Evaluation of Twin Lakes for Use as a Regional Detention Facility
Covington Civil & Environmental, LLC (Covington) was engaged by the City of Diamondhead to evaluate the feasibility of designating Twin Lakes as a regional detention facility and, if so, how the Diamondhead’s Storm Water Ordinance should be modified to allow Twin Lakes to be used as a regional detention facility.
The evaluation looked at three scenarios:
1. Determining the detention capacity and water surface elevations of the Twin Lakes for existing storm water flows (Note: This will also represent the detention capacity and water surface elevations for future development within the drainage basin of the Twin Lakes assuming that the existing City of Diamondhead Storm Water Ordinance requiring on‐site detention of all future developments remains in effect).
2. Determining the detention capacity and water surface elevations of the Twin Lakes for storm water flows from future developments within the drainage basin, assuming the City of Diamondhead Storm Water Ordinance is modified to not require on‐site detention for
future developments and no modifications are made to the Twin Lakes and its drainage structures.
3. Determining the detention capacity and water surface elevations of the Twin Lakes for storm water flows from future developments within the drainage basin, assuming the City of Diamondhead Storm Water Ordinance is modified to not require on‐site detention for
future developments and a berm is constructed on the south side of Diamondhead Drive West to prevent flooding of Diamondhead Drive West.
The evaluation resulted in the following recommendations:
1. If Twin Lakes is not used as a Regional Detention Facility, the existing Storm Water Ordinance should remain in effect for this area and, if desired, the City of Diamondhead may want to investigate how to lower the 100‐year storm elevation in the upper lake to prevent flooding of Diamondhead Drive West (10‐inches) during a 100‐year storm.
2. If Twin Lakes is to be used as a Regional Detention Facility (Existing Storm Water Ordinance modified to not require on‐site detention within the Twin Lakes Drainage Basin), further evaluation is needed to determine the effect the increased flow from the lower lake (Table 7) will have on drainage under Kapalama Drive, costs for any necessary improvements to drainage under Kapalama Drive or construction of a berm south of Diamondhead Drive West and possibly the staging of any improvements based on future growth within the Twin Lakes Drainage Basin.
3. Under the City’s Flood Damage Prevention Ordinance, a Community Flood Hazard Area could be designated for the area around the upper lake (south lake) which will be inundated during the 100‐year flood. If the existing Stormwater Management Ordinance
remains in effect, the Community Flood Hazard Area Base Flood Elevation should be set at 40.5‐ft (1‐ft above the 100‐year flood elevation). If Twin Lakes is used as Regional Detention Facility but no improvements are undertaken, the Community Flood Hazard Area Base Flood Elevation should be set at 40.6‐ft (1‐ft above 100‐year flood elevation). The Base Flood Elevation should be set at 41.2 if the berm is constructed south of Diamondhead Drive West.
