Stormwater runoff: steep-site vs shallow-site strategies

One-line orientation

How you handle stormwater flips with terrain: steep sites are about slowing water down to stop erosion, flat sites are about detaining and soaking it in — and good design manages both the quantity and the quality of runoff.

Key points

• Steep site → slow the flow (erosion control):
  • Check dams: small barriers across a channel/ditch that cut water velocity and trap sediment (rocks, sandbags, logs).
  • Riprap: large angular stones on slopes/shorelines that absorb and deflect water energy.
  • Terracing and energy dissipation break a long fast slope into manageable steps.
• Shallow / flat site → detain and infiltrate:
  • Percolation & green space: permeable/porous paving, increased planting, green roofs to absorb stormwater.
  • Bioswales: vegetated channels that capture, filter, and convey runoff (plants that take wet and dry).
  • Detention / retention ponds: hold stormwater and release it slowly.
  • Underground vaults: plastic/concrete tanks under parking — for space-constrained urban sites; effective but costlier.
• Always two jobs: manage quantity (peak flow, flooding, erosion) and quality (filter sediment/pollutants).

Stormwater runoff: steep = slow it, flat = soak it

SCHEMATIC

One continuous site section, from a terraced steep run at left to shallow flat ground at right.

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Key

• Runoff — arrows shorten as each control slows the water
• Infiltration — water soaking in
• Water held on site — detention / retention pond
• Strategy divider — steep zone / flat zone

Always two jobs: quantity (peak flow, flooding, erosion) and quality (filter sediment / pollutants).

Confusions / comparison

	Steep site	Shallow / flat site
Goal	Slow flow, control erosion	Detain + infiltrate
Typical tools	Check dams, riprap, terracing	Permeable paving, bioswales, ponds, green roofs
Main risk addressed	Velocity → erosion	Volume → ponding/flooding
Water mostly	Conveyed downslope, decelerated	Held and soaked in on site

Related

→ Detention vs retention (this module): the pond pair in detail · Swales vs berms (this module): the bioswale’s relatives · SWPPP & erosion (this module): construction-phase versions of these controls · Site slopes (this module): the grades that drive the strategy.