Why C drops over time
The Hazen-Williams equation predicts head loss assuming a fixed pipe roughness:
h_f = 10.67 × L × Q^1.852 / (C^1.852 × D^4.87) [SI units]In US customary units, the constant becomes 4.52 with Q in gpm, D in inches, L in feet, h_f in feet:
h_f (ft) = 4.52 × L (ft) × (Q gpm)^1.852 / (C^1.852 × (D inches)^4.87)C is the friction coefficient. Higher C = smoother pipe = less head loss. Real pipe doesn't keep its commissioning C value over decades. Three mechanisms drive C downward:
1. Tuberculation — iron carbonate or calcium carbonate deposits roughen the interior wall in metal and lined pipes 2. Biofilm growth — organic films coat the pipe wall, especially in low-velocity sections of distribution mains 3. Sediment accumulation — gravity sewers, low-velocity force mains, and dead-end mains accumulate suspended solids
The combined effect: a 50-year-old water main has C-factor 10-30 points below its commissioning value.
Reference C-factor values
New pipe (commissioning):
| Material | C (new) | C (10-yr) | C (30-yr) | C (50-yr) | |---|---|---|---|---| | Cement-lined ductile iron | 140 | 138 | 130 | 120 | | Cement-lined steel | 140 | 138 | 130 | 120 | | Epoxy-lined steel | 145 | 143 | 138 | 130 | | PVC | 150 | 148 | 145 | 140 | | HDPE | 150 | 148 | 145 | 140 | | Cast iron (unlined) | 130 | 110 | 80 | 60 | | Galvanized steel | 120 | 100 | 80 | 60 | | Copper | 140 | 135 | 130 | 125 | | Concrete (smooth) | 130 | 125 | 115 | 105 | | Concrete (rough) | 110 | 100 | 85 | 75 |
Note: cast iron and galvanized steel age dramatically because they corrode internally; lined pipes (cement, epoxy) age slowly. PVC and HDPE age very slowly because they don't corrode at all.
What aging means for design
For new construction, design with the commissioning C value to verify the pump operates close to BEP at the design point. Then verify the operating point at the 30-year C value to confirm the pump still operates within AOR (Allowable Operating Region) at the end of asset life.
If the pump moves outside AOR at the 30-year C, either:
- Spec a steeper pump curve (more H-Q slope = more flow-margin tolerance)
- Spec a slightly oversized pump that operates near BEP at design AND remains in AOR at 30-year aging
- Accept reduced capacity at end-of-life
For a typical municipal water main:
- New C = 140 (DI cement-lined)
- 30-year C = 130
- Capacity reduction at constant TDH ≈ 12% (Hazen-Williams: Q ∝ C, so 130/140 = 93% capacity at fixed h_f)
- Actually since pumps move along their curves with system curve change, real impact is ~5-8% capacity reduction at the new operating point
Tuberculation specifically
Tuberculation forms in metal pipes carrying water with:
- Low pH (< 7.5)
- High dissolved oxygen
- Low calcium carbonate content (low Langelier index)
In the Northeastern US, soft groundwater is common — tuberculation is severe. In the Southwest, hard groundwater is more typical — tuberculation is less of an issue but scaling (calcium deposits) is.
The fix: corrosion control treatment at the water utility (zinc orthophosphate is a common additive). When this is in place from commissioning, tuberculation is suppressed and C drops slowly.
For lined pipes (cement-mortar lining inside DI), tuberculation can't form on the lining itself. C drops are due to biofilm + sediment, much slower.
Biofilm growth
Biofilms (microbial slime layers) form on any wetted pipe surface. In water distribution:
- Chlorine residual >0.2 mg/L suppresses growth
- Velocity > 3 ft/s helps shear film off (for pipes >12-inch)
- Dead-end mains accumulate biofilm rapidly because there's no flow
Biofilm adds maybe 0.5-2 mm of effective wall thickness in years. The hydraulic effect is modest — C drops 2-5 points over decades.
The bigger biofilm problem isn't hydraulic; it's water quality. Biofilm harbors bacteria, including pathogens. AWWA water-quality programs target biofilm as a public-health issue, not just a hydraulics issue.
