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 4 th 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 On-site Stormwater Detention Handbook the 4 th 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) SSR T is total volume (in m 3 /ha or in m 3 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 SSR T for an OSD storage is 455 m 3 /ha in which the maximum SSR for the extended detention storage is 300 m 3 /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 (SRD L ) is 40 l/s/ha. The SRD for the secondary (upper) orifice outlet (SRD U ) 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 SSR L and SSR T 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 m 3 /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 m 3 /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 (SSR L ); 100% of the available air space can be credited against the required overall detention volume (SSR T ); 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