October 2007

Handling Drainage Water By John W. Bartok, Jr.

Water can create problems inside and around the greenhouse. Inside, keeping floors dry helps prevent accidents and lowers disease potential. Outside, efficient handling of rain and surface water reduces the potential for erosion and flooding.

Inside Drainage

Floor drainage starts with a 6- to 8-inch gravel or stone base under the floor. This provides a collection area for overwatering if you are not planning to lay a concrete floor. To save on construction costs, many growers pave the walkways to make walking and cart movement easier. These should be sloped slightly to one side to allow the water to drain.

Full concrete floors are a better choice. Although there is generally very little excess water on a concrete floor, sloping the floor to a drain is important. A slope of 1Ú8-inch per linear foot is usually the standard. Although more expensive, a trench drain along the sidewalls or post lines makes installation of the floor easier and floor drainage quicker. Trench drains are available in several widths, typically 4 and 8 inches wide. The length of sections can vary from 3 to 8 ft. Today most are made of polyethylene and have anchor tabs to attach to the concrete.

In small greenhouses, the drain can lead to a drywell. In larger houses, the drains are usually connected together and lead to daylight. If the greenhouse is in an aquifer area, regulations may require that the floor water be collected in a sump. In some areas, a vinyl liner may be required below the floor to prevent fertilizer and pesticides from getting into the water table.

Curtain Drains

If you need to lower the water table around the greenhouse or intercept water flowing from a hillside, a curtain drain may be needed. This is frequently the case where there is an impermeable soil layer below ground level. Rainwater percolates through the topsoil to the hardpan layer and then travels laterally until it reaches daylight.

Construction consists of a trench being excavated to a depth of the impermeable soil layer, geotextile placed in the trench and a perforated drainpipe placed on 3-6 inches of gravel or peastone at the bottom. The trench is then backfilled with gravel or stone. Shoring should be used if the trenches are over 4 ft. deep. The drainpipe should be sloped about 1Ú8-inch per foot to grade or to a detention pond.

Greenhouse Roof Drainage

As we build more and larger greenhouses, roof drainage becomes a factor that needs to be addressed. Rainfall that normally soaks into the soil and becomes ground water is converted to runoff. Paved driveways, parking and storage areas also add to this total.

How much water are we talking about? 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. A 1-inch rainfall on an impervious area will produce 27,154 gals. of water. As much as 95 percent of this water runs off, the rest will evaporate.

Designing the Drainage System

Greenhouse gutters, downspouts, drain pipes and swales must be designed to handle runoff. Although some greenhouses are designed to allow the water collected in the gutters to run off at the end into a swale, most are designed with a piping system that collects the water and directs it to a pond or drainage area. Downspouts carry the water to lateral pipes that are connected to a larger main. These are usually placed about 50 ft. apart along the gutter. Spacing them too far apart could result in water flowing over the gutter and into the greenhouse.

Correct sizing of the piping system is important to handle the volume of water collected. The chart can be used as a guide in selecting the pipe size. For example, a 6-inch lateral drain is needed to take the water from a gutter downspout that drains a 25 x 100-ft. section of roof if the slope of the pipe is 6 inches in 100 ft. If a lateral pipe is needed to drain eight of the above-greenhouse sections, it would have to be 10 inches in diameter if the same slope is used (8 sections x 2,500 sq.ft. per section = 20, 000 sq.ft.).

The water from individual, free-standing greenhouses is frequently drained through a grass- or stone-covered swale in the space between the greenhouses. The area should be graded so that it is V-shaped with the sides having a 1- to 5-ft. vertical to horizontal slope. The bottom of the V should be sloped toward one or both ends of the greenhouse depending on where the water will be discharged. Where larger installations are involved, it may be desirable to install a catch basin at the end of each greenhouse and carry the water in a below-ground piping system.

Detention Ponds

To prevent water from curtain drains, paved areas or the greenhouse roof from flooding neighboring property, a detention pond can frequently be 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 several days. It also allows sedimentation, organic matter and other pollutants to settle out before the water is released. The pond is dry between storms.

The pond is usually designed by an engineer who takes into account the amount of area from which the water will be collected, the soil type in the area, the peak flow rate and the allowed discharge rate. An emergency spillway should be included to handle storms that exceed design flow. A fence around the pond will keep animals out. Typical cost is about $1 per cu.ft. of storage capacity.

Drainage Regulations

In most states, laws or regulations govern the discharge of water onto another person's property. In areas covered under zoning regulations, a plot plan showing building locations and drainage measures is usually required. Some locations are covered by inland wetlands regulations. A permit is needed to disturb or discharge into protected areas, such as marshes or swamps. State statutes also protect landowners, and an easement may be necessary. Where large lows are expected, it may be necessary to have an engineer check to see that downstream waterways and road culverts are large enough to handle the extra water.

Excess water removal both inside and outside the greenhouse is important. Before building new facilities, consideration should be given to how the water will be handled.