Sediment accumulation
In gravity sewers and low-velocity force mains, suspended solids settle out at flat-grade portions, low points, and bends. The accumulated sediment:
- Reduces effective pipe diameter
- Creates a rough flow boundary (lower C)
- Can cause backups during peak flow events
Designers control sediment by:
- Maintaining minimum scouring velocity (≥ 2 ft/s typical for sewers)
- Cleaning periodically (jet vactor truck)
- Avoiding very-flat-grade sewer sections
Force mains specifically should run at ≥ 2 ft/s most operating hours to keep solids in suspension. A force main that pumps intermittently at high velocity, then sits stagnant, is worse than one that runs continuously at lower velocity — the stagnant periods let solids settle.
When to re-line, repair, or replace
Three options when an aging pipe is reducing capacity:
1. Re-lining (cement-mortar in place)
A cement-mortar slurry is sprayed onto the inside of the existing pipe via a "lining train" pulled through. The lining hardens to ~3/8" thick.
- Cost: $20-50/ft of pipe (depending on size)
- Capacity recovery: C jumps back to 130-140
- Service life of new lining: 30-50 years
- Disruption: pipe out of service for 24-72 hours, plus connection re-tapping
Best for: ductile iron / steel mains with significant tuberculation but otherwise intact.
2. Sliplining (HDPE inside existing)
A new HDPE liner is pulled through the existing pipe. The new pipe is smaller (typically 80% of nominal ID) but C is now 145-150.
- Cost: $40-80/ft
- Capacity: depends on diameter reduction; sometimes slightly worse, sometimes much better
- Service life: 50+ years for HDPE
- Disruption: pipe out 24-48 hours
Best for: pipes that need both surface repair AND pressure rating upgrade.
3. Pipe-bursting / replacement
Replace the existing pipe entirely with new HDPE, PVC, or DI.
- Cost: $80-200/ft (depending on conditions)
- Capacity: design choice (can size larger if right-of-way allows)
- Service life: 50-100 years for DI, 50+ for PVC, 50+ for HDPE
- Disruption: significant — pipe out for days to weeks
Best for: deteriorated pipes (leaks, breaks) where re-lining would just defer the inevitable.
How to estimate aged C without testing
Three sources, in order of preference:
1. Field measurement — install pressure transducers at known segments, run a known flow, back-calculate C from the headloss. Most accurate but requires shutdown coordination. 2. Historical records — utilities sometimes track C-factor evolution from periodic flushing and pressure surveys. If your utility has this data, use it. 3. Reference tables — use the table above as a rough screen.
For high-stakes design (large transmission, fire-flow systems), do option 1. For routine distribution-system design, option 3 with a 30-year-aged C is usually adequate.
A common error: using new-pipe C for replacement projects
A municipality plans to replace a 12" cast-iron main from 1955 with a new 12" PVC main. The hydraulic study uses C=150 (new PVC) and concludes capacity will improve. They don't check what current capacity actually is — and they're surprised when the new pipe delivers exactly what the old one did, because the bottleneck was downstream.
The lesson: when sizing a replacement, the new C value matters, but so does the rest of the system. If the bottleneck is at a downstream pump or further-downstream pipe, replacing one segment helps less than you'd hope.
A reverse common error: assuming severe aging where it didn't happen
A lined-DI main has been in service 30 years. The engineer assumes C dropped from 140 to 110 ("standard 30-year aging"). Re-lining is proposed for $1.2M.
But cement-lined DI in well-treated water often retains C = 135 at 30 years. The actual capacity loss is 3-4%, not the 20% the engineer assumed. The re-lining is unnecessary.
The lesson: validate aging assumptions with field measurement before committing to capital projects.
How the calculator handles it
The pipe-material selector in the Headloss Calculator includes pipe-age options:
- New (commissioning C)
- 10 years
- 30 years
- 50 years (for asset-life-end checks)
Selecting an age sets the appropriate C value automatically per the table above. The system curve and operating point recalculate with the aged C. This lets you stress-test designs against end-of-life conditions before commit.
For mixed-age systems (e.g., new force main connecting to a 40-year distribution network), use multiple pipe segments each with appropriate age + material settings.
References
- AWWA Manual M11 — *Steel Pipe: A Guide for Design and Installation* (chapter on aging and roughness).
- AWWA Manual M9 — *Concrete Pressure Pipe Manual* (concrete-pipe aging).
- Williams, G. S., and Hazen, A. *Hydraulic Tables.* (Original 1905 reference, still cited.)
- Lamont, P. A. "Common Pipe Flow Formulas Compared with the Theory of Roughness." J. AWWA, 1981.
- AWWA Research Foundation. *Distribution System Tuberculation: Its Cause and Cure.*