Stormwater Calculations - Mathematics and Statistics for the Environment - page 779

• Group A —Soils with low runoff-potential due to high infiltration rates even when thor-
oughly wetted. These soils consist primarily of deep, well to excessively drained sands and
gravels with high water transmission rates (0.30 in./hr.). Group A soils include sand, loamy
sand, or sandy loam.
• Group B —Soils with moderately low runoff potential due to moderate infiltration rates
when thoroughly wetted. These soils consist primarily of moderately deep to deep and
moderately well to well drained soils. Group B soils have moderate water transmission
rates (0.15 to 0.30 in./hr.) and include silt loam or loam.
• Group C —Soils with moderately high runoff potential due to slow infiltration rates when
thoroughly wetted. These soils typically have a layer near the surface that impacts the
downward movement of water or soils. Group C soils have low water transmission rates
(0.15 to 0.15 in./hr.) and include sandy clay loam.
• Group D —Soils with high runoff potential due to very slow infiltration rates. These soils
consist primarily of clays with high swelling potential, soils with permanently high water
tables, soils with a claypan or clay layer at or near the surface and shallow soils over nearly
impervious parent material. Group D soils have very low water transmission rates (0 to
0.05 in./hr.) and include clay loam, silty clay loam, sandy clay, silty clay, or clay.
Any disturbance of a soil profile can significantly alter the soil's infiltration characteristics.
With urbanization, the hydrologic soil group for a given area can change due to soil mixing, intro-
duction of fill material from other areas, removal of material during mass grading operations, or
compaction from construction equipment. A layer of topsoil may typically be saved and replaced
after the earthwork is completed, but the native underlying soils have been dramatically altered.
Therefore, any disturbed soil should be classified by its physical characteristics as given above for
each soil group.
Some jurisdictions require all site developments to be analyzed using an HSG classification that
is one category below the actual pre-developed HSG. For example, a site with a pre-developed HSG
classification of B, as determined from the soil survey, would be analyzed in its developed state
using an HSG classification of C.
25.6.3.3.2 Hydrologic Condition
Hydrologic condition represents the effects of cover type and treatment on infiltration and run-
off. It is generally estimated from the density of plant and residue cover across the drainage
area. Good hydrologic condition indicates that the cover has a low runoff potential, while poor
hydrologic condition indicates that the cover has a high runoff potential. Hydrologic condition
is used in describing non-urbanized lands such as woods, meadow, brush, agricultural land, and
open spaces associated with urbanized areas, such as lawns, parks, golf courses, and cemeteries.
Treat ment is a cover type modifier to describe the management of cultivated agricultural lands.
Tables 25.5A and 25.5B provide an excerpt from Table 2-2 in TR-55 which shows the treatment
and hydrologic condition for various land uses. When a watershed is being analyzed to deter-
mine the impact of proposed development, many stormwater management regulations require
the designer to consider all existing or undeveloped land to be hydrologically good conditions .
This results in lower existing condition peak runoff rates which, in turn, results in greater post-
development peak control. In most cases, undeveloped land is in good hydrologic condition unless
it has been altered in some way. Since the goal of most stormwater programs is to reduce the peak
flows from developed or altered areas to their pre-developed or pre-altered rates, this is a reason-
able approach. In addition, this approach eliminates any inconsistencies in judging the condition
of undeveloped land or open space.