As previously noted in our September posts, PA DEP has convened a BMP Manual Revision Committee. In their notification to attendees, they noted that

… recommended updates may include edits to existing portions, complete omissions, or additions of new material.  All changes much be justified through acceptable principles of engineering or science.  If unable, to do so, subgroups may bring new ideas before the bigger committee for consideration and feedback.

Following is PA DEPs statement on the focus of this  Sub-Group, as presented at the first Committee Meeting:

The BMP Manual needs more emphasis on Volume Reduction Computations for vegetated systems such as swales, rain gardens, green roofs, constructed wetlands, naturalized basin, soil amendments, landscape restoration, floodplain restoration, etc.

Evapotranspiration accounts for more than half of the annual water budget in natural areas.  However, it is seldomly included in SWM designs or computations.  Can the  group develop an acceptable method or model that takes ET and the other benefits of vegetated systems into account.  (Also see Subgroup Topic B)

• Does the BMP manual provide sufficient info for native plants and their incorporation into vegetated BMPs?   Where are the gaps and how can they be addressed?   Should the planting recommendations for rain gardens and other vegetated BMPs be modified?  (i.e. plant density, spacing, species selection, etc.)  How and why?
• How can the BMP Manual be updated, as a whole, to provide a better focus on green infrastructure?  Are there areas that conflict with Green Infrastructure initiatives?
• What additional incentives should be incorporated into the manual to encourage green infrastructure and vegetated BMPs?
• What additional credits (or modification to existing credits) should be incorporated in the manual for “green” and non-structural BMPs?

Vegetated swales vs. underground piping.  Intuitively, we know that runoff traveling across a stable and vegetated channel has more opportunity for volume control and water quality treatment than with piping.  Since there are no incentives for volume reduction, practitioners seldomly make the effort to provide vegetated swales unless they are trying to meet water quality protection guidelines.

• What specific volume reduction criteria or other credits can the subgroup develop for vegetated swales?  (above and beyond the credit that is given with check dams)
• What has been done in other states and can be utilized in PA?  (i.e. Washington State, Delaware, etc.)

Some vegetated BMPs require a different modeling approach.  For instance, Green Roofs.   CN methodology has been utilized for Green Roofs, but this is more of a “bastardization” of the CN method since green roofs do not behave like natural ground.   (e.g., S, Ia, etc.)   There has been a lot of research done with green roofs in recent years which hopefully can shed light on this issue and provide designers with an approach for computing both volume and peak attenuation - as well as water quality.

admin BMP Manual Revision BMPs, constructed wetlands, floodplain restoration, green infrastructure, green roofs, naturalized basins, non-structural BMPs, PA BMP Manual, rain gardens, vegetated swales, volume control