Area Drainage: Working Outside the Building Footprint

By Peter A. Kraut, P.E., CPD

Area drainage has traditionally been the task of the civil engineer, but with atriums, courtyards and green roofs blurring the lines of responsibility, plumbing engineers often work more than 5-ft. outside of the building footprint and can even design site drainage. Signs of poor drainage include standing water, accumulation of leaves and debris, silt deposits on solid surfaces, water intrusion and even structural damage. In these areas, pipe sizing becomes secondary to grading and drain selection. Determining the tributary area becomes much more complex and coordination is key.

Rainfall

What is the 100-year, 1-hour storm? Simply put, it is the worst 1-hour period of rain experienced in the last 100 years. Can a storm in the next 100 years bring a greater amount of rain? Absolutely.

From Table 1 we can see that if a building lasts 50 years, it is 39% likely to experience a rainfall greater than the 100-year storm. The need for overflow drains on roofs becomes clear. When sizing area drains, the same thoughtfulness should be applied. If an area drain is overloaded, will the accumulated water run across a walkway or into the building? If thresholds are low, consider over sizing drains or putting in secondary drains.

Another commonly overlooked characteristic of rainfall seen in storm drain design is that rain does not fall straight down. More often, it is blown by wind and comes down at an angle. For this reason, vertical areas must be included when determining the tributary area that will collect in a drain. Fifty percent of all vertical walls and 35% of adjacent vertical walls should be added to the horizontal area, but opposite vertical walls can be excluded. Figure 1 illustrates this requirement.

Tributary Area

Determining the area of an irregular space in a graded yard can be a bit more difficult. One must consider the path that water will take when flowing across such areas. To do this, a topographic site plan is needed. A topographic plan has contour lines that indicate various layers with the same elevation. Concentric lines indicate increasing or decreasing grades. Every five or ten units, a darker line is used to assist in reading. As the grade lines get closer together, the slope is steeper. As they get farther apart, the slope approaches flat. Water of course flows downhill, always perpendicular to the contour lines. By tracing the path that water will flow, the area that feeds each drain can be determined.

Often, these topographic maps have spotted elevations that indicate high points, low points and other important elevations. They indicate these elevations in feet above sea level, using decimal feet in lieu of inches. Multiply the decimal by 12 to get inches or by 96 to get eighths of an inch. So, the civil notation 1506.87 is the same as the architectural notation 1560'-10 1/2". When correlating the civil elevations to the architectural elevations, exercise care. While civil engineers work almost exclusively with elevations above sea level, architects often use an arbitrary 100 ft. for the elevation of the first floor of a building. If you stop and think, you can foresee the problem when your location is, let's say, 99 ft. above sea level.

Another factor to consider in determining tributary area is absorption. While all water that hits a roof is supposed to make it to the drain, this is not true on site. Water is absorbed by soil, vegetation and even pavement. The percentage of water that does not get absorbed is referred to as the coefficient of runoff. As the slope of a surface increases, so does the coefficient of runoff. Use this factor to reduce the area considered for each drain. Table 2 indicates commonly used values for different surfaces:

One final thought to consider when determining tributary area is contributing flows. In many cases, roof drains and gutters spill through downspouts to grade. This flow must be added to the surrounding area when sizing the drain. Having determined the tributary area, considered thresholds, added walls, subtracted absorption and added contributing flows, where do we put the drains? One in each low point; where are the low points? This is a job for the architect, however some assistance from the engineer may be needed. An architect needs to coordinate the walkways with surrounding elements while keeping the running slope - the slope in the direction of travel, at least 1:50 but less than 1:20. If the slope is too shallow, the water will pond. If it is too steep, it may not be accessible to the handicapped. For accessibility, the cross slope, perpendicular to travel, must be less than 1:50. These challenges lead to the many ramps and retaining walls required on today's projects. When setting area drains with bar grates in walkways, the bars must be separated by spaces no larger than 1/2 in. and must be placed perpendicular to the path of travel.

