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DRAFT_ImperviousAreaAnalysisMemoDRAFT 08505-003-02 1 November 2018 Impervious Area Drainage Impact Analysis TECHNICAL MEMORANDUM Impervious Area Drainage Impact Analysis TO: Scott Williams, Public Works Director FROM: Brian Icerman, PE; Jarrod Hirneise, PE DATE: November 8, 2018 SUBJECT: Impervious Area Drainage Impact Analysis Jones Edmunds Project No. 08505-003-02 1 EXECUTIVE SUMMARY The City of Atlantic Beach has experienced an increasing amount of impervious area on residential lots, particularly in the area east of Sherman-Puckett Creek and south of Saturiba Drive where larger lots are commonly being subdivided into multiple lots with additional impervious area. The City is concerned with how this increase in impervious area and runoff will impact the existing stormwater collection system in these areas. The City’s development code currently allows residential lots to be 50 percent covered by impervious area. We analyzed the impact of increasing impervious coverage on the City’s stormwater collection system and how mu ch of that impact could be mitigated by reducing the allowable impervious area on residential lots to 40 percent. Local-scale surface flooding worsens due to increases in runoff volume and runoff rate from additional impervious surface coverage. The additional runoff adds stress to local-scale drainage features that have already met or exceeded their hydraulic capacities. Systems that are already at or exceeding capacity are prone to creating nuisance flooding. Ultimately, surface flooding will occur more frequently and at greater depths as impervious surface coverage increases. The additional stress on the drainage system will cause more frequent instances of nuisance flooding in roadways and on private properties. With a development code allowing 50 percent impervious area versus 40 percent, the City’s stormwater system will require more frequent maintenance because of the displacement of sediment created from increased flows and the deposition of that sediment in the stormwater system. DRAFT 08505-003-02 2 November 2018 Impervious Area Drainage Impact Analysis 2 INTRODUCTION The City of Atlantic Beach has experienced an increasing amount of impervious area on residential lots, particularly in the area east of Sherman-Puckett Creek and south of Saturiba Drive where larger lots are commonly being subdivided into multiple lots with additional impervious area. Additional impervious area reduces the ability for stormwater to infiltrate into subsurface soil storage, increasing the volume and rate at which stormwater runoff is generated. The City is concerned with how this increase in impervious area and runoff will impact the existing stormwater collection system in these areas. The City’s development code currently allows residential lots to be 50 percent covered by impervious area. We analyzed the impact of increasing impervious coverage on the City’s stormwater collection system and how much of that impact could be mitigated by reducing the allowable impervious area on residential lots. Jones Edmunds previously developed a hydrologic and hydraulic (H&H) model of the City’s stormwater system using Streamline Technologies Inc. Interconnected Pond and Channel Routing Version 4.03.02 (ICPR4) as part of the City of Atlantic Beach Stormwater Master Plan Update project. The model was developed at the scale appropriate for evaluating potential stormwater improvement projects that will be included in the City’s Capital Improvement Plan (CIP). The original model did not contain enough detail to quantify the impacts of increasing impervious areas on local-scale stormwater collection systems. To analyze the impacts of redevelopment we selected a representative basin within the existing model, increase the modeled level-of-detail within that basin, and run model simulations to show the impact of increased impervious area on the stormwater system. The following sections outline the steps taken to evaluate the impacts of additional impervious area, the results of the analysis, and recommendations. 3 EXISTING CONDITIONS MODEL UPDATES Figure 1 shows the representative area of interest (AOI) selected by Jones Edmunds from the existing ICPR4 model to increase the model level-of-detail within. This area was selected primarily because it contained several larger parcels with relatively low existing impervious surface coverage that have potential to be redeveloped with significantly more impervious surface coverage. DRAFT 08505-003-02 3 November 2018 Impervious Area Drainage Impact Analysis Figure 1 Study AOI 3.1 HYDROLOGIC UPDATES Jones Edmunds subdivided this area into 22 local-scale stormwater basins with an average contributing area of 0.84 acre. We characterized the hydrologic conditions in the new model basins using TR-55 curve number methodology with directly connected impervious area (DCIA) percentages. We digitized the existing impervious surfaces using Nearmap’s high-resolution imagery collected on May 4, 2018. City permit data was also reviewed from the previous 5 years for parcel-specific permitted impervious area calculations and plans sets. We incorporated this data into our impervious area dataset and calculations where available. The aerial imagery and permit