Northwinds Parkway Pond Analysis Report_Final.pdf

NORTHWINDS PARKWAY

POND ANALYSIS

City of Alpharetta, GA

MAY 15, 2025

SUBMITTED BY

Dewberry Engineers Inc.

2835 Brandywine Road, Suite 100

Atlanta, GA 30341

678.530.0022

SUBMITTED TO

City of Alpharetta, GA

1790 Hembree Road

Alpharetta, GA 30009

May 15, 2025

Dewberry Engineers Inc.

COA#: PEF002398

Expires: 6/30/2026

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 1

NORTHWINDS PARKWAY POND

ANALYSIS

Executive Summary

The City of Alpharetta engaged SWP&E and Dewberry to evaluate the existing culvert and downstream detention pond located on Tributary 11 to Big Creek at Northwinds Parkway in Alpharetta, GA as part of a detailed stormwater

MS4 inspection. The purpose of this study is to evaluate if upgrades to the existing culvert or existing detention pond could reduce the backwater flooding experienced at Northwinds Parkway. Under current conditions, the City’s culvert under Northwinds Parkway cannot be inspected or maintained due to this backwater flooding. For this study, Dewberry leveraged the effective HEC-HMS hydrologic and HEC-RAS hydraulic modeling into a PCSWMM stormwater model for both existing and proposed conditions. Dewberry also reviewed the original stormwater design report for the detention pond to incorporate additional detail for the pond and outlet control structure and quantify the available water quality volume. Field survey collected by Terramark on April 24, 2024 was incorporated in the

PCSWMM stormwater model for the culvert at Northwinds Parkway as well as the pond storage volumes and outlet control structures.

Six proposed capital improvement options were evaluated:

1. Option 1: Non-Structural Alternative

2. Option 2: Clear and Maintain Culverts as Fully Functional

3. Option 3: Increase Orifice Sizing at Northwinds Detention Pond OCS

4. Option 4: Lower Orifice at Northwinds Detention Pond OCS

5. Option 5: Convert to Dry Extended Detention Facility

6. Option 6: Replace Existing Box Culverts with Span Bridge

A summary table highlighting the level of service for each option are included in Tables ES-1. Only Option 6 provides an improvement to the level of service. A summary of the ponding depth at Northwinds Parkway is included in Table

ES-2. Option 5 provides the greatest reduction in ponding at the Northwinds Parkway culverts during non-storm events, which is the most critical concern for this project scope. Concept level cost estimates are included in Table

ES-3.

Table ES-1: Level of Service Comparison

Option

Level of Service

Northwinds Parkway Culvert

Northwinds Detention Pond

Existing Conditions 25-Year 5-Year

Option 1: Non-Structural Alternative 25-Year 5-Year

Option 2: Clear and Maintain Culverts as Fully Functional 25-Year 5-Year

Option 3: Increased Orifice Sizing at Northwinds Detention Pond OCS 25-Year 5-Year

Option 4: Lowered Orifice at Northwinds Detention Pond OCS 25-Year 5-Year

Option 5: Convert to Dry Extended Detention Facility 25-Year 5-Year

Option 6: Replace Existing Box Culverts with Span Bridge 50-Year 5-Year

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 2

Table ES-2: Ponding Depths at Northwinds Parkway Culverts

Table ES-3: Concept Level Cost Estimates

By comparing the improvements to level of service, ponding depth of the Northwinds culverts, and total project cost, Option 5: Extended Dry Detention Facility is the most effective option to address the concerns of frequent ponding of the existing culverts at Northwinds Parkway. This option does not improve the level of service of the culverts, however it provides the greatest decrease in ponding during non-storm events to allow for better inspection and maintenance of the culverts. Any redesign efforts to implement Option 5 should include an analysis of the water quality volume requirements and ensure that the GSWMM standards are still met for any pond retrofits. Since this pond is privately owned, coordination with the City and the property owners is required for this option. The City may also consider requiring the implementation of Option 5 prior to using the facility for any future stormwater management measures.

