Debits & Credits Make Dollars & Sense

April 10, 2008

About the author: Vincent W. Davis, P.E., is storm water engineer for DelDOT. Davis can be reached at 302.760.2180 or by e-mail at [email protected].


In 1991, DelDOT found itself subject to a new state law and regulations requiring the treatment of storm water pollution (7 Del. C., Ch. 40) as well as the control of flooding associated with the effects of development, highway widening and increased impervious surfaces.

The new environmental law was to be implemented through the Delaware Department of Natural Resources and Environmental Control (DNREC), but a provision in state regulations allowed the delegation of most aspects of this regulatory program to local governments and other state agencies. With DNREC oversight, delegated agencies were granted the authority to design, review and permit their own projects provided internal checks and balances were put in place.

Why Bank on Banking?

DelDOT quickly got on board with the notion of self regulation and applied for delegated agency status. It had its detractors both inside and outside the department, but the ability to determine review times and implementation strategies won the support of upper management and elected officials clamoring for projects to move forward. By 1993, DelDOT had added storm water management ponds to almost all of its projects to mitigate the effects of highway runoff for quantity and quality; this had an immediate and stark impact on project cost and public acceptance.

In some cases, the public lined up against storm water ponds, and in others they demanded more be done. This was especially true in areas with a history of flooding. On projects with developed rights-of-way (ROW), DelDOT purchased homes, relocated residents and razed buildings in order to make room for storm water ponds, bringing about exorbitant economic and social impacts.

Storm water impacts associated with highway projects were posing unique challenges. Because long linear projects cut across multiple watersheds, they create many small drainage areas that, under state regulations, need individualized storm water management control. Retention ponds, being the preferred best management practice (BMP) under the regulations, proliferated.

By 1995, DelDOT had accumulated enough documentation to make a strong case to DNREC that the new program may not be working as well as intended on highway projects. After considerable discussion and negotiation, DelDOT and DNREC signed a 1996 memorandum of agreement (MOA) regarding storm water banking.

The MOA is often referred to as storm water banking because implementation is tracked using a system of debits and credits as accrued per watershed. In Delaware, DNREC has officially delineated and defined 45 watersheds. Each watershed is a separate bank account.

The MOA established criteria so that if DelDOT demonstrated it could not provide storm water quality management on a given project, the deficit thus created would be mitigated on another DelDOT project or an existing road, highway or bridge within the same watershed. It also states that the deficit can be mitigated in another watershed determined by DNREC to be in greater need of water quality control.

The MOA is for water quality purposes only. Water quality increases have to be controlled on site, in accordance with the latest version of Delaware sediment and storm water regulations.


Under the MOA, DelDOT immediately found it easier to deal with certain types of projects—those with confined ROW and minor roadway widening occurring within an existing ROW. Meeting storm water management requirements was no longer a stumbling block for this type of work.

DelDOT dutifully kept track of its debits from difficult projects and built credits into others. To date, the department has an overall credit of 39.15 acres. Seven watersheds have credits and six have debits. For the watersheds that have debits, none is greater than 0.55 acres.

Credits and debits are counted on a per-project basis, with DNREC maintaining oversight of the MOA. Account reporting of credits and debits is audited by DNREC every three years, when the DelDOT storm water section has to renew delegation authority.

Storm water banking has proven a useful tool to help keep projects moving forward and batch the impacts from many small projects into larger, more meaningful retrofits. For instance, it is standard practice that, for safety purposes, DelDOT add a narrow-paved shoulder when it paves and rehabilitates rural roads. Shoulders added through the Pave and Rehabilitation Program are typically 1 to 2 ft wide, fitting within an existing ROW.

Under the MOA, it is more feasible to bank the added impervious area as a debit and treat it at another time rather than build storm water ponds every time a road is paved. In turn, the Pave and Rehabilitation Program sets aside a portion of its funds each year to support retrofit and restoration work.

Banking in Action

DelDOT has constructed many highway projects incorporating storm water credits to account for debits taken on other projects (the conventional approach to banking). The department has participated in five stand-alone projects aimed solely at improving water quality; they would not have been possible without the MOA.

One undertaking was a 2,500-ft stream restoration of Pike Creek in New Castle County, Del. This project was done in conjunction with DNREC; improved agency partnerships focused on resource protection are another MOA benefit.

The selected reach of Pike Creek was experiencing downcutting and excessive bank erosion from increased storm water flows. The degradation contributed to high sediment loads, which were compromising an aquatic habitat and negatively impacting water quality. The project entailed a Rosgen-type stream channel redesign. The goal was to construct a stable channel to restore the aquatic habitat and mitigate adverse effects of rainfall event runoff. The project was completed in 2001 and has proven successful.

Another project involved the design and construction of a sand filter at a park-and-ride in Bear, Del. This was approximately a 2.3-acre impervious site with high traffic volumes. It was drained by a closed system with no storm water quality treatment. With funds available from a storm water banking, plus National Pollutant Discharge Elimination System (NPDES) MS4 Permit Program funds, a cast-in-place Delaware sand filter design was installed in 2003 to treat runoff before it entered a local stream.

10 Years Later

Delaware’s storm water regulations have been updated several times since the inception of the storm water banking MOA. Under reviews of the DNREC sediment and storm water regulations from 1991 to 2002, the preferred way to treat for water quality was through the use of wet extended detention ponds. From 2002 to 2005, DNREC encouraged other storm water quality treatment methods, including bioswales and bioretention areas.

It has long been known by practitioners that swales and filter strips work to remove pollutants, but there have not been good quantitative tools available to help designers meet regulatory requirements. In January 2004, DNREC introduced the Delaware Urban Runoff Management Model, which provided a computational tool allowing designers and regulators to implement green-technology BMPs (i.e., bioretention, bioswales, infiltration trenches and filter strips) in lieu of ponds. With this program, designers could input more detailed and site-specific information for a particular BMP, making it easier for the designer and reviewer to determine which green-technology BMP would be effective.

In 2005, DNREC updated its regulations and required the use of green-technology BMPs, making them the preferred approach. The effect on storm water banking has been profound; reliance on the practice has been reduced because green-technology BMPs tend to fit within available ROW, whereas ponds do not. It has also brought DelDOT into conformance with standing regulations in contrast to the storm water banking approach, which is a method of last resort requiring the issuance of a variance from the regulations.

Into the Future

DNREC is in the process of drafting major revisions to storm water regulations, which should be completed by 2009. One of the probable outcomes is that storm water management will move from a site-specific approach to a watershed approach. Since the MOA already addresses storm water quality banking on a watershed-wide basis, it is assumed there will not be any significant changes in this area.

One area that will most definitely change is the way in which storm water quality is measured. Presently, it is measured as 80 percent removal of total suspended solids. With the implementation of NPDES Phase II requirements, pollution control strategies will be implemented for each watershed, mirroring total maximum daily load limits for certain nutrients.

This could affect the MOA in that storm water quality may only be applied to a particular watershed based on specific pollutant removal needs. Likewise, balancing storm water credits and debits across watersheds would be complicated by a need to have comparable pollutant removal needs in both watersheds.

In the future, it is certain that storm water pollution will persist, and our tools to deal with this will continue to evolve. Storm water banking has proven a useful tool for DelDOT and will likely remain a resource to turn to for difficult projects.

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