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tall.
<br />';, i i 56 DESIGN AND CONSTRUCTION OF STORM WATER MANAGEMENT
<br />., ~l' ~! ~ (d) Should detention/retention storage be used to reduce the required size
<br />~ of storm sewers?
<br />Vii, ) f~, (e) Whal construction problems are likely to be encountered and how will
<br />y they affect casts?
<br />Cj •- ~j (() Where will the system discharge? Water quality may be an issue worthy
<br />~~ jk~ of attention.
<br />`~ (g) Will downspouts/foundation drains be connected to the storm sewers?
<br />j;~ il'~( This has been common practice, primarily because the sewers provide
<br />~~ II a convenient means of disposal, but this conflicts with the notion of
<br />I ~ minimizing the impervious area directly connected to the storm sewers,
<br />reduces sewer capacity, and, in the case of foundation drains, tan
<br />increase the risks of basement Gooding. This issue should be carefully
<br />li looked at when retrofitting existing systems.
<br />~.';~i
<br />-); I I H. Detention Facilities
<br />(a) Is detention required? If yes, what are the appropriate regulations? Is
<br />capacity dictated by local ordinance or policy?
<br />(b) What are the design discharge frequencies?
<br />(c) What kinds of detention are appropriate? Is on-site ponding necessary,
<br />or should regional (large) ponds be relied upon?
<br />(d) How will contemplated detention facilities address erosion/sediment
<br />control, runoff quality enhancement, creation of attractive and safe park
<br />areas, groundwater recharge, and other multi-use mnsiderations?
<br />(e) How will detention fit into the regional drainage system? For example,
<br />could on-site detention actually aggravate, rather than reduce, down-
<br />stream peaks?
<br />(f) If a dam is required, then standard dam design considerations and
<br />studies are necessary including geotechnical evaluations, adherence to
<br />state regulations (or design Good spillway capacity, freeboard, hazard
<br />classification, etc.
<br />I. Wafer Quality Mitigation Measures (Other Than Detention)
<br />(a) What local requirements prevail (how much of~which pollutants must
<br />be removed, and how frequently).
<br />(b) What mitigation measures can be adopted, and how can they be op-
<br />timized.
<br />J. Other Special Structures
<br />(a) How many special structures (outlet and inlet protection, Gow splatters,
<br />multiple channel lining types, diversion boxes, etc.) will be necessary?
<br />(b) What design considerations are associated with them?
<br />(c) Can same be eliminated or modified?
<br />It is characteristic of drainage system design problems (and water
<br />engineering in general) that the design considerations mentioned above
<br />cannot be reduced to simple rules. This is why imagination, experience
<br />and mature judgment play equally important roles in the conceptual
<br />design phase of successful urban drainage projects (see Chapter 9 for
<br />further details).
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<br />DESIGN CONCEPTS AND MASTER PLANNING 57
<br />VII. RISK ANALYSIS
<br />An essential step in the design of urban drainage and flood control
<br />systems is the selection of the recurrence frequencies (probabilities) of
<br />the runoff events for which the major and minor systems are to be
<br />designed, which in turn will determine the sizing of the various com-
<br />ponents of the systems. A clear understanding of the risks and costs
<br />associated with alternative drainage designs leads to better drainage
<br />systems and to wiser public and private investment. The process by
<br />which this understanding is achieved is risk analysis, the elements of
<br />which are described briefly herein. The additional cost of risk analysis
<br />(engineering analysis, regulator education, and negotiation) may restrict
<br />its application to larger projects.
<br />There are no hard and fast rules regarding recurrence frequencies for
<br />design. The engineer must ascertain what local policy is regarding
<br />design return frequencies, and then ask if such standards are appro-
<br />pdate for the particular setting. Common sense and judgment on the
<br />part of the designer and local authorities should supersede uniform but
<br />arbitrary standards. Local regulators may accept (or at least carefully
<br />consider) proposed deviations based on principles of risk assessment.
<br />A. Definitions
<br />Risk is the expression of potential adverse consequences measured
<br />in teens of inconvenience, damage, safety, or even professional liability
<br />or political retribution. Risk analysis is the quantification of exposure,
<br />vulnerability and probability. Risk analysis involves the evaluation of
<br />alternative means to reduce risk and, EinaBy, the determination of ac-
<br />ceptable levels of risk (Wiggins 1978):
<br />In a risk analysis context the design runoff event is the event the
<br />drainage system must handle without permitting an unwanted con-
<br />sequence (unacceptable risk). [t implies that uncertainty has been Ne-
<br />fined (the return period or the probability of the event), acceptable risk
<br />has been determined (no event smaller than the design event will exceed
<br />the capacity of the primary drainage system), and the vulnerability of
<br />the finite number of exposed improvements has been quantified and
<br />found Consistent with the acceptable risk.
<br />Because some of the criteria related to drainage design have come to
<br />be accepted as principles, there is confusion about thew meaning and
<br />application. For example, the "700-year runoff event" is the event that
<br />has a probability of occurence of 0.01 in any given year. It is often
<br />taken to mean that the event will occur only once in one hundred years,
<br />which although true on the average, may not be true for a particular
<br />100-year period. Furthermore, within the context of risk analysis, the
<br />unwanted consequence is not the runoff but the damage which results.
<br />It is the uncertainty of this consequence which is of concern, and not
<br />the occurrence of the event.
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