Option Peak 100-Year

Ponding Depth (ft) Non-Storm Event Ponding Depth (ft)

Existing Conditions 17.4 6.7

Option 1: Non-Structural Alternative 17.4 6.7

Option 2: Clear and Maintain Culverts as Fully Functional 17.4 6.7

Option 3: Increased Orifice Sizing at Northwinds Detention Pond OCS 17.4 6.6

Option 4: Lowered Orifice at Northwinds Detention Pond OCS 17.4 5.9

Option 5: Convert to Dry Extended Detention Facility 17.4 2.4

Option 6: Replace Existing Box Culverts with Span Bridge 16.7 2.8

Alternative Concept Level Cost

Option 1: Non-Structural Alternative $0

Option 2: Clear and Maintain Culverts as Fully Functional $30,000

Option 3: Increased Orifice Sizing at Northwinds Detention Pond OCS $55,800

Option 4: Lowered Orifice at Northwinds Detention Pond OCS $55,800

Option 5: Convert to Dry Extended Detention Facility $97,800

Option 6: Replace Existing Box Culverts with Span Bridge $3,109,650

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 3

Background

The culvert at Northwinds Parkway in Alpharetta, GA is located on Tributary 11 to Big Creek, approximately 850 feet upstream of Highway 400 as shown in Figure 1. This tributary is mapped as a Zone AE floodplain with floodway based on the effective FEMA modeling and mapping. The existing culvert is a double barrel 8’x8’ reinforced concrete box (RCB) culvert, which outlets into a private detention pond servicing the Northwinds development. The total drainage area draining to the culvert is approximately 404 acres and approximately 462 acres to the Northwinds

Parkway Detention Pond. The basins are mainly commercial and medium to high density residential land use.

FIGURE 1. The culvert at Northwinds Parkway and the Northwinds Detention

Pond are located on Tributary 11 to Big Creek in Alpharetta, GA.

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 4

The culvert at Northwinds Parkway has a history of inundation and overtopping of Northwinds Parkway. As shown in

Figure 2, this culvert frequently appears fully submerged with limited available conveyance capacity, resulting in increased risk of road overtopping. As part of more extensive MS4 stormwater inspections and reporting, Dewberry is providing the City of Alpharetta with an analysis of the existing hydrologic and hydraulic conditions of the culvert and downstream pond, as well as proposed recommendations to improve the conveyance capacity and access for routine maintenance and inspection.

Northwinds Detention Pond Stormwater Report Review

As part of this analysis, Dewberry reviewed the “Stormwater Management Study & Hydrology Study for Northwinds”, developed by Associated Engineering Consultants, Inc. and dated September 3, 1996. This study utilized HEC-1 hydrologic model with Soil Conservation Service (SCS) and Modified Puls routing methods to show that proposed

Northwinds Pond and primary spillway structure would produce proposed peak flows that were lower than the existing flows for the 2-, 5-, 10-, 25-, 50, and 100-Year storm events.

This report also documented that the pond would meet the water quality volume requirements at that time, which included treating the first half inch of rainfall for 93.6 acres of upstream impervious area that required treatment. This resulted in a required water quality volume of 3.9 acre-feet, which could be captured and retained with the available

6.4 acre-feet of volume below the wet pond normal pool elevation according to the 1996 report. Based on Georgia

Stormwater Management Manual (GSWMM) updated in 2016, current standards require the first 1.2 inches of rainfall to be treated for water quality volume. This would result in a total of 9.7 acre-feet water quality volume if the pond were designed to present day standards, which is larger than the volume of the original pond design wet pool volume.

This also does not account for any sedimentation that may have occurred over the past 27 years since construction.

The report also briefly discusses the new road to be constructed upstream of the Northwinds Detention Pond, which is assumed to be Northwinds Parkway, although this is not explicitly stated. The report indicates that this new road and its culverts had been included in the 1996 analysis, however the report does not provide any details on the water surface elevations at the new culvert or the effects of the detention pond on the culvert’s level of service.

FIGURE 2. Upstream face of Northwinds Parkway culvert obtained as part of citywide floodplain modeling indicated limited conveyance capacity for the double barrel culvert.

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 5

Existing Conditions Survey Data

Terramark collected survey data for the project location on April 24, 2024 including the culvert at Northwinds

Parkway, the storage volume of the downstream detention ponds, and its outlet control structure information. This survey indicates that the existing conditions invert of the pond is approximately elevation 1006 feet, while the constructed inverts of the upstream double barrel box culvert are elevations 1004 feet and 1002 feet for the upstream and downstream inverts, respectively. Since the ground elevation of the downstream pond is higher than the inverts of the culverts at Northwinds Parkway, the current pond grading results in a decreased conveyance capacity for the existing box culverts. This survey data included in Attachment A was incorporated into the PCSWMM stormwater model to evaluate the existing conditions and effectiveness of proposed conceptual options.

