Knowledge Base

Attenuation Design Tutorial

Full worked example: 100-dwelling housing scheme from greenfield rates to orifice sizing

📋 Project Brief

Site Type
100-dwelling residential scheme
Total Area
2.8 hectares
Location
East Midlands (River Trent catchment)
Impermeable Area
1.4 ha (roads, roofs, drives)
Soil Class
WRAP class 4 (slowly permeable, clayey)
Infiltration Rate
2×10⁻⁶ m/s (BRE 365)
Receiving Water
Surface water sewer (S1 hierarchy)
Design Approach
Attenuation (infiltration ruled out)
1
Greenfield Runoff Rate

Calculation Method

Establish the baseline greenfield runoff rate (QBAR) using the Greenfield Runoff Calculator, which implements the simplified IH124 formula:

QBAR = SPR × SAAR × AREA^0.89

Input Parameters

  • SPR (Standard Percentage Runoff) for WRAP class 4 ≈ 0.47
  • SAAR (Standard Annual Rainfall) for East Midlands ≈ 650 mm
  • Site area = 2.8 ha = 0.028 km²

Results

Return Period QBAR (l/s) Growth Factor
1-in-1 year 2.1 0.40
1-in-30 year 8.5 1.64
1-in-100 year 5.2 1.00

Baseline QBAR = 5.2 l/s (1-in-100 year event)

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2
Climate Change Uplift

Regulatory Framework

The Environment Agency provides catchment-specific climate change allowances. For flood risk assessment, apply the upper end allowance to peak rainfall intensity.

River Trent Catchment (2080s)

  • Catchment: River Trent (Midlands)
  • Epoch: 2060–2080s (assumed development lifetime)
  • Upper end allowance: +40% peak rainfall intensity

Design Rainfall Uplift

Standard 1-in-100 year design rainfall for East Midlands (60 min duration): 52 mm

Applied uplift: 52 mm × 1.40 = 72.8 mm

Event Rainfall (60 min)
Baseline 1-in-100 52.0 mm
+40% uplift 20.8 mm
Design rainfall 72.8 mm
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3
Developed Site Runoff Volume

Runoff Calculation

Calculate the runoff volume from impermeable surfaces (roofs, roads, drives) under the climate-adjusted design rainfall.

Parameters

  • Impermeable area: 1.4 ha = 14,000 m²
  • Design rainfall: 72.8 mm = 0.0728 m
  • Runoff coefficient: 0.9 (residential roofs and sealed surfaces)

Volume Calculation

Runoff volume = 14,000 × 0.0728 × 0.9 = 917 m³

Urban Creep Allowance

Allow 10% increase to account for future impermeable intensification:

917 × 1.10 = 1,009 m³

Component Volume (m³)
Base runoff volume 917
Urban creep (+10%) 92
Total design volume 1,009
4
Storage Volume Required

Attenuation Principle

The detention basin must store the difference between inflow and permitted outflow during the critical storm duration. The critical duration is typically the 6-hour event for UK design storms, but check multiple durations.

Calculation Basis

  • Inflow volume: 1,009 m³ (from Step 3)
  • Permitted discharge: 5.2 l/s (greenfield QBAR)
  • Storm duration: 60 minutes (critical for this site)

Storage Requirement

Outflow during 60-min storm:

5.2 l/s × 3,600 s = 18,720 litres = 18.7 m³

Required storage:

1,009 − 18.7 = ~990 m³

Parameter Value
Inflow volume (1-in-100+CC) 1,009 m³
Permitted discharge rate 5.2 l/s
Outflow in 60 min 18.7 m³
Storage volume ~990 m³
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5
Orifice Sizing

Flow Control Design

An orifice plate at the basin outlet restricts discharge to the permitted greenfield rate. Use the orifice flow control calculator to size the opening for your head conditions.

Input Parameters

  • Target discharge: 5.2 l/s = 0.0052 m³/s
  • Maximum head above orifice: 1.5 m (basin depth at storage)
  • Orifice type: Sharp-edged opening in concrete base slab

Orifice Calculation

Using the orifice equation: Q = Cd × A × √(2gH)

Where:

  • Cd (discharge coefficient) ≈ 0.61
  • g = 9.81 m/s²
  • H = 1.5 m

Required orifice diameter ≈ 75 mm

Diameter (mm) Area (cm²) Discharge (l/s)
70 38.5 4.7
75 44.2 5.2
80 50.3 5.8
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6
Interception Check (S2)

Regulatory Requirement (S2)

The first 5 mm of rainfall falling on impermeable surfaces must be managed on-site through infiltration, reuse, or evapotranspiration. This volume is provided by interception storage.

Interception Volume Required

  • Impermeable area: 14,000 m²
  • Interception depth: 5 mm = 0.005 m
  • Volume: 14,000 × 0.005 = 70 m³

Interception Provisions

Feature Area (ha) Storage (m³)
Permeable paving (driveways) 0.35 17.5
Swale network (avg. depth 150mm) 0.35 52.5
Total provided 0.70 70.0

Result: S2 requirement met ✓

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7
Treatment Train (S4)

Pollution Hazard Assessment

Residential roofs and roads are medium pollution hazard sources. Design a treatment train that removes sediment and pollutants progressively.

Proposed Treatment Train

  • 1st stage: Permeable paving (removal of fine sediments)
  • 2nd stage: Vegetated swale (biofiltration, settlement)
  • 3rd stage: Detention basin (settlement and polishing)
  • 4th stage: Outfall through orifice

SuDS Index Assessment

The SuDS Index (SIA) score evaluates treatment effectiveness. For residential medium hazard, a score ≥ 14 is compliant.

Treatment Component SIA Points
Permeable paving 5
Vegetated swale 6
Detention basin 5
Total SIA 16

Result: SIA ≥ 14 requirement met ✓

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✓ Design Summary

Parameter Value Basis
Greenfield QBAR (1-in-100) 5.2 l/s IH124 simplified; WRAP class 4, 650mm SAAR
Design rainfall (1-in-100+CC) 72.8 mm (60 min) +40% River Trent climate change allowance
Developed runoff volume 1,009 m³ 1.4 ha impermeable @ 0.9 coefficient; +10% urban creep
Attenuation storage required ~990 m³ Inflow − outflow during 60-min critical event
Orifice diameter 75 mm Sharp-edged opening @ 1.5m head = 5.2 l/s
Interception volume (S2) 70 m³ 5mm from 1.4 ha impermeable; permeable paving + swales
Treatment train (S4) SIA = 16 ✓ Permeable paving → swale → basin (medium hazard compliant)
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Last reviewed: 2026-05-10 · Sources cross-checked against the canonical registry at the time of publication. Standards may be updated; verify currency against the publisher before relying on any citation.