Storm Sewer Network Analysis and Design

Storm Sewer Network Sizing

Rational Method ($Q=CiA$)

1. System Parameters

Analysis Mode	Design (Auto-Size) Assessment (Existing)
Manning's Roughness ($n$)	Presets 0.013 (Concrete) 0.010 (PVC/Plastic) 0.012 (HDPE Smooth) 0.024 (CMP Metal) 0.015 (Vitrified Clay)
Min. Pipe Size (mm)

2. IDF Curve Parameters

$I = \frac{A}{(T_c + B)^C}$

Intensity (mm/hr) vs. $T_c$ (min)

A
B
C

How to find A, B, C: These regional coefficients vary by Return Period. You can derive them directly from your local gauge data by using our Rainfall Frequency & IDF Analysis Worksheet.

Rational Coefficient (C)

Surface Type	C Range
Asphalt & Concrete	0.70 - 0.95
Roofs	0.75 - 0.95
Lawns (Flat, Sandy)	0.05 - 0.10
Lawns (Steep, Clay)	0.15 - 0.35
Residential (Single)	0.30 - 0.50
Residential (Multi)	0.40 - 0.75
Commercial/Ind.	0.50 - 0.90
Parks / Cemeteries	0.10 - 0.25
Forest / Woodlands	0.10 - 0.25

Network Topology Schematic

Network Cycle Detected! Ensure flow is strictly unidirectional (upstream to downstream).

A Physical Catchment & Pipe Inputs

#	Pipe ID	Up Node	Dn Node	Area A (ha)	Runoff C (0.0 - 1.0)	Local Tc (min)	Dia D (mm)	Up Invert (m)	Length L (m)	Slope S (m/m)	Act

B Hydraulic Routing & Sizing Results

Topology			Rational Routing ($Q = CiA$)					Pipe Sizing (Manning's Full Flow)
Pipe ID	Up Node	Dn Node	Σ (CA) (ha)	Sys Tc (min)	Intensity I (mm/hr)	Design Q (m³/s)	Req'd Dia (mm)	Pipe D (mm)	Cap Qfull (m³/s)	Vel Vfull (m/s)	Time Tt (min)

C HGL / EGL Longitudinal Profile

Select Flow Path:

Pipe Conduit Physical Water Hydraulic Grade Line (HGL) Energy Grade Line (EGL)

Hydraulic Theory: Rational Method Network Sizing

1. The Rational Method ($Q = CiA$)

Calculates the peak surface runoff from a specific drainage area. In metric units, a conversion factor ($1/360$) is applied to maintain standard unit compatibility. For catchments with mixed land uses, a composite runoff coefficient is used:

$$C_{comp} = \frac{\sum C_i A_i}{\sum A_i}$$

2. Network Routing & Time of Concentration ($T_c$)

The system time of concentration ($T_{c,sys}$) at any given node is the maximum of the local area's $T_c$ or the cumulative travel time from the most remote upstream point:

$$T_{c,sys} = \max(T_{c,local}, \max(T_{c,up} + T_t))$$

* Note: Most design standards enforce a minimum $T_c$ (typically 5 to 10 minutes) to prevent physically unrealistic, infinite rainfall intensities at the very top of the network.

3. Pipe Sizing (Manning's Equation)

Pipes are sized assuming full flow capacity using Manning's equation. The theoretical required diameter is calculated and then rounded up to the nearest standard commercial pipe size.

$$D_{req} = \left( \frac{Q \cdot n}{k \cdot S^{1/2}} \cdot \left(\frac{4}{\pi}\right)^{5/3} \cdot 4^{2/3} \right)^{3/8}$$

4. Topological Sorting (DAG Algorithm)

The pipe network is modeled computationally as a Directed Acyclic Graph (DAG). The algorithm automatically sorts the pipes from upstream to downstream to guarantee that a pipe is only sized after all of its tributary inflows and accumulated travel times have been processed.

5. Velocity Constraints (Self-Cleansing)

Storm sewers must maintain a minimum velocity to prevent sediment deposition, but not exceed maximum velocities to prevent pipe scouring.

Constraint	Metric (m/s)	Imperial (ft/s)
Min (Self-Cleansing)	0.75 - 0.90	2.5 - 3.0
Max (Desirable)	3.0 - 4.5	10.0 - 15.0

6. Typical Manning's Roughness ($n$)

Manning's $n$ varies by pipe material and age. The values below are typical standard design values for storm sewer networks.

Pipe Material	Typical Design $n$
Concrete Pipe (RCP)	0.013
PVC / Smooth HDPE	0.010 - 0.012
Corrugated Metal (CMP)	0.024
Vitrified Clay	0.013 - 0.015

References & Sources

• Federal Highway Administration (FHWA). (2009). Urban Drainage Design Manual (HEC-22). Chapter 7: Storm Drain Design. (Standard procedure for applying the Rational Method to pipe networks and self-cleansing velocity criteria). [Download HEC-22 PDF]
• Chow, V. T., Maidment, D. R., & Mays, L. W. (1988). Applied Hydrology. McGraw-Hill. (Definitive text on the Rational Method and flow accumulation principles). [View Archive]
• ASCE / WEF (1992). Design and Construction of Urban Stormwater Management Systems. ASCE Manuals and Reports on Engineering Practice No. 77 / WEF Manual of Practice FD-20. (Provides standard Manning's n values and C-factor lookup tables).
• Manning, R. (1891). On the Flow of Water in Open Channels and Pipes. Transactions of the Institution of Civil Engineers of Ireland. (Original formulation of Manning's equation for gravity pipe sizing).

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CivilSheets by Sothearith Seak