Required Storage (Max S)

—m³

Storage Available

—m³

Critical Storm Duration

—min

½ Drain Time

—hr

K design (K/FS)

—m/s

Infiltration Area a_s50

—m²

Method

Core equations S(t) = A · C · R(t) · uplift − (a_s50 · K_design · t) / 2
K_design = K_fs / FS

Rectangular: a_s50 = 2 · (L + W) · (D / 2) · · V_avail = L · W · D · void
Circular: a_s50 = π · Ø · (D / 2) · · V_avail = π · (Ø/2)² · D · void

t_{½ drain} = (V/2) / (a_s50 · K_design) ≤ 48 hr

Base area excluded per BRE 365 — conservative, accounts for siltation. a_s50 based on side area to 50% of effective depth.

Iteration Table — Storage Required by Storm Duration

Duration (min)	Rainfall (mm)	w/ uplift (mm)	Inflow (m³)	Outflow during storm (m³)	Storage Required S (m³)

Critical case = maximum S row (highlighted). Iterate geometry inputs until Storage Available ≥ Max S and ½ drain time ≤ 48 hr.

Sensitivity Analysis (K × 0.5 and K × 2)

Per EGBC enforcement precedent — document K sensitivity when input data limited. Required storage at degraded and enhanced K values.

Scenario	K design (m/s)	Required Storage (m³)	½ Drain Time (hr)	Status vs trial geometry

Saved Iterations

Disclaimer: This calculator implements BRE Digest 365 (2016 rev) iterative method per Metro Vancouver Stormwater Source Control Design Guidelines (2023) and BC Sewerage System Standard Practice Manual V3 (2014) testing protocols. Results are for preliminary sizing only and require review, verification, and sealing by a P.Eng. licensed in BC. Site-specific K_fs testing per SPM V3 minimums is required — assumed K values are not defensible per EGBC enforcement precedent (Lilles Consent Order, November 2025). User assumes all responsibility for design decisions.

Calculating…

What this method does

BRE 365 sizes a soakaway by finding the worst storm duration where rainfall coming in beats water seeping out by the largest margin. That margin is the storage your pit must hold.

It's not a single calculation. It's a search — across many storm durations — for the one that's hardest to handle.

The core idea: water balance

At any moment during a storm, the pit is filling from runoff and emptying through its walls. Whichever is bigger wins.

Rainfall × area

A · C · R

−

OUT

Seepage through walls

a_s50 · K · t / 2

STORE

Volume pit must hold

S (m³)

Why "÷ 2" on the outflow?

The pit fills gradually during the storm. Wetted area on the walls grows from zero (empty) to full (peak). Average wetted area over the storm = roughly half of peak. So outflow rate × time gets halved.

The variables — what each one means

Impervious area

Hard surface that generates runoff — parking lot, roof, driveway. NOT the soakaway footprint.

m²

e.g. 1,000 m² parking lot

Runoff coefficient

Fraction of rainfall that becomes runoff. 0.9 for asphalt, 0.95 for concrete, 0.2–0.4 for grass.

unitless

e.g. 0.90 for asphalt parking

Rainfall depth

Total rain that falls during the storm duration (from IDF curve). Changes for every storm duration you test.

m (convert from mm)

e.g. 24 mm in 1 hr

K_fs

Field sat. hydraulic conductivity

How fast water seeps through the soil. Measured by permeameter, median of all tests (per EGBC). Gravels fast, silts slow.

m/s

Sand: ~10⁻⁴ · Silt: ~10⁻⁷

Factor of safety

Divider on K to account for siltation, soil variability, construction disturbance. Metro Van 2023 says 2–8 by judgement.

unitless

4 typical · 8 for silts/limited data

K_des

Design K (used in calc)

K_fs divided by FS. This is what actually goes into the equation — the conservative seepage rate.

m/s

K = 10⁻⁵, FS = 4 → K_des = 2.5×10⁻⁶

a_s50

Infiltration area at half-depth

Side wall area of the pit, measured to half the effective depth. Represents the average wetted area during a storm. Base ignored — siltation risk.

m²

Rect: 2(L+W)·(D/2) · Circ: π·Ø·(D/2)

Effective depth

Depth of pit BELOW the inlet invert — the storage zone. Not total dig depth.

e.g. 2.0 m of usable storage

Storm duration

How long the design storm lasts. You test many values — 10 min through 24 hr — and find the worst.

seconds (convert from min)

60 min = 3,600 s

Required storage

Volume pit must hold for that storm duration. The OUTPUT of the equation. Calculated for each duration tested.

m³

Bigger storms ≠ always bigger S

V_avail

Storage available

Volume your trial pit actually holds = pit volume × void ratio of fill. Compare against max S.

m³

Rock fill: void ~0.35 · Chamber: 1.0

t_½

Half-drain time

Time for the pit to drop from full to half-full once rain stops. Must be ≤48 hr (BC). Otherwise pit is full when next storm hits.

hours

(V/2) / (a_s50 · K_des)

Why iterate? The "critical duration" concept

This is the part that trips people up. You can't just plug in a single design storm and solve.

