Storm Drainage Systems: Surface and Subsurface Storm Water

Fundamentals of Storm Drainage Systems

The storm drain is a specific unit of the plumbing system designed to convey rain or storm water to a suitable terminal.
Storm water is typically discharged into street gutters and then conveyed by a public drain system to natural drainage terminals such as canals, rivers, and lakes.
Under general plumbing rules, storm drains are not permitted to discharge into a septic tank or the main sewer line.
Collection and disposal of storm water is a critical phase of plumbing design that must not be ignored to prevent structural and environmental hazards.

Consequences of Improper Storm Drainage

Failure to properly divert roof water can lead to several significant issues:

Settlement of the structure caused by erosion or the washing away of soil from the foundations.
Subjecting basement walls and floors to unnecessary ground water pressure and potential leakage.
Run-down water creating leaks in walls and windows.
Water spilling onto people passing by or approaching entry doors.
Erosion of surrounding grounds and disfigurement of landscaped areas.

Government Regulations and Flood Control

The disposal of storm water is a major concern for local and national governments.
Large appropriations are regularly incorporated into annual budgets specifically for drainage purposes.
Flood control is a priority program for government infrastructure.
The modern trend is to provide storm sewer lines for commercial, industrial, and residential establishments.
Laws and ordinances often make the connection of storm drains to storm sewer lines compulsory.

Splash Pan Mechanics

A splash pan is a specialized collector for water coming down from a downspout.
It functions by leading the accumulated water away from the house at a relatively low rate of flow to prevent erosion near the foundation.
It is typically made of concrete and leads water toward a dry well or other drainage points.

Classifications of Storm Drains

Storm drains are classified into three primary types based on their location and installation method:

Inside Storm Drain: These are located under the basement floor or within the walls of the building. This type is common in congested business districts or for buildings occupying the entire lot frontage. For large buildings, multiple lines are used to convey water from the roof as well as inside courts or open areas.
Outside Storm Drain: This system is installed outside the foundation walls of the building. It is suitable for locations where the building does not occupy the entire lot.
Overhead Storm Drain: This is utilized when the street drainage elevation is higher than the basement floor of the building. It relies on gravity flow. Pipes are fitted and suspended inside the ceiling using suitable hangers spaced at close intervals.

Sizing Considerations for Storm Drains

Several factors determine the required size of a storm drain pipe:

Rainfall Intensity: Gauging rainfall over a specific period, noting if it is constant or consists of exceedingly heavy showers of short duration.
Roof Characteristics: Consideration of varying roof areas, the slope (pitch), and the total distance water must travel before reaching the roof conductors.
Pitch Effect: Water drains faster on high-pitched roofs compared to flat roofs, requiring larger drainage pipes.
Building Height: The height of the building significantly contributes to the velocity of water falling inside vertical pipe conductors. This velocity fall accelerates the flow rate entering the storm drain.
Fittings: The use of improper fittings and short offsets, which can negatively affect water flow, must be avoided.

Rainfall Specifics and Pipe Capacities

A conservative estimate for maximum rainfall in the Philippines is approximately $20\,mm$ in a $5\,minute$ interval.
This data is used to compute the approximate volume of water accumulated on a roof per minute.

Table 9-1: Size of Storm Drain

This table indicates the maximum drained roof area in square meters based on pipe diameter and slope percentage:

Diameter of Pipe (mm / In.)	$2\%$ Slope ( $sq.\,m$ )	$3\%$ Slope ( $sq.\,m$ )	$4\%$ Slope ( $sq.\,m$ )
$75\,mm$ ( $3"$ )	$114$	$142$	$170$
$100\,mm$ ( $4"$ )	$242$	$315$	$388$
$125\,mm$ ( $5"$ )	$438$	$566$	$694$
$150\,mm$ ( $6"$ )	$700$	$903$	$1,105$
$200\,mm$ ( $8"$ )	$1,463$	$1,888$	$2,313$
$250\,mm$ ( $10"$ )	$2,563$	$3,309$	$4,055$
$300\,mm$ ( $12"$ )	$4,100$	$5,290$	$6,480$
$350\,mm$ ( $14"$ )	$5,576$	$7,203$	$8,830$

Installation Grades and Change of Direction

Storm drains should be installed with a slope of not more than $2\%$ per meter run.
Changes in direction should use a combination of a $Y$ and a $\frac{1}{8}$ bend, or a long-radius fitting (long sweep elbow).

