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Infiltration plays an increasingly important role in meeting stormwater runoff mitigation regulations. One of the primary considerations for infiltration feasibility is the runoff routing.

Infiltration systems must drain in a reasonable amount of time to avoid creating mosquito habitat and to recover storage volume for subsequent storms. Drain down of water stored on the surface within 72 hours is commonly referenced as a benchmark for West Nile Virus control. Drain down of the design volume within 48 hours is typically required to avoid bypassing a significant amount of rainfall in successive storms. Where longer drain down times are required due to slow-draining soils, the infiltration system storage volume can be increased to maintain a high annual runoff capture volume percentage. The relationship between the storage volume, annual capture percentage, and drain down time depends on local rainfall patterns and is best determined using continuous simulation modeling.

Soil characteristics are the most important factor influencing infiltration rates.  The Natural Resource Conservation Service has grouped soils that have similar runoff characteristics into four classes. A common rule of thumb for infiltration feasibility is that hydrologic soil groups A and B are suitable, C soils are questionable, and D soils are not good candidates.

  

Table 1: Characteristics of NRCS hydrologic soil groups

Soil Class

Infiltration

Runoff Rate

Infiltration Rate

Soil Types

A

High

Low

>0.3 in./hr when wet

Sand or sandy loam

B

Moderate

Moderate

0.15 to 0.3 in./ hr when wet

Silt loam or loam

C

Low

Moderate to High

0.05 to 0.15 in./hr when wet

Sandy clay loam

D

Low

High

0.0 to 0.05 in./hr when wet

Clay, silty clay, clay loam

 

It is also prudent to remember that infiltrated water does not go "away." A good understanding of subsurface geology, especially horizontal confining layers, can help avoid the mistake of infiltrating water in one area only to have it reemerge as a seep in a neighboring property. Care must also be taken to avoid underground features like contaminated soils and groundwater, utilities, groundwater extraction wells and septic tanks. Common routing related infiltration feasibility criteria are as follows:

  • Drain storage within 48 hours or increase storage volume in poor-draining soils 

  • Drain down surface storage within 72 hours for vector control

  • Native soil infiltration rate should be at least 0.5 inches per hour for reliable long term infiltration

  • Avoid infiltrating over horizontal confining layers, such as ledge, caliche, or clay

  • Avoid infiltrating near utility trenches or other linear deposits of permeable fill that may accumulate and conduct groundwater

This is an excerpt from the Professional Development Article: Introduction to Infiltration Best Management Practices (BMP). Read the article and take the quiz for free PDH credits.

Click here to view additional hydrologic soil group info from the NRCS. 

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