 |
       |
      |
   |
|
Aquatic Hydraulics: Critical Depth
|
 |
Critical depth is the depth of flow over an obstruction. Critical depth is typically two-thirds the normal depth of a stream, and it occurs over all freely overflowing zones in the streambed (Newbury). Typical locations where critical depth may occur include abrupt changes in channel slope, channel constrictions, unsubmerged outlet of a culvert on a sub-critical slope discharging into a wide channel or with a free fall at the outlet, and at the crest of an overflow dam or weir. Critical depth is dependent on channel geometry and discharge (http://il.water.usgs.gov). The channel slope produces critical depth and velocity for a given discharge. For a waterway of rectangular cross section, equation one can be used to calculate critical depth. King's "Handbook of Hydraulics" is useful in computing critical depths for waterways with circular and trapezoidal cross sections (http://il.water.usgs.gov). |
|
|
(Eq. 1)
|
dc = (q²/g)^ 1/3 |
Where
|
|||
|
|
Flow and velocity at critical depth are called critical flow and critical velocity, respectively. Critical flow and velocity occur as water passes over an obstacle in a channel or drops over an abrupt downward break in the channel bed resulting in a change in channel velocity and depth (Newbury). For example in riffles, a free-overflow condition is created and water is forced to accelerate (Newbury). Critical velocity over the crest of an obstruction (in the previous example, a riffle) is related to the depth in the following relationship:
|
|
(Eq. 2)
|
Vc = (9.8hc) |
Where
|
|||
|
|
Stream flow passes through critical depth as it proceeds from sub-critical to super-critical flow as can be seen in the above illustration. Sub-critical flow occurs where the depth of flow is greater than the critical depth. This flow is considered laminar, streaming or tranquil. Super-critical flow occurs where depth of flow is less than the critical depth. This is where flow is turbulent, rapid or shooting (Hwang and Houghtalen). Super-critical flow can result in air-entrainment, which may cause an increase in the depth of flow (http://il.water.usgs.gov). This type of flow also can cause a reduction in channel friction resulting in an increase in velocity. When flow occurs at critical depth the following relationship must be satisfied:
|
|
(Eq. 3)
|
A³/T = Q²/g |
Where
|
|||
|
|
|
||
|
Home | About Us | Services | Restoration Process | Project Profiles | Stream Ecology Educational Services | Links and Resources | Current Restoration Efforts | Contact Us
Oxbow River & Stream Restoration; 2905 Klondike Road; Delaware, OH 43015; (740) 362-4134. © 2008 Oxbow River & Stream Restoration, Inc. Last updated July-2008. |
|
 |