Page 406 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

366 Cha pte r T e n

There are also components to a BMP that provide additional pretreatment or water
quality protection. Some of these are referred to as water quality (WQ) inlets, and they
may take many forms and names. Some common pretreatment units are forebays or
sedimentation basins before a pond, or specialty premanufactured items such as oil/
grit separators or cyclonic catch basin inserts. There are also structural additions to the
BMP systems whose primary purpose is to decrease the flow rate of runoff, which is
important for erosion and damage protection in the receiving structure or water body.
Many of these are referred to as level spreaders or energy dissipators, although
sometimes they are just called by the name of the material they are made of such as
riprap. Figure 10.1.4 depicts a detention pond where additional riprap has been used
for erosion control.
Table 10.1.6 lists many of the structural BMPs that have been segregated by
functional unit operations by the NCHRP (2006). Many of the BMPs serve
multifunctional purposes. Figure 10.1.5 depicts a detention pond with added
retention which aids in stormwater quality control. Figure 10.1.6 is of a retention
pond. The main differences between detention and retention ponds are that the
former detain most of the stormwater, releasing it at controlled rates to downstream
structures or waterbodies, while the latter retain most of the stormwater, allowing it
to eventually infiltrate into the ground or evaporate. Each also usually provides
overflows for larger storm events.
In addition to the BMPs already listed in the tables, there are many new or combined
variations on all these themes. For instance, there is a BMP called an ecology ditch. It can
best be described as a combination of a swale and a bioretention cell.

FIGURE 10.1.4 Detention pond with riprap erosion control around the inlets and banks in Irmo,
S.C. (Photograph taken July 2007.)