Drainage Area Calculator

Understanding Drainage Calculations

Proper drainage design is essential for water management and flood prevention.

  • Runoff Coefficients Different surfaces have varying water absorption rates.
  • Slope Effects Steeper slopes increase runoff velocity.
  • Rainfall Intensity Local weather patterns affect system requirements.
  • System Capacity Must handle peak flow conditions.

Hydrologic Analysis

  • Runoff Calculations Rational method: Q = CIA. Time of concentration: Tc = L^0.8((1000/CN)-9)^0.7/(1900Y^0.5). SCS method: Q = (P-0.2S)²/(P+0.8S).
  • Design Storm Return period: 2-100 years. Intensity duration: 5-60 min. Distribution type: I-IV. Peak intensity: i = a/(t+b)^c. IDF curves: regional.
  • Basin Characteristics Shape factor: 0.1-0.3. Drainage density: 3-5 mi/mi². Relief ratio: 0.002-0.02. Bifurcation ratio: 3-5. Stream order: 1-6.

Surface Parameters

  • Land Cover Impervious ratio: 0.2-0.9. Manning's n: 0.011-0.24. Depression storage: 0.05-0.3". Infiltration rate: 0.1-4.0 in/hr.
  • Soil Properties HSG classification: A-D. Porosity: 0.3-0.5. Field capacity: 0.15-0.3. Wilting point: 0.05-0.15. Bulk density: 1.2-1.8 g/cm³.
  • Topographic Factors Slope range: 0.5-10%. Aspect ratio: 1.5-3.0. Contour interval: 2-10'. Flow path: 100-5000'. Channel gradient: 0.001-0.05.

Design Standards

  • System Requirements Collection efficiency: 90-95%. Inlet spacing: 200-400'. Bypass flow: <25%. Spread width: 6-8'. Ponding depth: 3-6".
  • Performance Criteria Capture ratio: >80%. Hydraulic grade: 0.5-2%. Flow velocity: 3-10 ft/s. Freeboard: 20% depth. Safety factor: 1.25-1.5.
  • Quality Control TSS removal: 80%. Peak reduction: 25%. Volume capture: 90%. First flush: 0.5-1.0". Treatment rate: WQv = (P)(Rv)(A)/12.

Maintenance Protocol

  • Inspection Schedule Visual check: monthly. Sediment depth: quarterly. Structure integrity: annual. Vegetation control: semi-annual.
  • Performance Monitoring Flow measurement: continuous. Water quality: monthly. Erosion check: quarterly. Infiltration test: annual.
  • Remediation Triggers Sediment buildup: >3". Flow reduction: >20%. Structural damage: any. Ponding time: >48 hrs. Erosion depth: >2".

Design Considerations

Safety Factor

Include 25% capacity margin for storms.

Pipe Sizing

Account for minimum slopes and velocities.

Collection Points

Strategic placement of drains and gutters.

Maintenance Access

Include cleanouts and inspection points.

Surface Types Guide

  • Roof Surfaces High runoff, requires immediate collection.
  • Paved Areas Significant runoff, needs proper slopes.
  • Grass/Lawn Natural absorption, slower runoff.
  • Gravel Moderate absorption, filtered runoff.