Storm drain compliance

Check every storm drain against the CPHEEO Manual on Storm Water Drainage Systems (2019): rational method flows from your IDF, velocity, Froude number, freeboard, cover and manhole spacing.

Design > Storm Compliance (SWM)... opens the Storm Drain Compliance (SWM) window. It works out the rational method flow in every drain from your IDF curve and the subcatchments, takes SWMM's peaks when there is a valid run, and checks every drain against the storm water manual. It lists every drain with a Pass, Warn or Fail verdict and colours the drains on the map by verdict while it is open.

SWM in DrainPilot means the CPHEEO (MoHUA) Manual on Storm Water Drainage Systems, Volume I Part A, 1st edition, August 2019. Some choices also cite SM, the CPHEEO Manual on Sewerage and Sewage Treatment Systems, Part A, 2013, and CL-D, the storm drainage DPR checklist in SWM Appendix A 2.1.

Run the check

The Storm Drain Compliance (SWM) window on its Drains tab, with peaks from the last SWMM run. Return period Class I cities: 5 yr, runoff C 0.75, IDF form Talbot I = a / (t + b) with a 5000 and b 20. Summary: drains 13, pass 13, warn 0, fail 0. A table lists each drain with its shape (RECT_OPEN), size, length, Manning n, inverts, slope, area, C x A and time of concentration. Below, the checks for drain D-J4: flat or adverse slope, overtopping, velocity between 0.6 and 3 m/s, Froude number and freeboard, each marked Pass with its Storm Water Drainage Manual reference. — The Drains tab: each drain checked against the Storm Water Drainage Manual.

Run the model first if you want SWMM's peaks; without a valid run the check uses the rational method flows. The line at the top of the window says which.
Open Design > Storm Compliance (SWM)...
Choose the Return period for the town: Class I cities 5 years, other cities 2 years, or 50 or 100 years for airports and critical infrastructure.
Choose the Runoff C: a land use, and the C within its range on the slider, or From imperviousness.
Choose the IDF form and type your city's coefficients, as described in Rainfall and design storms.
Set the rule choices: Storm drain cover at starting manhole, Plastic pipes in hills (6 m/s), Hill inlet time (3 min) and Keep t0 within 5 to 30 min.
After a valid run, choose SWMM run or Rational flows for the flows the drains are checked at.
Read the chips (drains, pass, warn, fail, return period, catchments) and the table on the Drains tab. Click a row to see every check on that drain with its value, limit and source.
Export the statement with CSV or Excel.

The checks

A drain's verdict is its worst check: Fail over Warn over Pass. A check that cannot be made (for an unsupported shape, say) is shown as Not checked.

Check	Applies to	Limit	Verdict if breached	Source
Flat or adverse slope	Every drain	Fall greater than 0	Fail	Gravity flow
Flow above full-bore capacity, or Overtops the channel	Every drain	Flow within capacity	Fail	not in india-norms.md (unverified)
Velocity below 0.6 m/s	Drains carrying flow	0.6 m/s	Fail	SWM Table 5.2 p.70
Velocity above 3 m/s	Every drain	3 m/s; 6 m/s with Plastic pipes in hills	Fail	SWM Table 5.2 p.70
Froude number above 0.8	Open drains	0.8	Warn	SWM s.5.3 p.69
Freeboard under the band value	Open drains	See the freeboard table	Fail	SWM Table 5.3 p.70
Cover under 0.6 m at starting manhole	Closed conduits at the head of a line	SWM 0.6 m or SM 0.5 m	Fail	SWM s.5.4 p.79 or SM (per conflict list in india-norms.md)
Reach longer than the manhole spacing	Closed conduits	See the spacing table	Fail	SWM Table 11.5 p.215
Catchment beyond the rational method limit	Drains whose catchment exceeds it	8 km2	Warn	SWM s.4.4.1.3 p.51

With rational flows, the capacity check compares the design flow with the drain's full capacity by Manning's equation. With SWMM peaks, a closed conduit fails it when its peak depth reaches the full height; for an open drain under SWMM peaks, the freeboard check is the one that catches a drain running too full. No d/D limit for storm conduits is confirmed in the norms, so the capacity check is marked unverified and only flags flow above full capacity.

