Archive for November, 2007

Geohazards: Rivers and Floods

Wednesday, November 7th, 2007

Hazard Characterization

Floods are high-water stages where water overflows its natural or artificial banks onto normally dry land, such as a river inundating its floodplain. Floods occur at more or less regular intervals in riverbeds and floodplains but also beyond them. Besides storm surges the two main types of floods are river flood and flash flood. Floods occur as natural phenomena when the volume of river runoff is so high, and the riverbed too small to contain the water masses. Floods, or high water stages, are most regular in springtime. Strong floods happen irregularly, in so-called re-occurrence intervals of 10, 50 or 100 years. But these intervals are only statistical averages. Heavy summer rainfalls can also lead to floods.

Floods have become an increasing problem for man’s environment since urbanization has altered natural drainage ways, straightened and even relocated river beds within their natural flood prone areas. Also impacting runoff rates of flood prone areas is the habit of increased soil sealing (asphalt parking areas, highways, commercial urban development, etc.) leading to a higher than normal flood hazard. This occurs as rainwater more quickly runs off directly into the streams and the water mass inflow via paved drainage ways and underground pipes to rivers, storm water runoff which is no longer delayed by natural soil retention induces “flash flood” like storm events.

Flash floods are the fastest-moving types of floods. A flash flood is a specific type of flood that appears and moves quickly across the land, with little warning. Heavy rainfall concentrated over an area, thunderstorms, hurricanes and/or tropical storms cause most flash flooding. Dam failures can also cause flash flood events. When a dam or levee breaks, a gigantic quantity of water is suddenly discharged downstream, developing strong destructive forces which may reach elevations previously undamaged by flood waters.

Flash floods can contribute to river floods, or can be caused by river floods, for example if an embankment collapses. Flash floods can happen anywhere but are mostly bound to river and stream drainage areas and are thus integrated into most government agency flood zone maps and the delineation of flood zones.

Risk Management

The most important part of flood risk identification and management is the flood-prone area delineation (extent). Flood-prone areas are those areas subject to inundation as a result of flooding with certain known frequency. The determination of a flood prone area requires considerable collation of historical data, accurate digital elevation data, and hydrologic discharge data calculated along a number of cross-sections located throughout a watershed. In addition to taking past flood events into account, it could be possible to derive river flood prone areas by area elevation modelling with the assistance of satellite imaging. Research institutions can develop a “flood prone area map” based on digital terrain models, river runoff, flood data and climate models. These are available to the public.

Flood Zone Determination

Flood Zone Determinations are usually provided by the state and local government engineering departments. Certificates of Elevation are provided by Professional Engineers, Professional Land Surveyors or Registered Architects from the private sector on a fee basis.

Flood Insurance

Flood insurance is recommended in flood prone areas. There are two types of coverage, structural and contents. Renters can buy contents coverage only. Flood insurance is available through the National Flood Insurance Program (NFIP) and information about this insurance is available through you local insurance agent. Don’t wait for the next flood there is a 30 day waiting period for coverage to take effect.

Dominick Amari , P.G.

Tractor / Trailer Turning Maneuvers and Turn Times in Night-Time Accidents

Wednesday, November 7th, 2007

Tractor trailers usually take three times as long or longer to accelerate as a regular passenger car. That coupled with the large bulky structure of a tractor trailer unit makes for a very long acceleration time to clear an intersection once the tractor trailer starts to pull out onto a roadway. The fact that the trailer wheels do not track directly behind the tractor wheels when a turn is being made means that a wider then normal turn has to be made in many circumstances. This can lead to the tractor going partially off of the roadway as it’s making the turn before straightening up in its intended lane. All of these factors need to be considered when trying to analyze just how the accident occurred and just what would be visible to the oncoming motorist during night time accidents. The following discusses the approximate turning times and tractor paths throughout the turn and the headlight visibility and orientation to the oncoming motorist through the turn.

 Studies show that average acceleration factor for a tractor trailer is approximately .05. That is an acceleration rate of 1.6 feet / second / second. This means that the first second the truck accelerates a distance of 0.8 feet, after 2 seconds it has accelerated a distance of 3.22 feet and after 3 seconds it has accelerated a distance of 7.25 feet. This shows that as times goes on the vehicle is accelerating to a higher speed and is gaining speed and covering a greater distance each second. So as seen in the table below if a truck accelerates for 10 seconds from the time that it starts until it reaches the point of impact it travels a total distance of 80.5 feet. The first 3 seconds it only travels 7.25 feet but the last 3 seconds it travels a distance of 41.06 feet.












Distance Total

From Start

1 =



2 =



3 =



4 =



5 =



6 =



7 =



8 =



9 =



10 =




 Depending on the roadway configuration and the amount of available sight distance the oncoming motorist may only see a portion of the truck’s total turn time prior to the impact occurring. What is important to know, in an accident where the oncoming vehicle runs into the side of a trailer, is where the oncoming motorist is located and where the tractor / trailer is positioned when the motorist could first see it. For instance if at a certain speed the motorist can see the tractor / trailers acceleration for the last 6 seconds from the time it travels from 12.88 feet to 80.5 feet, the motorist should be able to see that a tractor / trailer is entering the roadway and should start to slow down and be more attentive to the roadway. If, however, the sight distance limits the oncoming motorist’s view to only seeing the last 3 to 4 seconds of the tractor / trailer acceleration, the angle of the headlights might be such that the glare could veil the side of the trailer so that it might not be very conspicuous. It may appear to the oncoming motorist that there is just another vehicle approaching in the opposite direction not realizing the impending danger of a trailer angled across the their lane just beyond the headlights. This is where a thorough investigation determining the roadway geometry, the tractor / trailer acceleration characteristics, and the approaching motorist speed becomes necessary in properly analyzing this type of accident.

 A scene investigation using a total station or some other acceptable means of measurement to make a scale diagram is needed along with information concerning the acceleration characteristics and speeds of the involved vehicles. An acceleration test with the same or similar type of tractor / trailer and load that was involved in the accident can help determine the acceleration rate of the vehicle involved. A speed calculation can usually be done of the approaching vehicles speed by either crush damage analysis, speed from skidding or a combination of the two. Remember the important factor is what each of the motorist could see and at what point they could see it.


Todd Hutchison