Storm Drain (Pipe) Sizing
General Goals:
• List and describe drainage design steps
• Size storm drains in a drainage system
• Judge adequacy of drainage system design
Specific Objective:
• Determine (quantify) size of pipe needed to
accommodate peak discharge to a given drain
Pipe
Objectives Procedure Constraints Hydraulic Design
Characteristics
1. Determine contributing area to upstream end of
drain
2. Determine tc and runoff coefficient for
contributing area
• Determine design rainfall intensity for tc
• Determine peak discharge (typically using
Rational Method)
• Size pipe for peak discharge and given
constraints (typically using Manning’s equation),
assume pipes will flow full under design
discharge but not be placed under pressure
head
• Repeat process for downstream drains (account
for increasing contributing area)
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Pipe
Objectives Procedure Constraints Hydraulic Design
Characteristics
Velocities should be > 2.0 ft/s to avoid
siltation and < 10-15 ft/s to avoid damage
to system components
Minimum slope of 0.5% is often required
Head losses should be minimized
Minimum drain size is also usually
specified in local drainage ordinances;
12-15 inches is a common minimum
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Pipe
Objectives Procedure Constraints Hydraulic Design
Characteristics
Local ordinances often require the size of storm
drains to not decrease in the downstream direction
When making a
transition at a
junction, the
vertical
alignment of the
incoming and
outgoing pipes
should be such
that the crowns
line up (not the
inverts) Invert = elevation of the lowest level of the pipe at a specific point along the run of the pipe
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Pipe
Contributing Area and tc
Objectives Procedure Constraints Hydraulic Design
Characteristics
Side note:
Ensure that
you account
for tc and Area 2
contributing
area
properly!
Area 3
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Pipe
Objectives Procedure Constraints Hydraulic Design
Characteristics
After the peak discharge has been computed
you must select an appropriate pipe material,
shape, slope, and length
One approach:
Assume a pipe size, then determine the capacity
of that pipe using standard design charts
(charts 35 - 51 in file “hds3.pdf” - downloadable
from
http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_num
ber=4&id=9)
If the capacity is inadequate alter the size of the
pipe and recheck the capacity
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Pipe
Objectives Procedure Constraints Hydraulic Design
Characteristics
OR:
Given a design flow, the exact pipe dimension to
convey the flow can also be determined using
Manning’s equation, Darcy-Weisbach equation, or
numerous other equations, or software:
Manning:
Darcy-Weisbach:
where d = minimum design pipe diameter
n = Manning’s roughness coefficient
Kn = constant; 1.0 for SI, 1.49 for USCS
Qp = design peak discharge
So = local slope
f = friction factor
g = gravitational constant
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Pipe
Objectives Procedure Constraints Hydraulic Design
Characteristics
The final step in designing a storm drain system is
to check the HGL
Computing the HGL will determine the elevation,
under design conditions, to which water will rise
in various inlets, manholes, junctions, etc.
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Pipe
Objectives Procedure Constraints Hydraulic Design
Characteristics
1. When hydraulic calculations do not consider minor losses
(e.g., expansion, contraction, bend, junction), the elevation
of the HGL for the design flood should be at least 1.0 ft
below ground elevation
2. If all minor losses are accounted for, it is usually
acceptable for the HGL to reach the gutter elevation
3. Minor losses should be included when the velocity
exceeds 6 ft/s
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
Pipe
Objectives Procedure Constraints Hydraulic Design
Characteristics
In-Class Exercise…see you in class!
Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology
