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GI presents numerous opportunities and challenges in an urban setting. Green roofs are effective in capturing perhaps the first inch of rainfall on one of the largest impervious areas by percentage in a city. They are, however, expensive, relative to other options and are not always possible due to structural issues. Infiltrating street and driveway runoff is very effective and, as shown in Philadelphia, can often handle even relatively large storms of 2 to 3 inches. The complication there is that maintenance costs are relatively high in relation to initial capital costs, and must be permanently  funded. We have found that, if GI is placed at least 10 feet from the nearest building or basement, that basement flooding is usually not an issue. There will be, however, some cases where a buried conduit will lead to basement flooding even with separations of more than 10 feet, and this will have to be one of the risks that are prepared for.  Finally, if a large scale GI program is contemplated, it is important to do some groundwater modeling to assess the cumulative effects of moving from a low infiltration situation to one of relatively high infiltration. There may be a general increase in the groundwater table.

Despite all this, GI is an attractive option when compared to pure storage options like tunnels and tanks if the additional benefits of greening are considered in making a decision to pursue GI as part of a CSO reduction program.

GI presents numerous opportunities and challenges in an urban setting. Green roofs are effective in capturing perhaps the first inch of rainfall on one of the largest impervious areas by percentage in a city. They are, however, expensive, relative to other options and are not always possible due to structural issues. Infiltrating street and driveway runoff is very effective and, as shown in Philadelphia, can often handle even relatively large storms of 2 to 3 inches. The complication there is that maintenance costs are relatively high in relation to initial capital costs, and must be permanently  funded. We have found that, if GI is placed at least 10 feet from the nearest building or basement, that basement flooding is usually not an issue. There will be, however, some cases where a buried conduit will lead to basement flooding even with separations of more than 10 feet, and this will have to be one of the risks that are prepared for.  Finally, if a large scale GI program is contemplated, it is important to do some groundwater modeling to assess the cumulative effects of moving from a low infiltration situation to one of relatively high infiltration. There may be a general increase in the groundwater table.

Despite all this, GI is an attractive option when compared to pure storage options like tunnels and tanks if the additional benefits of greening are considered in making a decision to pursue GI as part of a CSO reduction program.