The tension

Short storms = high intensity but small total volume → low S
Long storms = lots of total rain but soil has more time to drain → low S
Somewhere in the middle = the worst combination → max S = critical duration

The critical duration depends on your soil. Permeable gravels drain fast → short critical duration. Tight silts can't drain → long critical duration.

Required storage S vs storm duration. The curve peaks at the critical duration. Position depends on soil K.

Reading the iteration table

The table runs the equation once for each storm duration. Each row is one storm scenario. The largest S value is your design number.

Duration	Rainfall	w/ uplift	Inflow	Outflow	Storage S
10 min	10 mm	12 mm	10.8 m³	0.4 m³	10.4
30 min	18 mm	21.6 mm	19.4 m³	1.1 m³	18.3
60 min	24 mm	28.8 mm	25.9 m³	2.2 m³	23.7
120 min	30 mm	36.0 mm	32.4 m³	4.4 m³	28.0
360 min	42 mm	50.4 mm	45.4 m³	13.2 m³	32.2
720 min	52 mm	62.4 mm	56.2 m³	26.4 m³	29.8
1440 min	65 mm	78.0 mm	70.2 m³	52.8 m³	17.4

Illustrative only — actual values come from the live Results tab. In this example the 6-hour storm governs (32.2 m³).

Reading row-by-row

Duration — storm length being tested
Rainfall — historical IDF depth for that duration
w/ uplift — rainfall × climate uplift factor
Inflow = A × C × R (volume entering pit)
Outflow = a_s50 × K_des × t / 2 (volume infiltrating during storm)
Storage S = inflow − outflow (what the pit must hold)

Common confusion

The longest storm has the highest rainfall total, but NOT always the highest required storage. As the storm gets longer, outflow grows (more time to drain). Eventually outflow grows fast enough that S starts to drop. That's why short-to-medium storms often govern in permeable soils, and longer storms govern in tight soils.

Reading the result cards

Required Storage (Max S)

23.7 m³

What you need. Largest S from the iteration table — the critical case. Your pit must hold at least this much.

Storage Available

31.5 m³

What you have. Trial pit volume × void ratio. Compare to Required Storage. If smaller → enlarge geometry.

Critical Storm Duration

60 min

Which storm governs. The storm length that produced max S. Useful to know — confirms whether short or long storms control your site.

½ Drain Time

36 hr

Recovery check. Time to drop from full to half. Must be ≤48 hr in BC. If too slow, pit may overflow when back-to-back storms hit.

K design (K/FS)

2.5×10⁻⁶ m/s

The conservative seepage rate. Your measured median K divided by FS. This is what's actually doing the work in the equation.

Infiltration Area a_s50

18 m²

Effective wall area. Side area of pit to half-depth. Bigger pit → bigger a_s50 → more outflow → smaller required storage.

Pass / fail indicators

Two checks must both pass for the trial geometry to be acceptable:

Volume: PASS Storage Available ≥ Required Storage
½ Drain ≤ 48 hr: PASS Pit recovers before next storm

If either fails (FAIL) → adjust geometry. Larger footprint (L, W, Ø) helps both. More depth helps volume but slightly hurts drain time.

The design workflow — what to actually do

Input fixed values — catchment area, runoff coef, measured K, FS, climate uplift, IDF rainfall depths
Try a starting geometry — pick L/W/D or Ø/D based on available footprint
Check both pass/fail indicators
- Volume PASS + drain PASS → done, but try smaller to optimize
- Volume FAIL → enlarge footprint or depth
- Drain FAIL → enlarge footprint (not depth — depth doesn't help drain time)
Iterate geometry until comfortable — typically aim for V_avail ~1.2–1.5 × required S to leave reserve capacity
Check sensitivity table — if K × 0.5 still passes, design is robust. If fails, document the risk or add more FS
Verify all checklist items — GWT clearance, frost, setbacks, pre-treatment, climate uplift documented

Depth vs footprint trade-off

Adding depth increases storage roughly linearly, but only marginally increases infiltration area (sidewall area at half-depth is D/2). Adding footprint (L, W, Ø) increases BOTH storage and infiltration area. For drainage time problems, always go wider not deeper.

