PERMFILE103025

• Permit No: M2002004
• IBM Index Class Name: Permit File
• Doc Date: 7/8/2002
• Doc Name: Response to Preliminary Adequacy Review APPENDIX B E F
• From: GCC Rio Grande Inc
• To: DMG
• Media Type: D
• Archive: No

• • i
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58 DESIGN AND CONSTRUCTION OF STORMWATER MANAGEMENT
The extent to which risk analysis is employed in the design and
selection of a drainage system depends on the nature and extent of the
unwanted consequences. For simple situations, mere recognition of
risks, uncertainty and possible adverse consequences may be sufficient
to permit informed judgments. When public health and safety are at
stake, a more rigorous risk analysis should be undertaken. In any event,
however, the elements of risk analysis should be systematically con-
sidered in formulating design parameters and guiding the decision-
making process.
B. Methodology
The determination of risk-causing factors and unwanted conse-
quences requires an assessment of the probability of occurrence and
value of potential damage. Four determinations must be made. First,
the critical events (i.e., the design events) must be defined and their
magnitudes estimated. For drainage design, these events should be
those that produce the unwanted consequences. Only the events that
would cause damage need to be evaluated, and a probability of occur-
rence determined for each.
Next, the vulnerability of the exposure (the property that can be
damaged) should be determined. The cottsequence, the third element,
might range from public safety through inconvenience to severe dam-
age. Finally, a value {such as dollars or time delays), for all exposed
properties, is associated with each of the consequences.
For a given design, the probability of flooding, expected damage and
system cost can be assessed. The resulting estimated probability, dam-
age and cost give one point on the risk analysis curve. Similar analyses
for altemative designs provide additional points. Information provided
by the curve then allows assessment of risk avoidance or acceptance.
Alternative strategies should be pursued to the extent that their mar
ginal costs of implementation are equal. This is to say that each addi-
tional dollar spent on avoidance should lower the risk an equivalent
amount. The selected design should be the one that balances avoidance
and acceptance at the point of acceptable cost and risk.
VIII, DESIGN ECONOMICS
While elements of design are important, information on the costs
associated with stormwater management is also necessary to judge the
feasibility of a program. Costinformafion is important to planning
agencies and policy makers because the information can be used in
setting user fees and comparing alternative basin-wide drainage strat-
egies. Cost information is important to land developers because
the information is basic to the determination of the feasibility of
development.
DESIGN CONCEPTS AND MASTER PLANNING 59
Costs for stormwater control are not simple to quantify because of
the uncertainties involved. The simplest estimation method consists of
a table of mean values, one for each type of development. Such tables
are usually applicable only for the jurisdiction for which they were
developed. The mean value is the cost per acre of development and is
limited to certain cost categories such as construction costs. Costs far
such items as engineering, land, and annual operation and maintenance
would be estimated separately, possibly by using either mean Lost
per acre values, or proportionality Constants multiplied by the cost of
construction.
There are empirical equations for predicting costs associated with
other aspects of stormwater control such as erosion and sediment con-
trol. Mean cost estimates tan be used for such erosion and sediment
control methods as small sediment basins, interceptor berms, chemical
rnntrol, and seeding with fertilizer and mulch (Schueler 1987).
Recognizing that many drainage systems include some form of storm-
water runoff control, such as detention, statistical relationships for pre-
dicting the construction costs (per unit volume) of detention storage
are useful. Similar methods are useful for estimating other costs in-
cluding land costs, planning, design and supervision costs. Further
discussion of economic evaluation is provided in Chayter 8.
IX. DRAINAGE MASTER PLANNING
Master planning is one of the most widely used and frequently mis-
understood terms in drainage practice. There are (ew published defi-
nitions, but a master plan typically addresses such subjects as charac-
terization of site development, grading plan, peak rates of runoff and
volumes for various return frequenries, locations, criteria and sizes of
detention ponds and conveyances, measures to enhance mnoff quality,
pertinent regulations and how the plan addresses them, and consist-
ency with secondary objectives such as public recreation, aesthetics,
protection of public safety, and groundwater recharge. ~ ~'
In its simplest form, a master plan may only identify the essential
elements, alignments, and functions of a drainage system. Even at this
conceptual level, the master plan should be based upon estimates of
peak and total discharges for some selected runoff recurrence inter-
val(s). These recurrence intervals, in turn, should be selected based on
local standards and risk assessment, as discussed earlier in this chapter.
The next level of master planning should establish specific criteria
consistent with acceptable risk, including design discharges and water
surface profiles and elevations. Head losses at waterway crossings and
other Constructions or obstructions should be recognized in develop-
ment of the water surface profiles. This level of master planning defines
the ultimate drainage system components desired and provides infor-
mation for their preliminary design and cost estimation. More impor-