The Lake Asbury Municipal Services Benefit District (LAMSBD) is a Dependent Special District organized pursuant to Chapter 86-392, Laws of Florida. LAMSBD was created to assume partial ownership of three lakes and three dams in Central Clay County, Florida.

The three earth dams were constructed during the period from 1965 to 1969 as part of the Lake Asbury Development. The three dams are designated as the North Lake Dam (NLD), the South Lake Dam (SLD) and the Lake Ryan Dam (LRD). The dams impound Lake Asbury, South Lake Asbury, and Lake Ryan, respectively. These earthen dams have served satisfactorily since their construction with only routine maintenance and repairs.

In 1978, in the aftermath of the failure of the Teton Dam, the three dams were inspected by the U.S. Army Corps of Engineers, Jacksonville District. The reports prepared for each dam generally characterized the dams as falling into an "unsafe condition" category. The majority of unsafe conditions identified by USACE involved redundant capabilities of the control structures on each dam.

Since that inspection, it has been the goal of the community to correct the deficiencies identified by USACE and subsequent engineering firms. In the past, funding for the projects has been an obstacle. During the 2009 Legislative Session, LAMSBD asked the Florida Legislature to amend the District Charter to allow for a local referendum to raise the assessment in order to fund improvements. A Local Bill was passed unanimously in the Legislature, however the Governor vetoed the measure. LAMSBD repeated its attempt to pass similar legislation in 2010 and was successful in getting the measure signed into law.

During the summer of 2010, a referendum was held in Lake Asbury and the voters of the District voted overwhelmingly to increase their assessment to fund improvements to the dams.

The LAMSBD Board of Trustees has not formally adopted a scope of work for the improvements, but Legacy Civil Engineers has recommended the following improvements:

1. Construction of an Emergency Overflow Spillway on the South Lake Dam
2. Construction of an Emergency Overflow Spillway on the Lake Ryan Dam
3. Modifications and improvements to the existing control structures at the South Lake and Lake Ryan Dams
4. Installation of additional seepage control devices on the South Lake Dam
5. Installation of erosion and scour protection of the upstream face of the South Lake Dam

These improvements will help to ensure the safety and durability of the dams in Lake Asbury for many decades to come.

Dams create large impoundments of water, such as lakes and reservoirs, by receiving fast-flowing streams of water, storing volumes of water, then releasing the water slowly downstream. During rain storms at any of the lakes in Lake Asbury, stormwater enters the lakes quickly and leaves the lakes a much slower rates, which helps keep the lakes full. With dams, you care less about how fast water enters the lake than how much storage volume is available in the lake before the storm. This is a concept that is often poorly understood by regulatory and county staffs that don't routinely deal with dam impoundments.

When the lakes in Lake Asbury were first built, there was only open land in the entire basin that drained through what has become Lake Asbury. There were no homes, there were no large parking lots, there was no network of paved roads. Back then, rainfall either soaked into the ground (infiltration), or flowed through a network of small creeks to reach Black Creek. When the dams, lakes and control structures were designed, nobody envisioned the housing bubble that would lead to massive construction of homes, businesses, and roads in the area. The construction boom that ensued in the early 1990s led to more impervious area which reduced infiltration and increased volume of water entering the lakes.

When the lakes, dams and control structures in Lake Asbury were designed and constructed, it appears that they were well-designed to the best standards of the day, and could safely contain and control the runoff from the 100-year storm event. The construction of Seminole Village, west of Lake Asbury, greatly increased the basin area that drained to Lake Ryan, which reduced the capacity of that system to barely control runoff from the 25-year storm event. Increased impervious areas in the subdivisions along Henley Road compromised the ability of the South Lake Dam to control runoff to the older standards. Since Lake Asbury is controlled by multiple control structures, including a spillway, the North Lake Dam has always exceeded the requirements of the old standards.

