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 Curve Number Method is the currently established methodology for volume control (i.e. up to the 2-year 24-hour storm)   What other comparable methodologies exist, if any?   What is the feasibility of these methods being used/accepted by the engineering community?  (i.e. the Small Storm Hydrology Method has not caught on)

Assuming that CN Methodology remains as the established event-based methodology, does the group recommend any changes to how it is currently utilized.

1. Continue not utilizing a composite CN.
2. Continue to use a meadow assumption for existing conditions, or better.
3. Initial Abstraction:  (Ia = 0.2S)  Does it make more sense to switch to Ia = 0.05S?  (Which would lead to the need for a new curve number table)   Compare and contrast the differences.
4. Any recommendation for adjusting curve numbers for disturbed land, which is later revegetated (such as lawns)

Alternate Methodology:

It has been said that continuous modeling is better than event-based modeling, particularly for trying to mimic natural conditions.

Are we there yet? (continuous modeling)   Is there a practical way of doing this?  Accepted by engineering community?  Can we develop another control guideline (such as CG-1) that uses continuous modeling?   Can this be done in a way that most practicing engineers can understand and implement?

Possible concerns:

• Unintended ponding - WNE
• Sufficient storage for next storm
• Undersizing infiltration systems.

Rational Method (and Modified Rational Method).

The Rational Method is still very popular.  It is still being used by many engineers and municipal ordinances for peak rate designs and used in conjunction with the SCS Method for volume control.  What are the group’s general feelings/experiences with the rational method/MRM?    Many municipalities have limited its use to drainage areas of 20 acres or less.    Does the group feel that limiting its use to smaller drainage areas addresses concerns with its use?

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