Existing Conditions Modeling

For this analysis, Dewberry leveraged effective hydrologic and hydraulic floodplain models developed for the City of

Alpharetta to evaluate the existing conditions performance of the pond and determine its impacts on the upstream culvert at Northwinds Parkway. Using the effective FEMA HEC-HMS and HEC-RAS models for the Big Creek watershed, Dewberry imported the model geometries into PCSWMM, which is a hydrodynamic stormwater modeling program. PCSWMM was selected to model the pond outlet control structure in more detail and to more accurately evaluate proposed updates to the outlet control structure.

Drainage Areas

The basins for the Northwinds Parkway Pond analysis shown in Figure 1 were leveraged from the effective HEC-

HMS modeling for the Big Creek watershed. These basins were reviewed against the City of Alpharetta stormwater inventory and refined to capture more detail based on the stormwater pipe network. Additional basin breaks were added for the Northwinds Pond and several other culverts along Tributary 11 to Big Creek.

Curve Number

The curve number for each revised basin was updated using the same methodology as the effective HEC-HMS model, which utilized the NRCS Curve Number method based on soils and land use. Soils data was obtained from the Natural Resources Conservation Service’s (NRCS) Soil Survey data and City of Alpharetta land use data. A curve number value was assigned to each land cover-Hydrologic Soil Group (HSG) combination using the values provided in Table 1 below, and an area-weighted curve number was developed for each subbasin in the PCSWMM model.

Table 1: Landuse-HSG Curve Number Values

Land Use Hydrologic Soil Group*

TR-55 Cover Description A B C D

Barren Land 77 86 91 94 Newly Graded Urban

Deciduous Forest 36 60 73 79 Woods (Fair condition)

Evergreen Forest 36 60 73 79 Woods (Fair condition)

Pasture/hay 70 80 87 90 Row Crops (poor condition)

High Intensity Urban 77 85 90 92 Residential (1/8 acre or less)

Low Intensity Urban 51 68 79 84 Residential 1 acre lot

Mixed Forest 36 60 73 79 Woods (Fair condition)

Paulustrine Forested Wetland 100 100 100 100 Assume similar to impervious cover

Estuarine Forested Wetland 100 100 100 100 Assume similar to impervious cover

Open Water 100 100 100 100 Assume similar to impervious cover

Agriculture/Forestry 36 60 73 79 Woods (Fair condition)

Offices 89 92 94 95 Commercial and business

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 6

Large Lot Single Family Detached 51 68 79 84 Residential (1 acre lot)

Manufacturing/Warehousing/Wholesale 81 88 91 93 Industrial

Multi-Family/Apartments 75 85 90 92 Residential (1/8 acre or less)

Park/Recreation/Conservation 49 69 79 84 Open space (Fair condition)

Public/Institutional 81 88 91 93 Industrial

Retail Sales & Services/Hotels 89 92 94 95 Commercial and business

Single Family Attached Residential 77 85 90 92 Residential (1/8 acre or less)

Single Family Detached Residential 57 72 81 86 Residential (1/3 acre)

Transportation/Communication/Utilities 83 89 92 93 Streets and roads (paved)

Undeveloped 43 65 76 82 Woods-grass (Fair condition)

Time of Concentration

The time of concentration for each revised subbasin was updated using the same methodology as the effective HEC-

HMS model, which utilized the regression analysis method of lagtime as documented in U.S. Geological Survey

Water-Resources Investigation Report 00-4049 “Lagtime Relations for Urban Streams in Georgia” by Ernest J.

Inman. The lagtime is considered to be the time from the center of mass of the rainfall even to the time of peak runoff discharge. The lag time was determined using the equation:

𝒕𝒍 = 𝟕. 𝟖𝟔𝑫𝑨𝟎.𝟑𝟓𝑻𝑰𝑨−𝟎.𝟐𝟐𝑺−𝟎.𝟑𝟏

Where, tl = lag time (hours)

TIA= total impervious area (%)

S= main channel slope in ft/mi

DA=drainage area (sq. mi.)

