FLOOD00281

• County: Larimer
• Community: Greeley
• Title: Design Criteria and Construction Specifications volume 2 of 5 Storm Drainage Criteria
• Date: 10/1/1997
• Prepared For: Greeley
• Prepared By: Department of Public Works
• Floodplain - Doc Type: Educational/Technical/Reference Information

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STEP 10: Add the total loss in Column 20 to the energy grade
at the downstream end (sta 0+00) to compute the
energy grade at the upstream end (Sta 1+10) for
'this example.

E.G. (U/S)

= E.G. (O/S) + TOTAL LOSS
= 100.58 + 0.21
= 100.79 (Column 11)
= energy grade
= upstream
= downstream

Where: E.G.
UjS
DjS

STEP 11: Enter the new invert (Column 2), pipe diameter D
(Column 3), pipe shape (Column 5), pipe area A
(Column 6), then compute the constant ~ from Step 1
in Column 7, the computed velocity V in Column 8,
the new Q (Column 9), and the computed velocity
head Hv (Column 10).

STEP 12:

Compute the new water surface,
upstream station (Station 1+10 for
w.s. = E.G. - Hv
= 100.79 -0.58
= 100.21 (Column 4)

W.S., for the
this example).

STEP 13: Repeat Steps 1 through 12 until the design is
complete. The hydraulic grade line and the energy
grade line are plotted on the profile (see Figure
6-5).

DISCUSSION OF RESULTS:

The hydraulic grade line (HGL) is at the crown of the pipe from
Station 0+00 to 2+48. Upstream of the transition (station 2+55.5),
the 54-inch RCP has a greater capacity (approximately 175 cfs) at
that slope than the design flow (145 cfs). The pipe is therefore
not flowing full but is substantially full (i. e., 145/175 = 0.84,
greater than 0.80). The computed HGL is below the crown of the
pipe. However, at the outlet, the actual HGL is higher, since the
outlet of the 54-inch RCP is submerged by the headwater for the 66-
inch Rep. To compute the actual profile, a backwater calculation
would be required; however this accuracy is not necessary for
storm sewer design in most cases.

At the junction (Station 4+55.5), the HGL is below the top of the

pl.pe. However, in this case, the full flow capacity (100 cfs)
the same as the design capacity, and the HGL remains parallel
the top of the pipe. A similar situation occurs at the junction
station 5+65.5 except that the HGL remains above and parallel
the top of the pipe.

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