Side Entry Pit Spacing (Back-of-the-Envelope) Calculator

Not to be used for real-world calculations. This is tool is unchecked. A simplified tool using the Rational Method + Manning’s equation for gutter flow

Pit Spacing (m)

Distance between pits

Lane Width (m)

3.5

Effective width draining to gutter

Longitudinal Grade (%)

Grade

2.0

e.g. 2% => 0.02 slope

Road Crossfall (%)

Cross

3.0

Typically ~2%-5%

Manning’s n

0.015

~0.013 - 0.020 for concrete & gutter flows

Runoff Coeff (C)

0.90

Urban ~0.8–0.95

Rainfall Intensity (mm/hr)

100

Refer to BOM data

Pit Efficiency (%)

Eff

Capture efficiency of pit

Allowable Flow W (m)

MaxW

2.5

Max allowable gutter spread

Parameter	Value / Units
Q (Rational Method)	-- L/s
Req. Flow Width (W)	-- m
Depth at Kerb (y)	-- m
Flow Capacity (Manning)	-- L/s
Velocity in Gutter	-- m/s
Pit Captures This Flow	-- L/s
Check vs. Allowable W	--

Methodology & Assumptions

1. Rational Formula for Inflow

Q_in (L/s) = ( C × I × A ) / 3600

C = Runoff Coefficient
I = Rainfall Intensity (mm/hr)
A = Catchment Area (m²) = (Pit Spacing × Lane Width)
3600 is to convert mm·m²/hr to L/s

2. Triangular Gutter Flow (Manning’s Equation)

We assume a simple triangular cross-section formed by the crossfall and a top flow width W.

y = i_x × W
A = (1/2) × W × y
P = √( W² + y² )
R = A / P
v = (1 / n) × R^(2/3) × S_L^(1/2)
Q_gutter = A × v

i_x = crossfall as a fraction = Crossfall% / 100
S_L = longitudinal grade as fraction = Grade% / 100
n = Manning’s roughness

3. Iteration to Find Required Spread

We have a design flow, Q_in, from the Rational Formula.
We increase W from 0 to some max (8 m in increments) until Q_gutter(W) ≥ Q_in.
The first W that satisfies that capacity is the required W.
Compare to user’s Allowable Flow Width.

4. Pit Efficiency

Pit Capture Flow = Pit Efficiency × Q_in
(Simplified assumption that if gutter flow arrives at the pit, that fraction is captured.)

For real designs, refer to local drainage manuals (e.g., BOM for rainfall data) and more detailed gutter-flow guidelines.

UNCHECKED Side Entry Pit Spacing (Back-of-the-Envelope) Calculator DRAFT