Use of passive stormwater samplers in water sensitive urban design

The application of WSUD practices requires information about stormwater runoff quality from urban surfaces. Passive samplers are not powered and rely on the physical flow of stormwater to obtain a sample and have been used in past studies to collect samples from small urban catchments. Various types of samplers are available and can be classed in terms of the main hydraulic principle that is applied in their design. These principles are gravity flow, siphon flow, rotational flow, flow splitting and direct sieving. This paper reviews each class of passive sampler to establish their strengths and weaknesses. Factors to be considered in the use of these devices to sample runoff from urban surfaces are discussed. Desirable attributes of a passive sampler to determine Event Mean Concentrations are also identified, leading to a set of design criteria. The criteria includes recommendations on minimum catchment area to be sampled, the design storm frequency used to determine hydraulic capacity, the sample flow volume ratio (SFVR) and sample storage requirements. As particles are conveyed in stormwater as washload, bedload or floating, it is important to clearly state the type and upper size limit of the particles to be sampled. It is concluded that samplers that use the principles of rotational flow or flow splitting offer the greatest potential in terms of sampler performance. A concept design of a flow splitter device is provided in this paper. The intended use of the device is to obtain stormwater samples from small urban catchments (approximately 200 m2 in area) in order to determine unbiased concentrations of organic and inorganic particles smaller than 500 um in size