CE 3372 Storm Sewer Design and Analysis
Fall 2014
Assumptions/Problem Statement
Figure 1 is a map of a subdivision in Houston, Texas. Your team has been selected
to design a storm sewer system for the subdivision, and demonstrate the design will
function for several design rainfall events in Harris County, Texas.
Figure 1. Lazybrook Subdivision Layout
The streets are all curb-and-gutter sections with 8-inch curbs. The residential lots
all drain towards the streets with slope of 0.005. The map shows elevations in feet
above MSL. The outfall for the storm sewer is to be a 8-foot diameter RCP whose
invert elevation will be 44 feet (discharging into the bayou, whose water elevation
during non-storm conditions is 35 feet).
The outline on the map is the area to be drained – on the south of the drawing the
portion of W 18th street outside the outline drains elsewhere, as does the area north
of Brooktree Dr.
Storm inlets are to be located at least every 200 feet along the streets, and each
intersection must have an inlet. Use SWMM to demonstrate the hydraulics is
satisfied by your teams design, and plot the hydraulic grade line for your system for
the two design storms in the table below. The maximum velocity in the sewer
should not exceed 9 feet/second – use vertical drops as needed to control velocity.
Duration
Frequency
3-hour
2-year
3-hour
25-year
Performance
All runoff contained in
storm sewer – no storage
in streets
Surcharge OK – up to 8inches in storage in
streets OK
As part of the engineering design-and-analysis the following exhibits will probably
be useful:
1) A topographic map of the land surface (as indicated by the elevations shown
on Figure 1).
2) A topographic map of the upper elevation of the storm sewer (soffit
elevations) – the minimum cover in the subdivision is to be 6-feet (that is the
storm sewer lines are to be buried at least 6 feet below the land surface).
3) A cumulative rainfall time series for the 2-year design storm, with rainfall
depths every 10 minutes.
4) A cumulative rainfall time series for the 25-year design storm, with rainfall
depths every 10 minutes.
5) A map showing the locations of the inlets. You may assume all inlets are 10foot curb inlets.
6) A map showing the drainage areas to each inlet, and approximate flow-path
for the drainage area.
7) A profile plot showing the hydraulic grade line for the sewer system (along
the longest alignment) for the two design storms at the largest flow.
8) An outflow hydrograph at the outfall for the two design storms.
9) A profile plot showing the hydraulic grade line for the sewer system for the
two design storms if the bayou water elevation is at 58 feet.
Project Report
Provide a report to the client using the following format:
1. Letter of Transmittal
2. Executive Summary (1 page)
3. Introduction (background information, methods and procedures, etc.)
a. Topographic map(s)
4. Hydrologic Analysis
a. Design storms (include data sources in appendix)
b. Determination of Curve Numbers for inlet drainage areas
5. Hydraulic Analyses of System under various conditions
a. SWMM model construction
i. Inlet locations (final iteration)
ii. Modeling street and sewers (using offsets)
iii. Sewer dimensions (diameters)
b. Hydraulic grade line profiles for different conditions
i. 2-yr storm – free outfall
ii. 25-yr storm – free outfall
iii. 2-yr storm – 58 foot tailwater
6.
7.
8.
9.
iv. 25-yr storm – 58 foot tailwater
c. Hydrograph at outfall
d. Inlet capacity check (external to SWMM, do the inlets have
capacity at estimated flows? -- if not, then add inlets as needed)
Cost Opinion
a. Plan and profile drawing (similar to the Rocksprings Storm
Sewer Drawings, stationing important, pipe diameters
important, pipe invert elevations important. Inlet details can
be omitted – inlet locations should be shown!)
Summary and Recommendations
References
Appendix
a. Data sources for design storms.
b. One representative SWMM input file.
c. Edited SWMM summary report for each hydraulic simulation.