As noted in mid-March, we’ve been reaching out to experts for contributions on key issues — and are looking forward to featuring their commentaries as they come in.

As always, feel free to comment, and/or get in touch with us if you’d like to join our team of Guest Contributors.

Guest Commentary, submitted by

John A. Miller, P.E., CFM
Water Resources Engineer
Certified Floodplain Manager

To be perfectly blunt, I am very glad that some site engineers did not become structural engineers.  Why?  Because, if they had, we would have buildings, bridges and other structures falling down around us.  Too many site engineers do not consider Factor of Safety.  Let me explain.

Factor of Safety in structural engineering is used to compensate for variations of materials, oversight in production and assembly, construction workmanship, simplification in design and future unknown loads that a structure may experience beyond its initial design.  Think of a commercial building, where a room is converted from office space to file storage.  The dead load or weight of the contents of the room dramatically increases with the addition of dozens of file cabinets, filled to bursting.  In most cases this change of use does not lead to failure, as the original design anticipated, using a Factor of Safety, that an alteration might be made at some time in the future.    (NB: As a professional engineer, I suggest that if you are making this magnitude of change, you consult a structural engineer to verify the structure’s ability to support the increased load).

A site engineer may say that using a Factor of Safety in stormwater management design is “over-engineering,” since the consequences of failure or partial failure are not as significant.  And to boot, I have often heard a site engineer testify that his or her design produces “better than before conditions” when meeting the applicable regulations (unfortunately, for many reviews I have conducted, in truth, the design doesn’t even meet the minimum standards.)

There are a number of consequences for failing to design stormwater management systems with a proper Factor of Safety.  For structural engineers, injury or death and major financial loss can occur at the design site.  “Luckily” for site engineers and their clients, the consequences may be less clear.  Stormwater runoff is not so easy to see as a building falling down, and the damage can occur further downstream, maybe in another municipality or even another state.  Unintentional results such as stream channel erosion, degradation of water quality, or a drop of the groundwater table can all arise from disregarding the incorporation of a Factor of Safety.  In the most extreme cases, flooding can result in death and major financial loss on downstream properties.

To design with a Factor of Safety, the reviewing agency or board would be more confident with the inaccuracies in the Curve Numbers or runoff coefficients, time of concentration, the accounting of the change in soils with construction compaction and other variables used in modeling the design or the modeling technique itself.  One has to ask, has the engineer considered changes in rainfall intensity and totals and antecedent watershed conditions?  Is the design fully mimicking the preexisting conditions for peak runoff, water quality treatment and groundwater recharge through the range of future precipitation?  And what about excess runoff volume - has that been managed?  Is there sufficient Factor of Safety if one or a few of the engineer’s generalizations is off the mark or misunderstood?

So, the next time you hear testimony, even by a licensed professional engineer, that the design is “better than current conditions,” take a moment and ponder if sufficient Factor of Safety is built in to make that so.  Don’t take the regulations for granted.  Ask yourself and then the engineer - does the design adequately consider and mitigate the water resource impacts from the current proposed and future land use changes and variability in precipitation?  Is there a Factor of Safety incorporated into the design to protect the areas downstream of the site?

John A. Miller, P.E., CFM is an Associate Water Resources Engineer with Princeton Hydro, LLC in Ringoes, New Jersey.

admin Guest Commentary BMPs, peak rate, stormwater best management practices, Stormwater Regulations, stormwater system design, volume control, water quality

Since 2001, USEPAs Office of Research and Development (ORD)  has been developing a new simulation model/decision-making tool designed to answer some critical stormwater management questions, including how to determine selection and placement of BMPs at strategic locations, including:

How effective are BMPs in reducing runoff and pollutant loadings?

Where, what type, and how big should BMPs be?

What are the most cost effective BMPs for meeting water quality and quantity objectives?

The tool, SUSTAIN (System for Urban Stormwater Treatment and Analysis Integration Model) is designed for use by watershed and stormwater practitioners to develop, evaluate, and select optimal BMP options for various watershed scales with a major focus on cost and effectiveness.

SUSTAIN is designed to integrate GIS analysis and visualization, watershed/BMP simulation, cost, and system optimization.  It incorporates an innovative concept of developing a cost-effectiveness curve for watershed decision making.  In addition, an equally innovative multi-tier concept was incorporated for performing staged optimizations from smaller watersheds to a larger watershed.

