Rain Garden Project
A Litte Fall Of Rain
The traditional techniques of storm water management are concerned with volume flow rates. The difficulty with dealing with storm water is that while average rainfall on any given day of the year or in any given month is consistent, the amount of rain on any given day or at any hour of the day varies widely. For Evansville over the last three decades (1971 to 2000), the average daily rainfall has been between 0.08 and 0.17 inches; highest in the spring, lowest in the winter. Most precipitation occurs in small quantities, and most of the time it is not raining at all. In the past one year period (June 2008 to May 2009), the average daily rainfall was 0.11 inches. The average yearly rainfall in Evansville has been 44.27 inches; the total rain fall in the past year was slightly less than 42 inches. However, in the past year there were eleven days that had more than one inch of rain, and one day that saw more two inches of rain (May 25, 2009, with 2.25 inches). The record rainfall in the last century occurred in October of 1910, when 6.50 inches fell in one 24 hour period. So while average rainfall on a yearly or monthly scale is predictable, the amount of rain that will fall in any one day or in one hour is not predictable.
A plot of daily rainfall over the past year, compiled from NOAA weather data (http://www.noaa.gov/climate.html), can be seen below.
These amounts might not seem like very much rain, but in terms of volume of water they can be quite significant. One-quarter inch of water falling on a football field results in 7,500 gallons of water. You can fit 50 or 60 football fields inside the area taken up by a parking lot and buildings of an average mall. Parking lot surfaces are impervious, which means that the water cannot go through them, so most of that water that falls on a parking lot needs to be drained or it will become a puddle. If the runoff goes into the sewer system and to a water treatment plant, the plant will see an increase in water volume as impervious surfaces are being created.
Storm water management needs to be concerned with the volume of water, but more importantly by the rate of rainfall, or how quickly rain is falling. A light rain falling steadily can produce a quarter inch of water in a day, but that same quarter inch of rain falling in just half an hour would be a monsoon-level storm. One-quarter inch of rain in half an hour is almost 4 gallons of water per second, or 2000 half-liter water bottles emptied on the ground every minute.
A storm drain and sewer system needs to be large enough to handle the volume of water from a storm, but it also needs to drain the water quickly enough to account the flow rate of rainfall during a storm. If the gutters along the side of a roadway cannot remove rain water as it is falling, puddles will form on the side of roadways, which can cause hydroplaning and accidents. And if rain water is sent into the sewer systems faster than the sewers can carry it away, there is risk of backing up sewage into houses and basements. However installing larger diameter sewer pipes is more expensive and is usually not needed, since the heavy rains do not happen frequently.
One technique of dealing with high flow rates is storm water retention. By providing areas where water can collect during the storm and slowly release at smaller flow rates, the required size of the storm drains can be brought down. Retention ponds are usually required for commercial developments such as strip malls, and retention is sometimes achieved by burying concrete tanks under parking lots. The water from the impervious surface runs into the retention facility where it is slowly released into the storm drain. This is especially necessary where the storm drains pre-date the urban development. However, if the water is released into the storm drain, this still does not address the reduced infiltration that the urban development causes the ground water. A rain garden acts as a retention space, diverting runoff water from returning to the sewer system, and also allows the runoff to infiltrate back into the ground, re-hydrating the underground water sources.