Freeboard, open drains (SWM Table 5.3 p.70), by drain depth:

Drain depth	Freeboard	Status
Up to 300 mm	0.1 m	Verified
Over 300 up to 900 mm	0.15 m	Verified
Over 900 up to 1500 mm	0.3 m	Verified
Over 1500 mm	0.9 m	Verified. SWM gives up to 90 cm for larger drains; the checker applies 90 cm, the conservative reading.

Manhole spacing, closed storm drains (SWM Table 11.5 p.215), by size. The table overlaps and leaves gaps; a size it does not name takes the band below it, so it is never allowed a longer spacing:

Size	Maximum spacing	Status
Up to 299 mm	40 m	Unverified: Below 300 mm is not in the table; 40 m is carried down.
Over 299 up to 600 mm	40 m	Verified
Over 600 up to 699 mm	40 m	Unverified: 600 to 700 mm is a gap in the table; the 40 m band applies.
Over 699 up to 1050 mm	100 m	Verified
Over 1050 up to 1400 mm	150 m	Verified
Over 1400 up to 1499 mm	150 m	Unverified: 1400 to 1500 mm is a gap in the table; the 150 m band applies.
Over 1499 mm	300 m	Verified

Cover at the starting manhole. SM and SWM disagree; SWM (2019) is newer. The window offers SWM 0.6 m (SWM s.5.4 p.79) and SM 0.5 m (SM (per conflict list in india-norms.md)); the newer SWM value is the default. Cover is ground level minus the pipe crown at the head of the line. A SWMM model has no separate ground level, so DrainPilot takes the manhole rim (invert plus maximum depth).

The Manning n, lined CI or DI choice (CL-D 0.013 or SWM 0.011) applies only to pipes known to be lined CI or DI. A SWMM .inp file does not record pipe material, so for models opened in DrainPilot this choice currently changes no verdict.

How the rational flows are worked out

The Catchments tab: for each subcatchment SJ1 to SB3, the outlet, area in hectares, runoff C 0.75, overland length 60 m, t0 by formula 2.69 min and t0 used 5 min, intensity 200 mm/hr, and the peak Qp in m3/hr and L/s (666.7 L/s for the 1.6 ha catchments, 525 L/s for the 1.26 ha ones). — Rational method flows for each subcatchment.

Peak flow Qp = 10 C I A, in m3/hr, with I in mm/hr and A in hectares (SWM s.4.4.1.3 p.51), for each subcatchment draining to a node.
The intensity I is read from your IDF at the time of concentration tc = t0 + tf.
The inlet time t0 = 0.994 (1.1 - C) L^0.5 / S^0.333 (SWM s.4.4.1.6 p.54 to 55). L is the overland flow length, the subcatchment area divided by its width; S is its slope. The manual gives no units for this formula: DrainPilot takes L in metres and S in percent, and badges it unverified.
With Keep t0 within 5 to 30 min ticked (the default), t0 is held between 5 and 30 minutes, or from 3 minutes with Hill inlet time (3 min).
Going downstream, C x A adds up, and each point takes the longest time of concentration among the paths reaching it, adding the travel time through each drain at its full-bore velocity (1 m/s when that is unknown). At a node with two outgoing drains the flow is split equally.

The Catchments tab lists each subcatchment: outlet, area, C, overland length, t0 from the formula and t0 used, intensity and peak flow. The runoff coefficient ranges are from SWM Table 4.2 p.53:

Land use	C range
Commercial	0.7 to 0.95
Industrial	0.6 to 0.9
Institutional	0.7 to 0.95
Residential, high density	0.6 to 0.75
Residential, low density	0.4 to 0.6
Recreational	0.1 to 0.25
From imperviousness	C = 0.9 x impervious share + 0.15 x pervious share, using each subcatchment's % Imperv. These two values are DrainPilot defaults, not from the manual, and are badged unverified.

What the window can and cannot check

Shapes checked: circular (and force main), closed rectangular, open rectangular and trapezoidal. A drain of any other shape is listed with a warning and its checks are Not checked.
A conduit with several barrels is checked as one barrel; the window warns which ones.
Models in US units are converted to metres and m3/s before checking.
A loop in the network makes the flows below it incomplete; the window names the conduits that close the loop.

A clean statement means the drains meet the limits for the IDF, C and rules you entered. It does not check that the IDF is your city's, or that the levels and catchments in the model match the site.