Coal Brook Stream Restoration Terence J. Ostrowski, P.E. Engineer – Stormwater Management and River/Stream Hydraulics Section Borton-Lawson Engineering Abstract The Coal Brook watershed has a history of uncoordinated and haphazard development. In the lower portions of the watershed, the stream channel has been significantly altered from its original course by prior coal mining activities and development. At a point approximately half way through the stream reach, the channel had been completely filled in by a 40- foot high embankment, thus preventing flows from the upper watershed from reaching the lower portions. During significant storm events, runoff from the upper watershed ponds behind the blockage and slowly percolates into the ground, contributing to the underground mine pools. A recent development boom has brought this problem to light since several businesses are interested in locating on lands upstream of this blockage. Due to the presence of numerous undersized culverts and bridges, removal of the blockage would threaten downstream areas with flooding. Several potential solutions to this problem were investigated. It was found that replacement of downstream culverts with adequately sized structures was improbable due to dense development in this area. Therefore, a solution to open the blockage and divert excess storm flows via underground conduit to an available constructed wetland area was investigated. This solution provides several benefits, which include not only the elimination of the ponding area, but also reduction of infiltration to the mine pools with a resultant decrease in Acid Mine Drainage (AMD) to the Susquehanna River, reduction of the flooding hazard to downstream areas, and enhancement of a hydrologically undercharged wetland area. Existing Field Conditions The Coal Brook watershed is typical of many watersheds in the coal mining regions of Luzerne County. The headwaters of the 2.6 square mile watershed begin as undisturbed forest lands in the Wilkes-Barre Mountains outside the limits of the coal measures. However, before the stream reaches developed areas on the northwest side of Interstate 81, the stream enters the coal measures and all base flow is lost due to infiltration into the deep mine pools. In this area, normal stream flow vanishes either through cracks in the streambed or subsidence created by weakening mine workings. From this point on, the channel flow is intermittent and limited to period of stormwater runoff from the developed areas downstream of Interstate 81. Along the lower Coal Brook reach, the channel 1 Figure 1 - Coal Brook Watershed Map has been severally impacted by prior strip mining and commercial development. Diverted long ago from its natural course, the channel has been enclosed it some parts by long reaches of culvert running under parking lots and buildings. In many cases, these culverts are significantly undersized, since water was rarely seen in the channel before the commercial development occurred. 2 Approximately 0.6 miles downstream of Interstate 81, more than half of the drainage area (1.5 sq. miles) is impounded behind a forty foot fill embankment, apparently created when a large debris pile was level in the early 1980s. Due to this blockage, all runoff from the upper watershed eventually infiltrates into the underground mine pools. The mine pools are connected by a series of underground mine tunnels which make up the major mine pool complexes of the Wyoming Valley. Prior studies have determined the water that infiltrates in this portion of the valley the Southeast Complex Lower Mine Pool. Coal Brook d Figure 2 - Wyoming Valley Mine Pool Complexes Outflow from the lower mine pools are controlled primarily by a series of boreholes located in the adjacent Solomon Creek watershed. The boreholes were originally drilled into the deep mine workings as a way to control large fluctuations in mine pool levels. These fluctuations, due in part to an influx of surface water during storm events, create pressures in the underground mine workings that potentially lead to ground subsidence in the areas underlain by the workings. While the boreholes do their part in controlling mine pool fluctuations, the discharged mine water is highly acidic and contains large concentrations of metals, such as iron and manganese. This discharge is commonly referred to as Acid Mine Drainage (AMD). Solomon Creek, the receiving body of the borehole discharge, is encrusted with orange iron precipitate downstream of the outfall. Below this point, the water quality in the receiving stream is severely degraded. 