CIV E 482 - Design Lab Stromwater Management: What Is The Importance of Today's Lab?
CIV E 482 - Design Lab Stromwater Management: What Is The Importance of Today's Lab?
CIV E 482 - Design Lab Stromwater Management: What Is The Importance of Today's Lab?
Stromwater Management
Stormwater is rainwater or melted snow that runs off streets, lawns and other sites.
When stormwater is absorbed into soil, it is filtered and ultimately replenishes
aquifers or flows into streams and rivers. (Source: www.epa.gov)
This lab will give you exposure to the impacts of stormwater and give you practice on
how to size culvert and basic BMPs.
For this class, you will only need to mitigate for the roadway runoff.
Merging surfaces allows you to see the final grading, which is useful when assessing your proposed
earthwork and drainage.
Within the Toolspace, right click on the Existing Surface, and select Surface Properties… and
change the Surface Style to _No Display
Repeat the above step for the Proposed Roadway Surface and change the Surface Style to _No
Display
You should now see the Merged-With-RDWY surface clearly
Turn off the sample lines as detailed below:
o Expand Corridors from within Toolspace > right click on the corridor that you would like
to edit > select Properties…
o Click on the Codes tab > copy the current code set style and rename it to “Revised”
o Click on the Information tab, and change the name to “Display No Sample Lines”
o Click on the Codes tab > click on the edit Code set style > expand Link > on the default
row, click Standard under the Style column > Copy, and Rename it to “No Display”
under the Information tab, and turn off the visibility of the Link under the Display tab,
click Apply and OK > click standard under the Style column and change it to No Show,
and click OK > click Apply and OK > click Apply and OK again.
Drainage Assessment
Notice where there are flow paths that end adjacent to or flow along the edge of your daylight. Those are
the areas that you should consider designing a culvert to channel the upstream water under the road.
Create a new layer called C-DCV
and set it current. Based on the
Watersheds, Slope Arrows and
the Rain Fall Paths - sketch out a
boundary (using a Polyline) for
each of the watershed areas, but
only draw the watershed within
your property boundary (for this
class). Make sure that each of
the polylines are closed and
have an elevation of “0”.
From this point, you will be able
to determine the total number of
drainage areas within the project
boundary, and you will also be
able to determine the area of each.
Fill Area
Cut Area
Where the water flows under the roadway, you will need to design a culvert. Where the water flows
over the roadway, you may want to design a drainage ditch, a roadside channel, and add median cuts
while the road is super elevated to avoid having to install inlets and to allow the water to move off the
road to a safe area for treatment.
Culvert Design
Determine the 100-year discharge (Q100) for all of your culverts. Consider using an excel spreadsheet so
that you can easily repeat the operations. If done in an excel spreadsheet, BE SURE to show ALL of
your hand calculations for at least one (1) of the culverts.
You will need to consult the Highway Design Manual Chapters 800 through 870, and the 6 HOUR
For your Information
Rainfall Isopluvial maps in the San Diego County Hydrology Manual to complete this section.
Culvert Lengths
Based on the actual length of your
designed/sketched culverts. The culverts
should extend from daylight line to daylight
line
Culvert Length(s):
1) 2)
3) 4)
5) 6)
7) 8)
Culvert Length
Where:
∆E = change in elevation along he effective slope line (in feet)
L = Length of the longest flow path (in feet using Figure 3-4)
I = 7.44 * P6 * Tc (-0.645)
Where:
P6 is the adjusted 6-hour storm rainfall amount
Tc is the time of concentration in minutes Note: P6 must be within 45% & 65% of
P24, otherwise make adjustments to P6
Areas
Expand Surfaces (in Toolspace) and right click on the Merged-With-RDWY Surface
Click Edit Surface Style…
o Click the Display tab
o Make the Watershed & Slope Arrows Component Types invisible
Area #1
As an example: Catchment Area #1 = 388,747 SQ FT
Convert to acres
Inlet Control – Inlet control occurs when the culvert barrel is capable of conveying more flow than the
inlet will accept.
Outlet Control – Outlet control flow occurs when the culvert barrel is not capable of conveying as much
flow as the inlet opening will accept.
