CENE 333L – LAB #8 Stormwater

CENE 333L – LAB #8 Stormwater
All new development causes changes to the stormwater characteristics including; increased
surface runoff flow, locations of concentrated flow, volume of runoff, stretching or narrowing
the hydrograph, changing the timing of the peak flow, erosion problems, and pollution runoff. As
such the design for and control of these stormwater changes has become a significant part of civil
and environmental engineering (CENE) projects that change the conditions of site. For this lab
we will be verifying and designing infrastructure to handle flows generated for parking lots P62,
P62A, and P62B.
For this lab you will be designing a system to handle the stormwater generated on site as well as
offsite stormwater that needs to be routed around your project area.
 Coconino County Engineering Design and Construction Manual and Drainage Design
Manual; http://www.coconino.az.gov/index.aspx?NID=838
 City of Flagstaff Low Impact Development (LID) Requirements
 Maricopa Association of Governments Uniform Standard Details found here
1. Utilize existing design standards for analysis
2. Explain what the rational method and its parameters are
3. Understand what LID is and why it is used
4. Understand the criteria needed for a storm water pipe design
Part 1: Stormwater Runoff Analysis
Using the data provided in the “Sub-Basin Information” spreadsheet you will determine the
runoff from each sub-basin for the 5, 10, 50 and 100-yr storms. To determine these flows you
will use a modified rational equation found in the Coconino County Drainage Design Manual
which is already setup in the “Sub-Basin Information” excel file. To complete the analysis you
will need to determine the area of each sub-basin using the AutoCAD drawing, the rainfall
intensity using the NOAA 14 data for 10-minute duration (based on minimum time of
concentration), and curve number values as determined from the Coconino standards.
You report must address the process used to determine runoff, why a weighted curve number
was generated, what the weighted curves numbers were, limitations of rational equation, how
and why the rational equation we used was modified, what NOAA 14 is and why we used it,
what the time of concentration is and why we used 10-minutes, and the total flow for each storm
generated for the entire watershed.
Part 2: Onsite Low Impact Development
LID is a fairly new stormwater design requirement for CENE projects. LID is necessary to help
reduce two things; changes to the hydrograph, and capturing the first flush to reduce pollutants
that enter streams. Since LID is a new item it is not yet required by all municipalities but it is
quickly becoming required by the majority of municipalities and is also required for any LEED
certified projects. Typically LID is applied as a retention design to capture a specified rainfall
depth of the first part of a storm falling on any increased impervious area. For Flagstaff we need
to design for the first 1” of rainfall falling on all new impervious surface. For the sake of this lab
we will use the impervious surface identified in the “Sub-Basin Information” spreadsheet and
determine the volume of runoff generated by the first 1” of rainfall on 10% of this area.
Impervious surfaces are surfaces that prohibit the majority of the rainfall from penetrating into
the surface, instead it runs off into storm water management infrastructure, examples are metal,
asphalt, concrete, plastic, clays, masonry, etc.
The runoff volume is technically going to be detained but shall be detained in such a way that the
flow increase from the impervious area will not enter local stream ways until the peak flow of
the hydrograph has passed. It is typically accepted that if water is detained for 12 hours then the
increase in flow will not enter streams until after the peak flow therefore the flow will not
contribute to potential flooding peaks.
For the design of your LID system you will need to design three (3) identical basins to hold
100% of the volume, with 1 foot of free-board. You will need to calculate your volumes by hand.
The basin needs to utilize an underdrain system with a perforated pipe that will drain the entire
volume of water in the basin in 12-hours. To determine this you need to use your volume of
water in the basin, 12-hour requirement and convert those to a cfs flow. The pipe design can be
done by hand or in the software of your choice. The basins shall be trapezoidal in shape, have a
minimum bottom width of 4 ft, banks sloped at 4:1, and cannot have a ponding depth greater
than 18”. Final design must also comply with the City of Flagstaff LID standards.
You report must address your final design volume, provide the final design volume calculations
showing volume required and verifying design meets those requirements, the overall depth,
ponding depth, length, ponding volume, total basin volume and plan view area. You also need to
discuss the City of Flagstaff LID requirements and how your design meets them.
Prepare a lab report individually following the provided guidelines. Lab report is due by
the start of class on 05/04/2016 by 5 pm to your instructor’s office for both sections

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