How much water can your swale carry?

What a swale has to do

A swale is a shallow, wide, grass-lined ditch that carries storm water across a yard or field without cutting a gully. It has to hold two limits at once: enough area to carry the flow, and a velocity low enough that the grass is not torn out. A narrow, deep ditch can carry the same flow as a wide, shallow one, but faster, which is why it erodes.

How do I calculate the capacity of a swale?

Use Manning's equation on the cross-section. For a trapezoid with bottom width b, side slope z (z feet across for each foot of rise) and flow depth d, the area is b·d + z·d², the wetted perimeter is b + 2d·√(1 + z²), the hydraulic radius R is the area divided by the perimeter, and velocity is V = (1.49 ÷ n) × R^(2/3) × S^(1/2). Flow is velocity times area.

What Manning n do I use for grass?

North Carolina's grassed swale manual uses n = 0.033 for grass and tells designers to assume the grass is not mowed regularly, since long grass is rougher. Its table also lists 0.035 for riprap and 0.038 for turf reinforcement matting. Other agencies publish other values, and we found no agreed range in the sources we read, so the page uses 0.033 and lets you type your own n.

How fast can water flow in a grass swale?

The same manual allows 4 ft/s for grass and 6.5 ft/s for riprap, and a matting limit that varies with the product (about 9.5 ft/s in its table). It also says to multiply the calculated velocity by 1.3 as a safety factor before comparing it with these limits. Faster than that and the grass tears out.

How much freeboard should a swale have?

The manual calls for at least 0.3 foot of freeboard, the height of the bank above the water, during the 10-year storm. Enter the full depth of your swale in the optional field to check it.

How steep can the sides be?

The manual says a side slope of 4:1 (horizontal to vertical) or flatter if you want to mow it with a riding mower, no steeper than 3:1 in most cases, and never more than 2:1. Triangular swales are for small catchments under 2 acres, it adds.

What design flow should I compare it with?

The manual designs swales to carry the peak flow of a 10-year storm, found with the Rational Method: Q = C × i × A. The culvert size calculator shows how to get that flow from the area, the runoff coefficient and the rain. Enter the result in the optional field.

Does a swale have to be straight and uniform?

The manual wants a fairly constant slope from start to end and gradual changes where a change is necessary. A steep section needs separate treatment; check each reach with its own slope and cross-section.

Worked example

A swale with a 2 foot flat bottom, 3:1 sides and 0.5 foot of flow on a 2 percent slope has an area of 1.75 ft², a wetted perimeter of 5.16 ft and a hydraulic radius of 0.339 ft. With n = 0.033 the velocity is 3.10 ft/s and the flow 5.43 cfs, about 2438 gpm. Multiplied by the 1.3 safety factor the velocity is 4.04 ft/s, a hair over the 4 ft/s limit for grass. On a 1 percent slope the same swale carries 3.84 cfs at 2.20 ft/s, 2.85 with the factor, which passes. A V-shaped swale (bottom width 0) with 4:1 sides and 1 foot of flow on 1 percent carries 11.15 cfs.

Reading the result

The big number is the flow at the depth you entered. The table shows the flow at a quarter, half and three quarters of that depth, to see how the capacity grows: depth enters the formula strongly, so a few inches more depth adds a lot of flow. If the velocity check fails the fix is geometry, not just a lining: a wider bottom or flatter sides spread the same flow over a bigger area, and a flatter slope or a check dam slows it. Matting or stone is the last resort.

Limits of this method

Manning's equation assumes steady, uniform flow in a channel of constant shape. Real swales have bends, culvert entrances, driveway crossings and changes of slope. The n of grass changes with its height and the depth of flow: it is much higher in shallow flow through tall grass than in deep flow over mown turf, which is why the manual says to assume the grass is not mowed. This is the standard of one state's agriculture department; check your county and state erosion-control rules.

Common mistakes

Mixing the side slope: 3:1 means 3 feet across for 1 foot up, so z = 3, not 0.33. Typing the bank height where the flow depth belongs. Using the bed slope of the whole ditch when one reach is much steeper. Measuring the slope in percent but typing it as a fraction. Judging a swale by its capacity alone while ignoring its velocity.

Sources: North Carolina Department of Agriculture and Consumer Services, Soil and Water Conservation Division, Community Conservation Assistance Program, Grassed Swale Design Manual, chapter 8, Vegetated Swales (equations 8-1 and 8-2, tables 8.1 and 8.2); the 1.49 coefficient is that manual’s, FHWA’s HDS 5 uses 1.486. Related: culvert size calculator, French drain calculator, rain garden size calculator, pipe slope calculator. How we check our calculators.