The SUSTAIN Version 1.0 was released in October 2009.  Prior to this release, four workshops were held in selected EPA Regions, and additional workshops are being planned.  ORD has committed additional resources to showcase applications and perform needed enhancements to better facilitate the setup and application of SUSTAIN while improving output accuracy and credibility.

The software and technical report are available for download here.

SUSTAIN includes four key components:

(1)  a BMP site evaluation tool which uses the ArcGIS platform and user-guided rules to determine site suitability for various BMP options,

(2)  a watershed module that integrates locally derived data with watershed simulation models to predict flow and pollutant loading,

(3)  a stand-alone BMP module that performs process-based simulation, and

(4)  an optimization module that performs cost estimating and combines performance and cost data of various design alternatives.

SUSTAIN is designed to link with external models for inputs of flow and pollutant time series.  It also has internal stand-alone sub-modules for performing watershed/landscape runoff simulation, BMP process simulation, conduit routing, and stream conveyance and pollutant routing.  The framework includes a suite of system and interface capabilities.  System features emphasize the use of GIS-based visualization and support for developing watershed simulation networks that may include sequences of land parcels, management practices, and stream reaches.

Various practitioners, municipalities, and watershed groups at the regional and local levels can use SUSTAIN to address a variety of stormwater management planning questions on:

(1)  developing TMDL implementation plans,

(2)  identifying management practices to achieve pollutant reductions in an area under an MS4 stormwater permit,

(3)  determining upstream source control strategies for reducing volume and peak flows to CSO systems, and

(4)  evaluating the benefits of distributed BMP implementation on water quantity and quality in urban streams.

That’s the background and set-up.  We hope you go to the website and attempt to familiarize yourself with this remarkable new tool.

First Reaction/Review

EPA hosted a free 2-hour webcast on March 11, 2010 that provided an overview of SUSTAIN, descriptions of the major system modules, and an example application in using the SUSTAIN framework.  Speakers included Dennis Lai (USEPA), Leslie Shoemaker, and John Riverson (both from Tetra Tech, EPAs consultant on the project).  The webcast targeted watershed management professionals wanting to learn how to use and apply SUSTAIN framework for evaluation and decision-making of stormwater management options.

We participated in the webcast and have a variety of comments and questions:

PROS

1.  In so many ways, EPAs SUSTAIN (and the bright minds at Tetra Tech) constitutes a giant leap forward in the science and art of stormwater management.  Not long ago, stormwater management in most Pennsylvania municipalities either was nonexistent or consisted of figuring out how many detention basins to excavate.  SUSTAIN is a vivid demonstration of how far we have advanced.  SUSTAIN and its amazing family of linked models is awesome in its complexity and sophistication, essential if it is to accomplish its ambitious mission.

2.  Although stormwater management guidance, including PADEPs guidance in the December 2006 BMP Manual has grown much more comprehensive and complete, there remains much ”irrationality” in the selection of BMPs across Pennsylvania development projects.  Although we continue to argue that stormwater management planning/engineering/design involves both science and art, some sort of mechanism like SUSTAIN would be enormously helpful, especially in certain less “experienced” portions of Pennsylvania.  SUSTAIN could help us do better.

CONS

1.  The challenge in such complex modeling efforts is to somehow capture the full complexity of the nation’s natural systems, watersheds, and subtleties of every site, together with the full variability of so many different development programs.  This seems like an overwhelming task! As we speak, we confess that we have not been able to download and study every modeling element of SUSTAIN and assess the myriad assumptions which are being made, including assumptions which may not be valid in the modeling, depending upon natural system and watershed and site, as well as assumptions which simply may not be included in the modeling, but which might be important when designing and engineering a stormwater management plan for Development Proposal A at Site X.

2.  In one sense, our comment above is premature.  EPA and Tetra Tech have been developing this modeling tool for nearly 10 years with enormous effort.  The problem for us users is that it takes vast numbers of hours to assess the remarkable modeling package, including the vast number of assumptions which have gone into model development.  The 2-hour webcast was extremely compressed, even rushed in our minds.  We simply did not have time to digest everything that Tetra Tech was saying. Our sense is that even developing a basic understanding of SUSTAIN requires a full day of explanation and presentation.

