UFC 3-210-10
25 October 2004
CHAPTER 7
COMPARISON OF LID TO CONVENTIONAL PRACTICES
7-1
INTRODUCTION. Conventional stormwater management practices focus on
providing an efficient site drainage system that rapidly conveys runoff away from
buildings and off pavement, and then attenuates the peak runoff rate at a large
stormwater management facility downstream. In contrast, LID provides runoff
management as far upstream as possible where it originates and if necessary, also
at multiple points along each flow path. LID and conventional practices can be further
compared in a variety of ways:
7-2
COMPLIANCE VS. WATER RESOURCE OBJECTIVES. While conventional
stormwater management is primarily concerned with attenuating the peak runoff rate
from a developed site, the principal goal of LID is to ensure maximum protection of the
ecological integrity of the receiving waters by maintaining the watershed's hydrologic
regime.
7-3
WATER QUANTITY CONTROL. Conventional drainage practices effectively
reduce peak runoff rates, but do not reduce runoff volume. Instead, conventional
drainage practices increase runoff volume by not mitigating the effects of the increased
impervious area. The LID features that facilitate infiltration, by comparison, help to
reduce runoff volume directly. Runoff volume reductions using LID features can be
significant when infiltration is increased over a sufficiently large area.
Conventional drainage reduces the amount of subsurface water available to
the base flow in nearby streams. LID features that enhance infiltration can have the
beneficial effect of helping to maintain those base flows. Other LID features allow the
strategic use of stormwater on-site, while conventional drainage designs focus on
moving the water rapidly off-site.
A conventional stormwater management facility has a limited ability to
manage water quality because it is limited to removal by settlement of pollutants. An
LID approach, by comparison, takes advantage of a variety of mechanisms that filter
water either overland or via infiltration to the subsurface.
7-4
CONSTRUCTION COSTS. Construction costs for LID will vary depending on
the characteristics of predevelopment site features, the density of development, the
particular LID features selected, and their size and design. For example, the cost of
bioretention areas will be a function of the depth of porous backfill and the degree to
which underdrains are utilized. Case studies for commercial, townhouse, and detached
home residential areas in Prince George's County, Maryland, have demonstrated that
not simple to generalize. The scale of the project, availability of materials, and skills
and training of staff are all factors. IMPs involving landscaped areas are often simple to
maintain because work can often be performed by landscaping crews or residents; hard
17
Greenhorne and O'Mara, 1998.
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