Box Gutter Sizing — AS/NZS 3500.3

Project record Step 3 — site data

Design process

1. 1
   Design ARI

   Determine the design ARI from Clause 3.3.4 (Table 3.3.4). The flow chart assumes 100 years for Australia and 50 years for New Zealand — if other ARI are selected, adjust the flow chart to suit. The Appendix I design graphs are plotted for a 100 year ARI, 5 min duration rainfall intensity.

   Australia — 100 year ARI, 5 min (100I₅) New Zealand — 50 year ARI, 10 min (50I₁₀)
2. 2
   Rainfall intensity for the site

   Determine the rainfall intensity for the site from Clause 3.3.5. (For guidelines, see Appendix D.) Read it off the BOM IFD tool for the site coordinates.

   100I₅ intensity (mm/h)

   Find site by place name

   Site latitude

   Site longitude

   Rainfall source — recorded on the print record

   Open BOM IFD tool for this site ↗

   Australian place names and coordinates from GeoNames (CC BY 4.0). The BOM IFD grid is 0.025° (≈2.8 km), so the grid cell may differ slightly from the site.
3. 3
   Site data, dimensions and layout

   Obtain dimensions and other relevant data from physical observations and measurements, plans or both (see Paragraph J2 for the rainhead example, J3 for sump/side overflow, J4 for sump/high-capacity overflow). Recorded in the project record above.
4. 4
   Position of box gutter, expansion joints and outlets

   Select position of box gutter, expansion joint(s) and outlet(s) based on the approximate maximum catchment from Figure I1 for 16 L/s flow and the site rainfall intensities.

   Gutters discharging to this outlet 1 (single gutter) 2 (two gutters)
5. 5
   Catchment area A_c

   Determine catchment area (A_c) for each section of box gutter and each outlet (see Clause 3.4).

   Catchment area A_c per gutter (m²)
6. 6
   Design flow Q, and total flow through the outlet

   Determine the design flows (Q) from Figure I1 using the rainfall intensity and A_c. Design each box gutter and device separately. Note 2: the total design flow is the summation of the design flow for each box gutter and the section of roofing discharged directly into the sump.
7. 7
   Decision — is the design flow > 16 L/s?

   If yes on the first trial, A_c needs to be reduced: review the selections at Step 4. An increase in the number of outlets may require sumps rather than rainheads. If still yes on subsequent trials, the design is beyond the scope of the general method.
8. 8
   Sole width w_bg and gradient

   Determine sole width (w_bg) and gradients of box gutter from Figure I1, using the design flow.
9. 9
   Box gutter depth

   Determine the actual minimum depth of the box gutter from Figure I1, using Q, w_bg and the gradient.

   Adopt depth mm
10. 10
    Vertical downpipe and rainhead depth h_r

    Select a size of vertical downpipe and total depth of rainhead (h_r) from Figure I3.
11. 11
    Check h_r against Figure I2 Note 1

    Check if the total depth of rainhead (h_r) needs to be adjusted as required by Note 1 of Figure I2 — the Figure applies only where h_r ≥ 1.25 D_e or 1.25 D_i.
12. 12
    Length of rainhead l_r

    Determine the length of rainhead (l_r) from Figure I3. Note 2: Figure I3 is for a gutter gradient of 1:200; for steeper gradients determine the equivalent 1:200 depth from Figure I1 first.
13. 13
    Size of the rainhead

    Determine the size of the rainhead from Figure I2, using h_r and l_r. The width of the rainhead equals the width of the box gutter.
14. 7
    Sole width w_bg and minimum gutter depth d_bg

    Determine sole width (w_bg) and minimum depth of box gutter (d_bg) for free flow conditions from Figure I1, for the box gutter with the maximum flow.
15. 8
    Decision — is the total design flow through the outlet > 16 L/s?

