Civil and environmental engineers are noticing some unintended consequences of the updated Fairfax County Soils Map (See Field Notes, Vol. 19 No. 4). These include:

• Certain existing Stormwater Facilities may now be too small to accommodate    all of the development planned in their watershed, and;

• Some Allowable Stormwater Runoff rates have increased using the new soils    map vs. the old soils map.

In developed areas that were previously mapped, the new soils maps typically reflect that the original soil profiles have been disturbed by grading. This increases its run-off rate characteristic due to compaction and removal of the upper, more well-drained, portion of the soil profile. In such situations, the new NRCS Soil Survey for Fairfax County shows areas in which the Hydrologic Soil Group¹ has shifted one or more groups (e.g. C to D) to higher runoff potential. (See Figure 1 below)

Interestingly – there are cases where this makes it easier, as well as harder for new development. In the case where a stormwater pond was designed and constructed and most of its watershed developed – the last infill area may now need additional detention capacity because analysis using the new soils maps show higher runoff volumes than previously calculated with the old maps. However on infill sites without existing detention, the existing flow rates are now higher than when using the old maps, so the allowable discharge rate is now higher. Thus, such infill sites could have lower stormwater control requirements.

For more information, please contact Mike Rolband or Dan Lucey.

¹Hydrologic Soil Group defined