15A NCAC 02H .1051 MDC FOR INFILTRATION SYSTEMS
The purpose of this Rule is to set forth the design requirements for infiltration systems that are constructed to meet the requirements of this Section.
(1) SOIL INVESTIGATION. A site-specific soil investigation shall be performed to establish the hydraulic properties and characteristics of the soil within the proposed footprint and at the proposed elevation of the infiltration system.
(2) SEPARATION FROM THE SHWT. The lowest point of the infiltration system shall be a minimum of two feet above the SHWT. However, the separation may be reduced to no less than one foot if the applicant provides a hydrogeologic evaluation that demonstrates that the water table will subside to its pre-storm elevation within five days or less.
(3) SOIL SUBGRADE SURFACE. The surface of the soil subgrade shall have a slope of less than or equal to two percent. Terraces and baffles may be installed to achieve a level subgrade.
(4) PRETREATMENT. Pretreatment devices shall be provided to prevent clogging. Pretreatment devices may include measures such as sumps in catch basins, gravel verges, screens on roof and patio drains, filters, filter strips, grassed swales, and forebays. Rooftop runoff that is discharged to the surface of an infiltration system shall not require pretreatment.
(5) DRAW DOWN TIME. Infiltration systems shall be designed to dewater the design volume to the bottom of the infiltration device within 72 hours or less. In-situ soils may be removed and replaced with infiltration media or infiltration media may be placed on top of in-situ soils if the applicant provides a soils report that demonstrates that the modified soil profile allows for infiltration of the design volume within 72 hours or less.
(6) OBSERVATION PORT. For infiltration devices located under the ground surface, a minimum of one inspection port shall be provided.
History Note: Authority G.S. 143-214.7B; 143-215.1; 143-215.3(a);
Eff. January 1, 2017.