BS 8515:2009 pdf free download – Rainwater harvesting systems – Code of practice.
4.1.2 Calculation methods
COMMENTARY ON 4.1.2
The three approaches to sizing recommended in 4.1.2 are based on methods given in DIN 1989-1.
4.1.2.1 General
The storage capacity of the rainwater harvesting system should be determined using one of the following methods:
a) a simplified approach for residential properties, where there is consistent daily demand, for which no calculations have to be carried out (see 4.1.2.2);
b) an intermediate approach which uses simple formulae to calculate a more accurate estimation of storage capacity than the simplified approach (see 4.1.2.3);
c) a detailed approach for non-standard systems, where there is variable demand through the year (see 4.1.2.4).
NOTE 1 The simplified approach is not suitable for commercial premises as the assumptions relating to demand are not applicable. The intermediate approach may be used for certain commercial and industrial premises, such as schools and offices.
NOTE 2 For larger rainwater harvesting systems, the size of the system needs to be analysed using a detailed approach to ensure a cost-effective solution is developed, as seasonal variations in rainfall can affect sizing requirements even where demand is relatively predicta ble and consistent.
Once the storage capacity has been determined, storage tanks should be selected on the basis of working capacity, rather than the total capacity of the container.
The size of the tank should allow for rainfall variation; however, it should be noted that construction above a certain size based on rainfall for that area provides very limited additional benefit unless stormwater attenuation is intended.
NOTE 3 The size of the tank will also affect how often the stored water overflows. Occasional overflowing can be useful for maintenance and might have benefits for water quality.
4.1.2.2 The simplified approach
To apply the simplified approach to sizing the rainwater harvesting system for non-potable domestic use, storage capacity should be estimated using the following method.
First, the roof plan area draining to the storage tank should be established in accordance with BS EN 12056-3, and the annual average rainfall depth for the location of the site should be determined from Figure 2.
In most cases, the storage capacity should then be read from the Y axis of Figure 3, using the appropriate diagonal line for the rainfall depth. However, where the site has a large roof plan area and/or is in a region with high annual rainfall, the storage capacity should be determined in relation to the population in the house.
Where the system is to provide both non-potable water for domestic use and stormwater control, the integrated sizing approach given in A.1 should be used to estimate the additional storage capacity needed.