NOTE: Legacy Civil Engineers uses the naming standards for the dam structures that have been used by every engineer that has studied and reported on the dams. Hence, the North Lake Dam (aka the Lake Asbury Dam or North Dam) is called the North Lake Dam in order to maintain historical engineering consistency.

When the Teton Dam failed in the 1970s, there was a nationwide movement to improve dam safety by increasing the standards for impoundments. Passive (e.g. spillway) control structures were mandated as redundant (backup) control devices. Design storm runoff was increased from the 100-year storm to the Probable Maximum Precipitation (PMP). The 100-year storm in the Lower Basin involves 11.0 inches of rainfall in a 24-hour period and has a 1% probability of occurrence. The PMP storm in the Lower Basin involves over 31 inches of rainfall in a 24-hour period and has an occurrence probability of one-in-a-million.

Legacy Civil Engineers does not believe, at this time, that it is cost -effective to improve any of the District's dams to the modern PMP standards, although Legacy Civil will provide a formal analysis of this as part of the Preliminary Engineering Report. Our initial Benefit Cost Analysis (BCA) of retrofitting the dams to a PMP standard indicate that the costs would be beyond the means of the District and that the benefits gained under a return-probability of one-in-a-million would be very low on a Present-Worth basis. Nonetheless, it is a prudent decision-making practice to offer this alternative.

During meetings with SJRWMD staff, attended by Trustee Pertucci, Legacy Civil expressed concerns about permit conditions that might require retrofitting the existing dams to current SJRWMD standards. SEE ATTACHED MEMORANDUM. The senior SJRWMD staff in attendance agreed that the public safety benefits of designing and permitting improvement projects to standards less than the PMP exceeded the risk of the "do-nothing" alternative that might arise if project costs exceeded the District's funding ability. The agreement that arose from that meeting was that the District would provide risk-analysis of delivering improvements that were less than the PMP standard.

A more complicated issue with regulatory standards arises as a result of discharge attenuation standards: the so-called pre-post rules. Under these rules, stormwater discharge after construction (measured in cubic feet per second) cannot exceed the stormwater discharge prior to construction for a design 25-year storm. Here is why this is a problem that will require some thought in the District's decision-making process:

1. The Normal Water Level (NWL) in South Lake Asbury is about 44.3 feet NAVD 88 (North American Vertical Datum 1988).
2. The lowest point on the top of the South Lake Dam is 49.1 feet NAVD 88.
3. Under existing conditions, the South Lake Level will rise to about 46.3 feet NAVD 88 during a 25-year storm.
4. In order to meet the pre-post rules, the bottom of a new spillway would need to be set at, or above, 46.3 feet, which leaves only 2.8 feet of vertical clearance between the top of the road and the bottom of the spillway. In that 2.8 feet of vertical clearance, you would need to construct a sturdy top for the spillway, and a suitable base for the road, which leaves very little vertical opening for a spillway.

Legacy Civil Engineers brought these concerns to the attention of the SJRWMD staff, who suggested an option of elevating the top of the dam. This is another alternative that will need to be discussed in the Preliminary Engineering Report. However, Legacy Civil prepared SJRWMD to accept that this might not be a viable alternative due to structural concerns with the existing earthwork.

Trustee Huggins presented yet another alternative to Legacy Civil Engineers, where the bottom of the spillway would be set at the existing NWL. His point is valid: a spillway at this elevation would create a completely "passive" overflow system that avoids the potential of flooding if somebody fails to open the valve on the existing control structure. The problem with this alternative is that SJRWMD might not consider permitting this alternative, since the post-construction runoff would be huge.

During the August meeting, SJRWMD did agree to issue a permit with a 10-year duration. This would allow LAMSBD to construct the projects on a pay-as-you-go basis, should they elect to do so. During the Preliminary Engineering Report phase of this project, decisions on project phasing need to be made by the Board of Trustees, since these decisions will need to be incorporated into the project design and into the permit conditions.

In the next page, we are going to discuss existing conditions that will affect project decision-making.

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