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A coarse layer drains easily when wet, yet can resist entry when it is dry. Beneath an unsaturated fine layer, its large pores require the interface pressure to rise enough before water enters. This temporary hydraulic contrast creates a capillary barrier. The barrier stores or diverts water; it is not an impermeable seal, and breakthrough depends on entry conditions, layer geometry, flux and defects.
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Chapter contents
Choose the route closest to the decision, or read from Chapter 1. The chapters remain open; these grouped routes are a navigable index.
Route 1 · Define the decision
Route 2 · Work the ledgers
Route 3 · Transfer the method
Route 4 · Check limits and evidence
Route 5 · Ask and continue
CHAPTER 1 OF 12 · DEFINE THE DECISION
1. Start with the counter-intuitive dry interface
Unsaturated conductivity is controlled by connected water pathways, not saturated conductivity alone. A dry coarse material may have very low conductivity because its large pores empty at modest suction. Water arriving through finer pores can therefore accumulate above the interface.
The layer order is essential. Fine over coarse can form a capillary break; coarse over fine usually does not create the same entry barrier. State the wetting direction and initial water state before using the term.
CHAPTER 2 OF 12 · DEFINE THE DECISION
2. Define entry and breakthrough operationally
Breakthrough begins when the pressure state at the interface permits a connected conducting pathway into the coarse layer. Experiments may detect it from first drainage, a pressure threshold or visible wetting below the interface.
Those definitions need not give the same instant. First local leakage can precede bulk outflow, and sensors may average over a front. Report the chosen endpoint and its detection limit.
CHAPTER 3 OF 12 · WORK THE LEDGERS
3. Work a layer-storage ledger
Consider a 0.40 m fine layer whose mean volumetric water content rises from 0.12 to 0.29 before breakthrough. Ignoring lateral flow and volume change, stored water depth is thickness times water-content change.
| State | Fine-layer water content | Stored depth above initial | Observation |
|---|---|---|---|
| Initial | 0.12 | 0 mm | Dry profile |
| After 20 mm input | 0.17 | 20 mm | No drainage |
| After 48 mm input | 0.24 | 48 mm | Interface accumulation |
| Breakthrough | 0.29 | 68 mm | Coarse layer begins conducting |
CHAPTER 4 OF 12 · WORK THE LEDGERS
4. Link pore contrast to the entry condition
The coarse layer’s water-entry suction reflects the pressure needed to invade its largest connected pore network. Greater contrast between fine and coarse retention curves can increase storage above the interface, but the entire conductivity and retention functions matter.
A coarse layer with a broad particle-size distribution or fine contamination may contain smaller pores that bridge the interface, weakening the barrier. Contact quality and packing are therefore part of the hydraulic design.
CHAPTER 5 OF 12 · WORK THE LEDGERS
5. Change the slope and redirect flow
On an inclined interface, water can move laterally within the fine layer while entry into the coarse layer is delayed. The diversion length depends on slope, supply, hydraulic functions and layer thickness. Once local storage is exhausted, water breaks through or emerges downslope.
| Change | Likely effect | Competing risk | Evidence check |
|---|---|---|---|
| Steeper interface | More lateral gravity component | Erosion or outlet concentration | Flux along slope |
| Higher rainfall flux | Earlier local saturation | Runoff and ponding | Complete water balance |
| Finer contamination below | Lower entry contrast | Preferential bridges | Interface sampling |
| Root or crack crossing | Local early breakthrough | Biological redistribution | Replicated tracers |
CHAPTER 6 OF 12 · TRANSFER THE METHOD
6. Distinguish storage capacity from service life
Storage capacity is a state difference: how much extra water the fine layer can retain before the coarse layer conducts. Service life also depends on repeated storms, drainage between storms, evaporation, vegetation and lateral outlet capacity.
A test that stops before breakthrough estimates short-term retention, not indefinite protection. After breakthrough, hysteresis can make the return path different from the original drying state.
CHAPTER 7 OF 12 · TRANSFER THE METHOD
7. Connect the established soil-water routes
The Richards-equation guide keeps storage and boundary fluxes consistent. The unsaturated-conductivity guide explains why a dry coarse layer can transmit less than a wetter fine layer.
Use the hysteresis guide for repeated wetting and the wetting-front guide when unstable fingers or overshoot may bypass a nominally uniform interface.
CHAPTER 8 OF 12 · TRANSFER THE METHOD
8. Design a defensible column or field test
Characterise particle-size distribution, porosity, water-retention and unsaturated-conductivity curves for both layers. Build replicated interfaces with documented compaction and contact. Measure input, storage profile, interface pressure, lateral flux and drainage.
Apply more than one flux and initial water state. A field design should also test roots, settlement, freeze–thaw where relevant, extreme events and outlet blockage. Validate the model against withheld storms rather than fitting every parameter to one breakthrough trace.
CHAPTER 9 OF 12 · CHECK LIMITS AND EVIDENCE
9. Know the limits and common misconceptions
A capillary barrier is not waterproof and does not rely on the coarse layer having lower saturated conductivity. Its performance is an unsaturated entry phenomenon. Once the coarse layer wets, its conductivity may rise sharply.
One-dimensional columns can overstate uniformity and cannot establish lateral diversion length. Scaling a laboratory suction threshold to a field cover requires geometry, heterogeneity and boundary conditions.
CHAPTER 10 OF 12 · CHECK LIMITS AND EVIDENCE
10. Use an evidence and source routine
Record layer order, thickness, slope, texture, density, porosity, retention and conductivity curves, initial water, flux history, interface construction, pressure sign, breakthrough definition, storage, lateral drainage, defects, replication and model residuals.
Stormont and Anderson show that the coarse lower layer controls breakthrough through its water-entry condition. Their storage-capacity method links overlying texture, thickness and entry head. Kämpf and Montenegro study inclined capillary barriers as landfill covers.
CHAPTER 11 OF 12 · ASK AND CONTINUE
11. Frequently asked questions
Why does gravel stop water when gravel drains quickly?
Dry gravel lacks connected water-filled pores until the interface pressure rises enough to enter its large pores.
Does breakthrough mean the whole interface failed?
Not necessarily. It may begin locally, so spatial sensors and drainage patterns matter.
Can a flat barrier divert water?
It can store water and delay vertical flow, but sustained lateral diversion generally needs a slope and an outlet path.
CHAPTER 12 OF 12 · ASK AND CONTINUE
12. Continue with a storm transfer check
Use the storage ledger for a second storm that begins at water content 0.20 rather than 0.12. Holding the breakthrough water content at 0.29, calculate the remaining storage. Then add 10 mm of lateral diversion and state the rainfall amount at which drainage would be expected under the simplified balance.
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