Investigation of approach flow around drain pipe with rice husk envelope (case study: Ran-Behshahr project)

This paper studies the hydraulic performance of rice husk envelope in subsurface drainage system in Behshahr, Iran. Seven piezometers were installed between two subsurface drains designated as S3PD14 and S3PD15. Subsurface drainage system was monitored in rainy season. Parameters such as daily midpoint depth of water table, drain discharge rate, and head loss of water table in the vicinity of the drain including approach-flow head (hap) and entrance-flow head (he) were recorded. Parameters such as approach-flow resistance (Wap), approach-flow constant (αap) and approach-flow head loss fraction (Fap) were estimated in both S3PD14 and S3PD15 drains. The results showed that the performance of rice husk was better in S3PD15. Also, the proper performance of rice husk depends on the rate of envelope decay and the installation conditions of the trench backfill, which is reflected in its hydraulic conductivity. The great difference between approach-flow head and entrance-flow head could be related to clogging of the external part of rice husk envelope.