Worked example — start to finish

Site: 1,000 m² parking lot in Cranbrook. Glaciofluvial sand. Median K_fs = 1×10⁻⁵ m/s from 4 permeameter tests.

Step 1 — Design K K_des = K_fs / FS = 1×10⁻⁵ / 4 = 2.5×10⁻⁶ m/s

Step 2 — Try rectangular pit 6 × 3 × 2 m a_s50 = 2 · (6 + 3) · (2/2) = 18 m²
V_avail = 6 · 3 · 2 · 0.35 = 12.6 m³

Step 3 — Test 60-min storm (R = 24 mm hist · 1.20 uplift = 28.8 mm = 0.0288 m) Inflow = 1000 · 0.90 · 0.0288 = 25.9 m³
Outflow = 18 · 2.5×10⁻⁶ · 3600 / 2 = 0.081 m³
S = 25.9 − 0.081 = 25.8 m³ required

Result: trial geometry FAILS

Required = 25.8 m³ but Available = 12.6 m³. Need to enlarge. Try 8 × 4 × 2.5 m: V_avail = 28 m³, a_s50 = 30 m². Re-run.

Step 4 — Verify ½ drain time t_½ = (28/2) / (30 · 2.5×10⁻⁶) = 186,667 s = 52 hr ✗ FAIL
Drain time too slow. Go wider, not deeper. Try 10 × 5 × 2 m: V_avail = 35 m³, a_s50 = 30 m²
t_½ = (35/2) / (30 · 2.5×10⁻⁶) = 65 hr — still fails. Need higher K or larger footprint.

What this teaches

With K = 1×10⁻⁵ m/s and FS = 4, you may need either: (a) lower FS if data is robust, (b) larger footprint than expected, or (c) shallower-and-wider geometry. The iteration shows you the constraint before construction.

Equation sources & references

The equations and conventions used here are taken from the following published sources:

Primary source — the BRE digest itself

BRE Digest 365 — Soakaway Design (DG 365)

Building Research Establishment, Revised February 2016. ISBN 978-1-84806-918-6. First published 1991 (replacing Digest 151), revised 2007 and 2016. Author: Stephen L Garvin.

Core equation: I − O = S
Where: I = A · R (inflow from impervious area × rainfall depth); O = a_s50 · f · D (outflow = side area to half-depth × infiltration rate × storm duration); S = required storage.
a_s50 definition (verbatim): "the internal surface area of the soakaway to 50% effective storage depth: this excludes the base area which is assumed to clog with fine particles and become ineffective in the long term"

PDF copy hosted by Unda Ltd · Original sold by BRE Press / IHS

Secondary references — methodology context

CIRIA C753 — The SuDS Manual

Woods Ballard, B. et al. (2015). Chapter 13 "Infiltration systems" expands BRE 365 with climate-change uplift, half-drain time ≤24 hr requirement, and void ratio guidance for fill materials.

CIRIA, London. ISBN 978-0-86017-760-9.

BS EN 752 — Drain and sewer systems

European standard referenced by BRE 365 §3.1. Provides general infiltration design principles, including soakaway-to-building setback (5 m minimum).

Metro Vancouver SSCDG 2023

§1.6.2 — factor of safety on K (2–8 range). §1.7.3 — GWT and bedrock setbacks. Adapts UK methodology to BC context.

Direct PDF

BC SPM V3 (2014)

§III-8.3.3.3 — constant-head borehole permeameter testing methodology. Minimum 4 tests per dispersal area. Source of "median K_fs" convention used by EGBC.

Direct PDF

Notation alignment

BRE 365 uses British notation. This walkthrough uses notation aligned with North American / BC practice for clarity. The mapping:

BRE 365 symbol	Used here	Meaning
f	K_des	Soil infiltration rate (BRE) / Design hydraulic conductivity (BC)
D	t	Storm duration (BRE uses D; renamed t here to avoid conflict with pit depth)
a_s50	a_s50	Internal surface area to 50% effective depth (unchanged)
I, O, S	Inflow, Outflow, Storage	Volumes (unchanged)

Departures from strict BRE 365

This walkthrough adds three things BRE 365 does not include but BC practice requires:

1. Factor of Safety on K. BRE 365 uses measured soakage rate f directly. BC convention divides K_fs by FS = 2–8 (Metro Van 2023). This is more conservative.
2. Climate uplift on rainfall. BRE 365 mentions climate change; CIRIA C753 adds explicit uplift multipliers. EGBC Climate Change Action Plan V1.1 (2023) makes it mandatory in BC. Applied here as a direct multiplier on IDF depth.
3. ½-drain time ≤48 hr. BRE 365 cites 24 hr (UK). BC practice typically allows 48 hr. Adjust the threshold in your design report based on AHJ requirements.