3 Figure 3 - South Wilkes-Barre Borehole Figure 4 - Receiving Stream below Borehole Below the fill embankment, the Coal Brook channel was diverted from it natural course to flow parallel Mundy Street. The channel slope is very steep in this section, ranging in slopes from 3 to 4 percent. Culm banks (refuse piles of unusable coal and rock), remnants from the old coal workings, extend from the channel overbank and reach heights of over 50 feet. Along this reach, the channel carries only local storm runoff from Mundy Street and the culm banks, until it combines with discharges from heavily developed commercial areas along the east side of Mundy Street. The confluence of the two sources occurs midway through an existing 72- inch corrugated metal pipe near the Kranson Men’s Clothing Store. Immediately below the outlet of this culvert, the stream crosses Mundy Street through an arch culvert that has been severely clogged by rock and debris from the upstream culm banks. Below this point, the channel is open for another 100 feet before it is enclosed in a 150 foot underground system beneath the existing Jiffy Lube parking lot. This system begins as an old stone arch culvert and transitions to a 60- inch corrugated metal pipe about midway through. Figure 5 - Channel Along Mundy Street Figure 6 - Mundy Street Culvert (Culm banks on left) 4 At Spring Street, the channel enters a dense residential section of Wilkes-Barre City. The channel banks in this reach are steep, however the profile is very mild, which results in heavy deposition of silt and rock washed off from the upstream culm banks. Two bridges cross the channel at Spring and Scott Streets. Silt accumulation beneath the Spring Street Bridge has severely reduced the capacity of this structure. The Scott Street Bridge is currently being replaced by a structure sized to convey the 100- year flow. In the open channel between Spring and Scott Streets, a sanitary sewerline follows the channel with several manholes located at the bottom of the stream bed. Below the Scott Street Bridge, the channel enters a 5ft by 5 ft box culvert which encloses the channel underground for a length of approximately 1000 feet. The majority of this pipe lies underneath the parking lot of McCarthy Tire and it is presumed that a portion of the pipe runs beneath an existing garage. Part way through this system, the box section transitions to 60- inch corrugated metal pipe. The bottom of the corrugated pipe has been corroded significantly over the years, most notably near its outlet, where the pipe has partially collapsed. Figure 7 - Channel Below Spring Street Figure 8 - Box Culvert Below Scott Street After the channel exits the corrugated pipe, it is again diverted along the south side of Wilkes-Barre Boulevard. From this point on, the channel has adequate slope and capacity to convey storm flows within its banks until it joins the larger Laurel Run. Approximately 0.3 miles downstream of this point, Laurel Run joins the main stem of Mill Creek before draining into the Susquehanna River. Approach to Alternatives The pressure of impending development in the Coal Brook Watershed required a review of short- term solutions to the capacity problems of Coal Brook. In theory, the short-term alternatives, if cost effective and technically feasible, would allow development to proceed while funding for the longer- term solutions was obtained. Any long-term alternatives presented would accommodate a 100-year flood for Coal Brook. These alternatives would ideally reduce the seepage of Coal Brook flows into underground voids and the mine pool. This will stabilize future groundwater conditions and reduce potential hazards such as 5 acid mine drainage and subsidence potential. Furthermore, the water presently ponding in the area of the fill embankment is immediately upstream of a newly built 8,500-seat arena complex. Ponding of water behind the embankment is actually higher than the Arena concourse and event floor and potentially threatens the stability of the structure. Short Term Alternatives Many short-term alternatives were considered. Three were developed for detailed evaluation. Basic engineering considerations that governed the evaluation of alternatives are as follows: • Alternative 1 - Channel Diversion to the Highland Park Basin. An overflow storm sewer would convey stormwater ponding above a specified elevation to a nearby stormwater basin near the newly built arena. When the capacity of the overflow sewer is reached, a secondary overflow pipe would convey the excess to the downstream Coal Brook channel. This overflow would only occur during very large storms. To relieve the excess volume in the receiving stormwater basin, a pump would be installed to convey the diverted flow back to the original channel, below the fill embankment. The pump discharge is limited due to capacity problems in the downstream reaches. COST - $3,000,000 • Alternative 2 – Upgrading of Downstream Bridges and Culverts This option replaces the most severe restrictions in Coal Brook, the Jiffy Lube culvert and the Kidder Street system to allow a 25 year storm event to pass. The fill embankment would be utilized as a detention basin where most storm flows would be stored. Larger storms would discharge overflow to a proposed overflow pipe through the embankment to the lower Coal Brook channel. A special condition of this approach would be lining the slopes of the detention basin with clay to prevent horizontal migration of water downstream to the area of the Arena.