Nomograph solutions provide reliable designs for many applications. It should be remembered that
velocity, hydrograph routing, roadway overtopping, and outlet scour require additional and separate
computations beyond what can be obtained from the nomographs. The following example is for the
(basic) design of a concrete pipe culvert.
The following design procedure requires the use of inlet and outlet nomographs
(Step 1)
List the design data needed:
Q = discharge (cfs) L = culvert length (ft)
S = culvert slope (ft/ft) TW= tailwater depth (ft)
Ke = inlet loss coefficient (grooved-end) D = pipe diameter (in)
V = velocity for trial diameter (ft/s) dc = critical depth
HW= allowable headwater depth for the design storm (ft)
(Step 2)
Determine trial culvert size by assuming a trial velocity between 2 to 10 ft/s & computing the
culvert area
A = Q/V
Determine the culvert diameter
(Step 3)
Find the actual HW for the trial size culvert for both inlet and outlet control.
For inlet control, use the inlet control nomograph with D and Q, and find HW/D
for the proper entrance type.
Compute HW and, if too large or too small, try another culvert size before
computing HW for outlet control.
For outlet control, use the outlet control nomograph with the culvert length,
entrance loss coefficient, and trial culvert diameter.
To compute HW, connect the length scale for the type of entrance condition and
culvert diameter scale with a straight line, pivot on the turning line, and draw a
straight line from the design discharge through the turning point to the head loss
scale H. Compute the headwater elevation HW from the following Equation
HW = H + ho - LS
where:
H = head loss, ft
ho = ½(critical depth + D) or tailwater depth, whichever is greater (maximum =
D)
L = culvert length
S = culvert slope
Civ E 482 LAB
San Diego State University 9
[EXAMPLE] Culvert Design Calculation
Example provided by the Knox County Tennessee Storm Water Management Manual
Input Data
Discharge for 2-year flood = 35 cfs Discharge for 25-year flood = 70 cfs
Allowable headwater (HW) for 25-year discharge = 5.25 ft Length of culvert = 100ft
Culvert slope = 0.012 ft/ft Entrance type = Groove end with headwall
Calculations
Step 1 Assume a culvert velocity of 5 ft/s. Required flow area = 70 cfs/(5 ft/s) = 14 ft2 (for the 25-yr
recurrence flood).
Step 2 The corresponding culvert diameter is about 48 in (4 ft). This can be calculated by using the
formula for area of a circle: Area = (3.14D2)/4 or D = (Area times 4/(3.14))0.5
Therefore,
D = 9(14)(4)/(3.14))0.5(12in/1ft)
D = 50.7 in
Step 3 A grooved end culvert with a headwall is selected for the design. Using the inlet control
nomograph, with a pipe diameter of 48 inches and a discharge of 70 cfs; read a HW/D value of
0.93
Step 4 The depth of headwater (HW) is (0.93) x (4) = 3.72 ft, which is less than the allowable headwater
of 5.25 ft. Since 3.72 ft is considerably less than 5.25, try a smaller culvert.
Step 5 Using the same procedures outlined in Steps 3 and 4 the following results were obtained.
Step 6 The culvert is checked for outlet control by using outlet control nomograph
With an entrance loss coefficient Ke of 0.20, a culvert length of 100 ft, and a pipe diameter of 36
in, an H value of 2.8 ft is determined. The headwater for outlet control is computed by the
equation: HW = H = ho - LS
Therefore:
ho = 3.5 ft
Step 7 Since HW for outlet (5.10 ft) is greater than the HW for the inlet control (4.98 ft), outlet control
governs the culvert design. Thus, the maximum headwater expected for a 25-year recurrence
flood is 5.10 ft, which is less than the allowable headwater of 5.25 ft.
Step 8 Estimate outlet exit velocity. Since this culvert is based on the outlet control and the discharge
into an open channel downstream with tailwater is above the culvert, the culvert will be flowing
full at the flow depth in the channel. Using the design peak discharge of 70 cfs and the area of a
36-inch or 3.0-foot diameter, the exit velocity will be:
Q = VA
With this high velocity, consideration should be given to provide an energy dissipater at the
culvert outlet.