Greenfield sites are typically located on the periphery of existing developments and are sites with no urban development and hence can be assumed to be completely rural (fully pervious).
The parameters Tp and BL depend on the size of the catchment and, for larger catchments, the geometry of the catchment. The plot scale equations for Tp and BL are hence sensitive to the catchment area specified for use in the equations.
The SuDS guidance recommends that for plot scale areas of less than 0.5km² (50ha) in size, greenfield runoff calculations are estimated based on an area of 0.5km² and then rescaled to the actual size of the catchment. ReFH2 can be used to implement this procedure using the following steps to calculate greenfield runoff rates and volumes. Recommendations are then made for maintaining an audit trail.
Greenfield Runoff Rate
The first steps are to identify the model parameters that are appropriate for an area of 0.5km² (50ha). The approach will differ slightly depending on whether you either use a catchment or point export file from the FEH Web Service. The first three steps are as follows:
Step 1. Design equations: Select to use plot scale equations on the catchment/point import screen if you are using a catchment import file. If you are using a point data import file these are selected automatically.
Step 2: Area: On catchment/point import set the area to the default development area of 0.5km² (50ha) if your development area is less than 50ha and to your development area if greater than 50ha.
Step 3. Rainfall Parameters: On the rainfall screen accept the initial rainfall defaults as all return periods for drainage design are included by default. Set the Areal Reduction Factor (ARF) to 1.0 and accept the default seasonality of a winter storm.
The software will then estimate the Tp and BL parameters based on the 0.5km². You will need to record these parameters. The rainfall event duration is also dependent upon Tp but the software will automatically set this for a given value of Tp. The final step, step 4, is to configure ReFH2 for the actual extent of the site to enable both runoff rates and volumes to be estimated for the site.
Step 4. Configure ReFH2 for the actual extent of your development area: on the modelling screen reset the catchment area to the correct area of your development area and override the Tp and BL parameters with the values you recorded when the catchment area was set to 0.5km².
Step 4 will also automatically set the recommended duration to that for the 0.5km² extent. This effectively rescales your results as recommended by the SuDS guidance on model parameters if your site is less than 50ha.
Having completed steps 1-4, the greenfield peak flow can either be read from the “as rural” results presented in the software or exported via the export button saves the summary statistics of peak flow and direct runoff volume for all modelled return periods to a CSV file. The default return periods encompass all the design return periods specified within the SuDS guidance. For a given AEP the peak flow is reported with units of m³/s. The peak flow can be converted to a runoff rate with units of litres per second per hectare (L/s/ha) using:
Where the runoff rates is in (l/s/ha), the peak flow is in (m³/s) and the catchment area is in (km²).
Greenfield Runoff Volume
The next step is to estimate the allowable greenfield runoff volume that can be discharged (at the greenfield flow rates) during an event. This runoff volume for a development site is usually defined as the 1:100 year 6 hour duration design event based on research by Kellagher (2002). The final step, Step 5, in the ReFH2 procedure to achieve this after estimating the runoff rates is as follows:
Step 5. Reset the recommended duration: in the rainfall modelling screen override the duration for the 0.5km² parameter set by setting it to 6 hrs.
Having completed Step 5, the greenfield volume, with units of mega litres, can be read from the “as rural” results presented in the software or exported using the Export button.
The hydrograph from which the volume is estimated can also be exported. This has units of m³/s. If you wish to convert this hydrograph to a volume within each time step it is necessary to multiply the ordinates of the hydrograph by the number of seconds in the model time step. The time step is specified in the rainfall modelling screen and the tubular results with units of hours:mm:ss and thus you will need to calculate the number of seconds in the model time step.
Audit Trail Recommendations
It is recommended that the project file is saved after step 4 and then under a different file name under step 5. The file name for the save at step 4 should specify that this file relates to greenfield runoff rate calculations and the file name for the save at step 5 should specify that the file relates to greenfield runoff volume calculations. This audit trail will ensure that you will always be able to recreate your results and you will also be able to pass generated report and project files across to third parties as part of quality assurance procedures.