Surface Water Drainage – Understanding the Common Enemy Doctrine Part 2

Today, senior litigation attorney, Robert Enos, continues his explanation of the common enemy doctrine as it relates to surface water drainage. This is a complex and technical analysis to understand just how water flow works and the proof model required to make a case.

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Surface Water Drainage – Understanding the Common Enemy Doctrine – Part 2
By: Robert J. Enos, Esq.

As we all know, rain is a normal phenomenon for many mid-latitude areas. It creates runoff and that runoff creates a potential damage to neighboring properties, especially when that runoff has been altered by the actions of another. Previously, we introduced the common enemy doctrine and liability created by steering or directing the flow of water on to another’s property. Today’s article will be a little more technical than normal as we unpack the Rational Method, a mechanism for determining the water movement across property. A common question we get, is why is this important? Litigation and water cases are no different, the plaintiff must show a harm, that is an injury that is suffered by a plaintiff. If grading is changed but the water impact is not changed, then there is no harm. The plaintiff must prove the injury that is suffered as a result of the defendant’s actions. There are a multitude of factors that go into this analysis ranging from land use types, slope, and soil that all play a direct role in surface drainage issues.

As important as the diversion of rain runoff is to a claim; the surface type and slope over which rain-water runoff flows is a significant factor in determining the water flow changes. A runoff coefficient is a number that relates the rainfall rate and runoff rate. Using the runoff coefficient, scientists and hydrologists can calculate how much water passes over a given area per second. Runoff coefficient is a dimensionless factor that is used to convert the rainfall amounts to runoff. It represents the integrated effect of catchment losses and hence depends upon the nature of land surface, slope, degree of saturation, and rainfall intensity. It is also affected by the proximity to water table, degree of soil compaction, porosity of soil, vegetation, and depression storage.

One of the ways you can find the runoff coefficient is by using the Rational Method. The “Rational Method” is used for areas less than 50 acres. If the area you are assessing is more than 50 acres, you will have to use a coefficient table. Write down the following formula: Q = CiA. Find the appropriate values for each variable. In the Rational Method equation, “C” is the runoff coefficient, meaning it is your unknown. “Q” is the value for the peak rate of runoff. This value is in CFS. One CFS is equal to one cubic foot of water passing over a particular point over the course of a second. “A” equals the size of the area you are measuring. “A” is measured in acres.

Calculate the value for “i.” This is a measurement of rainfall intensity. It is measured in inches per hour. This value is calculated using a Seelye chart and an IDF chart. To use a Seelye chart, you need to have the length of the land and its angle of grade. Once you have this value, you can use an IDF chart to determine “i.” IDF charts vary from area to area and are based on average rainfall intensities. IDF charts are typically available from the state government. The actual agency varies from state to state.

The major factors affecting the rational method runoff coefficient value for a watershed are the land use, the soil type and the slope of the watershed. The physical interpretation of the runoff coefficient for a watershed is the fraction of rainfall on that watershed that becomes storm water runoff. Thus, the runoff coefficient must have a value between zero and one.

Land Use: Surfaces that are relatively impervious like streets and parking lots have runoff coefficients approaching one. Surfaces with vegetation to intercept surface runoff and those that allow infiltration of rainfall have lower runoff coefficients.

Slope: All other things being equal, a watershed with a greater slope will have more storm water runoff and thus a higher runoff coefficient than a watershed with a lower slope.

Soil Type: Soils that have a high clay content don’t allow very much infiltration and thus have relatively high runoff coefficients, while soils with high sand content have higher infiltration rates and low runoff coefficients. The U.S. Soil Conservation Service (SCS) has four soil group identifications that provide information helpful in determining watershed runoff coefficients. The four soil groups are identified as A, B, C, and D. Classification of a given soil into one of these SCS groups can be on the basis of a description of the soil characteristics or on the basis of a measured minimum infiltration rate for the soil.

This is important in California because so much of the land is comprised of uneven terrain and with differing surface types. Not only is the ability to develop that land impacted or impeded based on the runoff coefficient that developer may have to address — but the potential plaintiff who suffers from altered grading has to understand the change in the water flow to prove harm and the Rational Method is a calculation method that shows how water flow can change based upon the alteration of the grading.

The attorneys at BPE Law Group have years of experience related to real estate issues that may affect your rights. No two situations are the same and each requires its own independent legal analysis of the facts. If you have a real estate or surface water issue the attorneys at BPE Law Group are here to help.

The attorneys of BPE Law Group, PC. have been advising our clients on real estate, business and estate planning issues for over 20 years and have assisted numerous clients in business and real estate matters and have represented and advised brokers on their professional obligations as well as consumers on their rights. If you have questions concerning legal matters, give us a call at (916) 966-2260 or e-mail Keith at Our flat fee consult for new clients may get you the answers you need for the questions you have.

The information presented in this Article is not to be taken as legal advice. Every person’s situation is different. If you are facing a legal issue of any kind, get competent legal advice in your State immediately so that you can determine your best options.