data were used to identify locations where pavers have been installed. According to City development codes, pervious pavers are considered 50 percent pervious when calculating the total impervious area of a parcel. We identified approximately 28,000 square feet of pavers in the AOI, which is approximately 4 percent of the total impervious surface. After reviewing the permit data provided by the City and determining that most of the pavers in the AOI were permitted as 50 percent pervious, we decided to model all of the paver surfaces as if they were 50 percent pervious. We also considered pool footprints to be pervious based on current City development codes. We used the impervious surface spatial dataset to determine the unconnected and directly connected impervious area percentages for each of the new basins. We developed a composite curve number representing the hydrologic response from the non-DCIA area for the basins. We used an open land curve number of 65 for all of the basins based on soil data obtained from the Natural Resource Conservation Service’s (NRCS) Web Soil Survey. DRAFT 08505-003-02 4 November 2018 Impervious Area Drainage Impact Analysis 3.2 HYDRAULIC UPDATES Jones Edmunds added 26 nodes, 20 pipe links, and 34 overland flow weir links to model the local-scale hydraulics in this area. The City’s stormwater asset database was used to determine pipe shape, size, inverts, and material. We used a 5-foot by 5-foot digital elevation model (DEM) created using the 2007 Florida Department of Emergency Management (FDEM) Light Detection and Ranging (LiDAR) data to parameterize the overland flow weir connections and assign stage-area relationships to the nodes. 4 FUTURE CONDITIONS HYDROLOGIC ANALYSIS Jones Edmunds used the impervious surface spatial dataset, permit data, and Duval County Parcels spatial data to estimate the existing impervious surface percentage of each parcel within the AOI. We determined that approximately 35 percent of the parcels within the AOI were at or above the 50 percent allowable impervious surface limit and approximately 70 percent of the parcels were above 40 percent impervious surface coverage in existing conditions. The existing average impervious surface percentage was approximately 45 percent. Section 24-66 of the City’s Land Development Code requires that the difference between the pre-development and post-development volume of stormwater runoff be stored on-site for the 25-year storm event when 400 square feet or more of new impervious area is added to a parcel. Assuming that the provided on-site storage facilities are maintained and in working condition, this means that any additional runoff volume generated by new impervious area should not impact the City’s drainage system if more than 400 square feet of new impervious area is added. No on-site storage is required if less than 400 square feet of new impervious area is added, and the additional runoff volume from the new impervious area will impact the City’s stormwater system. We developed hydrologic parameters for four future conditions build-out scenarios to simulate the potential impacts of new impervious area on flood conditions in the AOI. For the first two scenarios we assumed that all on-site storage facilities provided for new impervious area are maintained and in working condition in perpetuity. If this is the case, the greatest impact to the City’s stormwater system would occur if all lots with new impervious capacity added the lesser of 400 square feet of new impervious or the remainder of their allowable impervious area coverage and were not required to provide on-site storage. Scenario 1 assumed that the allowable parcel impervious area was 40 percent with maintained conditions, and Scenario 2 assumed that the allowable parcel impervious area was 50 percent with maintained conditions. In the maintained 40 percent coverage scenario (Scenario 1), we added approximately 13,500 square feet (0.3 acre) of impervious area to the AOI. This increased the basin impervious percentages by approximately 2 percent on average with a minimum increase of 0 percent and a maximum increase of 6 percent. In the maintained 50 percent coverage scenario (Scenario 2), we added approximately 28,500 square feet (0.65 acre) of impervious area to the AOI. This increased the basin impervious percentages by approximately 4 percent on average with a minimum increase of 0 percent and a maximum increase of 9 percent. Figures 2 and 3 show the spatial variation of basin impervious area percentage increases. Basins with the larger percentage increases contain parcels with low existing impervious percentages and basins with smaller percentage increases contain parcels DRAFT 08505-003-02 5 November 2018 Impervious Area Drainage Impact Analysis with high existing impervious percentages, which limited the amount of impervious area that could be added. Figure 2 Basin Impervious Area Increases for 40 Percent Impervious Maintained Scenario Figure 3 Basin Impervious Area Increases for 50 Percent Impervious Maintained Scenario DRAFT 08505-003-02 6 November 2018 Impervious Area Drainage Impact Analysis For Scenarios 3 and 4 we assumed that all on-site storage facilities provided for new impervious area were unmaintained and not in working condition. Currently, the