10.2 Stormwater & Clearwater Systems (SPS 382.36)

Definitions and separation

Storm water is wastewater from a precipitation event (SPS 381.01 (246)). Clear water is wastewater other than storm water with no impurities, or impurities below concentrations the department considers harmful, such as noncontact cooling water and condensate from refrigeration compressors and air conditioning (SPS 381.01 (56)). SPS 382.36 (4) (b) keeps them out of the sanitary system:

• Storm or clearwater piping may not connect to a sanitary drain system, except where a combined sanitary-storm sewer serves the site.
• Gravity storm drains may not be combined with clearwater drains before the storm building drain unless the department approves.
• Table 382.38-1 limits clearwater to a municipal sanitary sewer to 50 gpd (section 10.3).

Design storm and peak flow (SPS 382.36 (3) and (5))

Systems upstream of detention are designed for at least the 10-year, 24-hour storm. Detention volumes are based on at least a 2-year, 24-hour storm and must avoid property damage in a 100-year, 24-hour storm (Type II distribution, which covers Wisconsin).

Area method for sizing conveyance piping:

$$Q_{\text{gpm}} = \frac{\text{roof area}}{26} + \frac{\text{paved area}}{32.5} + \frac{\text{lawn area}}{104}$$

The code's own example: 10,000 sq ft of roof / 26 = 385 gpm, or 0.85 cfs. The rational method and engineering analysis (SWMM, TR-55 and similar) are also allowed.

Conveyance sizing (SPS 382.36 (6))

Horizontal piping is sized by engineering analysis at full-flow capacity or by Tables 382.36-1 to 382.36-5 (by pipe type, diameter and pitch). Excerpt for PVC and ABS (Table 382.36-1, gpm):

Worked example. A 10,000 sq ft roof produces 385 gpm. At 1/8 inch per foot, a 6-inch PVC storm drain carries 340 gpm (too small), and an 8-inch carries 710 gpm, so use 8 inches. At 1/4 inch per foot, a 6-inch drain carries 480 gpm, which works. A vertical conductor may not be smaller than the largest horizontal branch discharging into it.

Roof drains (SPS 382.36 (10))

• Conventional roof, deck and balcony drains meet ASME A112.6.4. Strainers extend at least 4 inches above the roof, with inlet area at least 1.5 times the conductor area. Flat-deck strainers (sun and parking decks) are level with the deck, with at least 2 times the conductor area.
• Roof drain outlets are at least 2 inches (SPS 384.30 (3) (e)).
• Siphonic roof drains meet ASME A112.6.9 or ASTM F2021, are marked with baffle data, and are designed to ASPE 45.
• Controlled-flow roof drains: at least 2 drains on roofs of 10,000 sq ft or less and 4 or more above 10,000 sq ft. The 10-year, 24-hour storm may not be stored on the roof longer than 24 hours.
• Secondary roof drainage is sized like the primary, never connected to the primary system, and discharges where observable (Table 382.38-1 line 12). Openings and overflow rims are 2 to 4 inches above the primary drain's bottom, and no more than 5 inches above the adjacent roof.

Area drains and inlets (SPS 382.36 (9))

• Interior clearwater inlets end at least 1 inch above the floor.
• Exterior inlets have removable grates; grates on horizontal inlets over 6 inches block a 6-inch sphere and sit within 3 inches downstream of the inlet.
• Subsurface areas of 50 sq ft or less (other than stairwells) drain to the foundation drain through at least a 2-inch pipe or washed stone, or to the storm system through a 3-inch pipe.
• Larger subsurface areas and all weather-exposed stairwells need an area drain with a 3-inch fixture drain to the storm system, never to a foundation drain.

Traps, vents and backflow (SPS 382.36 (7) and (12))

• Traps are required on interior clearwater inlets, and on exterior inlets within 10 feet of an air inlet, door or openable window (loading-dock drains excepted). One trap may serve several inlets.
• A foundation drain discharging by gravity to a storm sewer is trapped, with cleanouts. Clearwater trap vents may not connect to sanitary or chemical vents.
• Subsoil and foundation drains subject to backwater get a backwater valve or a sump. Subsoil drain connections to the storm sewer are made above its horizontal centerline.
• Leaders discharging to a storm building sewer connect at or above finished grade (an indirect connection at grade needs a removable strainer).
• Tracer wire for nonmetallic storm sewers to municipal mains is brown.
• Underground stormwater reuse tanks are at least 8 feet from sanitary sewers.

Storm sumps and detention (SPS 382.36 (6) (g) and (8))