OCEAN OUTFALL REPAIR TECHNIQUE OFFERS REDUCED RISK AND ECOLOGICAL BENEFITS
Abstract:The City of Hollywood is responsible for the Hollywood Southern Regional Wastewater Treatment Plant, which is rated at 42 million gallons per day and discharges secondary treated effluent to the Atlantic Ocean via a 60-inch ocean outfall. The outfall is currently the primary method of effluent disposal for the treatment plant.
After the passage of Hurricane Andrew in 1992, the outfall required rehabilitation of its cover material over four discrete problem areas comprising a total of approximately 4,000 feet. An articulated concrete mat was chosen at that time to effect repair of the armor covering. Use of the concrete mats offered several advantages over the traditional armor rock, the most notable of which was a more stable cover material that was quicker to install. Adjoining mats were connected together with polyester cable, forming a contiguous repair. The rehabilitation was completed in late 1994. A subsequent inspection of the outfall was performed in May of 1997 to document the integrity of the cover material. At that time, no damage was found to either the original armor rock cover material or the rehabilitated cover material. Because Hurricane Irene, which passed through south Florida on October 15, 1999, caused an estimated 120 million in damage in Miami-Dade County alone, the City of Hollywood conducted another inspection of the outfall in March 2000.
The results of the inspection found a substantial amount of the original cover material was missing and that the outfall was visibly exposed in various areas. No visible leaks were found and all exposed pipe appeared to be in good condition. All areas previously repaired with the articulated concrete mat appeared in good condition and exhibited substantial marine growth. Based upon the inspection results, the exposure was not deemed to be extensive and totaled about 82 feet in length. The single largest area in need of repair was approximately 24 feet in length and contained one exposed pipe joint.
Options for the extent of repair/rehabilitation were developed and compared as a function of relative risk of continued cover material loss, cost and construction time. Selection of an alternative to implement depended upon available funding and judgment as to the value of the expense versus its corresponding reduced risk. The resistance of the previous repair to storm damage was highlighted. Due to the importance of the outfall as the primary means of effluent disposal, the City of Hollywood opted to rehabilitate all exposed areas as well as those with compromised cover material for a total of approximately 1,000 linear feet of proposed cover material. Again, the approach having the lowest degree of relative risk, the articulated concrete mat approach, was selected. The additional repair work was designed to tie into the previous repair work, which resulted in a contiguous repair almost 3,000 feet in length.
Proper installation of the armor mat requires a bed of gravel to protect the pipe during mat placement and to allow a gentle uniform sloping surface from the crown of the pipe down to grade. Therefore, removal of all existing large armor rock was necessary. Although the armor rock had shifted over the years, most contained several colonies of stony corals. Discussions with local regulatory officials required that a stony coral reconnaissance survey be performed prior to the start of construction. The survey found that over 3,000 stony coral colonies having a diameter greater than 10-cm would be impacted by the project. A parallel study was also performed on the concrete mats installed over seven years prior. This study found that the concrete mats had a greater density of stony coral colonies than surrounding natural reef areas and that they contained a greater variety of species per unit area.
As a result of the reconnaissance survey and taking into account the benefits of the prime habitat that would be added through the use of the articulated concrete mats, it was agreed that a total of 1,000 stony coral colonies would be transplanted from the armor rocks slated for removal to the proposed concrete mats using hydraulic cement to anchor them. The stony coral transplantation increased the cost of the project by approximately 15%. Use of the articulated concrete mat offered superior protection from future storm events for above-grade/exposed pipeline sections and provided a stable, prime substrate for stony coral growth. The transplanted corals will act as seed colonies speeding up their establishment. Should future work need to be done on the outfall pipe covered with articulated concrete mat, individual mats can be removed and later replaced after repairs have been made, leaving all marine growth intact.
Document Type: Research Article
Publication date: January 1, 2003
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