Reference document

Hydraulic Conductivity K — Literature Source Comparison

This reference compiles the two most-cited literature sources for saturated hydraulic conductivity values used in geotechnical and stormwater infiltration design: Das (2014) Table 7.1 and Freeze & Cherry (1979) Tables 2.2 and 2.3.

Both tables are valid for literature K assumption when no field data is available, but they classify and bin soils differently. For BC practice — particularly Kootenay glaciofluvial and glaciolacustrine sites — Freeze & Cherry should lead, with Das as cross-reference. Field-measured K_fs (permeameter, median of ≥4 tests) always supersedes literature values per EGBC enforcement precedent.

1. Source Documents

Primary citations · open-access links where available

Source A — most widely taught

Das, B.M. (2014). Principles of Geotechnical Engineering, 8th edition

Cengage Learning, Stamford CT. ISBN 978-1-133-10866-1. Table 7.1: "Typical Values of Hydraulic Conductivity of Saturated Soils"

Bins: Clean gravel · Coarse sand · Fine sand · Silty clay · Clay

Units: cm/s

Cengage product page (current edition)

Source B — definitive reference

Freeze, R.A. & Cherry, J.A. (1979). Groundwater

Prentice-Hall, Englewood Cliffs NJ. ISBN 0-13-365312-9. 604 p. Open-access via The Groundwater Project (rights returned to authors).

Table 2.2: Range of Values of Hydraulic Conductivity and Permeability — rocks and unconsolidated deposits, deposit-genesis classification

Table 2.3: Conversion Factors for Permeability and Hydraulic Conductivity Units

Units: darcy, cm², cm/s, m/s, gal/day/ft² (all in parallel)

Groundwater Project — full book, multiple languages
Hydrogeologists Without Borders mirror
Internet Archive scanned copy

2. K Values — Side by Side

Converted to m/s for direct comparison

Das (2014) Table 7.1 — converted to m/s

Soil type	cm/s (original)	m/s
Clean gravel	100 – 1.0	10⁰ – 10⁻²
Coarse sand	1.0 – 0.01	10⁻² – 10⁻⁴
Fine sand	0.01 – 0.001	10⁻⁴ – 10⁻⁵
Silty clay	0.001 – 0.00001	10⁻⁵ – 10⁻⁷
Clay	< 0.000001	< 10⁻⁸

Freeze & Cherry (1979) Table 2.2 — unconsolidated deposits, m/s

Deposit type	K (m/s)
Gravel	~10⁻³ – ~10⁰
Clean sand	~10⁻⁵ – ~10⁻²
Silty sand	~10⁻⁷ – ~10⁻⁴
Silt, loess	~10⁻⁹ – ~10⁻⁵
Glacial till	~10⁻¹² – ~10⁻⁶
Unweathered marine clay	~10⁻¹³ – ~10⁻⁹

Ranges read from Table 2.2 log-scale chart · Source: open-access via gw-project.org

Freeze & Cherry Table 2.2 — Rocks (additional coverage)

Rock type	K (m/s)
Karst limestone	~10⁻⁶ – ~10⁰
Permeable basalt	~10⁻⁷ – ~10⁻²
Fractured igneous and metamorphic rocks	~10⁻⁸ – ~10⁻⁴
Limestone and dolomite	~10⁻⁹ – ~10⁻⁶
Sandstone	~10⁻¹⁰ – ~10⁻⁶
Shale	~10⁻¹³ – ~10⁻⁹
Unfractured metamorphic and igneous rocks	~10⁻¹³ – ~10⁻¹⁰

Das Table 7.1 contains no rock entries. Freeze & Cherry uniquely covers fractured/unfractured bedrock — relevant for shallow bedrock sites in the Kootenays.

