HIDDEN IMPLICATIONS of DESIGN DECISIONS

PLEASE NOTE THAT MY PRIMARY SITE IS NOW LOCATED AT “THE BUILT DOMAIN.NET”. A link is provided in the upper right hand corner of the screen.


            At the present time, many design standards for the built environment serve a specific purpose, remain uncoordinated and have hidden implications. For instance, zoning setback standards have little in common with the separation requirements in a building code, and both are unrelated to the impervious cover percentages used to design storm sewer capacity, but the impervious cover percentage is a controlling factor that affects the other two.

I doubt that many cities have a map that includes the impervious cover percentage used to design each segment of their storm sewer system. This, however, is a critical piece of planning information that is rarely recorded; and when it is, the data rarely leaves the engineering division. Part of this may be due to the history of storm sewer evolution and our reluctance to examine technical engineering detail.

In the beginning, public health was concerned with the construction of sanitary sewers. Storm water was an environmental issue and sanitation was the first priority. Storm water could not be ignored as urban areas grew, however, since increased housing density produced increased runoff; and water is a force that demands respect. The problem was that an ideal sanitary sewer system is a closed system, but a storm sewer system must be open to receive runoff. Budgets are always an issue and compromise produced a combined sewer that was an open system. The result produced street and basement flooding with sanitary effluent when storm runoff exceeded pipe capacity. This was inevitable because storm water cannot be accurately predicted, and increased pipe sizes increase construction cost. The conflict was addressed by separating the systems at great expense, but combined sewers remain in many older cities and storm water capacity can still be exceeded.

Combined sewers are now unacceptable, but storm water remains unpredictable. Municipal budgets have increased to accommodate separate sewer systems, but budgets have unwittingly limited the size of storm sewer pipes. (Sanitary sewers require smaller pipe sizes and the capacity required is easier to predict.) We are now recognizing that storm water is an environmental problem that also constitutes a public health issue; and that expanding populations require additional shelter that increases runoff. In other words, storm water is not clean water discharging into our natural water supply. It needs sanitary treatment that represents another unanticipated public expense and the demand is increasing.  

            Behind all this detail is a critical engineering design decision. There are a number of factors that lead to the definition of a storm sewer system, but the amount of impervious cover anticipated should stand out to city planners and political decision-makers. For instance, an engineering consultant may use 30% impervious cover and a 100 year storm as sewer design criteria; but if this knowledge is unrecorded or limited to the engineering division there is a coordination problem. The 30% standard may be adopted to hold down pipe size and construction cost, but the decision means that everyone served by the line should pave and build on no more that 30% of his or her property. In other words, 70% open space is required. If the 30% allocation is exceeded by one property owner, the average allocation is reduced for all others who share the system, if its capacity is to be protected. This rarely happens for a number of reasons, including requests to vary from zoning regulations that are uncoordinated with the open space decision.

            I think it’s safe to say that zoning setback lines and building code separation requirements are not coordinated with the impervious cover limits adopted by storm sewer design engineers. In fact, these decisions are rarely recorded on storm system maps that have evolved over time. This means that building plan review and variance application requests can be unwittingly permitted to exceed storm sewer design capacity. Flooding is an inevitable long-term outcome if coordination is ignored, and we are slowly recognizing that this is a public health problem as well as an environmental threat that grows with the population served.

            Wherever storm sewers exist an open space decision has been made. This may not be apparent, but sewer capacity based on a 30% impervious cover assumption means that 70% open space is required. Many, if not most, of these decisions have been lost in time however, and can only be recaptured by reverse engineering the design. This places every system with an unknown open space value at risk of overdevelopment. In other words, wherever a storm sewer system is added or extended, an open space decision is made that directly affects the development capacity of the land involved, but many will never be aware of the limitation and continue to believe that there is an inalienable right to fully develop the property.

I should note that a 30% impervious cover limit can be a residential subdivision feature that may be inadequate to serve future requests for expansion when the total lot area is too small. When this is the case, the threat of overdevelopment increases as variance requests appear and the limit is unknown. The answer is to be aware of the relationships that require coordination and be prepared to explain their implications; since a tight budget means smaller infrastructure and limited building development capacity. These are leadership decisions that a city must live with for a very long time. They restrict its economic development potential and are a recipe for sprawl that has even greater consequences.

            The extension of sewers does not control growth. It encourages sprawl because the potential yield per acre is unknown, and can be an asset or liability to a city’s future economic health. It represents new money but can become a very old burden under these circumstances. This is not economic development. It is economic roulette with little concept of the odds involved. The only apparent solution is to plan with a grasp of land development capacity, a concept of the potential yield per acre from various land use activities, and projections of current and future government expense per acre. The result will be a plan in three dimensions, called urban form, which is capable of producing the yield needed to support a community’s average expense per acre over an extended period of time. Only then can cities extend sewers with the capacity and vision needed to live well within environmental limits imposed by a power we cannot challenge.

            The capacity of a storm sewer therefore is a function of the planning design decisions adopted. The rest is technical calculation, contract document preparation, bidding and construction execution. The difference between leadership and management in this sequence should be apparent, but leadership has been confused because essential goals, strategy and supporting intelligence have been missing. (I’ve previously mentioned that in my opinion the goal is to learn to live within limits and strategy must be based on an understanding of the physical, social and economic implications of land development capacity.) At the present time our efforts have produced a cancer of sprawl. This has happened in the name of freedom because the continent and planet appeared infinite, but a new level of awareness has arrived. The unlimited consumption of land in the name of freedom is not democracy. It is a threat to the people and the planet. The solution is to think in the three dimensions of urban form, with the intelligence needed to forecast and evaluate its physical, social and economic implications. Only this will produce the credibility and vision needed to lead private development toward the public goal of a sustainable future, and engineering systems to support this effort.

Author Note: In the preceding example, if 70% open space were required by the storm sewer system in place, the land development capacity of the remaining 30% could be predicted by choosing a forecast model from the Development Capacity Evaluation (DCE) software collection. (In fact, any percentage can be evaluated.) I’ve discussed this potential in previous essays and won’t repeat myself, except to note that the software and textbook are noted in the author profile.

Other Hidden Implications of Design Decisions

For those who have not read, "The Variance Trap", I would like to mention that it explains how overdevelopment is inadvertently encouraged by zoning regulations that are not anchored by an understanding of land development capacity. "City Design with Space" expands on the discussion.