ANSWAPPS: Model for the Analysis of Grass Swale-Perforated Pipe Systems Habib Abida 1 ; Jean Francois Sabourin 2 ; and Manel Ellouze 3 Abstract: A computer model for the analysis and design of grass swale perforated pipe systems is presented. The model, which was calibrated and validated using experimental as well as field data, performs detailed computations for flow through the system on a lot by lot basis i.e., from one catchbasin to another. Several parameters affecting the system performance are considered in the modeling approach. These especially included lot size and imperviousness, grass swale dimensions and its infiltration capacity, pipe length, number of orifices and their configuration, trench dimensions, and native soil infiltration capacity. The model was used to simulate the minimum trench depth required to capture runoff from a 25 mm storm for different native soils and different lot imperviousness ratios. Trench depths varied from 0.3 to 1.4 mm depending on native soil infiltration capacity and lot imperviousness. DOI: 10.1061/ASCE0733-94372007133:3211 CE Database subject headings: Stormwater management; Runoff; Infiltration; Calibration; Validation; Grasses; Computer models. Introduction While in the past, pollution of receiving waters was related mainly to wastewater systems, more recently surface water runoff has been implicated as a major cause of pollution. Varieties of pollutants accumulate within an urban area and are subsequently carried by stormwater runoff to rivers and lakes. This results in impairing water use for purposes such as water consumption, groundwater recharge, and recreational activities. Early stormwater management practices concentrated on reducing peak postdevelopment runoff levels to minimize down- stream flooding caused by urbanization. This was typically ac- complished by constructing stormwater detention ponds, which were designed as dry systems that would eventually discharge the entire detained volume of runoff to receiving waters. Therefore, such detention ponds just redistribute the rate of runoff over a period of time and do not reduce the total stormwater volume. More recently, and in response to growing concerns over the contaminating impacts of urban runoff on receiving waters, stormwater management alternatives started to address the prob- lem of water quality control. Recharge and mitigation of changes in the hydrologic budget also became prime objectives. The concentration of pollutants in stormwater runoff is gener- ally higher at the beginning of a storm and then decays as runoff continues Livingston 1988. This early runoff with high pollutant loads is typically referred to as the first flush. Several methods have been proposed for its evaluation. The first flush has been based on the relationship between the cumulative mass curve and the cumulative runoff volume curve. The percentage deviation of the curve from the diagonal was used as a measure for the strength of a first flush Gupta and Saul 1996. An additional approach calculates the correlation coefficients between the cu- mulative pollutant mass and the cumulative runoff volume Bertrand-Krajewski et al. 1998. Previous studies report that the occurrence of first flush events depends on various combinations of factors. For example, the occurrence of a first flush phenomenon has been related to the type of pollutant, catchment area, contributing impervious area, and rainfall intensity Lee et al. 2002. Kim et al. 2005 investi- gated the existence of first flush as a function of site-specific variables as well as storm characteristics. They suggested a new model that can be used to describe runoff concentrations and to estimate event mean concentrations and mass loading rates. Stormwater runoff has been identified as one of the leading causes of degradation in the quality of receiving waters, espe- cially during the first flush, responsible for the discharge of an enormous quantity of pollutants Lee and Bang 2000. Therefore, diverting the first flush of runoff from receiving waters results in the removal of the majority of total annual pollutant loadings. Several solutions for stormwater management have been adopted and applied. One of these solutions is to infiltrate a por- tion of the runoff, which results in groundwater recharge, low stream flow augmentation, water quality enhancement, and reduc- tion in the total runoff volume Schueler 1987; Stahre and Urbonas 1989; Horner 1999; Jan–Tai–Kuo et al. 2001; among others. Several modeling studies were performed to examine the per- formance of infiltration facilities. Kuo et al. 1989 developed a two-dimensional finite-element model to simulate the transient flow in a variable saturated porous medium for the study of infil- tration trenches. Routing is performed to find infiltration rate, water depth, and storage in the trench based on parameters such as soil properties, water table location, initial soil moisture con- 1 Assistant Professor, Dept. of Earth Sciences, Faculty of Science, Sfax, Route Sokra, km 3, 5 BP 802, 3018 Sfax, Tunisia corresponding author. E-mail: habib.abida@voila.fr 2 Chairman, J. F. Sabourin and Associates Inc., 142 rue de Varennes, Unité 11, Gatineau, Québec, Canada J8T 8G5. 3 Graduate Student, Faculty of Science of Sfax, Sfax, Sokra, km 3, 5 BP 802, 3018 Sfax, Tunisia. Note. Discussion open until November 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on May 5, 2005; approved on September 29, 2006. This paper is part of the Journal of Irrigation and Drainage Engineering, Vol. 133, No. 3, June 1, 2007. ©ASCE, ISSN 0733-9437/2007/3-211– 221/$25.00. JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING © ASCE / MAY/JUNE 2007 / 211