St. Joseph Hospital is in the middle of a $10 million expansion that will provide a state-of-the-art health-care facility for greater Bangor-area residents.

As part of this expansion, the hospital required engineering expertise to help obtain the necessary local and state environmental permits. Hospital officials contacted the James W. Sewall Co. to help with this process. In turn, the Sewall Co. contacted Bruce Crawford, P.E., to help expedite this important project.

This team started in earnest in late June 1995 and prepared all the site-related review and construction plans and secured the necessary permits by summer’s end. The City of Bangor, through its engineering and planning offices, was granted reviewer status by the Maine Department of Environmental Protection and was instrumental in streamlining the permitting process.

What made this project so challenging was that the site was essentially fully developed, with limited opportunities for siting new parking areas and storm-water detention facilities. The Maine Site Law requires that storm-water flows leaving a developed site shall not be greater in magnitude than the peak flows that existed in the predeveloped condition, or circa 1970 at the hospital.

Sewall Co. aerial photographs dating from 1970 were used to determine the predeveloped conditions, and these were compared to the proposed 1995-96 expansion. There were considerable changes in the past 25 years, the net effect being that most of the new site would be impervious and would tend to shed most rain water that fell on it. This created a challenging problem as to where to site the storm-water detention structures to reduce peak flows, but would not interfere with hospital operations or aesthetics.

The most challenging aspect of the project dealt with the northern section of the site. There was little room for storm-water detention structures, since most land was already occupied by parking lots, roads, and hospital buildings.

The solution to this problem was to divide the required detention volume into two separate areas and to redirect some existing storm sewers into different parts of the Broadway system. One detention area was designed as a long, shallow pond along the back edge of the parking lot near the Women’s Center, while the other one was to be sited near the main entrance to the hospital off Broadway.

Because of restricted space in the latter area, two low concrete-and-brick retaining walls were designed along the sides of the pond to increase the depth while minimizing the total footprint of the pond. To reduce the pond’s visual impact from the hospital’s main entrance, it was designed as a dry surge pond that will be landscaped and routinely mowed.

Instead of the conventional flow-through type detention pond that tends to remain wet much of the year, this detention area was designed to only receive storm-water surges during larger, less frequent storms. This innovative design required two procedures:

Modifying an existing storm-water catch basin and outfall by designing a hydro-brake into the existing 21-inch outfall pipe that drained to the Broadway system;

Connecting this existing basin to the proposed surge pond with a single 15-inch pipe.

During most storms, the 7-inch hydro-brake, which is a prefabricated stainless-steel insert fitted into the existing 21-inch outfall pipe, will handle the storm flows without restriction and allow all the flow into the Broadway system. During larger storms, the flows would be restricted by the hydro-brake and would back up through the new 15-inch pipe into the surge pond. Once the peak flows subside, this same pipe would then drain the excess water from the pond back into the basin and into the Broadway system.

This arrangement was more acceptable to hospital officials, since the pond will not be constantly wet as many flow-through ponds are, but will instead be landscaped and easily maintained. This is an aesthetically important point, since the detention area will be located next to the hospital’s main entrance.

Bruce Crawford, P.E., is a consulting engineer. John J. McCormack, P.E., is the engineering manager for the John W. Sewall Co.