Page 136 - Biosystems Engineering

Soil and W ater Conservation 117

erosion. It is the ratio of soil loss with a specific support practice to the
corresponding loss with upslope and downslope tillage (Renard et al.
1997). If farming operations are up and down a slope, then P should
be set to 1.0 (Fangmeier et al. 2005). If there is contouring only, the
P value is approximately 0.5; if there is contour strip cropping,
P = 0.25 (Fangmeier et al. 2005). The effects of terracing could be bet-
ter represented with the slope length factor, as terracing shortens the
overland flow distance and the slope length.

3.7 Best Management Practices
A best management practice, commonly referred to as BMP, is a practice
or combination of practices used in preventing or reducing nonpoint
source pollution or flow volume/peak in an economic and effective
manner. BMPs reduce pollutant concentrations and loads in runoff by
infiltration into the soil, physical infiltration by grass or other vegeta-
tion, adsorption onto soil and plants, bacterial decomposition, plant
uptake, and sediment deposition (Komor 1999).
There are two types of BMPs: structural and nonstructural. Structural
BMPs are essentially engineering solutions having physical mechanisms
or structures often designed and constructed to trap or filter pollutants
from runoff or reduce runoff velocities. Nonstructural BMPs focus on pre-
serving open spaces, protecting natural systems, and incorporating
existing landscape features such as wetlands and stream corridors into a
site plan. There are no physical structures associated with these types of
BMPs. Nonstructural BMPs can be achieved through education, man-
agement, and development practices. Selection of a BMP is a critical
issue. BMPs should be selected by considering such factors as the pollut-
ant to be treated, site conditions (land use, topography, slope, water table
elevation, geology, and climate), and, of course, economy.
Nonpoint source pollutions could be dealt with in two ways: pre-
vention/protection and treatment/control. Prevention is the most
desirable option because it is easier and more cost effective to stop or
avoid a problem before it ever happens. Examples of prevention
include low-impact designs (LID), controlled land use/cover, and
minimal disturbance such as preservation of wetlands. Although this
is the preferred option, it is not always possible to stop or prevent pol-
lution, and undesirable consequences are inevitable. In that case, treat-
ment comes into the picture. Various BMPs, especially structural ones,
should be considered. The goal in treatment is to reduce pollutant
loads/concentrations to acceptable levels through physical, chemical,
or biological processes. Trimming peak flows and storm flow volumes
through various detention and retention systems, or increasing infiltra-
tion rates by reducing imperviousness, are examples of flow treatment.
Most widely used watershed models are not yet equipped with
modeling BMP effects or have limited capabilities in that regard. The