SW3190506_Calcs_(HW-2 Riprap Outlet Protection)_20190701

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Designed By: FC Date:
Checked By: FC Date:
Company: Cardno
Project Name: Sealand Office
Project No.: SLND18001
Site Location (City/Town) Town of Midland
Culvert Id. HW-2
Total Drainage Area (acres) 5.9
Step 1. Determine the tailwater depth from channel characteristics below the
pipe outlet for the design capacity of the pipe_ If the tailwater depth is less
than half the outlet pipe diameter, it is classified minimum tailwater condition.
If it is greater than half the pipe diameter, it is classified maximum condition.
Pipes that outlet onto wide flat areas with no defined channel are assumed
to have a minimum tailwater condition unless reliable flood stage elevations
shoe- otherwise.
Outlet pipe diameter, Do (in.)
Tailwater depth (in.)
Minimum/Maximum tailwater?
Discharge (cfs)
Velocity (ft./s)
14.4
Max TW (Fig. 8.06b)
2.26
7.2
Step 2. Based on the tailwater conditions determined in step 1. enter Figure
8.06a or Figure 8.06b. and determine d50 riprap size and minimum apron length
(L.)_ The d50 size is the median stone size in a well -graded riprap apron_
Step 3. Determine apron width at the pipe outlet, the apron shape. and the
apron width at the outlet end from the same figure used in Step 2_
Minimum TW
Figure 8.06a
Riprap d50, (ft.)
0.5
Maximum TW
Figure 8.06b

Minimum apron length, La (ft.)
Apron width at pipe outlet (ft.) 4.5
Apron shape trapezoid
Apron width at outlet end (ft.) 9.5
Step 4. Derernime the nix:,a iium stone diameter-
dr,.3%='1.5xd5,
Minimum TW
Max Stone Diameter, dmax (ft.) 0.75
Step 5. Determine the apron thickness:
Apron Thickness(ft.)
Apron thickness = 1.5 x d,_
Minimum TW
1.125
0
4.5
1.5
Maximum TW
0
Maximum TW
0
Step 6. Fit the riprap apron to the site by making it le -,-el for the nrinimtuu
length, La, from Figure 8.06a or Figure 8.06b. Extend the apron farther
downstream and along channel banks until stability is assured. Keep the
apron as straight as possible and align it with the flog of the receiving stream.
Make any necessary alignment bends near the pipe outlet so that the entrance
into the receiving stream is straight.
Some locations may require lining of the entire clut unel cross section to assure
stability.
It may- be necessary to increase the size of riprap where protection of the
channel side slopes is necessary (Appendix 8.05). NI-lere overfills exist at
pipe outlets or flows are excessive, a plunge pool should be considered, see
page 8.06.8_

7/1 /2019