Drain Tech has recently completed a land drainage project requiring an installed sump and pump station for Tony and Catherine Krybolder at Brucknell in Victoria’s Southwest dairying district.
This Brucknell area is noted for its fertile clay loam soils but is often hampered by limited surface slope to allow surplus water to drain easily. For this project, the 18 hectares to be drained was nearly 500 metres from a suitable outfall. This would have required a large open drain construction or a significant investment in a large diameter drainage main. Drain Tech Director, Richard Gloyne was concerned that a lot of capital would be tied up in the drainage main and it would restrict how much area could actually be drained when given the budgetary constraints of the owners.
Sump and pump systems need considerable design and planning, because the outfall is below natural surface level and watertable control is dictated by pump cycling and capacity. As a general rule the larger the sump the more capacity for inflow before pumping is required. Of course extreme rainfall events are likely to cause system overload and the design needs to be based on several assumptions. The potential catchment area needs to be accurately determined and then an assumed 24 hour rainfall event needs to be struck in order to calculate an incoming flow rate.
For this project 4 ,1200mm diameter concrete culverts were joined together and stood on end side by side to create a sump approximately 4.5 metres deep with a storage volume below the inlet pipe of approximately 6000 litres and a flooded volume of 10,000 litres. For the 18 hectare catchment, a rainfall event of 15mm over 24 hours would create an inflow potential of 2,700 cubic metres or 2.7 million litres, 112,500 litres per hour or 31 litres per second. A 3 phase submersible pump is used to lift this volume 5 metres to the storage pond and is triggered by float switches.
This rainfall event of 15mm/24hr is often used as a guide in our drainage systems throughout the Southwest. Clearly higher rainfall totals are sometimes experienced but sump volumes and pump capacities can get expensive if built too large and under dairying conditions, brief water logging of the pasture (<48 hours) is an acceptable trade-off. Likewise if the sump is too small and the pump too slow the drainage system will always be in a “backed up state” and hence ineffective.
It is important to consider the potential to upgrade the system in the future. If it is observed that the pump capacity is too frequently being exceeded a second pump can be installed to commence pumping under extreme circumstances. In addition a pump using Variable Frequency technology could be programmed to ramp its flow rate up and down to better match the non-linear flow rates experienced in sub-surface drainage systems.
The complete system was commissioned in April 2015 and is being monitored to check the validity of our assumptions. The Krybolders have renovated a large portion of the 18 hectare project area and enter this winter with renewed confidence that new and existing pastures will now withstand the grazing pressure of their large milking herd.