Grass swale-perforated pipe systems for stormwater management

The performance and potential of perforated pipes for stormwater quantity and quality control is evaluated. The studied design consists of a grass swale underlain by a section of perforated pipe enclosed in an infiltration trench. The grass swale-perforated pipe system results in a pleasant curbless design, which may replace open ditch systems, in low density residential areas. Current design practices and construction methods were assessed by a Survey completed by 53 towns and municipalities in Ontario, Western Canada and the United States. The Survey of municipal experience showed that while 50% of the municipalities have experience with perforated pipes used as storm sewers, only few municipalities used a combined system of grass swales and perforated pipes for the purpose of urban runoff infiltration. Seventy six percent of the respondents however are Willing to use the system as an alternative to other stormwater management methods. Through series of laboratory tests, it was found that exfiltration rate out of pipe perforation is not directly proportional to the orifice area. The analysis of the results indicated that the orifice discharge coefficient varied with orifice size and the head above the orifice. Field tests were conducted to measure the infiltration rates of typical grass swales and existing pipe trenches. The infiltration rates obtained were found to be consistent with literature values. Finally, according to a 3 month field monitoring program, the total seasonal discharge for a properly designed perforated pipe system was found to be 13 times smaller than for a conventional stormwater system, which shows the favorable effect of the perforated pipe system on the hydrologic budget and pollutant loading.