Site Evaluation, Soils, and Drainage
Topography and water
Slope is expressed as percent grade = (rise ÷ run) × 100; degrees equal arctangent of rise/run. A 6% slope is about 3.4°. Grade must fall away from structures - typically a minimum of 2% (roughly 6" in the first 10 ft) - and paved swales need at least 1%. Map the watershed and identify off-site flows before drawing anything: water you redirect becomes someone else's problem, and drainage law generally protects the receiving property.
Soil
Separate texture (proportions of sand, silt, clay) from structure (how particles aggregate). Texture governs infiltration and water-holding capacity; structure governs aeration and root penetration. Field tests (ribbon, jar settle) give a rough class; a lab test gives pH, nutrients, organic matter, and soluble salts. Compaction from construction equipment destroys structure and is a leading cause of transplant failure. Match the drainage solution to the problem: swale or surface regrade for sheet flow, French drain or dry well for perched water, retention (holds water) versus detention (metered release).
Worked example
A swale drops 18 inches over a 25-ft run: 1.5 ÷ 25 = 0.06 = 6% slope. To hold a 2% minimum across a 12.5-ft span you need only 3 inches of fall - small errors in setting grades matter.
Traps
Confusing percent with degrees; assuming clay always drains poorly without testing; designing without a percolation or infiltration check; ignoring cut-and-fill balance on a grading plan.