3. Significant Differences

Where the sources diverge and why it matters

Issue	Das (2014)	Freeze & Cherry (1979)	Practical impact
Classification basis	Soil texture (USCS-aligned)	Deposit genesis (depositional environment)	F&C reflects that origin controls K more than texture alone
Glacial till	Not represented — would fall under "silty clay"	Explicit entry, range ~10⁻¹² to ~10⁻⁶ m/s	Critical for Kootenay sites. Das mis-classification can overestimate K by 5+ orders
Glaciolacustrine silt	Lumped into "silty clay"	Captured under silt/loess and till bands	F&C provides defensible range for Kootenay valley-floor sites (Cranbrook, Kimberley, Invermere)
Range width — clay	"< 10⁻⁸" (floor only)	Marine clay: ~10⁻¹³ to ~10⁻⁹ m/s	F&C resolves aquitard-grade values; useful for barrier design
Rock coverage	None	Karst, basalt, sandstone, fractured/unfractured igneous, shale	F&C usable at shallow-bedrock sites or fractured-rock soakaways
Units presented	cm/s only	darcy, cm², cm/s, m/s, gal/day/ft² in parallel	F&C eliminates conversion error step for SI reports
Distinction within fines	Silty clay vs clay only	Silt/loess, glacial till, marine clay all distinct	F&C lets engineer pick the correct genesis match
Ease of use	Simple, fits one screen	Log-scale chart, requires interpretation	Das wins for quick reference; F&C wins for defensible report

Where they agree

Clean gravel and clean coarse sand — converge in the 10⁻² to 10⁰ m/s range
General order-of-magnitude communication — both express that K spans ~13 orders across earth materials
Both are appropriate for preliminary literature K assumption; neither substitutes for field testing

4. Unit Conversion — F&C Table 2.3

Multiply row unit by column factor to obtain column unit

	Permeability k			Hydraulic conductivity K
From ↓ / To →	cm²	ft²	darcy	m/s	ft/s	U.S. gal/day/ft²
cm²	1	1.08 × 10⁻³	1.01 × 10⁸	9.80 × 10²	3.22 × 10²	1.85 × 10⁹
ft²	9.29 × 10²	1	9.42 × 10¹⁰	9.11 × 10⁵	2.99 × 10⁶	1.71 × 10¹²
darcy	9.87 × 10⁻⁹	1.06 × 10⁻¹¹	1	9.66 × 10⁻⁶	3.17 × 10⁻⁵	1.82 × 10¹
m/s	1.02 × 10⁻³	1.10 × 10⁻⁶	1.04 × 10⁵	1	3.28	2.12 × 10⁶
ft/s	3.11 × 10⁻⁴	3.35 × 10⁻⁷	3.15 × 10⁴	3.05 × 10⁻¹	1	6.46 × 10⁵
U.S. gal/day/ft²	5.42 × 10⁻¹⁰	5.83 × 10⁻¹³	5.49 × 10⁻²	4.72 × 10⁻⁷	1.55 × 10⁻⁶	1

* To obtain k in ft², multiply k in cm² by 1.08 × 10⁻³. Source: Freeze & Cherry (1979) Table 2.3.

Most-used conversion for BC stormwater reports: cm/s × 0.01 = m/s. Example: K = 1 cm/s = 1 × 10⁻² m/s = 864 m/day = 36,000 mm/hr.

RDEK / EGBC compliance

Design Checklist

Verify each item before sealing. References point to the governing document. Status column is for the GER's review notes.

Item	Reference	Status
Min 4 constant-head permeameter tests per drainage area	SPM V3 §III-8.3.3.3	☐ Verify
Median K used (not mean, not best)	EGBC Lilles 2025	☐ Verify
≥600 mm clearance above seasonal high GWT	Metro Van SSCDG 2023	☐ Verify
≥600 mm clearance above bedrock	Metro Van SSCDG 2023	☐ Verify
≥30 m setback from drinking water wells	Metro Van SSCDG 2023	☐ Verify
≥30 m setback from slopes >15% (or geotech review)	Metro Van SSCDG 2023	☐ Verify
Invert below community frost depth	BCBC 2024 App C	☐ Verify
Pre-treatment for parking lot runoff (OGS / swale / forebay)	BC USI 2014	☐ Verify
Climate uplift documented (scenario + horizon)	EGBC CCAP V1.1	☐ Verify
Overflow path to permitted outfall	RDEK Bylaw 1334	☐ Verify
Sensitivity analysis on K documented	EGBC Lilles 2025	☐ Verify
Letter of Assurance issued by GER (if required by AHJ)	EGBC Geotech PPG V2.1	☐ Verify

Disclaimer: This checklist is for working reference. Final compliance verification, sealing, and Letter of Assurance issuance rests with the Geotechnical Engineer of Record (GER). Local AHJ may add or modify items.