Underground Exfiltration Trench Systems

Exfiltration Systems-Why They Work

Civil Engineers ordinarily calculate stormwater retention ponds to meet the requirements of your land development project. Nevertheless, any stormwater pond commonly will take upwards of 10% to 20% from the total available land area. The particular size of these storwater management basins is largely based on the types of soils encountered onsite; all other factors being equal. Your land that’s utilized by it can’t be utilized for vehicular parking, or for construction of buildings. In this short article I will summary the exfiltration trench requirements from the City of Orlando, Orange County, Seminole County as well as the City of Sanford.

Provide For Greater Utilization Of Land Area

In order to allow a greater use of the land for its intended use, “exfiltration trenches”, or underground vaults could possibly be utilized. Underground stormwater management systems are a viable method of handling the stormwater management needs of the project, especially where availability of land is limited. Utilizing an underground system for managing stormwater enables the use of the area above it for parking and buildings. This then allows the same area to be utilized for two complimentary uses and increases the yield of the project.

Exfiltration trenches are created from drainage pipes with smalls holes throughout its length. Underground vaults, conversely, are manufactured from concrete structures with a large open area underneath and open along the bottom. Both of these systems provide a traffic bearing surface for vehicular traffic and parking.

In order to create the necessary volume exfiltration trenches are made up of perforated drainage pipe situated within a graven bed. This provides both a means for percolating the stormwater volume into the ground and a structural support for the pavement above.

How The Regulations Differ From Agency To Agency

Orange County is just about the most developer-friendly agency within Central Florida except when it comes to exfiltration trenches. Orange County will allow the use of exfiltration trenches, however, the criteria for its use makes them undesirable.

Section 30-281(3) Orange County Land Development Regulations state: “in the event the exfiltration system fails, the stormwater will be retained on-site for the full twenty-five-year, twenty-four-hour storm prior to any stormwater being permitted to leave the site.” This in essence necessitates the actual site to get designed like a bowl to ensure that all 8.6 inches of rain around the entire property is stored onsite. This demand requires the site to become a bowl for storing all of this rain. This may allow roughly One foot to Twenty-four inches of water over the property especially over the particular parking lot as well as landscaping areas. This is a very extreme requirement which makes the usage of this outstanding strategy for stormwater management unworkable as well as unlikely to be utilized. This is not shared by any of the nearby municipalities.

The City of Orlando, Florida provides advantageous conditions with regard to utilizing exfiltration trenches for stormwater or underground vaults. The City’s engineering manual is currently being revised to allow, civil engineers, to design these systems with a safety factor of 2. This is equivalent to the requirements of Seminole County, The City of Sanford and the St. Johns River Water Management District.

The particular exfiltration trench system needs clean-outs at one end and manholes at the other end. Clean-outs and/or manholes need to be spaced every three hundred feet in order to facilitate maintenance. Civil engineers find that this strategy adheres to good engineering practice without being over-kill..

Seminole County also offers positive requirements regarding the utilization of underground exfiltration systems.

The Seminole Code Appendix B, Chapter 4.2 c. (4) states: ” Exfiltration systems shall be designed with a safety factor of 2.0 (i.e., design using one-half of the permeability rate or one-half of the time for drawdown).”

This criteria meets good civil engineering practice and one that is conservative without having unnecessary negative consequences in the design of underground exfiltration stormwater management system.

The City of Sanford land development regulations includes equally beneficial criteria with regard to the use of underground exfiltration systems for stormwater management.

The municipality’s land development Schedule O, Section 2.4 states: “shall be designed with a safety factor of at least two… Furthermore, a sediment sump is required ahead of the exfiltration trench system in order to capture sediments that may clog the pores in the pipe and/or gravel.

The civil engineer designs this sediment sump to capture sediments in the stormwater runoff which may clog-up the pores inside the pipe or the rock bed. The City’s strategy is similar to that of Seminole County and without undue burden.

The regional agency in charge of stormwater management systems is the St. Johns Water Management District (SJRWMD). The District has criteria pertaining to exfiltration trench techniques which is actually very similar to that of Seminole County’s, the City of Orlando, and the City of Sanford. Orange County is the only municipality which discourages the utilization of these underground stormwater management strategies. Exfiltration trench methods tend to be a good effective means of managing stormwater and may assist increasing your property’s yield. This allows for a more cost effective stormwater solution which tends to increase the Return on Investment.

Increased Land Yield

Underground exfiltration systems allows an additional effective increase in the land area between 10% to 20%. Depending on the value of the land or its availability an underground exfiltration system may be the difference between a viable project and one that is not.

Leave a Reply

Your email address will not be published. Required fields are marked *