Attenuation storage sizing — discharge limited to greenfield rates (Defra NSTS S2), ≤24 h half-drain (CIRIA C753), EA climate-change allowances.
1
Site & Greenfield Data
2
Rainfall Data
3
Storage Calculation
4
Compliance Assessment
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PDF Export
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Site & Greenfield Data
Unique project identifier for documentation
Fields marked * are required
Unique site reference code
Total site area in hectares
Buildings, roads, hardstanding (m²)
Runoff Coefficients: These represent the fraction of rainfall that becomes runoff. Pervious areas (default 0.20) include gardens and planted areas. Impervious areas (default 0.95) include roofs and paved surfaces.
Range: 0.05–0.50 (0.20 typical for gardens)
Range: 0.70–1.00 (0.95 typical for hard surfaces)
Greenfield Rates: These are obtained from Calculator 1 (Greenfield Runoff Assessment). Enter or copy the calculated rates below.
Q1 from Calculator 1 — the Defra NSTS S2 frequent-event limit. If left blank, QBAR is used as a proxy (slightly generous).
Mean annual flood — reference figure. From Calculator 1.
100-year flood rate from Calculator 1
Calculated: QBAR ÷ Total Area. QBAR is the mean annual flood (~44% AEP, T≈2.3 yr on the annual-maximum series).
Defra NSTS S2: limit the 1% AEP discharge to the 1% AEP greenfield rate. Some LPAs require QBAR (stricter).
EA May 2022 peak-rainfall allowance epoch — pick the one spanning the development's design life.
Upper-end (95th) for essential/vulnerable development or where the LPA requires; central (50th) otherwise.
EA May 2022 Table 3: auto-populated from epoch+band above; edit to override.
EA May 2022 Table 3: auto-populated from epoch+band above; edit to override.
Selecting your catchment auto-fills the EA peak-rainfall allowance for the epoch/band above (EA May 2022 dataset, 90 catchments). Guidance: pick the epoch spanning your development's design life (2070s for a ~100-year residential lifetime); use the upper-end band for essential/vulnerable development or where the LPA requires, central otherwise. Note: the EA catchment dataset publishes only central (50th) and upper (95th) percentiles, so a "higher central" selection maps to the upper value.
Auto-filled from your catchment; editable. Cross-check on the EA peak rainfall map.
the urban-creep allowance (EA/Defra guidance): 10% uplift to account for future intensification within curtilages. Checked by default.
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Rainfall Data
Two entry modes available: Mode A uses FSR/FEH rainfall parameters (M5-60) for depth-duration conversion. Mode B allows direct entry of rainfall depths for multiple durations. Select the mode that suits your available data.
FSR/FEH Rainfall Parameters
Enter the 5-year 60-minute rainfall depth (M5-60) for your site. The calculator will convert this to other durations using standard depth-duration conversion factors.
5-year, 60-minute rainfall. Typical UK value: 20 mm. Check Flood Estimation Handbook or FEH CD.
Standard Depth-Duration Conversion Factors (relative to 60-min depth):
Duration
15 min
30 min
60 min
120 min
180 min
240 min
360 min
480 min
600 min
720 min
960 min
1440 min
Factor
0.56
0.75
1.00
1.32
1.54
1.71
2.05
2.27
2.45
2.59
2.85
3.31
Direct Rainfall Depth Entry
Enter your own design rainfall depths (mm) for each duration and return period, taken from FEH / FSR for the site. The greyed placeholders are only a formatting example (ungrown M5 base depths) and are not design values — in particular they are not grown for return period, so they under-state the 30-yr and 100-yr depths. Use Mode A if you want the tool to grow M5-60 for you.
30-year (3.3% AEP) Rainfall Depths
100-year (1% AEP) Rainfall Depths
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Storage Calculation & Results
Typical range 0.5–2.0 m. Used to calculate orifice diameter for fixed flow control.
Safety clearance above maximum water level (default 0.3 m). Range 0.1–1.0 m per UK standards.
