Quantifying the Danger

Getting caught in or pinned against an obstacle such as wood in a river is very dangerous.  Wood strainers are just that. Strainers, which means the water flows through the woody debris, but larger objects like swimmers or boats are pinned against the debris. The person is caught and held against the obstacle by the force of the current.

The force exerted on a person by the current is based on the body size of the person, position in the current, and the velocity of the current.  The force increases exponentially as the velocity of the current increases.  Many projects in rivers are constructed at very low flow levels, and surveyed for safety at those levels, while recreation on a river takes place at a much wider range of flow conditions.  Spring runoff or even summer storms can spike flow levels beyond what many river users are prepared to deal with.  See the attached chart for the force exerted on a child, an adolescent and an adult as velocities.  Self-rescue is difficult at moderate flows for a person who is pinned in a river.  At higher velocities, self-rescue is impossible.


 Flow Load Calculations - Background

  • Flow loads pinning a person against an obstruction in a river are a function of water velocity, water density, shape of the person, body position, area of the body presented to the flow and clothing worn.
  • Because there is a broad range of possible pin positions, body shapes, clothing worn and flow conditions, there is no single solution for the range of possible pinning load on any one person.
  • The calculations presented represent lightly clad persons, fully submerged, and pinned flat against a root wad or other structure.
    • Data to calculate body surface areas and determine geometry characteristics necessary to calculate drag coefficients come from:Physical Characteristics of Children As Related to Death and Injury for Consumer Product Design and Use, Highway Safety Research Institute, University of Michigan, UM-HSRI-BI-75-5, Final Report, 31 May 1975
    • Military Handbook: Anthropometry of U.S. Military Personnel, US Department of Defense, DOD-HDBK-743A, 13 February 1991

 

Hanging on to the downstream side of a boat, or floating downsteam from one is a very dangerous place when washed into an obstruction.  A swamped 16' canoe can contain one ton (2000 lbs) of water.  When that mass moving at the velocity of the current hits a stationary object, the impact force is tremendous. People have been crushed when caught between a boat full of water and an obstacle. 

The force on a pinned boat is much greater than that on a human because a boat has significantly more area exposed to the flow, especially when swamped.  Because of the magnitude of the force exerted against a pinned canoe - which can equal or exceed one ton - it is very dangerous to be pinned between a boat and debris in the river.


Canoe Flow Load Calculations ‐ Background
  • Flow loads pinning a canoe against an obstruction in a river are a function of water velocity, water density, shape of the canoe, orientation of the pinned canoe relative to the flow, and surface area of the canoe presented to the flow.
  • Because there are a broad range of possible possible pin positions, positions, canoe shapes and sizes, and flow conditions, there is no single solution for the range of possible pinning loads on any given canoe.
  • The calculations calculations presented presented are for generic generic river canoes ranging ranging from solo 12’ through 18’ tandem designs. Each is assumed to be on its side, fully submerged, pinned against a root wad or other ob t ti s ruction, with the open side f i ac ng the oncoming flow.
  • Data to calculate the generic canoe surface areas and determine geometry characteristics necessary to calculate drag coefficients were derived from characteristic measurements taken from different designs within each canoe length.