The concept of stormwater retention is to store temporarily, excess stormwater run-off. This is to avoid hydraulic overload of the sewer system, which could result in the flooding of roads and buildings with untreated wastewater or its release directly into the environment, causing pollution. When space is available in the sewer system, the retained water is released at a rate not exceeding the capacities of the sewer system, and the tank can then be cleaned ready for the next flush.
Meeting the challenges ahead
Climate change is increasingly the cause of extreme weather phenomena around the world, for example extreme rainfall. This challenges the existing stormwater drainage system, not least because an increase in rain intensity of 40-60% will reduce the return period of severe flooding events. Furthermore, increases in population in low-lying areas and near river deltas, and urbanisation, make the consequences of extreme weather worse.
However, flooding can also hit where least expected. Unusual weather patterns can, and do, cause storms and heavy rains in regions where historical data has offered no warning. In recent years, many examples of such unexpected floods have occurred around the world.
The number of flood scenarios has been increasing over the past decade. Any region faced with high annual rainfalls, increased populations and expanding cities will increasingly be called upon to focus on flood control and stormwater management.
Stormwater tanks are an effective way of reducing peak flow and equalising flow rates from stormwater runoffs in the sewer system. Stormwater tanks are a cost-effective solution, because sewer lines are already constructed and generally have a substantial remaining lifetime; replacing existing pipes in an urban environment is, in addition to being very expensive, troublesome and can take many years.
Stormwater tanks can be relatively easily adapted to the sewer system, and during heavy rain the sewer system is relieved, by guiding excess stormwater to the stormwater tank for temporary storage. All this illustrates the advantages of self-cleaning, pump-managed stormwater tanks.
Step-by-step guide
The Grundfos handbook ‘Design of stormwater tanks – recommendations and layout’ guides the reader step-by-step through every aspect of stormwater retention. Stormwater tanks can be built to almost any size and shape. The inlet and outlet can be managed by gravity, by a pump system or a combination of both, depending on the hydraulic conditions in the system.
The variation in stormwater tank design is often due to differences in construction, and means that the operation of each stormwater tank must be considered as a unique case. However, regardless of whether the tank is renovated or newly constructed, the determining factor for effective use and efficient cleaning is the hydraulics of the system, as it is the flow of water that cleans the tank.
The hydraulics of the system require the consideration of filling capacity, mixing, cleaning and emptying, as well as the retention time of the stormwater runoff. Equipment installed in the tank can directly influence the hydraulics, and the selection of equipment must address these issues.
Simulations using Computational Fluid Dynamics (CFD) to describe flow patterns in, for example, pumps, pumping stations and stormwater tanks have proven to be an extremely useful tool. Correctly applied, CFD catches design flaws early in the process, ensuring cost savings on operation, downtime, cleaning and maintenance well into the future.
Correctly dimensioned pumps ensure optimum delivery of wastewater to and from the stormwater tank, reducing the hydraulic load and ensuring available capacity. These can be pumps for dry or submerged installation and are able to handle the heavy flows required. Level sensor accessories can ensure that pump systems can control possible fluctuations in demand for performance.
Efficient mixing and cleaning of stormwater tanks is essential to minimise operation costs and avoid odour problems. The Grundfos range of Rain- Jets is especially designed for this purpose, offering robust construction, and easy service and maintenance. The Rain- Jets can work either alone or together with the robust and durable AMD mixer.
Stormwater handbook
The Grundfos handbook ‘Design of stormwater tanks – recommendations and layout’ covers every aspect of stormwater tank design, sizing, installation and positioning, control strategy and equipment, and shows how such measures ultimately offer savings on investments. Grundfos offers consultancy for stormwater tank design and optimisation as well as offering a full range of pumping and control solutions, mixers and cleaning equipment that, together, ensure reliable and automatic operation of the stormwater tank. Read ‘Design of stormwater tanks – recommendations and layout’ online as an iPaper or download as a PDF at www.grundfos.com/water-utility.
