CRMC Seeks Improved Management of Stormwater Runoff

The Rhode Island Coastal Resources Management Council (CRMC) is currently working with the Rhode Island Department of Environmental Management (DEM) to develop and implement a revised version of the State’s Stormwater Design and Installation Standards Manual. A draft version of the new RI Stormwater Manual can be found at the following website: http://www.dem.ri.gov/programs/benviron/water/permits/ripdes/stwater/index.htm. The primary intent of the revisions is to update the original manual that was adopted in 1993 with recent developments in the science and practice of stormwater management. Additionally, numerous engineered products have been developed, installed, and tested since the manual was originally published. Accordingly, the field of stormwater management has undergone rapid development since 1993, which requires a thorough reassessment of stormwater management practices used within the state.

Stormwater treatment devices and facilities are often referred to as Best Management Practices or BMPs. The CRMC will endeavor through planning and regulatory permitting to ensure that only the best available practices and technologies are used for the treatment of stormwater within the coastal zone that consider the limitations of land use, individual development sites, and the characteristics of the receiving waterbody. All parties preparing stormwater management plans must carefully consider these factors in preparing a plan since there is no “one size (or BMP) fits all” approach to stormwater management. Applicants developing stormwater management plans should consider a range of BMPs starting with practices that provide the highest level of treatment. They should carefully evaluate the characteristics of the receiving water body, site limitations, and development constraints using a “top down” approach to determine the best available BMP or preferably a series of BMPs for each site. Applicants are strongly encouraged to coordinate these efforts with CRMC staff in a pre-application meeting and consider any known impairments (e.g., nutrient, bacteria, dissolved oxygen, etc.) affecting the receiving water body.

In addition to the current stormwater manual revision efforts, the CRMC adopted in October 2006 stormwater treatment requirements focusing on Low Impact Development (LID) design within the Urban Coastal Greenways (UCG) Policy for the Metro Bay Region Special Area Management Plan (SAMP), which includes portions of Providence, East Providence, Pawtucket and Cranston (See: http://www.crmc.ri.gov/samp/metrobay.html). The CRMC is also working on a Suburban Buffer Design Manual which will supplement the Greenwich Bay Special Area Management Plan. Both SAMPs rely heavily on LID techniques for the management of stormwater runoff. All regulatory standards and requirements contained in CRMC’s Special Area Management Plans take priority over the standard requirements of the State’s stormwater manual or other CRMC regulations.

Interim Finding Regarding Hydrodynamic (Swirl) Separators

While revisions to the Stormwater Design and Installation Standards Manual and other CRMC regulations remain under development, the CRMC would like to advise applicants of the following policy. Through an evaluation of available studies that have field tested the efficiency of certain hydrodynamic separators, namely testing conducted by the Stormwater Center at the University of New Hampshire (See: http://www.unh.edu/erg/cstev/), the CRMC hereby concludes that the required 80 percent Total Suspended Solids (TSS) removal rate required under Section 300.6 of our coastal program is not being met by these devices. On this basis, pending the adoption of additional regulatory standards, the CRMC strongly discourages the use of hydrodynamic separators as the sole method of stormwater treatment for all new development and redevelopment projects. Applicants considering this type of treatment device are encouraged to discuss the use of these devices with CRMC staff prior to including them on any development plan. The CRMC, however, does not object to the use of these devices as an appropriate method of pre-treatment for other BMPs, such as infiltration facilities, vegetated BMPs, constructed stormwater filtration wetlands, or other approved methods. Hydrodynamic separators that include a filter component may be considered appropriate in certain circumstances, but only in situations which include a maintenance contract with a qualified professional trained and approved by the manufacturer. The CRMC will evaluate the efficiency of filter components as additional information becomes available. If you have questions concerning this statement of findings and interim policy, please contact CRMC staff at (401) 783-3370.