Site design in a Connecticut floodplain
Metcalf & Eddy|AECOM
3-D modeling using AutoCAD Civil 3D simplifies calculation of site flood water volumes.
Obtaining a permit to build within a floodplain requires a design that ensures the amount of water the site can hold is not decreased following construction. For example, adding fill to a site will flood out neighboring sites during a flood elevation. Consequently, civil engineers often need to compare the volume of water on a site from the 100-year flood before and after construction. Three-dimensional modeling of water volumes and water balancing can make these calculations easy, efficient, and visual.
The process is similar to an earthwork problem, except the "earth" being calculated is water. Using a software program such as Autodesk’s AutoCAD Civil 3D, an engineer can "flood" a site under both existing and proposed conditions and then compare the volume of water before and after construction. Calculations and figures can then be used for the permitting process.
Following is an example of the process used for a project in southwestern Connecticut in which water volumes were determined using AutoCAD Land Desktop and then calculated using Civil 3D.
Step 1: Delineate the 100-year foodplain—Determining whether a site is within the 100-year floodplain has never been easier. The Federal Emergency Management Agency (FEMA) Map Service Center website (http://msc.fema.gov) has an address fill-in box. After typing in the project address, users are given the option to purchase a map or view it online. Users can also type in the Map Panel ID, but that is not as useful because a street name is more commonly known than a Panel ID. Using the free map viewer option, copy and paste the map into an imaging program such as Microsoft Paint or Adobe Photoshop to make an overlay that can be used in Civil 3D. Pan to the bottom right-hand corner of the map to see the Map Panel ID for design documentation. Of course, community maps also need to be checked. The sample site in southwestern Connecticut has a Base Flood Elevation (BFE) of 10 feet (Figure 1 below).
Step 2: Build pre- and post-development terrains — To build terrains to flood out with water to the 10-foot elevation, an existing surface was created for the project using survey contours and survey elevation shots (Figure 2 below). A proposed surface was created using design contours (Figure 3 below). For the proposed surfaces, inner surface boundaries were defined for buildings to prevent the flood water from entering the buildings.
Step 3: Establish the water surface—The water surface was easy to create with only one entity by placing a polyline around the site and setting the elevation (z coordinate) to 10 feet. If community data provides a more stringent flood elevation, simply raise the water surface with one command by the given amount.
Step 4: Create pre- and post-development flood water volumes—Volume surfaces were made from the "Create Surface" pull down menu in Civil 3D. The base surfaces used were the before and after development surfaces created previously. The comparison surface selected for before and after development was the water surface created with one entity.
Water volumes for before and after development are displayed in the surfaces collection. In the initial site grading design, site water capacity decreased after development. To comply with permitting, proposed grading was adjusted to increase the site’s flood water capacity. Civil 3D updates the flood volumes for pre- and post-development and updates section cuts.
Working with Land Desktop requires users to redo much of the work for a simple rise in water level. After using Land Desktop, engineers can appreciate all of the automatic updating that Civil 3D performs.
Advantages of 3-D modeling
The great advantage of Civil 3D compared with Land Desktop is the use of styles. With Land Desktop, users must reissue commands or a series of commands to make changes. In Civil 3D, users simply change the style. The pre- and post-development depth maps were easy to create (Figures 4 and 5 below). Land Desktop was cumbersome and loaded the drawing with memory-hogging solids, whereas a handy analysis tab in Civil 3D lets users select a color scheme such as shades of blue. The ranges were easy to change with instant changes on screen, compared with Land Desktop where the old solids are erased and new solids inserted.
Styles allowed easy inclusion of colorful figures in a report, making flood calculations visual. The elevation analysis has a scheme of blues for water, with color bands representing water depth ranges. All cuts (negative elevations) were ignored since the fills represent the flooded site.
Another benefit of Civil 3D is keeping track of all the surfaces. The southwestern Connecticut project was a brownfields site, and analysis with Land Desktop comprised more than 30 surfaces and composite surfaces created for the DNAPL, LNAPL, PMC, DEC, groundwater, et cetera. Since Land Desktop was project based, it was necessary to always scroll through a long list of surfaces. On the other hand, Civil 3D has drawing-based data, so surfaces can be brought in from other drawings with data shortcuts. This provided a more organized method for the analysis.
Additionally, Civil 3D has a drop-down menu that imports an aerial image from GoogleEarth into an AutoCAD drawing. One needs merely to set the coordinate system in Civil 3D. While it is convenient, image quality is slightly degraded. A better aerial image can be obtained from most state GIS websites. Search online to find state GIS websites. Or, go to a website that has GIS links sorted by national and state organizations, such as the University of Arkansas library’s International Geospatial and Attribute Links (http://libinfo.uark.edu/GIS/us.asp).
Cutting sections in 3-D is also easier. Land Desktop had static cut sections, whereas Civil 3D has dynamic sections. In Civil 3D, users can move the section line and the section automatically updates. If surfaces are revised, the sections automatically update.
Calculating flood volumes with a dynamic 3-D modeling program is quick, and it creates colorful figures and tabular data. Learning how to use the software can be frustrating, and learning styles is like learning a new program. However, it is a breeze to use after that boot camp period and productivity gains are significant.
Salvatore Napolitano is a civil designer for Metcalf & Eddy|AECOM in Manchester, N.H. He has more than 16 years of experience with water and wastewater projects worldwide. He can be contacted at email@example.com.