SW6220603_Design Calculations (Riprap)_20220916

User Input Data
Calculated Value
Reference Data
Designed By:
JEH 5/23/2022
Checked By:
JEH 5/23/2022
Company:
Hilliard Engineering, PLLC
Project Name:
Harnett Self Storage
Project No.:
HC2202
Site Location (City/Town) Harnett County, NC
Culvert Id. Storm Outlet
Total Drainage Area (acres) 6.73
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 Urith no defined channel are assumed
to have a minimum tailwater condition unless reliable flood stage elevations
show otherwise_
Outlet pipe diameter, Do (in.)
Tailwater depth (in.)
Minimum/Maximum tailwater?
Discharge (cfs)
Velocity (ft./s)
36
6
Min TW (Fig. 8.06a)
53.77
7.61
Step 2. Based on the tailwater conditions determined in step 1- enter Figure
8.06a or Figure 8.06b. and determine duo riprap size and minimum apron length
(L) The d,,,., size is the median stone size in a well -graded nprap 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 ?.
Riprap dso, (ft.)
Minimum apron length, La (ft.)
Apron width at pipe outlet (ft.)
Apron shape
Minimum TW
Figure 8.06a
0.6
20
9
Trapezoidal
Maximum TW
Figure 8.06b
9
Trapezoidal
�D 0

Apron width at outlet end (ft.)
23
Step 4. Determine the maximum stone diameter:
d,,,ax = 1.5 x d5o
Minimum TW
Max Stone Diameter, dmax (ft.) 0.9
Step 5. Determine the apron thickness:
Apron Thickness(ft.)
Apron thickness = 1.5 x dr,,,
Minimum TW
1.35
3
Maximum TW
0
Maximum TW
0
Step 6. Fit the nprap apron to the site by making it level for the minimum
length_ L,. from Figure 8.06a or Figure 8.06b. Extend the apron farther
dostinstream and along chaimel banks until stability is assured. Keep the
apron as straight as possible and align it With the flow of the receiving stream.
Make any necessary alignment bends near the pipe outlet so that the entrance
into the receil-ing stream is straight.
Some locations may require lining of the entire channel cross section to assure
stability_
It may be necessary to increase the size of nprap where protection of the
channel side slopes is necessary (4ppendzr 8.05)_ Where overfalls exist at
pipe outlets or flows are excessive. a plunge pool should be considered, see
page 8-06-8-