Other architectural challenges may include pools and outdoor showers. Rain water should never drain into a pool. The surrounding drains should flow to the storm drain system while the showers should not. They need to have a small dedicated area drain, trapped and vented, connected to the sanitary sewer. Of course most plumbing codes prohibit allowing any rainwater from entering the sanitary sewer. The solution is different in each jurisdiction. Some allow this to drain to the storm system. Others require a roof over the outdoor shower. Still others require solenoid valves, activated by rain sensors to divert the water to the storm drain on rainy days while allowing bath water to drain to the sewer on sunny days.

In area drains outside the building footprint, it can be helpful to translate area to a flow rate in gallons per minute. In this form, it is easier to add flows such as sub soil drainage and sump pump discharges. In gallons per minute, one can more easily picture the amount of water being conveyed. To do this, use the following formula:

Q = 0.0104 x R x A x C

where:

• Q = Flow Rate (gallons per minute)
• R = Rainfall Intensity (inches/hour)
• A = Area (square feet)
• C = Coefficient of Runoff
• 0.0104 = Conversion factor G.P.M./sq. ft./inch/hr.

Example:

• R = 4 in./hr. rainfall
• A = 1,500 sq. ft. area
• C = 0.35, grass
• Q = 0.0104 x 4 x 1,500 x 0.35 = 21.84 GPM

Drain Selection

There are many drain grates available for use outside of the building footprint. Inexpensive PVC bar grates, seen often around pools for their resistance to chlorine, are not resistant to UV light and will break down over a short period of time. Other common alternatives in hard surfaces include bronze, nickel bronze and stainless steel. Additional products are available in virtually any material and can include custom logos and designs. Area drains for landscaped areas are most common in ABS, typically green to blend in with the surrounding surface. These offer better resistance to UV light and typically have much greater free areas than others. This allows for some level of conveyance even when the drain is partially blocked by leaves.

Trench drains are often used to minimize the rolling effect of spotted drains. They work especially well at the head of driveways and similar locations where slope in only one direction is desired. Whatever drain is used should be appropriate for the traffic expected. Light duty drains in service areas are often seen with dents from heavy cart traffic. Trench drains in driveways can be found crushed from the weight of cars. Careful specification is necessary. Drains are rated for light duty (up to 2,000 lbs.), medium duty (up to 5,000 lbs.), heavy duty (up to 7,500 lbs.), extra heavy duty (up to 10,000 lbs.) and special duty (over 10,000 lbs.). In general terms, light duty is recommended for pedestrian traffic, medium duty is recommended for service areas and heavy duty or greater is recommended for vehicular traffic.

Area drains should connect to pipe mains with lateral connections. Catch basins and drains with vertical connections disrupt the flow of water creating turbulence and reducing the self-cleansing effect of higher flow rates.

Piping

A major concern of pipe blockage is the location of pipes. Pipes routed under trees will likely be destroyed by roots in a few short years. Solutions include avoiding any path that falls under the canopy of a tree and transitioning to a stronger pipe material when unavoidable. Ductile iron (ASTM A 716) and concrete pipes (ASTM C 14 and ASTM C 76) are seldom used for small on site collection systems making hub and spigot cast iron (ASTM A74) or hubless cast iron (ASTM A888) the best choices. These too are seldom used outside of a building due to costs. Check the corrosivity of the soil before using any metal pipe, fittings or couplings below grade. PVC choices are many including PVC pressure pipe (AWWA C900), PVC water-service pipe schedule 80 and schedule 40 (ASTM D 1785), PVC sewer pipe (ASTM D 3034, SDR 35 and ASTM F 679, T-1, T-2) and PVC cellular-core pipe (ASTM F 891). PVC pipe will provide some degree of protection, while ABS, (ASTM D 2751) and corrugated PE (AASHTO M 252M) are the weakest, but most commonly used in open areas.

Pipes must also be located outside of the bearing footprint of foundations. This is generally considered to be an imaginary line projected 45-degrees down and away from the building footprint. Various foundations such as mat slabs, piles and spread footings all have different bearing footprints. Check with the structural engineer before routing near buildings and structures. If it is necessary to run adjacent to a structure or roadway, details for burial must be coordinated with the loads expected. They should include the depth of bury, the bedding under the pipe and the compaction around the pipe. Plastic pipe is typically installed in a bed of sand at least 4 in. deep and requires compaction up to the centerline of pipe. This keeps the pipe from flattening as it is buried. Metallic pipes often do not need bedding unless they are installed in rock and compaction around the pipe is generally not required.

