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.