3.  For example, if we understood Tetra Tech correctly in the webcast, it would seem that the water quality modeling in SUSTAIN very much “hangs” on work of the soil.  Although we are great believers in use of soil mechanisms for water quality renovation, the bulk of these mechanisms function for particulate-form pollutants, such as sediment and phosphorus.  For solubilized pollutants such as nitrogen forms, some organics, and others, soil-based mechanisms are not especially effective. Especially in the wonderfully expanding world of vegetative-based “green” BMPs, this aspect of water quality needs to be addressed and taken into account in this modeling (nitrogen is an important issue in so many Chesapeake Bay tributary municipalities).

Again, assuming we are understanding SUSTAIN correctly, it is incredibly important that all aspects of the modeling including all assumptions used in the modeling be comprehensive and complete. Like a chain, quality of modeling output cannot be expected to be any more accurate than the quality of its weakest link - than any of its many constituent assumptions.

4.  We need evidence of calibration and verification of this modeling.  We want to see how SUSTAIN applications would have treated existing sites which we feel have successful stormwater management plans.

5.  Lastly, and this comment is echoed above, users of SUSTAIN - whomever that might be in Pennsylvania - will have to invest substantial resources in this modeling tool for any kind of application for all the reasons discussed above. These days, who has these resources?  Certainly it’s hard to imagine individual MS4 municipalities being able to apply SUSTAIN in any context, although watershed groupings of municipalities might be able to consider SUSTAIN applications.  (In upcoming blogs, StormwaterPA hopes to report on both some interesting work being conducted by Pennsylvania Environmental Council in the Wissahickon as well as Chester County’s Brandywine Valley Association with the Chester County Water Resources Authority for the Brandywine Creek…)

Did anyone else attend the webinar?

Download the software and tinker?

Comments?

admin National, Stormwater BMPs BMPs, CSO, green infrastructure, MS4, PA BMP Manual, PA DEP, peak rate, TMDL, volume control, water quality

“Volume Based Hydrology” - A Much-Delayed Comment From the Editors

In the September issue of the Stormwater Journal, Andy Reese gives us an intelligent summary of stormwater management’s recent (and ongoing) paradigm shift - from peak rate control with detention basin solutions to control of total volumes of stormwater being discharged, using infiltration or evapotranspiration or capture and re-use and other management practice strategies including preventive Low Impact Development BMPs.

If you haven’t read the article, take the time and read it.  It’s enlightening and even entertaining (”As Mark Twain said, sacred cows make the best hamburger.”)  It’s gotten a lot of attention in recent months.  In “Volume-Based Hydrology” (or VBH, as he coins), Reese makes some provocative assertions (and know that we are barely even addressing the tip of the iceberg in our very summary comment here):

So it seems VBH is a water scarcity phenomenon

So, maybe VBH is a runoff volume reduction phenomenon.

VBH is really a stormwater pollution reduction phenomenon.

So it seems VBH is a channel erosion and habitat protection phenomenon.

So, VBH is a flood control phenomenon.

So…VBH is a floodplain management phenomenon.

Although we’re not sure that the technical arguments are as equally compelling for all of these stormwater management functions or benefits which Reese points out here (some do seem more important than others although relative importance may indeed vary with the specific situation, locality, watershed context, etc.), the point is that volume control is now known to provide much more bang for the management buck than previous strategies focusing exclusively on peak rate control.

This is a paradigm shift based on something of a revolution in our understanding of the stormwater problem itself and superior ways to prevent and mitigate these stormwater-linked problems.

It is important here to point out that the Pennsylvania Stormwater BMP Manual is substantially compatible with Reese’s VBH line of argument (he acknowledges this in the article).  The Manual’s Chapter 3, Recommended Stormwater Management Standards for Pennsylvania, sets forth an array of integrated peak-volume-quality standards which emerged as the Manual was being developed by PADEP, its consultant, and the Oversight Committee.  Assisted by the Villanova Urban Stormwater Partnership (VUSP) during the extended and arduous Manual development process, this set “recommended site control standards” emerged in many ways from thinking very similar to Reese’s VBH article discussion.  Reese develops a nice graphic showing it all as a kind of integrated set of objectives which work together to provide maximum management benefit:

This concept of integration is especially useful for so many developing Pennsylvania communities where this sudden paradigm shift in stormwater management, reinforced by state and federal mandates, is potentially too complicated and too costly and too politically challenging to embrace.  Yes, the bad news is that stormwater management is so much more complicated than we have conventionally understood it to be.

