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Street drain clearly clogged by 9 squares of 32 = 28 %

Grates and orifices[edit]

Debris in stormwater is often deposited in the narrowest parts of our low impact development systems. Garbage, leaf litter and sediment all contribute towards the clogging of these structures. It is often prudent to assume that maintenance will be intermittent and to include a clogging factor into engineering calculations to account for periods in which the debris has not been removed.

A suggested value of 0.5 is often adopted. This is equivalent to 50 % of the cross-section of the structure being clogged during the flow being calculated. Note though that in the orifice equation (for example) the clogging factor goes up to indicate that 100 % of the flow can pass when factor = 1 and the factor goes down as the device clogs.

e.g. In the image on the left 28 % of the grate is impassable. The clogging factor is 1.00 - 0.28 = 0.72.

Geotextile or filter fabric[edit]

  • Clogging of filter fabric has been observed in many field studies, e.g. "outflow ceases while water is still ponded above the drainage layer is most likely due to fines clogging the fabric and the need for more hydraulic head for the water to pass through the fabric"[1]
  • Laboratory research has demonstrated that the performance and clogging of maturing filter fabric can be predicted mathematically, based upon the media/filter material particle size distribution [2].
  • Elsewhere the mechanisms behind the clogging have been studied and characterised using CT-scanning technology [3].

Due to concerns about clogging, many types of facility may be constructed with limited or no filter fabric within:

  • The use of filter fabric is referred to as a practice in "older bioretention designs" In the Upstate Forever LID guide[4]. They go on to suggest that a choker course by used instead to separate the filter media and reservoir aggregate. Filter fabric may be used in side walls and should be placed directly over and within 2 feet of the perforated pipe drains when used in an underdrain.
  • Again, in Australia geotextile is only recommended around the sides of the practice :"Geotextile fabrics are not recommended for use between layers in bioretention systems due to the risk of clogging.", and :"Conventional bioretention systems have... ...a permeable geotextile liner around their sides (no liner along the base)" [5]. This practice is also seen in Massachusetts bioretention design documentation[6], indeed the side walls geotextile is also said to be "(optional)"[7].
  • In Finland the use of a choker course has been advocated for in place of filter fabric as replacing a clogged fabric layer would disturb established planting[8].
  • However, a recently revised document in Oklahoma suggests that geotextiles have a place in the base of the reservoir, but a choker course is best employed between layers inside the practice[9]
  • Filter fabric certainly has application in supporting backfield of native soils over infiltration and exfiltration trenches which comprise crates or clear stone[10].


Some clogging of perforated pipes is likely to occur between maintenance visits. As mentioned above a clogging factor of 0.5 is often applied. See Flow through perforated pipe. Pipes with smooth internal walls make for easier operation and maintenance. Various opinions are offered about how to mitigate the accumulation of sediment. The appropriate solution may depend on whether the water is flowing into the pipe (see underdrains), or out of the pipe as in exfiltration trenches.

Filter media[edit]

Salty water has been shown to cause degradation of bioretention filter media, and subsequent loss of the initial texture and flow conditions [11]

  1. Willard, L.L., T. Wynn-Thompson, L. H. Krometis, T. P. \ Badgley, and B. D. Neher. 2017. “Does It Pay to Be Mature? Evaluation of Bioretention Cell Performance Seven Years Postconstruction.” Journal of Environmental Engineering 143 (9).
  2. Palmeira, E. M. and Trejos Galvis, H. L. (2016). Opening sizes and filtration behaviour of non-woven geotextiles under confined and partial clogging conditions. Geosynthetics International. [1]
  3. Miszkowska, A., S. Lenart, and E. Koda. 2017. Changes of Permeability of Nonwoven Geotextiles due to Clogging and Cyclic Water Flow in Laboratory Conditions. Water 9(660). doi:10.3390/w9090660.
  4. Upstate Forever. 2005. “Bioretention - LID Fact Sheet.” Greenville, South Carolina.
  5. Water by Design. 2014. Bioretention Technical Design Guidelines (Version 1.1).
  6. Massachusetts Department of Environmental Protection. . “Bioretention Areas.” 1999.
  7. Massachusetts Department of Environmental Protection. 2014. “Bioretention Areas & Rain Gardens.” 2014.
  8. Tahvonen, O. 2018. Adapting Bioretention Construction Details to Local Practices in Finland. Sustainability 10(276). doi: doi:10.3390/su10020276.
  9. McLemore, A.J., J.R. Vogel, and S. Taghvaeian. 2017. “Bioretention Cell Design Guidance for Oklahoma.”
  10. Stormwater Management for Smart Growth, Allen P. Davis, Richard H. McCuen, Springer Science & Business Media, Aug. 16, 2005
  11. Kakuturu, S.P., and S.E. Clark. 2015. Clogging Mechanism of Stormwater Filter Media by NaCl as a Deicing Salt. doi: 10.1089/ees.2014.0337. [2]