Stormwater Detention Storage Design

Note 2 in the Design Notes series covering stormwater storage retention, issued in August 2012.
The design of stormwater detention storage systems within the City of Greater Geelong (COGG) shall comply with the following criteria.  This Design Note shall be read in conjunction with Sections 16, 17, 18 and 19 of the IDM.  Where the IDM and this Design Note are in conflict, this Note shall take precedence.


Estimation methods

Table 1 below summarises the methods to be used to estimate stormwater discharges and volumes, based on the catchment areas of the detention storage systems.

Table 1 - Estimation Methods for Stormwater Discharge/Volume

Catchment Area of Detention Storage
  Ac <= 5 ha
 Ac > 5 ha
Development Status of Catchment Fully developed, partially developed or fully rural Fully or partially developed Fully rural
Estimation of Rural or Pre-development Discharge Use Rational Method (f = 0.1 for fully rural areas) Use approved runoff-routing program (Table 2) with validation of discharge against Rational Method Use approved runoff-routing program (Table 2) with validation of discharge against VicRoads Method (rural discharge).  Discharge from VicRoads Method may be adjusted by +/-30% (overall) to reflect catchment slope, natural storage, surface roughness, soil type, etc.
Sizing of Detention Storage Volume Use runoff-routing program or simplistic method listed in Table 2 to size storage volume, including design of outlet (based on HGL and tailwater level) Use runoff-routing program listed in Table 2, to size storage volume, including storage geometry, design of outlet based on stage-storage and stage-discharge relationships, and tailwater levels Use runoff-routing program listed in Table 2, to size storage volume, including storage geometry, design of outlet based on stage-storage and stage-discharge relationships, and tailwater levels


Runoff coefficient

The COGG Runoff Coefficients (Rational Method) shall be used for the Rational Method, Modified Rational Method and Swinburne Method – See Table 3.

The runoff coefficient derived within the VicRoads Method (rural discharge Py) shall be used exclusively within the VicRoads Method.

The Mass Curve Method for sizing of storage volumes requires a volumetric runoff coefficient.  A volumetric runoff coefficient of 0.80 may be used for a residential area (f=0.6) as an alternative to detailed computation of the coefficient.


Time of concentration

The Rational Method flow time for overland sheet flows shall be based on the Kinematic Wave Equation.  Refer to AR&R 1987/1999 (or later as appropriate).

The time of concentration derived within the VicRoads Method (rural discharge tk) shall be used exclusively within the VicRoads Method.


Design rainfall

IFD charts may be downloaded from the Bureau of Meteorology website.  Refer to IDM website.


Methods for sizing of detention storage volumes

Table 2 - Detention Storage Sizing Methods
Type of Detention Sizing Method Area of Development Site (OSD) or RB Catchement Area (Ac) Comments
OSD Swinburne Method
Mass Curve Method *
Modified Rational Method
}  Ac < 2 ha * Volumetric Runoff Coefficient to be used for Mass Curve method.
Methods which assume a constant discharge from a gravity discharge system (eg Mass Curve) require a factor applied to the constant discharge to determine the detention volume.  The adjustment factor is 0.75 for above-ground storages and 0.6 for below-ground storages.
Modified Rational Method
XPRAFTS
DRAINS
}  2 ha <= Ac <= 5 ha Queries regarding COGG specific parameters for use within the various modelling software products to be discussed with COGG staff.
XPRAFTS
DRAINS (runoff-routing mode)
}  Ac > 5 ha Queries regarding COGG specific parameters for use within the various modelling software products to be discussed with COGG staff
Detention basins to be taken over by CoGG Modified Rational Method
RORB
XPRAFTS
DRAINS (Runoff-routing mode)
XPSTORM / XPSWMM (runoff-routing mode)
}  2 ha <= Ac <= 5 ha Queries regarding COGG specific parameters for use within the various modelling software products to be discussed with COGG staff
RORB
XPRAFTS
DRAINS (runoff-routing mode)
XPSTORM / XPSWMM (runoff-routing mode)
}  Ac > 5 ha Queries regarding COGG specific parameters for use within the various modelling software products to be discussed with COGG staff


Table showing runoff coefficients
Click to Enlarge Image


Notes

  1. Also for use in the Modified Rational Method and Swinburne Method
  2. Music Guidelines - Melbourne Water, 2010
  3. Example:

    Zone - R1Z - Ave lot size = 430m2
    a. Fraction Impervious
    (note that "f" based on Zone and on allotment size)
    =  0.75
    Design Return Period 10 years
    C10 = Ave (0.67+0.74)
      =  0.705




Page last updated: Thursday, 26 July 2018

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