basins and reservoirs in addition to evaporation losses and
conventional outflow. Three strata situations are presently covered, including (i) a Shallow
Water Table, (ii) Deep Water Table and (iii) Clogged Surface Layer. The equations utilize the
work of Bouwer (1978), Bear et al (1968) and Todd (1980) to define shallow water tables. The
methods prescribed are similar to those utilized by Main Roads (Western Australia) PC
SUMP(C) software.
WSUD On-site Detention - xprafts 2013
Page 8

Basin Infiltration Rate
Discharge through basin floor infiltration expressed (in m/hr): This value represents the
hydraulic conductivity of the basin floor.

Clogged layer
Thickness of clogged layer (in meters): The infiltration rate (in m/hr) should now relate to this
clogged layer.

Water table
Depth of water table below the invert of the basin (in meters): This option is only required if the
'Shallow water table' flag is turned ON.

Shallow Water table
Select this option to model a shallow water table at the given depth. If this flag is off then a
deep water table is implied with no interaction with the infiltration flow.
WSUD On-site Detention - xprafts 2013
Page 9
5. xprafts Rainwater Tank
Prior to 2001 the rainwater tanks was not considered as part of an OSD facility due to the
assumption that the tank would be full at the start of a major rainfall event. However, it has
been argued that a rainwater tank would not always be full at the start of a storm if its water is
used inside and outside the dwelling for non-potable purposes – toilet flushing, laundry, hot
water and garden watering.
As part of the detailed analyses of the cumulative impacts on peak discharges undertaken
in recent years the interaction of rainwater tanks and OSD tanks was investigated.
Analyses were also undertaken of rainwater tanks including the airspace that varies in
response to rainfall and water demands (internal and/or external). This led to a revision of the
proportion of a rainwater tank which can be counted as part of the OSD storage. This “credit”
reduces the SSRL and SSRT for an OSD system. The SSR is only adjusted if a rainwater
tank is included in the development / redevelopment and an airspace “credit” is considered
to partially offset the SSR.

Available Air Space Volume
This is the volume of allotment tank space between the orifice outlet and the spillway (in
m3/lot).

Available Water Space Volume
WSUD On-site Detention - xprafts 2013
Page 10
This is the volume of allotment tank below the orifice outlet invert (in m3/lot).
Note that rainwater tank data is only entered if the OSD designer is claiming an airspace
“credit” against the required OSD storage volumes. Based on the analysis results reported by
Cardno Willing (2004) the following reductions in the SSR values may be allowed subject to
Council approval:
 50% of the available air space can be credited against the required extended detention
volume (SSRL);
 100% of the available air space can be credited against the required overall detention
volume (SSRT);
subject to:
 a maximum Available Air Space Volume credit no greater than the ratio of the area of
roof discharging to the rainwater tank to the lot area times the overall site storage
volume that is required;
 the rainwater tank has an orifice outlet to ensure that the Available Air Space Volume
is recovered after a storm event and the maintenance schedule specifically
requires checking and cleaning of the outlet;
 the PSD for the rainwater tank orifice outlet is no greater than 40 l/s/ha;
 all outflows from the rainwater tank (outflows from the orifice outlet and overflows from
the spillway) are discharged to the OSD storage.

Roof Capture
Percentage of Roof Capture draining to a rainwater tank, i.e. represents the percentage of
allotment roof area directed into the rainwater tank.

Volume of Initial Water/Total Volume of Water Storage at start
Percentage of the initial water volume in Available Water Space Volume over the total
rainwater tank at which top-up commences.

Height of Outlet to Spill
This is the height (in meter) between the invert of orifice outlet and the spillway.

Spill Width/Developed Area
This is the spillway width represented in terms of meter per area of sub-catchment (m/ha)
times development ratio.
WSUD On-site Detention - xprafts 2013
Page 11