30-Year (3.3% AEP) Storage Requirement
Required Volume
0
m³
Critical Duration
0
minutes
Allowable Discharge
0
l/s
Volume per Hectare
0
m³/ha
Duration vs Storage Volume
Duration (min)
Rainfall Depth (mm)
Inflow Volume (m³)
Outflow Volume (m³)
Storage Required (m³)
100-Year (1% AEP) Storage Requirement
Required Volume
0
m³
Critical Duration
0
minutes
Allowable Discharge
0
l/s
Volume per Hectare
0
m³/ha
Duration vs Storage Volume
Duration (min)
Rainfall Depth (mm)
Inflow Volume (m³)
Outflow Volume (m³)
Storage Required (m³)
Drainage Performance & Compliance
Half-Drain Time (30yr)
0
hours
Half-Drain Time (100yr)
0
hours
Total Depth Required
0
m (incl. freeboard)
Urban Creep Adjustment
0%
applied
Half-Drain Time Compliance:
Additional 1% AEP storage:
Flow Control Orifice Sizing
Formula: Q = Cd × A × √(2gh), rearranged to find diameter d = 2√(A/π) where A = Q / (Cd × √(2gh))
Orifice Diameter
0
mm
Orifice Area
0
cm²
⚠️ WARNING: Orifice diameter is less than 75 mm. Risk of blockage: ensure sediment trap and maintenance access are provided.
🔴 CRITICAL: Orifice diameter is less than 50 mm. CIRIA C753 (small-orifice blockage protection) requires blockage protection (sediment trap, filter, strainer screen, or enhanced maintenance regime).
Orifice Calculation Details:
Coefficient of discharge (Cd) = 0.61
Acceleration due to gravity (g) = 9.81 m/s²
Maximum head (h) = 1.0 m
Allowable discharge (Q) = 0 l/s
Q (m³/s) = 0 m³/s
Calculated orifice area: A = Q / (Cd × √(2gh)) = 0 m²
Diameter: d = 2 × √(A/π) = 0 mm
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Compliance Assessment
Compliance Checklist: This assessment verifies that the attenuation storage design limits discharge to the greenfield rate (Defra NSTS S2, 2015), achieves ≤24 h half-drain (CIRIA C753 / SuDS Manual), and applies the EA Climate Change Allowances.
Requirement
Criterion
Status
Assumptions & Limitations
Climate change: Rainfall depths are uplifted by the selected allowance (%) per the EA Climate Change Allowances (upper-end recommended).
Urban creep: If enabled, impermeable area is increased by 10% to account for future intensification within curtilages (the urban-creep allowance (EA/Defra guidance)).
Minimum discharge: a ~5 l/s practical minimum flow control is often adopted for buildability; there is no per-hectare minimum in the national standards, and where this exceeds the greenfield rate it is a departure to be justified with the LPA / LLFA.
Orifice sizing: Based on circular fixed orifice with Cd = 0.61. Other flow control devices (weirs, restrictions) require bespoke design.
Storage volume: Calculated for discrete storm durations (15 min to 24 hr). Continuous simulation may yield different results.
50% AEP (2-year) flood: Managed within development; attenuation design should prevent adverse downstream impact.
Blockage risk: Orifice diameter < 50 mm requires blockage protection per CIRIA C753 (small-orifice blockage protection).
Additional Checks Required (Outside Scope)
Detailed site survey & topographic survey for storage location
Geotechnical assessment for permeability, groundwater, and infiltration testing
Maintenance accessibility, safety fencing, and surface inlet design
Detailed design of overflow routing and spillway (for storm > 1% AEP)
Detailed design of inlet structures, sediment traps, and pollution prevention
Post-development modelling if significant uncertainties remain
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PDF Export
Generate Professional Report: Create a complete PDF report suitable for submission to planning authorities and stakeholders. Includes all input data, calculations, results, and compliance assessment.
PDF Content
Cover page with project name, site reference, and date
Quinnsite branding and footer
Executive summary of key results
Input parameters and site data
Rainfall data and climate change uplift
Developed runoff calculation (weighted Cv, rainfall intensity, area)
Duration vs storage volume tables (30-year & 100-year)
Orifice sizing calculation with equation and working