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Grading Considerations In Land Development








Every successful project starts in the conceptual phase of design. Transforming the vision of the developer, land planner, or homeowner into reality is where design professionals earn their keep. The basics of any land development project start with the grading design. A residential architect may have a dream of a beautiful walkout basement with a brick laid patio and steps leading down to the in-ground pool. A real estate developer might envision a self-sufficient community away from the big city with mixed-use retail and residential. A thrifty real estate broker has found hundreds of acres of prime real estate for the taking at a great price. These are the types of issues that design professionals face every day. It is easy to jump on the project bandwagon with excitement for your client, but it is our responsibility to understand the project in every aspect so that the client can keep the dream alive.






Building an apartment community in a flood plain posed interesting concerns for the Landscape Contractors on this project. The grading is at a slope that is ideal for storm water runoff and drainage.


Let's look at why grading design is important in the three scenarios mentioned above. Answering the questions, "what, where, and why" will lead to "how."

Single Family Home

A large walkout basement with an in-ground pool sounds enticing to anyone and answers the "what" question. Where is the home to be built? This question raises the awareness of understanding the project. It also creates more critical thinking that may lead to further questioning – on a hill? In a valley? In the country? In the city? A synonymous question for "where?" in the eyes of land development becomes, "Is this property located where flooding can occur?" Why is this so important? Well, that's a loaded question that will be addressed later.

Why is the home to be built this way? The homeowner may have valid reasoning for positioning the home a certain way on the property due to the sun's position throughout the day, saving trees around the house, or merely curb side appeal. The homeowner is the customer and the customer is always right. Right?








The next question to ask is "how." It may be obvious to an experienced design professional that the best type of topography for a walkout basement is where the ground slopes from the front of the house (high side) to the back of the house (low side) – perhaps down to a low area or some type of drainage way. This textbook design may not be the case for the project. The land may slope from back to front thus changing the entire scope of the architectural design.

Multi-Family

The need for multi-family living is still strong. The outdated apartments of the eighties just aren't meeting the needs of today's young professional. Open space living, new appliances, garages, and recreational facilities are becoming necessities rather than options. Much of the same design constraints exist in this arena as with any other project. There is also the requirement to adhere to the Americans with Disabilities Act (ADA) site design requirements for accessibility.

The location of accessible parking spaces and sidewalk routes to buildings is one such design example. Accessible parking spaces cannot exceed a two percent slope within the parking space or the access aisle. Sidewalk ramps are constructed with slopes not to exceed a 1:12 slope. Accessible pathways and routes should be designed with slopes not exceeding 1:20 (five percent) and/or limiting the vertical rise to 30 inches.






Due to building in a flood plain, the builder was required to construct a finished floor two feet above the 100-year flood level. To do this, garages were built at "ground level," with the residential spaces in the complex sitting above the garages. Thus, in a 100-year flood, only the concrete flooring of the garage would become water-logged.


These design limitations are important to understand and implement in order for the client's vision to be a success.

Planned Residential Community

More and more people are being lured to planned developments outside of the densely populated urban areas to meet their needs.

The amenities within the community may include golf, tennis, walking trails, bike lanes, and swimming. Every need from dry-cleaning, home improvement, health care, dining, and entertainment is just around the corner. This is "what" the planned development strives to be.

Where is the best place to build? A real estate developer is in the business to develop land with minimal capital cost improvements, but this may be difficult to achieve when the customers wanting separation from the congested areas are driving the market. Infrastructure extended to the development may introduce various design constraints that must be investigated long before the first bulldozer starts pushing dirt.






This planned residential community features a man-made pond that was designed to lower the flood elevation onsite and serve as a depository for storm runoff (top 2). More than 110,000 cubic yards of dirt was moved during construction of the pond, with the final slope of the bank at a 3:1 ratio. The finished product (bottom 2) adds to the scenery of the community while serving an extremely functional and important purpose.


The design professional must be extremely considerate of the developer's vision and intent. There are reasons why the conceptual land plan is laid out the way it is. Bringing the element of creative design and jurisdiction approval requirements together is what makes design engineers such an invaluable resource.

How is this plan going to come to reality? Understanding the surrounding utility, drainage, environmental, geological, and existing topographic constraints will lead to the answers of how grading the land make the project's vision come to fruition.

Buyer Beware

A thrifty real estate broker has found hundreds of acres of prime real estate for the taking at a great price. What a great deal! - or is it? The broker has told us "what" it is we are looking for, but "where" is it located and "why" is it at such a bargain basement price? Answering these questions will once again lead us to "how" the grading design will drive the project.

Upon careful investigation of the property it is determined to lie within a flood plain. The price of the land actually may be too high based upon how deep the floodwaters may inundate the property. If the land would be twenty-five feet under water during a flood, you might want to renegotiate the purchase price to allow more money to be diverted to engineering design and construction costs for flood protection measures. Now we know "where" the property is located and "why" the property is so cheap.








