Apr 11, 2007

Creating an Alternative Water Supply Through Flood Mitigation

Orange City, Fla., explores alternative storm water management practices

With more than 1,000 new residents a day pouring into Florida, many communities are struggling with water issues as Florida’s prolific aquifer is strained by the additional population. Alternative water supply projects are constantly being explored and are proving to present many benefits over standard storm water management practices.

One such project involves pumping excess runoff from Mill Lake and Marshall Park Lake, both located in Orange City, Fla., to a water reclamation facility that would provide an alternative water supply as a source of reclaimed water for the growing Volusia County and surrounding communities.

Aside from providing these communities with an alternative water supply, the project also can mitigate flooding and provide a solution to storm water attenuation requirements, which have plagued those areas in recent years. Making this project more attractive is the fact that alternative water supply systems are supported by the water management districts under Florida Senate Bills 360 and 444 and the Florida Department of Environmental Protection (FDEP) through the State Revolving Fund (SRF), which provide possible sources of funding for such projects.

History

Mill Lake is a permitted surface water management system in Orange City owned and managed by the Florida Department of Transportation (FDOT). It receives runoff from its approximately 241-acre watershed as well as runoff from additional land that is pumped to Mill Lake. As a result of changing land use within the watershed and recent hurricane seasons, properties adjacent to Mill Lake and nearby Marshall Park Lake have experienced frequent flooding.

In October 1985, FDOT was authorized to dredge Mill Lake to increase storage volume and install a permanent pumping system to discharge excess floodwater to the St. Johns River near Blue Springs Creek, which is classified by FDEP as an impaired water body. The permit limits the pumping from Mill Lake to the St. Johns River to three weeks during the year’s wet season at a maximum rate of 1.5 cu ft per second (cfs) and requires FDOT to monitor water quality at this location.

In March 1995, the original permit was modified from a fixed pumping schedule to a schedule dependent on Mill Lake’s water level. Pumping initialized when the water level reached an elevation of 17.5 ft (NGVD 29) and stopped when the lake fell below 16.3 ft (NGVD 29). The elevation at the top of the berm around Mill Lake is 18.5 ft (NGVD 29), according to the existing construction plans. A maximum discharge of 1.5 cfs continued to limit the amount of pumping to the St. Johns River.

Major flooding in the area was observed in 2004, a likely direct effect of excessive rainfall from hurricanes that year, which totaled 11.26 in. between Aug. 25 and Sept. 8 during Hurricane Frances, according to the National Oceanic and Atmospheric Administration (NOAA). On Nov. 4, 2004, an Emergency Order allowed temporary pumping from FDOT-owned Miller Pond to Mill Lake to relieve the flooding at Miller Pond. Pumping was limited to 500 gal per minute (gpm) and a total of 36.5 million gal.

Between Dec. 25 and Dec. 30, 2004, one of the two pumps responsible for discharging water from Mill Lake to the St. Johns River stopped for approximately 30 days. Pumping from Miller Pond to Mill Lake, however, continued for 20 days until the problem was discovered, causing water levels in Mill Lake to rise significantly. On Jan. 20, 2005, pumping from Miller Pond to Mill Lake was terminated.

In January 2006, FDOT was authorized to construct a 6-in. storm water force main along U.S. 17/92 that will pump treated storm water from three FDOT-owned ponds to a gravity storm sewer system that ultimately discharges to Mill Lake. The anticipated runoff pumped to Mill Lake is 7.7 acre-ft, according to permit calculations.

Flood analysis

An engineering analysis identified six sub-basins within the Mill Lake Drainage Basin. All six sub-basins discharge directly to Mill Lake or to curb inlets connected to a gravity storm sewer system along U.S. 17/92, which outfalls to Mill Lake. Basin parameters for all six sub-basins are listed in Table 1. Maximum stages and discharges for Mill Lake as well as inflows to Mill Lake from the surrounding watershed are listed in Table 2.

According to Table 2, Mill Lake can contain the smaller 25-year, 24-hour storm event, but not the larger 25-year, 96-hour; 100-year, 24-hour; or 200-year, 24-hour storm events. The model demonstrates that Mill Lake was not designed to handle large storm events such as Hurricane Frances during the 2004 hurricane season, which was approximately equivalent to the 25-year, 96-hour storm event.

