# Presentation: 056144

## Steve Waters
Senior Hydrologist
Flood Warning / Water Quality Branch
Engineering Division

- Flood Control District of Maricopa County

- Flood Physics

**Steve Waters**
- Senior Hydrologist
- Flood Warning / Water Quality Branch
- Engineering Division

## Moving water exerts pressure on objects.
    Pressure = k(constant) x (Velocity) 2

Pressure on an area becomes a force.
    Force = (pressure x area)  
    water depth      greater area      greater the force

Surface becomes slippery      (reduced friction force)

-     Pressure = k(constant) x (Velocity) **2**
- Pressure on an area becomes a force.
-     Force = (pressure x area)
-     water depth      greater area      greater the force
- Surface becomes slippery      (reduced friction force)

- _Physics of Stream Forces_

## The buoyant force on an object is equal to the weight of the fluid displaced by that object

- _Archimedes’ Principle_

## Forces on Vehicles Crossing Streams

- Buoyancy Force

- Friction Force

- Stream Force

**The car will float downstream when:****Stream Force  > Friction Force**

- Vehicle Weight

## Water weighs 62.4 pounds/cubic foot, and cars displace a lot of it
The pressure exerted by moving water increases with the square of its velocity
Water, sand and mud reduce the frictional forces that hold a car in place
Water clarity and lighting conditions conceal the condition of the roadway beneath you

- The pressure exerted by moving water increases with the square of its velocity
- Water, sand and mud reduce the frictional forces that hold a car in place
- Water clarity and lighting conditions conceal the condition of the roadway beneath you

- _Some Water Physics Facts_

## From the FEMA Web Page

## Fighting a Losing Battle

- Width: 5.5 feet
- Length: 14 feet
- Ground Clearance: 10 inches
- Weight: 3,400 pounds

- 	However, 1 foot of water displaced by this vehicle weighs:
- (5.5’ x 14’ x 1’ x 62.4 lbs./cu.ft.) = 4,805 pounds

## Something a Little Bigger

- _Something a Little Bigger_

- Width: 6 feet
- Length: 18 feet
- Ground Clearance: 18 inches
- Weight: 5,040 pounds

- 1 foot of water displaced by this vehicle weighs:
- (6’ x 18’ x 1’ x 62.4 lbs./cu.ft.) = 6,739 pounds

## Stream Forces on Vehicles

**Feet/second**

**Water Velocity**

- * 14’ vehicle length and 10” clearance

**2.7**

**3.4**

**4.1**

**4.8**

**5.5**

**Miles per Hour**

- Water Depth (feet)

- F=AkV2, k=1.4 for square-cornered objects

## Water Depth: 1 foot

- Width: 5.5 feet
- Length: 14 feet
- Clearance: 10 inches
- Weight: 3,400 pound
- Net Depth = 0.17 feet

- Net Weight : 3,400 lbs - 817 lbs = 2,583 lbs
- Friction Force: 0.4 x 2,583 lbs = 1,033 lbs
- 1,033 lbs is greater than the 120 lbs of stream force, so the vehicle stays put

- 3,400 lbs

- 817 lbs

- 120 lbs

- 1,033 lbs

- 6 ft/sec

- 1.0 ft

## Water Depth: 1.5 feet

- Width: 5.5 feet
- Length: 14 feet
- Clearance: 10 inches
- Weight: 3,400 pound
- Net Depth = 0.67 feet

- 	Net Weight : 3,400 lbs - 3,220 lbs = 180 lbs
- Friction Force: 0.4 x 180 lbs = 72 lbs
- 72 lbs is less than the 473 lbs of stream force, so the vehicle moves downstream

- 3,400 lbs

- 3,220 lbs

- 473 lbs

- 72 lbs

- 6 ft/sec

- 1.5 ft

## Something Else to Think About

- 8 in.

- 12 in.

- Volume = 3.27 ft3/tire x 4 = 13.09 ft3
- 13.09 ft3 * 62.4 lbs/ft3 = 816.2 lbs

- _Buoyancy and Tire Size_

- 30 in.

- 30 in.

- Volume = 4.91 ft3/tire x 4 = 19.64 ft3
- 19.64 ft3 * 62.4 lbs/ft3 = 1,225.2 lbs

- Difference is 409 lbs. added buoyant force

## Stream Forces on Humans

- * Force on each leg

**fps**

**2.7**

**3.4**

**4.8**

**5.5**

**4.1**

**mph**

**Water Velocity**

## Flood Safety

- Judgement

- If You _Absolutely_ Must Cross a Flooded Stream...

- Enter slowly, watch the depth and back out if the front axle is submerged
- If your vehicle stalls in the wash, and upstream weather conditions are bad, abandon the vehicle and get to shore

## Flood Safety

- Remember...

- Floodwaters can conceal a damaged roadway
- Flash Floods rarely last more than an hour
- Don’t trade an hour for a lifetime