With the project moving forward, the design professional must provide the answer to "how" the grading design must drive the land plan. Bringing in thousands of cubic yards of fill dirt to raise the site above the flood elevation is the first step while consideration for offsite utility extensions such as sanitary sewer must be incorporated in the grading design. If the sound of endless truckloads of dirt doesn't appeal to the developer's pocket book, the possibility of a flood protection wall or levy can save the project from being under water in the future. This comes at a high price, long jurisdiction approval time, and a challenge to make aesthetically pleasing. Although, the popularity of walking trails along the top of levies can certainly be an opportunity to add to the beautification of the project.

Flood Plain Considerations

Considered more inland in the Midwest and Plains states rather than in coastal areas, the delineation of flood plains is an important step that shouldn't be missed in the conceptual design phase.

The Federal Emergency Management Agency (FEMA) along with your state's Department of Natural Resources regulates the construction and alteration of these areas. There are two types of identified flood zones - flood fringe and floodway. The flood fringe is the approximate limit of where the flooding would reach for the 100-year flood event. The floodway is the approximate location of the moving channel of water. The location of these flood zones can be found on the FEMA Flood Hazard Maps (http://www.fema.gov/mit/tsd/hm_main.htm).








Some of the flood-hazard maps have flood elevation cross sections marked on the maps. These elevations can be interpolated between cross sections and are considered the property's 100-year flood elevation. It is more common for major streams and rivers to have these delineated cross sections on the maps.

If the map for your project does not have a 100-year cross section delineated, then a bit more cumbersome road lies ahead in determining the 100-year elevation. First, contact your state Department of Natural Resources division of water to see if their flood plain management experts can help you in determining the 100-year elevation. If this becomes a fruitless adventure, then an economic decision needs to be made to either retain the services of a professional engineering firm to perform a hydraulic study of the waterway and determine the 100-year elevation or pay the added flood hazard insurance premium required by the lender.

For residential property owners the latter is probably the more reasonable economic solution. But for a residential real estate developer, the former may be the best economic solution to sell his product as well as a requirement by the local governing agencies. If the proposed building lies within the flood hazard area, then design measures need to be taken to keep the project moving forward. One such technique would be to raise the grade of the land around the building and the finished floor elevation.








FEMA considers the flood protection grade (FPG) for the finished floor of the structure to be sufficient protection when it is at least 1 foot above the 100-year flood elevation. Local jurisdictions may require a higher FPG such as two feet. In the case of a walkout basement, the basement floor elevation would need to meet the minimum FPG.

Offsite Utility and Drainage Limitations

Most great ideas are grandiose without limitations; however, the design professional must reel in the idea and apply the engineering limitations to thwart. In most new development scenarios the infrastructure must be extended to the site. Utilities such as gas, electric, telephone, water, and cable TV can traverse the land above or below without much thought to elevation constraints. Sanitary and Storm sewer are exactly opposite. Extending a gravity sanitary sewer system to the project site can certainly drive the grading design as well. If the sewer is extended from a lower elevation region, such as along a creek, and uphill to the project, there probably will not be a concern. But if the sewer is being extending from a higher elevation region or from a region at or near the same elevation as the project site, then it will surely control the grading design.






An existing stream sat directly where the builders wanted to construct multi-family housing.


The installed sewer must maintain minimum cover requirements over the top of pipe that varies in every part of the country. The sewer utility may have minimum flood protection elevations for manhole castings that would require the manhole lids to protrude out of the ground considerably higher than the grade around the manhole. The local building code may mandate that the lowest finished floor elevation must be a minimum of one foot above the nearest sanitary sewer manhole casting. The sanitary sewer can be the governing constraint for controlling the grading in any project.

Grading is the essence of drainage design. Water flows downhill and that is precisely what is working in our favor - most of the time. After looking at the flood plain restrictions, it is critical to identify the existing drainage watersheds affecting the site. This may include an offsite watershed that must be accounted for in the drainage system design. The lowest point of the site is usually the best location for a detention pond but it is not always the case.

In general, water is to be diverted from a high point and collected into lower areas where it can be channeled into the natural waterway of the watershed. This is accomplished in residential design by the use of swales, detention ponds, and storm pipe systems. It is good engineering practice to design the building at a high point so that water can be diverted away from the structure's foundation.






The stream was relocated to an existing portion of the site, allowing for construction of the complex.


The elevation at which water leaves the site will control the grading design throughout. Allowing for storm pipe cover, pipe slope, storm structures, inlet castings, swale slopes, and overland flow will all add elevation to the grading design. Concrete storm sewer pipe typically will need at least two feet of cover over the top of pipe. A street curb inlet will require a minimum of three feet of depth from casting to invert. A typical grass drainage swale requires a one percent slope but can be as low as 0.5 percent with a pipe underdrain below the flowline. Open grass areas should have at least a one percent slope to maintain positive drainage.

Summary

The ability to grasp the vision of the homeowner, developer, or architect will lead to asking the design related questions of "what, where, and why" leading to "how?" Grading design must include the in depth knowledge of what can affect the project. Flood plains, utility requirements, existing topography, ADA accessibility, and storm pipe system design are all examples of outside constraints that prevent the design to be completely free to the visionary. Transforming the idea into reality is the role of the design professional and being able to effectively mix the two can only lead to success.

Brian Cross is an associate at Roger Ward Engineering, Inc., based in Indianapolis, Indiana.



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June 18, 2019, 9:07 pm PDT

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