According to Figure 1, the total amount of water pumped to Mill Lake exceeded the amount of water discharged for two periods in December 2004 and January 2005 around the time when one of the two pumps responsible for discharging water from Mill Lake to the St. Johns River had stopped.

Alternatives analysis

As a result of the engineering analysis, several alternatives to mitigate flooding were recommended, including: 1) Excavate Mill Lake; 2) Acquire a new pond site; 3) Pump runoff from Mill Lake to the Southwest Regional Water Reclamation Facility (SRWRF), which also will create an alternative water supply of reclaimed water for Volusia County; 4) Pump runoff from nearby Marshall Park Lake, which also has experienced flooding, to Mill Lake and then from Mill Lake to the SRWRF; 5) Find a new discharge location for Mill Lake; 6) Increase pumping to the St. Johns River, which is an impaired water body; and 7) Install a drain well adjacent to Mill Lake, which involves the risk of polluting the groundwater. A cost analysis of the most feasible alternatives is shown in Table 3.

Additional studies were conducted to determine the amount of reclaimed water and revenue generated by applying alternatives 3 and 4. The estimated annual runoff volume for the Mill Lake and Marshall Park Lake watersheds was determined using the U.S. Geological Survey’s (USGS) average monthly streamflow information for Spruce Creek, Fla., which has similar hydrologic parameters, a relatively long period of record and close proximity to Mill Lake and Marshall Park Lake.

Assuming one-fourth of the annual runoff volume would be made available for reclaimed water due to infiltration and groundwater table elevations, it is estimated that the Mill Lake and Marshall Park Lake watersheds could annually produce 200 million gal and 100 million gal of reclaimed water, respectively, which could be sold to Volusia County. At an estimated $1 per thousand gal of reclaimed water, Mill Lake and Marshall Park Lake runoff could annually provide $200,000 and $100,000 of additional revenue, respectively, for Orange City.

These values are approximate and will likely vary from year to year based on the amount of precipitation and land-use changes within the watersheds. Therefore, alternative 4 provides the city with average annual revenue of $300,000, providing a 12-year return on the investment, assuming the city was to fund 100% of the project. In addition, pumping water from other storm water ponds to Mill Lake would likely produce additional reclaimed water and revenue.

Recommended alternative

Based on the analyses above, the project team recommended alternative 4. Although alternative 3 is less costly, alternative 4 would alleviate flooding at both locations, provide Volusia County with a more plentiful reclaimed water supply and provide the city with the most revenue.

In addition, redirecting the existing Mill Lake discharge from the St. Johns River to the water reclamation facility will reduce the river’s pollutant load at Blue Springs Creek. FDOT will likely transfer ownership of Mill Lake to Orange City, relieving FDOT of its responsibility to maintain Mill Lake’s discharge pumps to the St. Johns River and monitor the river’s water quality at Blue Springs Creek.

In January 2007, the City Council of Orange City approved proceeding with preliminary design for the recommended alternative to pump runoff from Marshall Park Lake to Mill Lake and from Mill Lake to SRWRF.

Funding

Several agencies may be willing to help fund the design and construction of the alternative water supply system. SJRWMD has recently initiated the Florida Water Protection and Sustainability Program, which includes a cost-share funding program under Senate Bills 360 and 444. Selected water reclamation projects are reimbursed up to 20% of their construction costs.

The Department of Environmental Protection (DEP) also offers the Drinking Water State Revolving Fund (SRF) Program, which provides selected applicants with low-interest loans to design, permit and construct public water facilities. The proposed water reclamation system in Orange City would be eligible for the SRF program.

Other agencies, such as Volusia County and FDOT, also may offer funding assistance. For instance, an agreement in which Volusia County compensates Orange City for the reclaimed water it receives from Mill Lake could provide a portion of these additional funds.

As populations increase, water resources become strained and environmental regulations tighten, alternative approaches such as Orange City’s provide a model for innovative storm water management.

About the author

Jeff Glenn is Water Resources Service Group leader and Elizabeth Bartell is Water Resources engineer at Reynolds, Smith and Hills, Inc. (RS&H). Jose M. Ortiz, PE (RS&H) and Paul Johnson (City of Orange City) also contributed to this article. Glenn and Bartell can be reached at 407/893-5800 or by e-mail at [email protected] and [email protected].

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