Stormwater and Zoning Plan Review

 

Impervious cover is building and pavement cover that increases the stormwater runoff produced by land in its natural state.

The plat approval process often neglects to record the impervious cover percentage(s) that apply to the storm sewer capacity introduced to serve the parcels created. These percentages may also be missing from the original civil engineering contract documents. I am also guessing that most cities do not have a storm sewer plan that records the storm sewer capacity of every branch and main line within its boundaries - in terms of the maximum impervious coverage percentages that apply. This does not present an immediate problem but introduces an Achille’s heel over time as cities grow and lifestyle standards change. The risk is called street and basement flooding. It can occur when too many room additions and miscellaneous pavement improvements are permitted to exceed the impervious cover percentage originally used to design storm sewer capacity for the lots created– and when Mother Nature does not respect the assumption that a 100-year storm will only occur once in a lifetime. We all understand unanticipated storms, but most are not aware of the relationship between storm sewer capacity and impervious cover limits that require correlation on given land areas.

I was once told by a civil engineer that single-family residential developers request or expect storm sewer capacity design based on 30% impervious cover. I recognize that this is a generalization, but I have not forgotten the comment. It helped me understand street and basement flooding as well as the need for relief sewers over time. A limited impervious cover percentage reduces the initial storm sewer pipe size and development cost involved but can require public expense for a new parallel system to increase capacity in the future.

The engineer’s comment was based on a separated storm and sanitary sewer system. A combined sewer serves both in the same pipe and receives stormwater through open street inlets. It was considered a public health innovation at the time, but ensuing street and basement flooding of combined waste revealed its weakness. The systems still exist in older neighborhoods but are now prohibited in most, if not all, new developments for rather obvious reasons.

The problem of sewer separation has been resolved. The cost of installed storm sewer capacity remains an issue involving the first cost of capacity to a developer versus a potentially long-term public demand for additional capacity. The demand represents a transfer of responsibility from the private to public sector involving significant public expense, disruption and inconvenience. It is a relatively easy problem to overlook. New development produces new revenue that appears to increase public income until its age begins to demand additional revenue for maintenance, improvement, and debt service. Excessive impervious cover that is not correlated with underground storm sewer capacity is simply one example of the problems that can occur when plan review efforts focus on land use compatibility rather than building design categories, design specifications, shelter capacity evaluation, and the context implications that emerge from the mathematical correlation involved.

To wrap up this brief storm sewer discussion, a community cannot easily compare a development request to the storm sewer capacity available unless: (1) It can calculate the total impervious cover percentage being proposed, and (2) It has recorded the storm sewer capacity installed throughout the city. If it can’t make the comparison between proposal and capacity, plan approval and economic development can create, or multiply, an invisible problem that may produce unexpected future public obligations.

The strategic issue in physical design is not regulation with independent “thou shall not” stipulations. These are tactical directions that are functions of strategic correlation and leadership direction. The term adopted by physical designers to indicate strategic correlation has been urban design and city design. Unfortunately, the leadership language required to lead the army of designers involved has been missing until now.

I won’t dwell on the language since I have discussed it many times in previous essays. I’m simply using a storm sewer example to describe its application. Several specification topics in a building design category forecast model involve impervious cover percentages. The most significant is the amount of unpaved open space present or planned since it determines the remaining buildable area, or impervious cover, that may be used for building cover, parking, and miscellaneous pavement. When the unpaved open space percentage is subtracted from 100%, the remainder should equal the storm sewer capacity present or planned.

This comment is meant to paint a broad picture and does not account for more detailed engineering calculations that can affect the final impervious percentage adopted. When the initial values do not match however, it is an indication that the topic deserves more attention. My point is that a variance departure from a forecast model specification should assume greater significance since it represents a breakdown in the correlation required to lead a single project toward a greater objective.

Storm sewer capacity is simply one of the more obvious systems that can be affected by promiscuous approval of variances from a zoning ordinance. They can easily disrupt a carefully defined leadership plan when it exists, but their effect is difficult to discern when there is no mathematically correlated plan in place. Reasons for variance disapproval can be agonizing when there is no correlated justification for a single regulation.

A storm sewer has simply been a convenient example in this essay since capacity is an established engineering calculation. The other specification topics in a forecast model do not benefit from the same amount of research and may involve acceptable ranges, but the point is not tactical mandates for single specification topics. It is the correlation required to lead physical design toward shelter capacity and intensity objectives that protect the activities of growing populations within geographic limits defined to preserve both their quality and source of life. It cannot be unconditional surrender of the planet. It must be symbiotic survival.

Walter M. Hosack: September 2024

Photo by Jondal