Category Archives: Reconstruction

Forensic Hydrology

Forensic Hydrology

 By Elvin Aycock, PE,  PLS, ACTAR,
I-ENG-A Advisor


Forensic hydrologists study the causes and effects of water damage in legal cases, including flooding, erosion, drainage problems, and hydroplaning. This paper offers background on hydrology and forensic hydrology. It also provides examples of real-world situations where forensic hydrologists used their expertise to aid attorneys in legal cases.

Hydrology is the scientific study of water and its properties, distribution, and effects on the earth’s surface, in the soil, and in the atmosphere.

The central theme of hydrology is that water moves throughout the Earth in a hydrologic cycle. The most vivid image of this is in the evaporation of water from the ocean, which forms clouds. These clouds drift over land and produce rain and snow. The rainwater flows into lakes, rivers, or aquifers, either evaporating to the atmosphere or eventually flowing to the ocean, completing a cycle.

Some of the water evaporates, some of it is intercepted by vegetation, and some of it travels over the land surface to streams. The streams flow into larger streams, which flow into rivers, finally flowing back to the ocean.

Water as a liquid or snow covers most of the surface of the Earth. By the process powered by gravity and the action of solar energy, an endless exchange of water, in vapor, liquid, and solid forms, takes place between the atmosphere, the oceans, and the earth’s surface. Water circulates in the air and in the oceans, as well as above and below ground.

The quantity of rainfall varies from region to region within the United States. At the same time of year, some regions are dry from lack of rain while other parts of the country experience flooding. It is not unusual for one part of the country to experience a hurricane, while another experiences a drought, and another has flooding.

Civil engineers and hydrologists study surface water—in particular, the measurement of its flow and volume. These studies are used to design the size of pipes for storm drainage systems and culverts. Other studies include designs for detention pond storage for development projects and water surface profiles of flood prone areas.

Local governments, for example, require hydrologic studies before the commencement of all significant building projects, and hydrology is applied when designating and managing flood plains. Hydrologists also are employed in the evaluation of water resources, wastewater systems, and irrigation projects. The public use of water for recreation and power generation also calls upon the work of hydrologists, who assist governments and private companies in controlling and managing water supplies.

Hydrologists use a variety of techniques. Some are simple and time-honored, while others involve the most cutting-edge modern technology, such as highly sophisticated computer models and satellite remote-sensing technology. Or, hydrologists may apply relatively uncomplicated methods for the measurement of snow depth or the flow of rivers and streams.

Hydrologists are particularly important in helping communities protect against flooding by identifying flood hazard areas and minimizing encroachment through the use of recommended state water buffers. By studying historical records, along with geologic maps and aerial photographs, hydrologists and engineers can make recommendations regarding the zoning laws for a particular area.

Forensic hydrologists investigate many issues, including the following:

  • Flooding of property from heavy rain events
  • Development of land that prevents water from flowing in its natural watercourses
  • Erosion of the earth’s surface caused by the flow of surface water across unprotected soil created by land disturbing activities
  • Hydroplaning on roadways caused by road defects
  • Uncontrolled discharge of surface water, which causes flooding downstream

Forensic is a term that is associated with hydrology in legal matters. According to the Merriam-Webster Dictionary, “forensic” means “relating to or dealing with the application of scientific knowledge to legal problems.” In this sense, “forensic hydrology” typically refers to investigations of water issues and the need to identify the cause and damage incurred by storm water. The forensic hydrologist uses a number of hydrologic tools to determine the history of an event, such as computer modeling. He or she may need to determine when flooding began or upstream factors that have recently changed due to clearing of land for land disturbance projects.

Forensic hydrologic investigations commence with flow paths, both current and historic, and flow velocities. Changes in water flowing across the ground surface, drainage ditches, and drainage pipes often create adverse conditions downstream. The conversion of wooded land to impervious surfaces increases the quantity of storm water and the velocity of the water. The impervious area decreases the time of concentration of the water, which increases the peak flow. All of these effects can cause damage to downstream property owners.

In cases where surface hydrology has caused damage to downstream property owners, hydrologists examine historic aerial photographs to determine locations of land disturbance projects. The timeline of the downstream damage is compared with the timeline of the land disturbing activities.

For example, consider a site where soil erosion has filled in a downstream lake over a period of years. The aerial photographs help identify areas of land disturbance and the time frame for each disturbance. Where several developments have occurred over a period of years, the aerial photographs help the hydrologist identify the size of each development and the period of time the land was susceptible to erosion. The damages can be allocated to the developers based on the data gathered.

Without close oversight by the governmental agencies, developers may omit many of the erosion and sediment control measures shown on engineering plans. Because developers may have gotten away with this practice in the past, they may feel they can skip some of the erosion control measures to save money. This creates a situation where heavy thunderstorms—without appropriate control measures—can cause severe damage to downstream property owners.

