Page 456 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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414 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
runoff quantity control, solids control, and other pollution control—are illustrated later on
in the chapter in Case Study 12.1 on the regulatory practices implemented by the city of
Austin, Texas.

12.5.1.1 Runoff Quantity Control Regulations addressing runoff quantity control can
be used to reduce the effects of land development on watershed hydrology. Hydrologic
control in turn results in pollution control, and can be accomplished through requirements
such as these:
• Open space: By maintaining specified levels of open space on a development site,
the total area of impervious surface is reduced and infiltration of precipitation is in-
creased. This leads to decreases in total pollutant discharge and potential down-
stream erosion by reducing total and peak runoff flows.
• Postdevelopment flow control: Many development regulations require that peak
runoff conditions from a site be calculated before and after construction. These
requirements specify that conditions after construction must reflect conditions
before construction. This control is typically accomplished through the use of
detention facilities, which can reduce peak runoff discharge rates, thereby de-
creasing downstream erosion problems. These regulations specify the desired
outcome; the approach for ensuring that outcome, however, is determined by the
developer.
• Runoff recharge: Regulations may specify that stormwater runoff be recharged on
site. Such regulations can reduce the runoff leaving a site, thereby reducing devel-
opment-induced hydrologic changes and pollutant transport. By directly promoting
infiltration, peak and total runoff rates can be decreased and pollutant discharges
and downstream erosion can be reduced. Such runoff recharge also might help
maintain surficial aquifer levels.

12.5.1.2 Solids Control Regulations addressing solids control could include require-
ments for control practices during and after construction, since such activity has been
shown to be a major contributor of solids. Construction activities can greatly increase the
level of suspended solids in stormwater runoff by removing vegetation and exposing the
topsoil to erosion during wet weather. Yet while communities have requirements for im-
plementing erosion control practices on construction sites, fewer communities require
erosion control after construction is complete. Because many other land uses can con-
tribute solids loadings, regulatory requirements can cover various types of industrial and
commercial activities.

12.5.1.3 Other Pollution Control Land development increases the concentrations of nu-
trients, pathogens, oxygen-demanding substances, toxic contaminants, and salt in
stormwater runoff. Development regulations, therefore, can be used to address some of
these specific pollutants. These regulations can take the form of special requirements for
limiting nutrient export in special protection districts or setting performance standards for
known problem pollutants.

12.5.1.4 Land Use Regulations Land use regulations can include zoning ordinances,
subdivision and site plan regulations and review requirements, and environmental resource
regulations such as wetlands protection. These practices are used as tools to promote de-
velopment patterns that are compatible with control of urban runoff discharges.