Filter Strips

Filter strips are grassy areas where runoff flows in a thin sheet. They’re not great at reducing peak flows but work well at removing solid pollutants during small storms (< 1 year). They’re most effective on gentle slopes (less than 5%) and with moderately permeable subsoil (0.15 to 4.3 mm/h or 0.006 to 0.17 in/h).

In VOSWMM, you can represent filter strips as porous sub-areas that receive runoff from upstream areas. These sub-areas, resembling field dimensions, collect water from impervious or semi-impervious surfaces and drain into a conduit representing a gutter or street. VOSWMM offers various methods to simulate filter strip infiltration.

Figure 4-2
Source: “Storm Water Management Model Applications Manual”

Infiltration Trenches

Infiltration trenches are excavated and filled with stone to capture and allow runoff to seep into the ground (Guo, 2001). Effective performance requires subsoil with a minimum permeability of 13 mm/h (0.5 in/h) (U.S. EPA, 1999). When simulating an infiltration trench in VOSWMM, it’s essential to consider infiltration, storage, and water flow. Figure 4-3 demonstrates a practical SWMM representation: a rectangular, fully porous sub-area with storage depth matching the trench’s pore space.

Figure 4-3
Source: “Storm Water Management Model Applications Manual”