Rainwater First-Flush Diversion Volume and Standpipe Length

ScopeA first-flush diverter keeps that water out of the tank, and it is the component a catchment built in a hurry most often leaves out. This calculator sizes a standpipe diverter by the Texas Water Development Board rule of thumb: 1 to 2 gallons per 100 sq ft of roof footprint, measured in plan rather than along the slope, with a minimum of 10 gallons per 1,000 sq ft, which is the low end. It converts the volume to the rain depth it represents and to the length of standpipe that holds it: 231 cubic inches per gallon over the pipe's cross-section, 32.7 in per gallon of 3-inch pipe, 18.4 in of 4-inch, 8.17 in of 6-inch. A 1,500 sq ft footprint draining to one downspout diverts 15 gallons at the low rate, the first 0.016 in of rain, which is 10.2 ft of 6-inch pipe but 23.0 ft of 4-inch -- why the manual favors 6 or 8-inch standpipes. At the upper rate the same roof diverts 30 gallons, 20.4 ft of 6-inch or 46.0 ft of 4-inch; a dusty site, a long dry spell, or ash on the roof argues for it. The manual says plainly that no exact calculation exists. Each downspout gets its own diverter sized from the area that drains to it. Diverting does not make rainwater potable, which takes filtration and disinfection. It does not size gutters, screens, the tank, or the harvest, or the weep-hole drain-down, and it uses nominal pipe diameter as the manual does. The TWDB manual, the local health department for potable use, and the AHJ govern.

Formula or tablediversion = roof footprint / 100 x rate (1 to 2 gal per 100 sq ft; minimum 10 gal per 1,000 sq ft); rain depth (in) = gallons x 231 / (footprint x 144); standpipe length per gallon (in) = 231 / (pi/4 x d^2); length = gallons x that.

Source and editionTexas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd ed. (2005), Chapter 2, First-Flush Diverters (pp. 7-8), by name. The local health department (for potable use) and the AHJ govern.

Free accessA free state manual's rule of thumb, restated; the standpipe length is cylinder geometry at 231 cubic inches per gallon.

What governsEstimate. Operator of record and primacy agency govern.

API field namesroof_footprint_sqft, diversion_rate_gal_per_100sqft, standpipe_diameter_in, diversion_gal, minimum_rule_gal, rain_depth_in, in_per_gal, standpipe_length_in, standpipe_length_ft, high_rate_gal, high_length_ft

• Roof footprint in plan horizontal projection of the area draining to the downspout, not the sloped surfaceTWDB Texas Manual on Rainwater Harvesting
• Nominal pipe diameter as the manual uses; actual inside diameters differ by a few percentTWDB Texas Manual on Rainwater Harvesting
• Not a treatment step diverted rainwater still needs filtration and disinfection for potable useTWDB Texas Manual on Rainwater Harvesting