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 the IDM. Where the IDM and this Design Note are in conflict this Note shall take precedence.
Design approaches
In general hydrodynamic performance should determine the optimum size and shape of flood and storm water detention basins (DB) rather than their catchment sizes or volumetric capacities.
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 detention basin:
- Identifying the suitable location and land area available for the basin.
- Concept planning of the basin by estimating size and shape of the basin.
- Hydraulic modelling of the basin and areas likely to be impacted by it.
- Understanding the retardation effect from inflow outflow and stage hydrographs.
- Assessing pre and post flood risks along with the impact of the basin.
- Repeating the above steps 1-5 until a satisfactory outcome is achieved.
Depending on the criticality of a detention system regarding its location in the catchment and various hydraulic influences designing of such systems 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 analysis 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 a basin size for the purpose of setting up hydraulic model. 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 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
| Computational elements vs Catchment area | Catchment area not over 10 hectares (ha) | Catchment area over 10 ha |
|---|---|---|
| Estimation of rural or pre-development inflow | Rational Formula or runoff routing program (without any hydraulic modelling at all) is acceptable. | For initial estimation use Rational Method or a runoff routing program. The size and shape to be finalised based on hydrodynamic performance of the system regarding flow retardation flood risk onsite and offsite impacts of the basin etc. |
| Size shape and storage volume of detention basin | Use a runoff routing program or method listed in Table 2 to size a storage basin including design of outlet (based on HGL and tailwater level). |
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) 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
| Type of storage | Site or catchment area A | Sizing method | Comments |
|---|---|---|---|
| On-site Detention (OSD) - DBs not to be taken over by the City | A not over 10 ha | Modified Rational Method Or listed tools below | Sizing based on hydrological routing is acceptable to the City. |
| Over 10 ha | XPSTORM HEC-RAS MIKE DRAINS RORB | The City expects hydraulic routing for finalising the size and shape of basins. Modified Rational Method or hydrologic routing may be acceptable depending on the hydraulic significance of the basin. | |
| Detention basins to be taken over by the City of Greater Geelong | A not over 10 ha | Modified Rational Method Or listed tools below | The City may accept sizing based on hydrological routing depending on the location and hydraulic significance of the basin. Please seek confirmation on this in the beginning of planning. |
| Over 10 ha | XPSTORM HEC-RAS MIKE DRAINS RORB | The City expects hydraulic routing for finalising the size and shape of basins. Modified Rational Method or hydrologic routing may be acceptable depending on the hydraulic significance of the basin. |
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 in turn is considered critical rainfall duration in Rational Formula) and can be based on the Kinematic Wave Equation unless impractical:
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 or a relevant software can be used to calculate flow velocity to estimate travel time which can be considered tc. In absence of a convincible determination of tc using formulas or software it should be estimated from professional experience and judgement by examining topography flow paths obstructions urbanisation infrastructures etc.
Design rainfall
The appropriate rainfall data and/or IFD charts to be downloaded from the Australian Rainfall and Runoff Data Hub webpage.