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402 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
Table 12.2 Relative Contribution of Nonpoint Source Loading

Relative Impacts, %
Source Rivers Lakes Estuaries
Agriculture 55.2 58.2 18.6
Storm sewers/urban runoff 1 12.5 28.0 38.8
Hydrologic modification 12.9 33.1 4.8
Land disposal 4.4 26.5 27.4
Resource extraction 13.0 4.2 43.2
Construction 6.3 3.3 12.5
Silviculture 8.6 0.9 1.6

1
Includes combined sewer overflows.
Source: U.S. Environmental Protection Agency (1990).

5. Prohibitions against developing floodplains
6. Utilization of porous pavements where applicable
7. Utilization of natural drainage features.
Construction controls such as minimizing the area and duration of exposure, protect-
ing the soil with mulch and vegetative cover, increasing infiltration rates, and construction
of temporary storage basins or protective dikes to limit storm runoff can significantly re-
duce receiving water impacts caused by the runoff.
No one single method is a panacea for all CSO or storm drain discharge problems. The
size and complexity of urban runoff management programs are such that there is a need for
an integrated approach to their solution. The type of problems associated with any given
community is dependent on a number of variables; as a result, the solution for a community
must be developed to fit the needs of that particular urban area. The solution is most often a
combination of various best management practices (BMPs) and unit process applications.

12.3.1 Description of the Planning Process
When considering stormwater management, the planner is interested in controlling the vol-
ume and rate of runoff as well as the pollutional characteristics. The goal is to preserve the
initial ecological balance so that expensive downstream treatment facilities can be mini-
mized. Because the size of storm sewer networks and treatment plants is quite sensitive to
the flow, quantity, and particularly the peak flow rate, a reduction in total volume or a
smoothing out of the peaks will result in lower construction costs.
The planning process for urban runoff management and control programs is based on
regulations that require such programs and on technical information about planning ap-
proaches. Table 12.3 compares planning approaches required by various regulations.
Despite the increasing complexities and uncertainties as one proceeds from left to right in
the matrix, the required planning approaches are similar. The process generally consists of
the following major components:

1. Determining existing conditions: Analyzing existing watershed and water resource
data and collecting additional data to fill gaps in existing knowledge.
2. Quantifying pollution sources and effects: Utilizing assessment tools and models to
determine source flows and contaminant loads, extent of impacts, and level of con-
trol needed.