STORMWATER DRAINAGE SYSTEM DESIGN
A. Design tasks
1. Determine quantity of stormwater--the peak flow discharge resulting from a storm of a
certain specified return period, using the rational method or TR55 method.
2. Establish a sewer capacity to pass the design peak rate of stormwater flow--the size of sewer,
using Manning's equation.
B. Design criteria and considerations
1. Design frequency--the return period of the design storm is 5 years for residential area.
2. Design flow depth--flowing full in pipe at a design slope, which generally is ground slope.
3. Design velocity
(a) The non-depositing (self-cleansing) velocity for silt and grit is vmin = 3 ft/s (0.9 m/s).
(b) The maximum velocity from scour consideration is vmax = 15 ft/s (5 m/s).
(c) The design velocity should satisfies vmin < vdesign < vmax.
4. Design slope
(a) Generally the ground surface slope.
(b) It may be adjusted to a larger slope than the surface grade from the non-depositing
consideration.
(c) Minimize the design grade (slope) for minimum excavation, still meeting the criteria
above.
5. Sewer line can be laid by the shortest route from cost consideration.
6. No closed loops are allowed in a pipeline system
7. Concrete circular pipes are most commonly used in stormwater drainage systems
8. Location of inlet (intercept point): stormwater is collected at low point of contributing area
and directed to a sewer pipe, considering runoff direction and flow pathway.
C.
Design computations
1. Compute the design peak storm flow discharge, Qdesign, using rational (or TR55) method for
a storm of a specified return period (of 5-year for residential area).
2. Estimate the maximum pipe size, Dmax, by the continuity equation
Qdesign = vminAmax, where vmin = 3 ft/s (the self-cleansing velocity) and
Amax = 3.14 (Dmax/2)2.
3. Calculate pipe diameter, D, by Manning's equation, from Qdesign, sewer slope equal to
ground surface grade, and roughness, n (Table 10.4).
4. If D < Dmax
(a) Select D as Ddesign (may be rounded to a next higher standard size) and
select the ground surface grade as the sewer slope, Sdesign.
(b) Check the design velocity, vdesign = Qdesign /Adesign, where Adesign = 3.14 (D/2)2.
If vdesign > vmax, recompute D for vmax by continuity equation from scour
consideration, and recompute sewer slope by Manning's equation.
In this circumstance, a slope less than the street grade is calculated and used.
5. If D > Dmax
(a) Select Dmax as design size for flushing of the pipe (non-depositing) and
(b) Recompute sewer slope by Manning's equation for Dmax.
In this case, a steeper slope than street grade is obtained and used to establish the sewer
capacity to convey the design peak flow.
D. Storm Sewer Design Process
General design procedure:
Design starts at the upper end of a storm sewer system and proceeds downstream, following
flow direction and pathway, in the order of pipe sections (between two manholes/intercepts) to
laterals, to submains, and to the main.
Design of pipe section (size D and slope So) alternates with computation of design flow (Q).
Step 1. Determine the total drainage area (sum of subareas) contributing to the intercept point
(inlets).
Step 2. Compute time of concentration, tc, for the intercept point (the longest of all possible routes),
including sewer flow time in upstream pipe section that has been already designed.
Step 3. Compute the peak flow, resulting from a design storm, at the intercept point, including flow
in upstream pipe section.
Step 4. Design the pipe section immediately downstream, i.e. between the current intercept point and
next one.
(a) Determine pipe slope (ground slope is first selected)
(b) Compute the pipe size (diameter) necessary to just flow full under uniform flow condition
and to carry the design flow capacity (Manning’s equation).
(b) Check flow velocity, using continuity equation, and
(c) Adjust pipe diameter and slope if necessary.
Step 5. Compute time of concentration and peak flow at next intercept point down slope
Design next pipe section by repeating steps 1 to 4.
Design all pipe sections in a sewer lateral.
Design all upstream laterals of a storm sewer submain.
Design all upstream submains of a storm sewer main.
Design the main of the storm sewer system
Step 6. Check ground cover above pipe (minimum 2-3 ft).