Basement Waterproofing Drainage Calculator
Estimate interior drainage pipe capacity, required sump pump flow rate, and perimeter drain sizing for basement waterproofing systems.
Formulas Used
1. Surface Inflow — Rational Method:
Q = C · i · A
where C = runoff coefficient (soil-dependent, 0.70–0.98), i = rainfall intensity (ft/s), A = basement footprint (ft²).
2. Groundwater Seepage — Darcy's Law:
Qgw = K · ih · Awall
where K = hydraulic conductivity (ft/s), ih = depth/(depth+5) (hydraulic gradient), Awall = perimeter × depth (ft²).
3. Pipe Capacity — Manning's Equation (full-flow circular pipe):
Q = (1/n) · Apipe · R2/3 · S1/2
where n = 0.013 (PVC), Apipe = πd²/4, R = d/4 (hydraulic radius), S = slope (ft/ft).
4. Sump Pit Volume:
Vpit = Qtotal × 600 s (10-minute peak storage)
5. Sump Pump Flow Rate:
Qpump = (Vpit / 5 min) × 1.25 (25% safety factor)
Assumptions & References
- Rational Method (Q = CiA) per ASCE 7 and local stormwater design standards.
- Manning's n = 0.013 for smooth PVC/HDPE perforated drain pipe (ASCE Manual of Engineering Practice No. 36).
- Darcy's Law hydraulic conductivity: Sandy/Gravel K ≈ 0.01 ft/s; Loam K ≈ 0.001 ft/s; Clay K ≈ 0.0001 ft/s (Freeze & Cherry, 1979).
- Hydraulic gradient approximated as depth/(depth+5) — conservative for shallow water table scenarios.
- Sump pit sized for 10-minute peak inflow storage at 24-inch standard pit depth.
- Sump pump sized to empty pit in ≤5 minutes with 25% safety factor per BOCA/IBC plumbing guidelines.
- Perimeter drain runs limited to 50 ft per segment to maintain adequate slope and cleanout access.
- This calculator provides estimates only. Site-specific soil testing, waterproofing design, and local code compliance are required for construction.