A recent case that caused severe damage to the downstream property owner was caused when the developer tied a new storm drainage pipe into an existing pipe that was not adequately sized to handle the drainage from the new development. The property flooded, and the downstream landowners brought suit against the developer.

In another case, a contractor tied the discharge pipe from the detention pond into an existing storm drainage system. The existing pipe was only adequate for storms up to the 25-year storm event. The flooding problem developed because the developer did not clean out the siltation in the detention pond. Silt from erosion was allowed to accumulate in the detention pond, which decreased the volume of the storage for stormwater. When the storm event occurred, the water overflowed the emergency weir of the detention pond and flooded the downstream homeowner. The homeowner sued the developer.

Few developers understand how detention ponds function. Often, the grading contractor will not build the pond to the designed volume. This prevents the pond from being able to function as designed, and water overflows the spillway and floods the downstream residents.

Outlet structure control devices are often improperly sized or are not installed at the correct elevation. This creates a potential problem of more discharge through the outlet structure than the design model allows.

Forensic hydrologists are also called on to work hydroplaning cases. The flow of water across the pavement surface can be critical and dangerous to the motorist if the roadway surface is not constructed correctly. Often, this is a roadway defect created by improper construction methods used by the contractor.

The transition of the roadway from a tangent section to a highly elevated section can create an area of ponding. Unless viewed during or after rainfall, the area of ponding cannot be identified by the naked eye. A detailed elevation survey is needed to determine the area of ponding and the flow direction of water.

In one hydroplaning case, it was determined that the water on the roadway ran at an angle across the travel lanes onto the paved median. The water turned back and ran across the roadway again. This created a dangerous condition and caused a vehicle to hydroplane and crash into the median barrier. This section of roadway ran downgrade and entered into a highly elevated section. No inlets were provided to collect the water in the median, and as the grade increased to create the elevation, the water flowed from the paved median across the travel lanes. The depth of water on the roadway caused the vehicle to hydroplane.

The forensic hydrologist with an inquisitive mind and the use of sound engineering principles can help identify the cause that creates these types of problems. He or she can provide a valuable service to the attorney handling storm water drainage cases.

Accident Reconstruction

Accident Reconstruction: Enhancing Results

Automobile accident reconstruction can be simplified by following some specific practices. Good documentation of the physical evidence leads to a thorough understanding of the vehicles’ motion before and after an impact. Poor documentation of the evidence may complicate and, in some cases, make a full reconstruction impossible. The accuracy of a reconstruction is largely dependent on the quality and preservation of the physical evidence.

There are three phases to most automobile accidents: pre-collision, collision and post-collision. When investigating an accident, there are distinct physical evidence patterns that should be documented for each phase. Measurement and photographs of the evidence is then used to assess vehicle motion.


Vehicles are often at their tire adhesion limit before a collision occurs, whether it is full braking, full yawing (cornering) or a combination. When tires are at their limits, they leave rubber on the road in the form of skid marks and/or yaw marks. Skid marks are straight (the tire is locked up and cannot turn the car). Yaw marks are curved (the car is fishtailing). Most modern vehicles have antilock braking systems, which prevent wheel lock and reduce the likelihood of finding pre-collision skid marks.

The length and character of this pre-collision evidence usually represents energy loss. In other words, when a vehicle locks its brakes, it loses energy and speed. When a vehicle is fishtailing it also loses energy, dependent upon its slip angle, i.e., the more sideways it gets, the more energy it loses. Published coefficient of friction values on different surfaces help understand the energy loss.


When a vehicle collides with an object or another vehicle, it deforms. The exact amount of this deformation also represents energy loss. Collision damage can often be compared to crash test data, in which the energy loss is a known quantity. For example, if a car crushes 20 inches during a frontal collision and that same model crushed 20 inches in a 30 mph barrier impact test, then the collision energy loss is equal, even though other dynamics may be different.

Measurement of the crush damage is compared to an undamaged vehicle. Measurement accuracy and detailed photographs can greatly influence the reconstruction. Some late model vehicles have black boxes onboard that record speed loss, which can be used to confirm crush energy calculations.

Post Collision:

After collision, damaged vehicles cause surface gouges/scrapes, drag marks, fluid trails and debris trails, all of which help understand post-collision motion. The nature of sliding contact with pavement and/or grass/dirt changes the energy loss. The harder a vehicle drags/scrapes the surface, the more energy it loses. Resulting surface damage and the corresponding vehicle component should be determined.

All three accident reconstruction phases and their associated energy loses are added up to give the initial vehicle sped (total energy). Careful documentation of each accident phase combined with the basic laws of physics will enhance auto accident reconstruction results.

By:  Elvin Aycock, PE, ACTAR, I-ENG-A Accident Reconstruction Advisor

Forensic Engineering Association