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Conveyance Analysis

Conveyance Analysis

(OP)
Hello I have been asked to help provide some Civil Engineering Design work, but my main area of experience is Structural. I am at a loss on how to begin or approach this assignment, so any comments/suggestions are appreciated. I have been asked to provide a Conveyance Analysis to a proposed development. Here is some description of the task:

A Conveyance Analysis is to demonstrate that the proposed development does not change the existing conveyance capacity of the project site.

Conveyance capacity of a site is understood to mean the ability of the site to convey water at the BFE from the upstream end to the downstream end of the site with a given total change in water surface elevation (total head loss). Conveyance calculations must be certified by a professional engineer licensed in the State of Texas that demonstrates that the development will not, at any time, change the conveyance capacity.

At a minimum, the Conveyance Analysis must include the following elements:

Develop a site plan that includes existing topography and structures with conveyance flow paths and directions indicated across the site. Conveyance flow paths may not necessarily align with the centerline of the modeled stream. Show areas of ineffective flow on the site plan, extending offsite as necessary.

On the existing conditions site plans, identify points of flow constriction for each flow path. Depending on the nature of the site, there may be only one flow path and one constriction point, or there may be several flow paths and constriction points.

Establish cross-sections at constriction points. Depending on the nature of the site, a single cross-section may intersect more than one flow path and constriction point.

For each cross-section, determine appropriate roughness, n, values, based on site-specific conditions. Choose the appropriate n value from the table that follows this discussion and document roughness selection with aerial and/or site photographs.

For each cross-section, calculate the conveyance, K, for existing conditions; where the depth of the cross-section is established from the BFE.

Develop a site plan that includes proposed topography, structures, conveyance flow paths and directions in a manner similar to that employed for existing conditions. Show areas of ineffective flow on the proposed site plan, extending offsite as necessary.

Identify proposed points of flow constriction and construct cross-sections for the proposed conditions. Identify site-specific proposed roughness values along the proposed cross-sections and document how these roughness values were determined. Calculate the conveyance for the proposed conditions.

Cross-sections should not span the entire floodplain; rather, they should be limited to the site boundaries so that the calculations compare existing and proposed conveyance across the site. If the profile of the site varies significantly in width perpendicular to the direction of flow, portions of the site may need to be compared independently of each other with conveyance maintained in each of the areas. Similarly, if the existing and proposed conditions vary significantly, the cross-sections used for comparison may not be in the exact same location, but should be representative of the flow in that portion of the site.

Compare the proposed conveyance capacity with the existing conveyance capacity. The proposed site must strive to maintain the existing level of conveyance.

Conveyance calculations must be included in the documentation. These calculations may be performed using a model such as HEC-RAS, in which case model outputs showing conveyance must be provided at each cross-section and model inputs must be provided.

A conveyance loss of no more than 0.5% of existing levels will be considered.

For complex sites, or if the conveyance increases more than 0.5%, the applicant should review the analysis and proposed conditions with the floodplain office.


I have a set of plan & profile plans in front of me. As I mentioned previously, my background is primarily structural, so any guidance is appreciated.

RE: Conveyance Analysis

This task is even more complicated than your last question. I had a longer answer typed up, but then deleted it. Not trying to be a jerk, but your firm really needs to hire a hydrologist.

If you're dead set on doing this, start by reading the HEC-RAS manual cover to cover, (it's long) and buy a good fluid mechs textbook to reference anything you don't understand.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Conveyance Analysis

(OP)
@beej67 - I was able to solve the other issue, so I am doing my best to tackle this problem as well. The feed back I received was helpful so I figured I would ask again.

RE: Conveyance Analysis

Yeah, this one's tougher. If you got the last one, pin your ears back and read the HEC-RAS manual. It has a lot of theory in it, so you might not need much more than that. But it's heavy stuff if you're not already familiar with the material.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Conveyance Analysis

(OP)
I am working on the Conveyance Analysis. I am only allowed to change the flow in the cross section by 0.5% (which is ridiculous). The problem I am having is that I am not allowed to change my "n" value because we are using earth fill to place on the FEMA area for mitigation. The area we are placing it on is earth as well. I am also trying to create a small hill in the area to be used for a future hotel. I am trying to see if I can just cut more of the existing condition away to make up for the fill hill I am creating, but I am having issues with my wetted perimeter. Has anyone been able to get by on the Conveyance Analysis by adding fill a not changing the n-value?

RE: Conveyance Analysis

Can you excavate a channel that would replace the lost cross sectional area at the same Manning's?

RE: Conveyance Analysis

It's a tough thing to do. Sometimes it's impossible. Sometimes the flood elevation in the fill area actually goes down due to "choking" effects and rises afterwards due to expansion and contraction losses. If you're not very familiar with the science, you'll see effects that confuse you.

If you can't get it to work, sometimes the best thing to do is just run the model pre/post, and then get easements or agreements with all the affected landowners.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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