Wastewater Flow, Hydraulics, and Capacity
What the exam expects
Hydraulics questions test whether you can reason about flow, velocity, slope, and capacity. Know the continuity equation (Q = A × V), Manning's equation conceptually, and the difference between gravity and pressure flow. Understand that velocity depends on slope and hydraulic radius, not just pipe diameter.
Concepts to master
- Flow units: mgd, gpm, cfs; conversions (1 cfs ≈ 448.8 gpm; 1 mgd ≈ 1.547 cfs).
- Peak flow factors and diurnal patterns; design flow vs. actual flow.
- Partially full pipe flow: velocity is highest near 80–90% full, not at full flow.
- Surcharging and the hydraulic grade line; backwater effects.
- Capacity limits: pipe size, slope, roughness (Manning's n), and obstructions.
- Infiltration/inflow (I/I) and its effect on capacity.
Traps
A classic trap: assuming doubling diameter doubles capacity - it roughly quadruples area but velocity also changes with slope. Another: confusing flow (volume per time) with velocity (distance per time). Also watch unit mismatches in word problems.
Worked example
A pipe carries 2.0 cfs. What is the flow in mgd? 2.0 cfs × 0.6463 mgd/cfs ≈ 1.29 mgd. (Since 1 mgd ≈ 1.547 cfs, divide 2.0 ÷ 1.547 ≈ 1.29.) Always check units before computing.