Stormwater Management

By: Jonathon Woodard, P.E.

In urban and suburban areas, stormwater systems are found everywhere.  From surface waterways, such as rivers, streams, and ditches, to subsurface drainage systems, such as storm sewers and culverts, stormwater systems are as ubiquitous as concrete, which is one of the reasons they are so important (Figure 1).  Impervious surfaces like concrete, pavement, and roofs prevent stormwater from naturally absorbing into the ground, and the resulting runoff can cause flooding, erosion, and subsequent structural damage.

Figure 1: Stormwater systems are found everywhere.

Stormwater is water that originates from precipitation (i.e., rainfall) and water that originates with snowmelt.  Stormwater that is not absorbed into the ground is called surface runoff.  Managing the collection and controlling of surface runoff is called stormwater management.

Typically, stormwater management design is based on the Rational Method.  The Rational Method estimates the peak rate of runoff at any location in a watershed as a function of the runoff coefficient, rainfall intensity, and drainage area.  In other words, the Rational Method is used to calculate how much water passes over a given area in cubic feet per second.

The runoff coefficient represents the ratio of runoff to rainfall.  It represents the interaction of many complex factors, including the storage of water in surface depressions, infiltration, antecedent moisture, ground cover, ground slopes, and soil types.

The rainfall intensity is the average rainfall rate in inches per hour for the period of maximum rainfall of a given frequency, with a duration equal to the time of concentration.  The time of concentration is defined as the time required for a drop of water falling on the most remote point of a drainage basin to reach the outlet.

The drainage area is defined as the drainage surface area in acres, measured in a horizontal plane.  The area is usually measured from plans or topographic maps.  The area includes all land enclosed by the surrounding drainage divides.

With this information, the designer can determine how much runoff is being conveyed to a particular point in a watershed, typically a drainage inlet such as a catch basin.  Storm sewer pipes are then sized based on the peak runoff rate compared to the calculated hydraulic capacity of the pipe (Figure 2).

Figure 2: Concrete storms sewer pipe.

In forensic engineering, we are sometimes asked to determine why an area floods when it historically has not.  For example, during a utility repair a contractor removes several trees from a homeowner’s yard.  The area is restored and covered with sod.  The homeowner alleges that after the trees were removed, their yard began flooding.  In this case, the drainage area encompassing the homeowner’s property consisted of several acres.  Relative to the size of the drainage area, the removal of three trees did not affect the runoff coefficient, and therefore the runoff rate had not been increased by their removal.

Figure 3: Erosion of soil around a drainage structure.

Sometimes there is no doubt that the runoff coefficient has changed, such as when a wooded area is redeveloped into a recreation area, with playing fields, picnic pavilions, and a parking lot.  The change in the runoff coefficient (from site grading and the addition of impervious areas) will increase the runoff rate.  It is not surprising then, when homeowners in an adjacent neighborhood experience flooding when it had never done so before.  Most municipalities require that the pre- and post-development runoff rate must be the same, which is typically accomplished by the use of stormwater basins.  Examining how stormwater runoff is managed is one way we can determine why an area might be flooding.

Other examples of stormwater management in forensic engineering include sewer backups from combined storm and sanitary sewer systems, and soil erosion over pipes and around drainage structures (Figure 3).  Stormwater systems are found everywhere, and understanding their design and function helps us determine their modes of failure and how that affects the surrounding property.