Handling rainwater runoff

As greenhouse businesses expand, handling runoff and drainage water becomes of more concern. Greenhouses create an impervious surface that collects rainwater. Auxiliary buildings and paved parking lots add to this area. The water from these areas can create serious situations downslope.

A 1-inch rainfall on an acre of impervious area will produce 27,154 gallons of water. As much as 95% of this water runs off. The rest will evaporate. In most parts of the country, a storm with an intensity of 3 inches per hour is used as the design standard. In a few areas, such as California, Idaho, Nevada, Oregon and Washington, the design rate is between 1 and 2 inches per hour. Where hurricanes occur — Florida, Alabama, South Carolina, Louisiana and Mississippi — a 4-inch-per-hour rate is used.

Rainwater collection from a roof is reasonably clean and frequently used for irrigation. For small- to medium-sized operations, plastic or corrugated steel storage tanks are used for collection. Click here for more information.

In larger operations, a retention pond may be the best choice. Where water is not needed but runoff has to be controlled for protection of downslope development, a detention pond can be used.

Both types of ponds are designed to hold and filter water. They are usually located in the lowest area on the property where normal runoff occurs. The incoming water can come through swales or from piping from the greenhouse gutters. The pond helps to control the sudden influx of water downstream.

These ponds are usually designed by an engineer who considers the amount of area from which the water will be collected, the soil type in the area, the peak inflow rate and the allowed discharge rate. State and local wetlands regulations will also have to be complied with. An emergency spillway is included to handle storms that exceed design flow. A fence around the pond may be installed to keep animals out.

Retention ponds

Retention ponds collect and store water for reuse. After filtering, many growers use the water for irrigation of indoor and outdoor crops. A forebay is usually added if the water comes from outdoor growing areas or from areas between hoophouses. This traps the larger particles and provides a structure that can be easily cleaned. Cattail, bulrush and phragmites are frequently planted in the water along the edge of the pond to remove nitrates and pesticide contamination. A drain at the bottom of the pond is included to allow for emptying when silt removal or other maintenance is needed. Adding a fire department dry hydrant should be considered if the pond is near the greenhouses. Ponds in areas with porous soil may require a vinyl or polyethylene liner.

Detention ponds

To prevent water from curtain drains, paved areas or the greenhouse roof from flooding neighboring property, a detention pond is frequently installed. You often see these adjacent to mall areas where there are acres of parking lot and roof areas that collect large amounts of rainwater. The detention pond is a control structure that collects this large volume of water and then slowly releases it over a day or two by limiting the flow through a small outlet pipe. It also allows sedimentation, organic matter and other pollutants to settle out before the water is released. The pond is dry between storms and usually has a grass bottom that is mowed during the summer.

Maintenance

The outlet and inlet to ponds need to be checked frequently for trash and debris.

Most ponds have vegetation on the sloped banks to prevent erosion. This needs to be maintained during the year.

Depending on the amount of particulate matter in the water source going into the pond, the sediment may have to be cleaned at 5- to 20-year intervals.