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2020

LMNO Engineering, Research, and Software, Ltd.

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2020
January 2, 2020. Focus on Hydrology - Impact of Culvert Replacement
January 22, 2020. Open Channel Uniform and Non-Uniform Flow


LMNO Engineering, Research, and Software, Ltd.
The fluid flow calculations website: https://www.LMNOeng.com
7860 Angel Ridge Rd. Athens, Ohio 45701 USA   (740) 707-2614
LMNO@LMNOeng.com

Newsletter. January 2, 2020.

Focus on Hydrology - Impact of Culvert Replacement

We recently completed a culvert design consulting project. Culvert design is a mature field, so it would seem that most of the nuances have been worked out. However, sometimes the impacts of culvert replacement are overlooked. A culvert may be replaced due to its having been partially crushed or having deteriorated so that it no longer properly conveys the intended design flow at the original design upstream head (water depth). It is tempting to simply replace the culvert with the same or larger size. While this is usually the strategy, it can have unintended downstream consequences.

Replacing a culvert with the same or larger diameter, and clearing brush/debris, results in less flow resistance - which is a desirable consequence. However, the failed culvert may have provided stormwater detention upstream of the culvert. Upon replacement, there is less flow resistance resulting in less upstream stormwater detention and an increased risk of downstream flooding.

Please let me know if questions. Thank you for your interest in the LMNO Engineering newsletter,

Ken Edwards, Ph.D., P.E. (Owner/Engineer)
LMNO Engineering, Research, and Software, Ltd.
https://www.LMNOeng.com     LMNO@LMNOeng.com

Culvert design calculator: https://www.LMNOeng.com/Pipes/hds.php


You received this free newsletter because you requested it at our website. If you no longer wish to receive it, send a message stating 
"Discontinue Newsletter" to LMNO@LMNOeng.com.

© 2020 LMNO Engineering, Research, and Software, Ltd.


LMNO Engineering, Research, and Software, Ltd.
The fluid flow calculations website: https://www.LMNOeng.com
7860 Angel Ridge Rd. Athens, Ohio 45701 USA   (740) 707-2614
LMNO@LMNOeng.com

Newsletter. January 22, 2020.

Open Channel Uniform and Non-Uniform Flow

The empirical Manning equation is the most commonly used equation for open channel flows. In open channel flow, the water surface is open to the atmosphere. For instance, open channel flow can be in a partially full pipe, in a natural stream or river, or in a manmade channel. The Manning equation is of the form:

Q = A k R2/3 S1/2 / n

where: Q = Flow rate (m3/s or ft3/s). A = Cross-sectional flow area (m2 or ft2). k = Unit conversion factor = 1 for SI units or 1.49 for English units. R = Hydraulic Radius (m or ft) = A/P, where P = Wetted perimeter (m or ft). S = Energy slope = Change in water surface elevation per unit length of channel (m/m or ft/ft). n = Manning roughness factor.

If the channel or pipe that water is flowing through is long with unchanging dimensions (a "prismatic" channel), then the water depth will be nearly constant along the channel's length. This situation is called uniform flow; the water surface is modeled as being parallel to the channel bottom. In this case, the slope of the channel can be used as the energy slope S. However, if the channel is not prismatic or the water depth changes with distance (near a channel drop-off for instance), then the energy slope S is the slope of the water surface, rather than the slope of the channel.

In many instances, we know the flow rate in the channel and need to determine the water depth. In this case, the Manning equation is solved for water depth numerically since S, A, and P (thus R) are functions of the water depth. If flow is uniform, then water depth is constant along the entire channel, so the Manning equation only needs to be solved numerically once for depth. If the water depth varies along the channel length (non-uniform flow), then the Manning equation is solved numerically for water depth, not just once, but over and over at small distance increments successively moving upstream or downstream along the channel.

More information can be found on our web pages: Uniform flow in pipe: https://www.LMNOeng.com/CircularCulvert.php
Uniform flow in trapezoidal channel: https://www.LMNOeng.com/Channels/trapezoid.php
Non-uniform flow in trapezoidal channel: https://www.LMNOeng.com/Channels/gvf.php (has demo mode and graph; note that non-uniform flow is also called gradually varied flow)

Please let me know if questions. Thank you for your interest in the LMNO Engineering newsletter,

Ken Edwards, Ph.D., P.E. (Owner/Engineer)
LMNO Engineering, Research, and Software, Ltd.
https://www.LMNOeng.com     LMNO@LMNOeng.com


You received this free newsletter because you requested it at our website. If you no longer wish to receive it, send a message stating 
"Discontinue Newsletter" to LMNO@LMNOeng.com.

© 2020 LMNO Engineering, Research, and Software, Ltd.


© 2020 LMNO Engineering, Research, and Software, Ltd. (All Rights Reserved)

LMNO Engineering, Research, and Software, Ltd.
7860 Angel Ridge Rd.   Athens, Ohio  USA   +1(740) 707-2614
LMNO@LMNOeng.com    https://www.LMNOeng.com