The design of stormwater detention storage systems within the City of Greater Geelong shall comply with the following criteria. 

This Design Note shall be read in conjunction with the IDM.  Where the IDM and this Design Note are in conflict, this Note shall take precedence.

For private lot scale stormwater detention use IDM Clause 19, table 13 to determine a permissible site discharge (PSD) at the legal point of discharge (LPOD).

Design approaches

In general, hydrodynamic performance should determine the optimum size and shape of a flood and storm water detention basin (DB) rather than its catchment size or volumetric capacity.  

That means, hydraulic routing (modelling), incorporating the intended detention systems, is required for designing such basins. Ideally, the City (City of Greater Geelong) expects undertaking the below or similar iterative process for determining the size and shape of a DB:

1. Identifying the suitable location and land area available for the basin.
2. Concept planning of the basin by grossly estimating size and shape of the basin.
3. Hydraulic modelling, incorporating the DB, of the areas likely to be impacted by it.
4. Understanding the retardation effect from inflow, outflow and stage hydrographs.
5. Assessing pre and post flood risks along with the impact of the basin.
6. Repeating the above steps 1-5 until a satisfactory outcome is achieved.

Depending on the criticality of a DB regarding its location in the catchment and various hydraulic influences, designing of such basin involving a simplistic process is acceptable. If the inflow catchment is reasonably small, limited to 10 hectares (ha), the simplistic process may involve flow estimation using Rational Formula (or composite form of it as required, that is, Q = i ∑CjAj) and Modified Rational Method or hydrologic routing (based on Continuity or Storage Equation). 

For catchments larger than 10 ha but up to 400 ha, the Rational Formula can be used for preliminary estimation of the DB sizes for the purpose of setting up hydraulic models. Various hydrologic computations (for example, Unit Hydrograph Method) and routing programs have their own parameters and requirements, discussion on which are beyond the scope of this Design Note except the Runoff Coefficient of Rational Formula.

Based on the catchment areas, Table 1 below summarises the computational approaches to be involved to estimate flow, volume, size and shape of a detention basin.

Table 1 - Summary of computational approaches

If the flow computations are based on hydrologic routing programs that mostly rely on non-deterministic, fictitious or ineffective evaluation of parameters and substantial assumptions on which results are very sensitive, the peak flows must be calibrated to the flows obtained from Rational Formula.

Methods and tools for sizing of detention storage

Table 2 lists the computational methods and tools to be used depending on the drainage area of two types of detention basins (DB) - On-site Detention (OSD) storages and the retarding basins (RB) - that are to be taken over by the City. The basin size is expected to be highly dependent on the design rainfall based on Annual Exceedance Probability (AEP) which is often adopted as 1 in 100, 20 or 10. The retardation effect of the basin should be reported as superimposed inflow, outflow and stage hydrographs.

Table 2 - Methods and tools for sizing of detention storages

Rational Method

Runoff Coefficient

Table 3 contains the runoff coefficients that shall be used for flow estimation using Rational Formula. The imperviousness as fraction of catchment or subarea (for composite analysis of Rational Formula) sizes shall be estimated utilising Melbourne Water’s MUSIC Guidelines 2018 (originally 2010) along with other available information, such as, satellite/aerial photos, maps, site visits etc.

Click to Enlarge Image

The blue shades above show how to find out runoff coefficient C10 for an f between 0.7 and 0.8

Time of Concentration

The time for overland sheet flows shall be considered as Time of Concentration tc (which is often considered as the critical rainfall duration in the Rational Formula) and may be based on the Kinematic Wave Equation unless found to be unrealistic:

tc = 6.94(L.n*)0.6 / (I0.4.S0.3) minutes

Where L is length of flow path in metre, n* is surface retardance coefficient, I is rainfall intensity (mm/h), S is slope (m/m).

If flow path is well-defined (naturally or by man-made), Manning Formula for Open Channel Flow computations or a relevant software can be used to calculate the flow velocity and then to estimate travel time which may be considered as the Time of Concentration, tc. In absence of a convincible determination of tc, it should be estimated from the professional experience and judgement by examining the topography, flow paths, urbanisation, infrastructures etc.

Design rainfall

The appropriate rainfall data and/or IFD charts to be downloaded from the Australian Rainfall and Runoff (ARR) Data Hub website to determine the design rainfall.