A Roof runoff management

Roof runoff should be diverted from feedlots and manure storage areas unless it is needed for some use, such as dilution water for waste storage ponds or treatment lagoons. This can be accomplished by roof gutters and downspouts with underground or open channel outlets (fig. 10-1). Gutters and downspouts may not be needed if the roof drainage will not come into contact with areas accessible to livestock.

Figure 10-1 Roof gutter and downspout

Figure 10-1 Roof gutter and downspout

Feedlot Runoff
to waste storage pond

The area of a roof that can be served by a gutter and downspout system is controlled by either the flow capacity of the gutter (channel flow) or by the capacity of the downspout (orifice flow). The gutter's capacity may be computed using Manning's equation. Design of a gutter and downspout system is based on the runoff from a 10-year frequency, 5-minute rainfall except that a 25-year frequency, 5-minute rainfall is used for exclusion of roof runoff from waste treatment lagoons, waste storage ponds, or similar practices.

Rainfall intensity maps are in appendix 10B. Caution should be used in interpolating these maps. Rainfall probabilities are based on measured data at principal weather stations that are mostly in populated regions. The 10-year, 5-minute rainfall in the 11 Western States was based on NOAA Atlas 1, and that in the 37 Eastern States was based on the National Weather Service HYDRO 35. Both of these publications state their limitations in areas of orographic effect. In the Western States, the 10-year, 5-minute rainfall generally is larger in mountain ranges than in valleys. Rainfall in all mountain ranges could not be shown on these maps because of the map scale and readability considerations. Many of these differences were in the range of 0.05 inch and fall within the contour interval of 0.10 inch.

A procedure for the design of roof gutters and downspouts follows:

Step 1—Compute the capacity of the selected gutter size. This may be computed using the Manning's equation. Using the recommended gutter gradient of 1/16 inch per foot and a Manning's roughness coefficient of 0.012, this equation can be expressed as follows:


qd = capacity of downspout, ft3/sec Ad = cross sectional area of downspout, in2 h = head, inches (generally the depth of the gutter minus 0.5 inch)

Step 3—Determine whether the system is controlled by the gutter capacity or downspout capacity and adjust number of downspouts if desired.

qd where:

Nd = number of downspouts

If Nd is less than 1, the system is gutter capacity controlled. If it is equal to or greater than 1, the system is downspout capacity controlled unless the number of downspouts is equal to or exceeds Nd

Step 4—Determine the roof area that can be served based on the following equation:


Ar = Area of roof served, ft2

q = capacity of system, either qg or qd whichever is smallest, ft3/sec P = 5-minute precipitation for appropriate storm event, inches

The above procedure is a trial and error process. Different sizes of gutters and downspouts should be evaluated along with multiple downspouts to determine the best gutter and downspout system to serve the roof area involved.


q = capacity of gutter, ft3/ sec

Ag = cross sectional area of gutter, in2

r = Ag/ wp, inches wp = wetted perimeter of gutter, inches

Step 2—Compute capacity of downspout. Using an orifice discharge coefficient of 0.65, the orifice equation may be expressed as follows:

(1) Design example 10-1—Gutters and downspouts

Mrs. Linda Worth of Pueblo, Colorado, has requested assistance in developing an agricultural waste management system for her livestock operation. The selected alternatives include gutters and downspouts for a barn having a roof with a horizontally projected area of 3,000 square feet. The 10-year, 5-minute precipitation is 0.5 inches. The procedure above is used to size the gutter and downspouts.

Step 1—Compute the capacity of the selected gutter size. Try a gutter with a 6-inch depth and 3-inch bottom width. One side wall is vertical, and the other is sloping, so the top width of the gutter is 7 inches. Note that a depth of 5.5 inches is used in the computations to allow for 0.5 inch of freeboard.

0.01184 x Ag x r0 67 0.01184 x Z6.6 x 1.760 67 0.46 ft3 / sec spouts. Determine number required to take advantage of gutter capacity.

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