Sanitary Sewer System Extension
Hydraulic Layout, Performance Analysis, and Trenching
By
TEAM NAME
Executive Summary
Executive Summary ............................................................................................................................... 2
Introduction .............................................................................................................................................. 4
Client Supplied Data .......................................................................................................................... 4
Problem Statement................................................................................................................................. 6
Methodology ............................................................................................................................................. 6
Average, Minimum, and Peak Wastewater Flow. .................................................................. 6
Results ...................................................................................................................................................... 10
Introduction
This report documents a sanitary system extension to an existing system. Client
supplied data are used, along with guidelines from various literature sources to lay
out the sewer plan view, and establish the sewer operating conditions.
The US EPA SWMM 5 computer program is used to simulate the sewer hydraulics
for different anticipated flow rates to compute the water surface elevation in the
sewer system, to select pipe sizes to maintain flow at desirable depths and to
evaluate lift station operations.
Sewer trenching and backfill requirements are supplied with the final layout and
sewer materials specifications.
Client Supplied Data
Figure 1 is a plan-view map of the existing and new system to be added. The main
street sewer line currently exists, and flows from North to South (downward in the
sketch). The map shows approximate land surface elevations in the project vicinity
Figure 1. Client Supplied System Layout.
The laterals along A, B, and C, street are not yet designed. The client provided the
sewage flow contribution from the D street sewer laterals along with the flow in the
main street line from the North. The numbers next to the dots on the figure are
manhole ID numbers, thus the main street line flows from manhole 5 to manhole 26
in the drawing.
The average wastewater flow in the main line at manhole 5 is 0.37 m3/sec. The
average wastewater flow at manhole 26 (current system, including the D street
contribution) is 0.376 m3/sec. The inflow and infiltration in the main line is
considered negligible. The existing main line is 1.065 meters in diameter,
reinforced concrete pipe, on a slope of 0.09%. The invert elevation at manhole 5 is
55.33 meters.
Table 1 is a list of the additional client supplied data for the sewer system extension.
The land elevations are elevations at the surface associated with the manhole
locations in the list. The contributing area is the portion of the service area that
contributes wastewater to the manhole. The lengths are the distances, in meters,
between the manholes.
Table 1: Geometric Data for Conceptual Design
Problem Statement
The problem is to design the sewer system extension between A street and C street
for a population density of 130 persons/hectare. The client has further specified
that sewer pipes must have a minimum cover of 2 meters, a minimum slope of
0.08%, and the designer is required to use a peak-flow factor of 3.0 and a minimumflow factor of 0.5. The minimum allowable pipe diameter is 150 mm.
The system design should include water surface elevation profiles in the system and
peak and minimum flow, evaluation of minimum and maximum velocities in the
context of current codes, and trenching and backfill recommendations. The design
must specify pipe dimensions and materials. Additionally, if the current line must
be resized, a lift station design is to be included to match the crown elevations of the
existing sewer south of D street.
The design goal is to produce a design that satisfies the institutional constraints,
using the smallest diameter pipes possible, that maintain sufficient cleansing
velocity at low flows, and does not surcharge at high flows.
Methodology
The system hydraulics are modeled in the US EPA SWMM program by constructing a
simulation model with the same geometric conditions as supplied by the client. The
analysts adjust pipe diameters and invert elevations to produce a system that
conveys the peak and minimum flows within established guidelines.
Average, Minimum, and Peak Wastewater Flow.
The average wastewater flow is estimated using a population-based approach. The
client specified a population density of 130 persons/hectare. The Texas
Administrative Code [CITE] provides the following formula for a population based
waste flow computation,
Q  A  (N  B)
(1)
where Q is flow in gallons per minute, A is a rate factor, N is the population served,
and B is factor of safety. The TAC provides default values of A and B as 0.15 and 20.

Converting Equation
1 into SI units the following is the average wastewater flow per
hectare:
1 ft 3
1m 3
Q  0.15 * (130 person /ha  20) *
*
 0.0876m 3 /min/ ha
3
7.48gal (3.25 ft)

(2)
The peak-flow factor is given as 3.0 and the minimum flow factor is given as 0.5.
Table 2 is a list of the minimum, average, and peak flow anticipated in the sewer
system calculated using Equation 2. These values are used in the SWMM model to
evaluate the system hydraulics. Of importance for design, the minimum flow is used
to help set slopes to maintain a minimum velocity of 0.6 meters per second (as per
ASCE MOP [CITE]); the peak flow is used to help establish diameters that contain the
flow without surcharge.
The main sewer flowing 75% full is the maximum flow depth acceptable because it
(the main sewer) is a large diameter sewer. This depth ratio is the value suggested
by the ASCE MOP [CITE] for a large diameter sewer. The laterals sewers are
Table 2. Waste Flows to Each Manhole
restricted to 50% full to reflect the greater flow uncertainty in smaller diameter
sewers.
Sewer Hydraulics
The main line excess capacity is checked using Manning’s equation for the predevelopment waste water flow, the computations are reported in the results section,
and these computations were used to guide the selection of an outfall condition in
the subsequent SWMM modeling.
The sewer hydraulics were modeled using SWMM. Figure 2 is a screen capture of a
SWMM model of the system that illustrates the network configuration that is used to
develop design conditions. In the screen capture the network is shown overlaid on
the basemap.
Each manhole is modeled as a storage junction, with about 1m2 junction base area.
The main line flow profile is depicted on the figure. The laterals are not carrying
flow in this figure, and this configuration was used as a base condition to develop
the system design.
Figure 2. SWMM sanitary sewer network.
In the SWMM model, the following modeling concepts are used:
(1) Invert elevations are set at 10 meters below grade, except for the main line.