The first thing to remember in sizing pipe is that if there is an occupied space below it, it's a roof. This includes underground parking, mechanical rooms and even storage spaces. As such, it will need to be drained in conformance with the code. This means primary and secondary drains, each sized to convey 100% of a 60-minute duration of the 100-year storm. This varies from 1 to 10 or more inches per hour depending on your location. Do not assume that two projects in the same vicinity will have the same rainfall rate as it may easily be double across the street in another jurisdiction. Always check with the local Administrative Authority before starting work. Most plumbing codes have charts that limit how many square feet can be conveyed through a given drain or pipe size at a given rainfall rate. This makes sizing by the square foot a relatively easy task.

Pipe outside of the building footprint can be sized using the plumbing code, manufacturer's data, or other engineered methods. When leaves and debris are a concern it is important not to oversize pipe. Full flow is necessary on an occasional basis to clean the pipe. This self cleansing velocity is generally considered to be between 2.0 and 2.5 feet per second. Pipes 3 in. and smaller will need a slope of 2% or greater to achieve this. Pipes between 4 in. and 6 in. will require a slope of 1% or greater. Pipes 8 in. and larger require only 0.5% slope. Pipes that receive more water than they can convey will surcharge. This practice is not allowed within the building footprint. On-site however, the water level can be allowed to back up in the drain tailpiece creating additional head pressure, which will in turn make the pipe flow faster. As such, pipe sizing is less critical except in areas where temporary ponding will create a hazard. Although actual internal diameters and material roughness can vary considerably, for pipe flowing full under gravity, Table 3 will be generally sufficient for sizing pipe.

Discharge

If possible, it is most desirable to connect to the storm drain in the street. Work outside of the property line has its own set of challenges and should be reserved for licensed civil engineers. In some cases, a street lateral conveys the water from the property line to the street main. Local utility companies and government agencies have specific requirements for this work. In other cases the on site storm drain can connect to the back side of a catch basin.

When a plumbing engineer finds himself doing the work of a civil engineer, he is most often told to take it through the face of the curb. This is easier said than done. Since the property line typically ends at the back edge of the sidewalk, the curb is often in a public right of way. Many local agencies will want input on this topic. Assuming the curb is on private property, there are still many questions to be answered. How tall is the curb? How much water is conveyed and how fast is it traveling? Where does the water go after it leaves the curb face?

In a typical 6 in. curb, a 3 in. or 4 in. maximum pipe is recommended. This leaves room for concrete and reinforcing above the pipe. If not detailed correctly, cracks will occur. Rectangular pipe can be used, keeping in mind that it too needs properly detailed reinforcement over the pipe. The transition from round to rectangular needs to happen far enough away from the curb that it is covered by grade without affecting the flow line of the pipe. The practice of discharging multiple smaller pipes through the curb face is also common. In this case, the smaller pipe systems must be independent all the way back to the drains they serve. Never place a wye fitting backwards in the direction of flow to branch a larger pipe into two smaller pipes.

Finally, check the flow rate and velocity of the discharge where it daylights. Will this flow be hazardous to pedestrians? When sump pumps are involved, this question is especially important. Their discharge must enter a storm drain system from the top through a wye fitting. The pipe must be sufficiently large enough to slow the velocity and return the water to gravity flow rates. The ultimate location of the discharge needs to convey the water to the local storm drain system. On more than one occasion, a sump pump discharge has flowed around a corner only to return to the same drain leading back to the sump pump.  Amusing, but not very effective.

Peter A. Kraut, P.E., CPD, is a licensed mechanical engineer in 19 states. He founded South Coast Engineering Group, near Los Angeles, in 2001. He designs plumbing and HVAC systems for commercial projects including high rises, hospitals, and even amusement parks. He can be reached at (818) 224-2700 or via e-mail at pkraut@socoeng.com.