Constructed Wetlands Design Calculator

Site & Catchment Parameters

Enter site-specific and rainfall data to define your drainage catchment

Project Information

Project Name

Site Reference

Engineer Name

Date

Catchment Area (ha) Total drainage catchment

ha

Impermeable Area (m²) Roofs, hardstanding, roads

m²

Dry Weather Flow (l/s) Base flow maintaining wetland habitat

l/s

Receiving Water Sensitivity Determines treatment stringency

Wetland Type & Configuration

Select wetland design approach and configuration parameters

Wetland Type Selection

Surface Flow (SF) – Open water with emergent and submerged plants. Best for habitat creation and secondary treatment.

Horizontal Sub-surface Flow (HSSF) – Gravel bed with reeds. High treatment efficiency, lower footprint.

Vertical Sub-surface Flow (VSSF) – Vertical percolation through media. Best aeration and nitrification.

Hybrid – Combination (e.g., VSSF + SF). Maximum treatment and habitat value.

Surface Flow Configuration

Permanent Pool Depth (m) 0.3-0.6m typical

Length:Width Ratio ≥3:1 treatment, ≥5:1 ideal

Inlet Configuration How flow enters the wetland

Sub-surface Flow Configuration

Media Type Substrate for root growth and treatment

Media Depth (m) 0.6-1.0m typical

Media Porosity Void ratio (0.30-0.45)

Sizing & Hydraulics

Calculate treatment volume and hydraulic residence time

Note: The calculator will compute the required wetland area and volume based on your design parameters. Hydraulic residence time (HRT) is critical for treatment effectiveness.

Design Rainfall Depth (mm) Storm volume to be stored

mm

Target HRT (hours) 24h basic, 48-72h advanced

Sizing Calculations

Treatment Volume Required

-

m³

Wetland Surface Area

-

m²

Actual Hydraulic Residence Time

-

hours

Design Compliance

PASS

Pollutant Removal Estimates

First-order kinetics: Ce/Ci = exp(-k × HRT)

Pollutant	k value (/day)	Inlet Conc. (mg/l)	Outlet Conc. (mg/l)	Removal %
TSS (Suspended Solids)	-	100	-	-
Total Phosphorus (P)	-	1.5	-	-
Total Nitrogen (N)	-	30	-	-
BOD₅	-	80	-	-
Metals (Cu, Zn)	-	0.5	-	-

Vegetation & Biodiversity

Plan planting zones and assess biodiversity compliance (Defra S6)

Defra S6 Guidance: Require minimum 3 vegetation zones, native species, open water habitat, and habitat feature provision for wildlife.

Planting Density & Species

Emergent Species Density (plants/m²) 4-6 typical

Marginal Species Density (plants/m²) 6-8 typical

Submerged Bunches/m² 3-5 typical

Biodiversity S6 Compliance Checklist

Native species only (no invasive exotics)

Minimum 3 vegetation zones

Open water area for birds/amphibians (≥20%)

Shallow margins for invertebrates (<0.3m)

Log/stone habitat features and refugia

Connectivity to existing habitat corridors

Recommended Planting Schedule

Zone	Water Depth	Example Species	Function
Zone 1: Deep	>0.6m	Potamogeton, Ceratophyllum, Ranunculus	Oxygen production, sediment stabilization
Zone 2: Emergent	0.3–0.6m	Phragmites australis, Typha latifolia, Iris pseudacorus	Primary treatment, nutrient uptake, bird habitat
Zone 3: Shallow	<0.3m	Juncus effusus, Carex nigra, Lythrum salicaria	Invertebrate habitat, secondary treatment
Zone 4: Banks	Above water	Alder, Willow, native wildflower mix	Riparian buffer, shelter, erosion control

Safety & Access

Define safety parameters and maintenance access requirements

Duty of Care: Assess water depth hazards in publicly accessible areas and implement appropriate control measures.

Depth & Slope Safety

Public Access Area Max Depth (mm) ≤600mm for accessible areas (Defra S5)

mm

Assess Fencing Requirement Required if actual depth > 600mm

Above-Water Side Slope Recommended ≤1:3

Below-Water Side Slope Recommended ≤1:4

Maintenance Access

Maintenance Track Width (m) 3m minimum to key areas

Freeboard Above Design Storm (m) ≥0.3m typical

Safety signage installed (deep water, no swimming)

Maintenance plan documented

Inlet/outlet protected from debris

Results & Compliance Summary

Review final design and export documentation

Wetland Dimensions

Surface Area

-

m²

Length

-

m

Width

-

m

Volume

-

m³

Compliance Assessment

Defra S4 Water Quality Compliance

Design criteria for this standard — the assessed result is shown below.

Water-quality treatment volume sized on impermeable area × design rainfall depth
TSS removal ≥80% (basic-treatment target)
HRT ≥24 hours for basic treatment
Inlet sediment forebay recommended
Design for NORMAL water sensitivity

✓ Defra S4 Status: COMPLIANT

Defra S5 Safety Compliance

Design criteria for this standard — the assessed result is shown below.

Max public area depth ≤600mm or fenced
Above-water slope ≤1:3
Below-water slope ≤1:4
Freeboard ≥0.3m above design level
Safety signage (deep water / no swimming)
Maintenance access ≥3m wide

✓ Defra S5 Status: COMPLIANT

Defra S6 Biodiversity Compliance

Biodiversity criteria — the assessed result (from the Step 4 checkboxes) is shown below.

Native species only
Minimum 3 vegetation zones
Open water habitat (≥20%)
Shallow margins <0.3m for invertebrates
Habitat features (logs, stones)
Habitat corridor connectivity

✓ Defra S6 Status: COMPLIANT (6/6 criteria)

CIRIA C753 §23 Compliance

CIRIA recommends HRT-based design with first-order kinetics modelling for pollutant removal.

Design criteria for this standard — the assessed result is shown below.

HRT ≥24 hours (minimum for 80% TSS)
Length:width ratio ≥3:1 for surface flow
Inlet sediment settling zone
Outlet structure controls water level
Vegetation zones
SIA mitigation index considered

✓ CIRIA C753 Status: COMPLIANT

Pollutant Removal Summary

Pollutant	Inlet (mg/l)	Outlet (mg/l)	Removal %	CIRIA Target

Maintenance Requirements Summary

Establishment Phase (Year 1): Fortnightly inspection, weed control, plug survival monitoring, hydraulic testing
Operational Phase (Years 2-5): Quarterly inspection, vegetation management, sediment monitoring, invert/outlet clearance
Long-term (5+ years): Annual inspection, silt removal (if depth <50% design), vegetation thinning, habitat enhancement
Emergency Response: Inlet blockage clearance, outlet failure response, algal bloom management

Engineer's Notes Optional, included in PDF report