The main line invert elevations is based on the client supplied invert
elevation at manhole 5 and the supplied slope. These elevations will be
adjusted as part of the design exercise.
(2) The lateral lines are all set at 0.5 meter diameter. The 6 laterals that connect
to the main line are offset 0.5 meters (crown elevation match). This
configuration decouples the laterals from the main lines, unless there is flow
in the laterals. The other lateral lines are adjusted (invert elevation and
offset) as part of the design process.
(3) The roughness coefficient is 0.015 for all lines --- reflective of good condition
concrete.
(4) The outfall node is set at invert elevation that preserves the slope of the
main line.
(5) The outflow condition for all simulations is NORMAL depth at the outfall.
This condition means that outfall control is based on Manning’s loss model
with the pipe slope as the energy slope --- a reasonable approximation to
current sewer design procedures.
Figure 3 is a closer image of the network model from SWMM with the network
shown on a base map built from Figure 1.
Figure 3. SWMM model network (closer view).
Figure 3 illustrates that the portion of the sewer system along D street is not
modeled in SWMM, instead values from the client were used to account for the flow
in this portion. Manhole 26 is part of the model and the D street flows are supplied
as lateral inflow to the manhole independent of the upstream inflows.
Two cases were examined: a pre-development (Case 1) and a post-development
(Case 2) simulation.
Case 1 is the system in its current condition with no flows from the lateral lines.
This base case is used to calibrate the main line model to conditions reported by the
client --- the client supplied flow is used, and friction terms are adjusted to produce
a flow depth consistent with the normal depth in such a sewer, at the peak-flow
discharge. In addition to checking depth, velocities are also checked for
minimum/maximum compliance.
Case 2 is the system, with the laterals carrying discharge. Lateral diameters and
slopes (actually invert elevations) are adjusted to produce flows at about 50% full in
the lateral lines. The friction terms from Case 1 are used for the lateral lines. As
with Case 1, velocities are checked for minimum/maximum compliance.
Trenching and Backfill
Need to describe methods for backfill specification. Pull from literature, state
guidelines, trenching will require added depth, want to know backfill required to
support street load.
Results
This section presents the results of the SWMM analysis, and backfill specifications.
Main Line Capacity
Prior to building the SWMM model, the main line was evaluated for excess capacity.
The main line currently carries about 1.13 m3/sec without the added burden of the
extension. The flow depth in the existing sewer is computed using Manning’s
equation for a circular section to determine if the main line has capacity for the
extension or if it also needs to be redesigned.
Table 3 lists the flow depth computations associated with Manning’s equation for
the current sewer. Current flow depth is about 49% full, therefore using the large
diameter guidelines this sewer has capacity.
Table 3. Main Sewer Flow Depth --- Current Conditions
Table 4 lists the flow capacity assuming the sewer flows at 75% full. The preextension excess capacity is about 0.96m3/sec. The sum of the peak flows in Table
2 is 1.17m3/sec, therefore the existing line has sufficient capacity.
Table 4. Main Sewer Flow Capacity --- 75% Depth
Sewer Design Flow Analysis – Case 1: Pre-Development
The pre-development conditions are a 1.065 meter diameter pipe at 0.09% slope
running south on Main Street as depicted on Figure 1. The capacity analysis
indicates that at normal depth (at the outlet) the flow should by approximately 0.5
meters deep. This depth was used as the calibration depth.
Figure 4 is a screen capture of the SWMM results for this test case. The flow depth
was 0.51 meters, essentially the target depth. The friction terms used were n=0.015.
Figure 4. Depth at outfall, Case 1, Average Flow
Figure 5 is a screen capture of the peak-flow case. The system inflows were
multiplied by the peak-rate factor of 3.0 and the simulation run. In this analysis, the
flow depth is 0.6 meters, which is below the upper limit of 75% full, so the sewer
has sufficient excess capacity for the extension flows. The flow velocity is about 1
meter per second (3.25 feet per second), which is an acceptable value, below an
upper limit (ASCE MOP [CITE]) of 3 meters per second.
Figure 5. Peak-Flow Results, Pre-Development Conditions
Figure 6 is a screen capture of the low-flow case for the pre-development
conditions. The system average inflows were multiplied by the low-rate factor of 0.5
and the simulation run. In this analysis, the flow depth is 0.35 meters, which is
below the upper limit of 75% full, so the sewer has sufficient excess capacity for the
extension flows. The flow velocity is about .7 meter per second (2.5 feet per
second), which is an acceptable value, above a lower limit (ASCE MOP [CITE]) of 0.6
meters per second.
Table 5 summarizes the relevant results for the pre-development hydraulics.
Table 5: Summary of pre-development hydraulics
Low-Flow
Flow Rate (cu.m./s)
Flow Depth (m)
Velocity (m/s)
0.19
0.35
0.74
Average
Flow
0.37
0.51
0.83
Peak-Flow
0.49
0.60
0.96
Figure 6. Low-flow results, pre-development case.
Sewer Design Flow Analysis – Case 2: Post-Development (Extension Added)
The post-development conditions are the lateral lines connected into the main
sewer system, with crown elevations matched, carrying their respective flows. In
this report only the recommended design is used. Substantial trial-and-error was
employed to size the pipes and generate the requisite invert elevations.
Trenching and Backfill Results
Conclusions and Recommendations
The existing main line has sufficient capacity to accommodate the anticipated
additional sewage flows without redesign. The added flows increase sewage depth
in the main line by [fraction of pre-development depth].
The lateral lines are all [material, slopes, diameters] and crown elevations are
matched [not matched].
The trenching and backfill recommended are [recommended]
References