Stormwater Systems
ARCH-433
Attendance
This water closet,
installed in Pullman,
Washington, flushes in
a counterclockwise
rotation. In what
direction would the
water rotate if it is
installed in Sydney,
Australia?
What You Need To Know
Understand how and why stormwater
systems operate
Know the differences between
stormwater and sanitary systems
Strategies available for reducing potable
water use
LEED credit points associated with a
reduction in potable water use
What You Need To Be Able To Do
Predict stormwater drainage problems
Be able to sketch a stormwater riser diagram
Correctly size a stormwater system
Calculate the amount of stormwater available
for non-potable uses
Make management decisions on the use and
implementation of stormwater systems
Terms
Stormwater systems
Roof drain (RD)
Overflow system
(OFL)
Leader
Flow control roof
drain
Downspout nozzle
(DSN)
Imperviousness
Stormwater
Harvesting
Area Drain
Purpose of Stormwater Systems
Collect stormwater from the roof and
move it to a point of disposal away
from the foundation
Collect stormwater from the site and
move it to an approved point of
disposal
Difference Between Sanitary
and Stormwater Systems
Absence of volatile
gases
No traps required
No venting required
Point of disposal
varies
Stormwater Discharge Points
Grade
Municipal storm sewer
system

Street
Stream, river, lake,
ocean or other suitable
point
Yes BUT, is this really
what we want!
New Code Requirements
Stormwater Systems
Roof drain systems


Flat roofs
Pitched roofs
Site systems

Qflow = Area (acres) x CR x IR = Ft3/Sec
Roof Drain Systems
Pitched Roofs

Gutters and leaders
Flat Roofs



Roof drains
Overflow drains
Flow control systems
Flat Roofs
Advantages?
Disadvantages?
Strategies - Roof Design
Strategies – Roof Design
Sizing Stormwater Systems
Variables for
commercial roof
drain systems




Area of the roof
Rainfall rate
Number of roof
drains
Slope of piping
Variables for
residential gutter
systems




Area of the roof
Rainfall rate
Number of gutters
Slope of gutter
What about Parking Structures
Floor Drains
Poor Design Consideration


Designed by an Architect
Designed by an Engineer
Concrete rot
Repairs
Roof Drains in a Flat Roof
Rainfall Rate (Table 8-15)
Pipe Capacity (Table 8-16)
Overflow Drain Systems
OFL
2”
RD
Beware of shortcuts!
DSN
Overflow Drain Systems
Required by virtually
every code

Prevents overloading
of the structure
Drain Installation
Overflows
Can also use
scupper drains


Advantages
Disadvantages
Strategies - Flow Control Roof
Drain System
Siphonic Drain Systems
Air Baffle
LEED SS Credit 6.1
Credit 6.1

Option 1 – Existing Imperviousness is ≤
50% (undeveloped sites)
 Implement a stormwater management plan
that prevents the post development discharge
rate and quantity from exceeding the predevelopment levels or:
 Implement a stormwater management plan
that protects receiving stream channels
LEED SS Credit 6.1
Credit 6.1

Option 1 – Existing Imperviousness is ≤
50% (undeveloped sites)
Protect stream channels from excessive
velocity
LEED SS Credit 6.1
Credit 6.1

Option 2 – Existing Imperviousness is ≥
50%
 Implement a stormwater management plan
that results in a 25% decrease in volume of
stormwater runoff from the two-year, 24 hour
storm
LEED SS Credit 6.1
Credit 6.1

Option 2 – Existing Imperviousness is ≥
50%
Bioswales
Cisterns, holding ponds
Green roofs
Rainwater harvesting
LEED SS Credit 6.2
Credit 6.2

Implement a stormwater management plan that
reduces impervious cover, promotes infiltration
and captures and treats stormwater runoff from
90% of the average annual rainfall using
acceptable best management practices (BMP)
 Remove 80% of the average annual post Total
Suspended Solids (TSS)
LEED WE Credit 2
Credit 2

Reduce the use of municipally provided
potable water for building sewage
conveyance by at least 50%
or

Treat 100% of wastewater on site to
tertiary standards
Water Reduction Strategies
Use of Stormwater/Grey water to:


Flush water closets and urinals
Water landscape
 Avoid non-native plants

Reduce the amount of water flowing across
site
Carpenter
How much stormwater (clear water) do
we need?

? gals per day to flush low flow water
closets
 How many work days per year?
How Much Rainwater is
Available?
Based upon .62 gallons/inch/sq. ft.
Efficiency varies between 75% and 90%
V
collected
= Rainfall x Area x Efficiency x .62 gals/inch/sq. ft.
Source: Texas Manual on Rainwater Harvesting, 3rd Edition, 2005
Rainwater Harvesting Capacity
Month
January
February
March
April
May
June
July
August
September
October
November
December
Lincoln
Maximum Minimum
1.59
0
1.63
0.06
6.65
0.06
7.21
0.26
10.09
0.91
7.67
0.63
12.5
0.37
8.57
0.07
8.28
0.29
5.4
0.01
3.81
0.01
3.42
0.04
Ave. Annual Total
50%
0.54
0.52
1.7
2.54
3.89
2.98
3
2.73
2.35
1.33
1.12
0.69
23.39
Source: National Oceanic and Atmospheric Administration, Department of Commerce
Calculating Water Storage
Month
Storage Capacity (Gals)
Starting Storage (Gals)
January
February
March
April
May
June
July
August
September
October
November
December
Water Demand
(Gals)
Irrigation
Demand
(Gals)
Total
Demand
(Gals)
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
0
0
0
0
0
0
0
0
0
0
0
0
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
8,710
Average
Rainfall
(inches)
Catchment
Area
(Sq. Ft.)
Collection Collection
Coefficient Efficiency
0.54
12,600
0.62
0.52
12,600
0.62
1.7
12,600
0.62
2.54
12,600
0.62
3.89
12,600
0.62
2.98
12,600
0.62
3
12,600
0.62
2.73
12,600
0.62
2.35
12,600
0.62
1.33
12,600
0.62
1.12
12,600
0.62
0.69
12,600
0.62
Total rainfall collected (Ave.)
90% of average rainfall (SSc6.2)
Difference
Flush amount required
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
Rainfall
Collected
(Gals)
3,586
3,453
11,288
16,866
25,830
19,788
19,921
18,128
15,604
8,831
7,437
4,582
155,314
139,783
15,531
104,520
End of
Month
Surplus
(Gals)
7,500
0
-5,124
-5,257
2,578
8,156
17,120
11,078
11,211
9,418
6,894
121
-1,273
-4,128
Stored
Amount
(Gals)
-5,124
-10,381
-7,803
353
7,500
7,500
7,500
7,500
7,500
7,500
6,227
2,099
What is Wrong with this
Picture?
Strategies
Two 7,500 gal tanks
installed to capture
rainwater


Rainwater is filtered with
a recirculation system
and used to flush water
closets
Capture system reduces
stormwater discharge
Green Roofs