    If yes, review the selections at Step 4. Possible measures include repositioning outlets and expansion joints, increasing the number of outlets, or a combination of measures. If still yes on subsequent trials, the design is beyond the scope of the general method.
16. 9
    Decision — are the gradients of the box gutters flatter than 1:200?

    If yes, the design is beyond the scope of the general method — the graphs chart only 1:40, 1:100, 1:150 and 1:200.
17. 10
    Downpipe and depth of sump h_s

    Select the downpipe from Figure I4 for the total design flow through the outlet, then determine the depth of sump (h_s).
18. 11
    Minimum distance between duct and sump sides ℓ_oc

    Determine the minimum horizontal distance between the sides of the overflow channel and those of the sump (ℓ_oc) from Figure I6(a), for the largest flow in any one box gutter.
19. 12
    Overflow channel width w_oc and depth d_oc

    Select the width of the overflow channel (w_oc) and determine the minimum depth of overflow channel (d_oc).
20. 13
    Height above the overflow weir crest h_t

    Determine the minimum height of the top of the box gutters above the crest of the overflow channel weir: h_t = [ℓ_oc + (d_oc + 30)] − (0.7ℓ_oc).
21. 14
    Decision — is the minimum depth of the box gutter ℓ_oc + (d_oc + 30) appropriate?

    If no, review the selection of overflow channel width w_oc at Step 12, or the selection of box gutter width w_bg at Step 7.

    Adopt depth mm
22. 15
    Size of the sump/side overflow device

    Determine the size of the sump/side overflow device and sump from Figure I6, using ℓ_oc, d_oc and h_s. Sump length w_oc + 2ℓ_oc, minimum 400 mm.
23. 7
    Sole width w_bg and maximum gutter depth h_a

    Determine sole width (w_bg) and maximum depth of box gutter (h_a) for free flow conditions from Figure I1, for the box gutter with the maximum flow.
24. 8
    Decision — is the total design flow through the outlet > 16 L/s?

    If yes, review the selections at Step 4 — reposition outlets and expansion joints, increase the number of outlets, or a combination. If still yes on subsequent trials, the design is beyond the scope of the general method.
25. 9
    Decision — are the gradients of the box gutters flatter than 1:200?

    If yes, the design is beyond the scope of the general method.
26. 10
    Downpipe and depth of sump h_s

    Select the downpipe from Figure I4 for the total design flow through the outlet, then determine the depth of sump (h_s), which shall be not less than 150 mm (see Clause 3.7.4).
27. 11
    Height of the overflow weirs ℓ_oc

    Determine the height of the overflow weirs (ℓ_oc) above the sole of the box gutter from Figure I6(a), for the largest flow in any one box gutter.
28. 12
    Height of box gutter above the weirs h_t

    Determine the minimum height of the box gutter above the top of the overflow weirs (h_t) from Figure I8, for the largest flow in any one box gutter. (The flow chart prints “Figure I6(a)” here, but I6 is the side-overflow graph — I8 is the design graph for the sump/high-capacity overflow device, and reproduces Appendix J Example 3.)
29. 13
    Decision — is h_a < (h_t + ℓ_oc)?

    If yes, the minimum depth of box gutter d_bg = (h_t + ℓ_oc). If no, d_bg = h_a.
30. 14
    Minimum depth of the box gutter d_bg

    Adopt depth mm
31. 15
    Decision — is ℓ_oc > 60?

    This sets the datum level for the depth of the sump.
32. 16
    Datum level for the depth of the sump

    If ℓ_oc > 60, the datum is the sole of the box gutter. Otherwise the datum is the downstream sole of the overflow channel, (60 − ℓ_oc) below the sole of the box gutter.
33. 17
    Size of the sump/high-capacity overflow device

    Determine the size of the sump/high overflow device and sump from Figure I7, using the required datum level, h_a and ℓ_oc. The depth of the sump shall be not less than 150 mm.

Design summary

Outlet arrangement

Validation — Appendix J worked examples

Standard detail

3D model