The good news is that this new “volume” focus which is designed to match pre-development and post-development hydrology allows us to accomplish so many stormwater management objectives in an integrated fashion and perhaps even reasonably cost-effective.

Use volume-based BMP strategies and related BMPs, and you can often go far in achieving volume and even peak rate (peak control for large storms is most challenging) and water quality and streambank erosion and flooding and floodplain objectives!  Many BMPs, (including many described in the PA Manual), give convincing performance across multiple objectives - clever design doesn’t require separate BMPs to satisfy management objectives.  With any degree of luck, you can kill two birds - perhaps even several birds - with one BMP stone.  Understanding the problem more completely is the first step in formulating a more comprehensive (and perhaps less complex and costly) solution.

Read the article.  Though some of this line of argument might vary a bit in other regions of the country with substantially different climate and physiography, we would argue that VBH and the discussion in the article is very applicable for so much of humid Pennsylvania.  Reese’s examples and interesting tables and graphs make sense in most of Pennsylvania.

Our only point of criticism is that this VBH paradigm shift article doesn’t in any direct way acknowledge the concept of prevention and its importance in this new management paradigm.  Preventive BMPs are given substantial attention in the Pennsylvania Manual.

The first step in comprehensive stormwater management - even before all of the vital objectives of VBH are wrestled with - needs to be ways to prevent runoff from occurring in the first place even as the same number of houses and square footage and cars are accommodated.

The complete article from the Stormwater Journal is available here.

admin Stormwater BMPs BMPs, capture and re-use, channel erosion, floodplain management, habitat protection, infiltration, LID, PA BMP Manual, peak rate, runoff prevention, stormwater best management practices, volume control

Sessions on Bay TMDL Scheduled in Six States, D.C

The U.S. Environmental Protection Agency will hold a series of public meetings to discuss the Chesapeake Bay Total Maximum Daily Load (TMDL) - a strict “pollution diet” to restore the Chesapeake Bay and its network of local rivers, streams and creeks.

EPA is scheduled to hold 14 public meetings throughout the watershed’s six states - Maryland, Virginia, Pennsylvania, Delaware, West Virginia, New York - and the District of Columbia from early November through mid-December 2009.

The Bay TMDL will establish the pollution reductions necessary for meeting Bay clean water standards. EPA officials will outline the Bay TMDL and receive public input during the sessions.  A draft TMDL will be issued in August 2010, followed by a public comment period. A final TMDL will be established by EPA in December 2010.

The PA meetings are scheduled for:

• November 17   Wilkes Barre, Pa.              2 p.m. - 4 p.m.
• November 18   Williamsport, Pa.             5 p.m. - 7 p.m.
• November 19   State College, Pa.              2 p.m. - 4 p.m.
• November 23   Lancaster, Pa.                   2 p.m. - 4 p.m.

For the specific location of a meeting in your area, visit: www.epa.gov/chesapeakebaytmdl

EPA Requests Comments on Survey for Stormwater Rule

The U.S. Environmental Protection Agency is proposing a survey to help strengthen stormwater regulations and reduce stormwater discharges from newly developed and redeveloped sites.  Stormwater discharges can harm water quality through increases in stormwater volume and pollutant loadings into nearby waterways.

Generally, as sites are developed, less ground area is available for rain to soak into, which increases stormwater volume.  This stormwater flows across roads, rooftops and other surfaces, picking up pollutants that then flow into waterways. The draft survey would require detailed information about stormwater management and control practices, local regulations, and baseline financial information.

EPA plans to propose a rule to control stormwater from newly developed and redeveloped sites and to take final action no later than November 2012.  In support of this rulemaking, EPA is proposing to require three different groups to complete questionnaires about current stormwater management practices: 1) the owners, operators, developers, and contractors of newly and redeveloped sites; 2) the owners and operators of municipal separate storm sewer systems; and 3) states and territories.

The proposed survey will be open for public comment for 60 days following publication in the Federal Register.

More information: http://www.epa.gov/npdes/stormwater/rulemaking

admin Chesapeake Bay Chesapeake Bay, flood control, peak rate, stormwater best management practices, stormwater runoff, TMDL, volume control, water quality

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?

admin BMP Manual Revision 2 year storm, curve number method, methodology, PA BMP Manual, peak rate, rational method, volume control