Roof Leaders and Downspouts

The roof leader, also known as a water conductor or downspout, connects the roof terminal to the storm drain.
They can be either the concealed type (inside walls or partitions) or the exposed type.

Table 9-2: Size of Roof Gutter and Roof Leader

Area of Roof ( $sq.\,m$ )	Gutter Top Dimension ( $mm$ )	Downspout/Leader Diameter ( $mm$ )
$1$ to $10$	$75$	$38$
$11$ to $25$	$100$	$50$
$26$ to $75$	$100$	$75$
$76$ to $165$	$125$	$90$
$166$ to $335$	$150$	$100$
$336$ to $510$	$200$	$125$
$511$ to $900$	$250$	$150$

Calculations and Illustrations

Illustration 9-1: Storm Drain Diameter

Find the size of a storm drain for a roof measuring $20 \times 30\,meters$ with a $2\%$ slope.

Calculate Area: $20\,m \times 30\,m = 600\,sq.\,m$ .
Refer to Table 9-1: Under a $2\%$ slope, the maximum area for a $150\,mm$ pipe is $700\,sq.\,m$ . Since $600$ is within this limit, specify a $150\,mm$ ( $6"$ ) diameter pipe.

Illustration 9-2: Downspout Sizing with Multiple Terminals

A roof has two sections: Section A ( $10 \times 20\,m$ ) and Section B ( $8 \times 20\,m$ ).

Section A Area: $10\,m \times 20\,m = 200\,sq.\,m$ . According to Table 9-2, this requires a $100\,mm$ ( $4"$ ) downspout.
Section B Area: $8\,m \times 20\,m = 160\,sq.\,m$ . According to Table 9-2, this requires a $75\,mm$ ( $3"$ ) downspout.
Refined Strategy: To avoid overloading long gutters ( $20\,meters$ ), it is standard practice to provide two or more terminals per leader.
Cross-Sectional Math: A $100\,mm$ ( $4"$ ) pipe has a cross-sectional area: $Area = 0.7854 \times 4^2 = 12.56\,sq.\,in$ . Dividing this into two terminals requires each terminal to have an area of $6.28\,sq.\,in$ . A $75\,mm$ ( $3"$ ) pipe has an area of $7.06\,sq.\,in$ , which is greater than $6.28$ . Therefore, specify $2$ pieces of $75\,mm$ diameter pipe for Section A.
Second Solution (by Area):
- Section A ( $200\,sq.\,m$ ) / $2$ terminals = $100\,sq.\,m$ per terminal. Table 9-2 shows $100\,sq.\,m$ fits within the $76$ to $165$ range, requiring a $75\,mm$ pipe. Specify $2$ pieces of $75\,mm$ .
- Section B ( $160\,sq.\,m$ ) / $2$ terminals = $80\,sq.\,m$ per terminal. Table 9-2 indicates $75\,mm$ is sufficient for $80\,sq.\,m$ . Specify $2$ pieces of $75\,mm$ .

Advanced Drainage Strategies

Flat vs. Pitched Roofs: Flat roofs often utilize interior leaders concealed by partitions. Pitched roofs require gutters and leaders.
Eaves and Trenches: Leaders can sometimes be omitted in one-story structures with wide overhanging roofs. A gravel-filled trench at the perimeter directly below the eaves catches roof water.
Subsurface Prevention: To prevent water from entering a basement without using waterproofing, perforated pipes and loose gravel can be used to direct water to a storm drain.
Non-Permeable Soil: If soil is not permeable, perforated pipes are essential for ensuring proper subsurface drainage.

Plumbing Symbols and Abbreviations

FD: Floor Drain
SD: Shower Drain
WC: Water Closet
LAV: Lavatory
KSK: Kitchen Sink
CO: Clean Out
LT: Laundry Tub
WH: Water Heater
WM: Water Meter
GV: Gate Valve
CV: Check Valve
VSTR: Vent Stack Thru Roof
SHO: Shower Head
D.S.: Downspout
S.T.: Septic Tank