City does not have a program to inspect the on-site storage facilities to ensure that they are maintained and kept in working condition. If these systems go unmaintained or are filled, the additional runoff from the new impervious surfaces will not be retained on-site and will impact the City’s stormwater system. The worst-case scenario for the City’s stormwater system would occur if all on-site storage systems were not maintained and all of the parcels with new impervious capacity in the AOI were built out to the maximum allowable impervious surface coverage. Scenario 3 assumed that the allowable parcel impervious area was 40 percent with unmaintained conditions, and Scenario 4 assumed that the allowable parcel impervious area was 50 percent with unmaintained conditions. In the 40 percent coverage unmaintained scenario (Scenario 3), we added approximately 32,500 square feet (0.75 acre) of impervious area. This increased the basin impervious percentages by approximately 3.5 percent on average with a minimum increase of 0 percent and a maximum increase of 16 percent. In the 50 percent coverage unmaintained scenario (Scenario 4), we added approximately 76,000 square feet (1.75 acre) of impervious area. This increased the basin impervious percentages by approximately 9 percent on average with a minimum increase of 0 percent and a maximum increase of 26 percent. Figures 4 and 5 show the spatial variation of basin impervious area percentage increases. Basins with the larger percentage increases contain parcels with low existing impervious percentages and basins with smaller percentage increases contain parcels with high existing impervious percentages, which limited the amount of impervious area that could be added. Figure 4 Basin Impervious Area Increases for 40 Percent Impervious Unmaintained Scenario DRAFT 08505-003-02 7 November 2018 Impervious Area Drainage Impact Analysis Figure 5 Basin Impervious Area Increases for 50 Percent Impervious Unmaintained Scenario 5 HYDROLOGIC AND HYDRAULIC MODELING AND RESULTS We simulated the FDOT 1-, 2-, 4-, 8-, and 24-hour storm events for the 2-, 5-, and 10-year return periods for the five model scenarios:  Revised Existing Condition Model with Local scale elements modeled.  Scenario 1 - 40 percent allowable impervious maintained on-site storage.  Scenario 2 - 50 percent allowable impervious maintained on-site storage.  Scenario 3 - 40 percent allowable impervious unmaintained on-site storage.  Scenario 4 - 50 percent allowable impervious unmaintained on-site storage. We compared peak flood stages from the four hypothetical scenarios to the revised existing conditions peak flood stages to determine how flood depths would be impacted by the additional impervious area. Changes in peak flood stages varied spatially depending on how much impervious area was added within the associated basin. Basins with lower impervious area percentage increases experienced smaller flood peak stage increases and basins with larger impervious area percentage increases experienced larger peak stage increases. Tables 1 and 2 summarize the maximum peak flood stage increases where surface flooding occurs in the four scenarios compared to existing conditions for the 2-, 5-, and 10-year return periods. DRAFT 08505-003-02 8 November 2018 Impervious Area Drainage Impact Analysis Table 1 Maximum Flood Depth Increases from Existing Conditions Scenario Return Period Average 2-Year 5-Year 10-Year Scenario 1: Maximum Flood Stage Increase 40 Percent Impervious Maintained (Inches) 1 1 1 1 Scenario 2: Maximum Flood Stage Increase 50 Percent Impervious Maintained (Inches) 2 3 1 2 Difference (Inches) 1 2 0 1 Table 2 Maximum Flood Depth Increases from Existing Conditions Scenario Return Period Average 2-Year 5-Year 10-Year Scenario 3: Maximum Flood Stage Increase 40 Percent Impervious Unmaintained (Inches) 3 3 1 2 Scenario 4: Maximum Flood Stage Increase 50 Percent Impervious Unmaintained (Inches) 6 4 3 4 Difference (Inches) 3 1 2 2 As shown in Tables 1 and 2, local-scale surface flooding will worsen due to the increases in runoff volume and runoff rate from additional impervious surface coverage. The additional runoff adds stress to local-scale drainage features that have already met or exceeded their hydraulic capacities. Systems that are already at or exceeding capacity are prone to creating nuisance flooding. Our modeling assumes all stormwater structures and roadside swales between structures are in a well-maintained condition. Any changes to this maintenance condition could adversely impact the adjacent parcels. Ultimately, surface flooding will occur more frequently and at greater depths as impervious surface coverage increases. The additional stress on the drainage system will cause more frequent instances of nuisance flooding in roadways and on private properties. More frequent maintenance of the City’s stormwater system will be required because of the displacement of sediment created from increased flows and the deposition of that sediment in the stormwater system. These results also show the importance of ensuring that on-site storage systems are maintained in proper working condition. If these systems are not maintained or are removed, impacts of additional impervious area on the City’s stormwater system will worsen.