Comprehensive Stormwater Management
Land Use | Runoff Coefficient |
Industrial & Commercial | 0.8 |
High Density Residential (>8 dwellings/acre) | 0.5 |
Medium Density Residential (4 to 8 dwellings/acre) | 0.4 |
Low Density Residential (<4 dwellings/acre) | 0.3 |
Open Space and Recreational Areas | 0.2 |
Where land use will be mixed, the runoff coefficient should be calculated using a weighted average. For example, if 60% of the contributing drainage area to the stormwater treatment structure is Low Density Residential, 30% is High Density Residential, and 10% is Open Space, the runoff coefficient is calculated as follows (0.6)(0.3)+(0.3)(0.5)+(0.1)(0.2) = (0.35) | |
Stormwater Control Measure | Drain Time of WQv |
Infiltration Basin or Trench1 | 48 hours |
Permeable Pavement - Infiltration1 | 48 hours |
Permeable Pavement - Extended Detention | 24 hours |
Extended Detention Facilities • Dry Extended Detention Basin2 • Wet Extended Detention Basin 3 • Constructed Wetlands (above permanent pool) 4 • Bioretention Area/Cell 5,6 • Sand and other Media Filtration5 | 48 hours 24 hours 24 hours 24 hours 24 hours |
• Pocket Wetland7 | 24 hours |
1 Practices designed to fully infiltrate the WQv shall empty within 48 hours to provide storage for subsequent storm events. 2 The use of a forebay and micropool is required on all dry extended setention basins. Each is to be sized at a minimum 10% of the WQv. 3Provide both a permanent pool and an extended detention volume above the permanent pool, each sized with at least 0.75*WQv . 4Extended detention shall be provided for the WQv above the permanent water pool. 5 The surface ponding area shall completely empty within 24 hours so that there is no standing water. Shorter drawdown times are acceptable as long as design criteria in Rainwater and Land Development have been met. 6 This includes grassed linear bioretention, which was previously titled enhanced water quality swale. 7 Pocket wetlands must have a wet pool equal to the WQv, with 25% of the WQv in a pool and 75% in marshes. The EDv above the permanent pool must be equal to the WQv. |
If the Percentage of Increase in Volume of Runoff is: | The Critical Storm will be: | |
Equal to or Greater Than: | and Less Than: | |
---- | 10 | 1 year |
10 | 20 | 2 year |
20 | 50 | 5 year |
50 | 100 | 10 year |
100 | 250 | 25 year |
250 | 500 | 50 year |
500 | --- | 100 year |
For example, if the percent increase between the pre- and post-development runoff volume for a 1-year storm is 35%, the Critical Storm is a 5-year storm. The peak discharge rate of runoff for all storms up to this frequency shall be controlled so as not to exceed the peak discharge rate from the 1-year frequency storm under pre-development conditions in the development drainage area. The post-development runoff from all less frequent storms need only be controlled to meet pre-development peak discharge rates for each of those same storms. | ||
Comprehensive Stormwater Management
Land Use | Runoff Coefficient |
Industrial & Commercial | 0.8 |
High Density Residential (>8 dwellings/acre) | 0.5 |
Medium Density Residential (4 to 8 dwellings/acre) | 0.4 |
Low Density Residential (<4 dwellings/acre) | 0.3 |
Open Space and Recreational Areas | 0.2 |
Where land use will be mixed, the runoff coefficient should be calculated using a weighted average. For example, if 60% of the contributing drainage area to the stormwater treatment structure is Low Density Residential, 30% is High Density Residential, and 10% is Open Space, the runoff coefficient is calculated as follows (0.6)(0.3)+(0.3)(0.5)+(0.1)(0.2) = (0.35) | |
Stormwater Control Measure | Drain Time of WQv |
Infiltration Basin or Trench1 | 48 hours |
Permeable Pavement - Infiltration1 | 48 hours |
Permeable Pavement - Extended Detention | 24 hours |
Extended Detention Facilities • Dry Extended Detention Basin2 • Wet Extended Detention Basin 3 • Constructed Wetlands (above permanent pool) 4 • Bioretention Area/Cell 5,6 • Sand and other Media Filtration5 | 48 hours 24 hours 24 hours 24 hours 24 hours |
• Pocket Wetland7 | 24 hours |
1 Practices designed to fully infiltrate the WQv shall empty within 48 hours to provide storage for subsequent storm events. 2 The use of a forebay and micropool is required on all dry extended setention basins. Each is to be sized at a minimum 10% of the WQv. 3Provide both a permanent pool and an extended detention volume above the permanent pool, each sized with at least 0.75*WQv . 4Extended detention shall be provided for the WQv above the permanent water pool. 5 The surface ponding area shall completely empty within 24 hours so that there is no standing water. Shorter drawdown times are acceptable as long as design criteria in Rainwater and Land Development have been met. 6 This includes grassed linear bioretention, which was previously titled enhanced water quality swale. 7 Pocket wetlands must have a wet pool equal to the WQv, with 25% of the WQv in a pool and 75% in marshes. The EDv above the permanent pool must be equal to the WQv. |
If the Percentage of Increase in Volume of Runoff is: | The Critical Storm will be: | |
Equal to or Greater Than: | and Less Than: | |
---- | 10 | 1 year |
10 | 20 | 2 year |
20 | 50 | 5 year |
50 | 100 | 10 year |
100 | 250 | 25 year |
250 | 500 | 50 year |
500 | --- | 100 year |
For example, if the percent increase between the pre- and post-development runoff volume for a 1-year storm is 35%, the Critical Storm is a 5-year storm. The peak discharge rate of runoff for all storms up to this frequency shall be controlled so as not to exceed the peak discharge rate from the 1-year frequency storm under pre-development conditions in the development drainage area. The post-development runoff from all less frequent storms need only be controlled to meet pre-development peak discharge rates for each of those same storms. | ||