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Manning Equation Calculator

The Modified Manning Equation is used to calculate flow velocities and volume (in cubic feet per second or cfs). This information is vitally important when selecting a grate type and size for a given project.

Where...

  • D = Depth of flow in feet
  • Q = Channel flow in cfs (calculated)
  • Z = Reciprocal of transverse slope (1/ST)
  • S = Longitudinal slope
  • N = Roughness coefficient at constant 0.016 (value for concrete and asphalt)

Instructions

Step 1

Enter the Longitudinal and Transverse Slopes. Enter one of the remaining variables and then press calculate:

  • Depth of Flow (D)
  • Total flow in cfs (Q)
  • Spread of flow in feet

Step 2

To Calculate flow capture of specific Neenah Grates, select a catalog number and grate style from the K Chart drop-down.

  1. Select the catalog number for the appropriate "K" chart and from the chart "Determine the "K" value based on the Longitudinal and Transverse slopes entered in Step 1.
  2. Derive the Grate Coefficient by plotting the Transverse Slope (ST) and Longitudinal Slope factors onto the K Chart.
  3. Enter the "K" value in the space provided.
  4. Press Calculate.

Step 1

  Alternate One Alternate Two Alternate Three
Depth of flow in feet (D): [?]
Transverse Slope in ft./ft. (ST): [?]
Longitudinal Slope in ft./ft. (SL): [?]
Roughness coefficient (N): (value for concrete or asphalt) 0.016 0.016 0.016
Total flow in cfs (Q): [?]
Spread of flow in feet: [?]
 

Step 2

Catalog numbers and grate types that have K-charts: [?]
Grate Coefficient from K-chart (K):
Grate capacity in cfs: [?]
Bypass flow in cfs:
Percent captured:
 

For additional information regarding Neenah Inlet Grate Capacities, please contact Steven Akkala P.E., at (920) 729.3653 or email at sakkala@nfco.com.

Program Methodology

If your inlet is not capturing enough of the flow, some improvement options include:

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Depth of Flow in feet (D)

Refers to depth of flow in the gutter immediately adjacent to the curb.

Range: 0.01 to 0.5 feet expressed as a decimal.

Most curbs max out at 6 inches, which would be expressed as 0.5 feet.

This value is normally calculated by the equation based on the Spread of Flow In Feet or Total Flow in CFS ( Q ).

Transverse Slope in ft./ft. (ST)

This is a required criterion.

Range: 0.005 to 0.06 expressed as a decimal.

Roadways are generally built with a crown to facilitate the flow of water from their surfaces. In most cases the centerline of the road is the high point with a transverse slope perpendicular to the centerline toward the gutter. Typical slope values are 1/4" per foot (this is a 2% transverse slope) which is expressed as 0.02.

Longitudinal Slope in ft./ft. (SL)

This is a required criterion.

Range 0.005 to 0.1 expressed as a decimal.

The longitudinal slope is the grade of the roadway centerline. For slopes of any consequence, vane grates are recommended because of their excellent hydraulic efficiency.

Total Flow In CFS (Q)

Expressed as cubic feet per second.

Range 0.005 to 0.1 expressed as a decimal.

(Q) refers to the water flowing down the street approaching an existing or proposed inlet.

This value can be calculated, or if known, can be used as a criterion to determine Depth of Flow (D) and Spread of Flow. Total Flow (Q) is based on using the rational equation Q = CiA where:

Spread Of Flow In Feet

The spread of flow refers to the water contained within the distance from the curb face toward the centerline of the roadway. Generally, maximum allowable spread of flow would be a design criterion allowing only a certain portion of the roadway to be covered with water; thus keeping the other portion of the road width free of water for safe vehicle traffic. However, this value can also be calculated by the program if either Depth Of Flow or Total Flow is known. Be aware that too great a spread of flow combined with high velocity traffic can lead to hydroplaning.

K Charts

These K charts are used to determine a K value, which is a factor related to both the transverse and longitudinal slopes. The slopes shown on the K Charts are given as a percent. As an example, Neenah's K Chart for the R-1879-A3G grate, yields a K value of approximately 23 where a transverse slope of 2% (ST=2) and a longitudinal slope of 2% (SL=2) intersects.

This K value, multiplied by the calculated depth of flow raised to the 5/3 power, provides the grate capacity in CFS.

Grate Capacity In CFS

The grate capacity is a calculated value. With 2% longitudinal and transverse slopes and a total flow of 2 cubic feet per second approaching the R-1879-A3G grate, the grate captures 1.13 cubic feet per second of flow. This is roughly 56% of the flow (2 CFS) approaching the grate. Between 30% and 60% is reasonable. Attempting to capture 100% of the flow at every inlet is inefficient and results in excessive use of inlets.