Impervious area for each subbbasin was calculated using percent impervious values from TR-55 Runoff Curve

Number tables, and the main channel slope was calculated as the difference in elevation, in feet, at the 10-percent and 85-percent points divided by the distance, in miles, between the points. These calculated lagtime values were converted into times of concentration assuming a 0.6 conversation factor and input into PSCWMM using the

Alternative Runoff Method (ARM) functionality to mimic the effective modeling hydrologic methodology. A summary of the revised subbasin parameters is included in Table 2.

Table 2: Summary of Revised Subbasin Parameters

Subbasin Drainage Area

(acres) Curve Number

Time of Concentration

(minutes)

B154 75.9 87.7 34.7

B155_1 167.0 86.5 53.9

B156 108.8 89.4 36.2

B157 58.3 80.6 26.4

B158 110.1 82.7 37.2

B159 124.3 81.7 59.5

B157_1 33.4 83.6 21.8

B159_1 24.4 80.9 25.1

B155 52.1 82.1 35.1

B154 75.9 87.7 34.7

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 7

Rainfall Data

All of the 24-hour rainfall events for the PCSWMM models were based on the NOAA Atlas 14 precipitation frequency estimates and centered over the study area. These values were updated to use the most recent precipitation estimates and were transformed to a time series using the MSE 5 distribution, while the effective HEC-HMS model and 1996 stormwater study utilized older precipitation estimates and an SCS Type II rainfall distribution. These updated NOAA values are smaller than the values utilized for the effective HEC-HMS modeling but are more current. Table 3 provides a summary of precipitation estimates for each storm event used for this analysis.

Table 3: 14-hour rainfall totals

Percent Annual Chance Event

24-hour Rainfall (in.)

2-Year 3.74

5-Year 4.46

10-Year 5.07

25-Year 5.94

50-Year 6.62

100-Year 7.33

Stormwater System

The storm drainage system for this watershed was imported into PCSWMM using the effective HEC-RAS model for structures located along Tributary 11 to Big Creek. All other subbasins and stormwater inventory were lumped into larger basins draining into Tributary 11 to Big Creek, following the same level of detail as the effective floodplain modeling. No additional stormwater inventory was added to this model. However as noted above, the subbasins were refined to take these pipes and drainage patterns into account. Pipe materials, sizes, and upstream and downstream measure down information were used from the effective HEC-RAS model, available detailed field survey collected at the time of initial model development in 2010, and 4/24/2024 field survey collected by Terramark. Headwall elevations and structure rim elevations were taken from 2010 and 2024 field survey and City of Alpharetta LiDAR data. Table 4 below summarizes the pipe size, material, and invert elevations for the critical inventory along Tributary 11 to Big Creek.

Table 4: Critical Stormwater Inventory along Tributary 11 to Big Creek

Location Inventory Link Number Pipe Size Material US Invert

(ft) DS Invert

(ft)

Nottaway Lane CJ12216.42 48” Round Pipe RCP 1058.31 1056.13

Westside Parkway CJ9176.389 10’ x 10’ Box Culvert RCB 1024.48 1023.80

Northwinds Parkway CJ7445.439 Two (2) 8’ x 8’ Box Culverts RCB 1004.14 1002.72

Northwinds Detention Pond CJ6909.280 OCS* & 72” Round Pipe RCP 999.23 998.82

Highway 400 CJ6514.552 Two (2) 6’ x 7’ Box Culverts RCB 998.94 996.89

North Point Parkway CJ5118.744 Three (3) 5’ x 8.7’ Box Culverts RCB 988.35 986.78

Rock Mill Road CJ4244.956 Two (2) 5’ x 5’ Box Culverts RCB 983.75 982.45

*Refer to Figure 3 and Figure 4.

Additional detail was added to the PCSWMM model to represent the outlet control structure (OCS) of the private

Northwinds Pond shown in Figure 3. The orifice and weir information was obtained from the April 2024 survey data collected by Terramark shown in Figure 4. It should be noted that differences in elevation were identified between the

Terramark field survey data and the 1996 “Stormwater Management Study & Hydrology Study for Northwinds”, developed by Associated Engineering Consultants, Inc. Elevation-area information for the pond was also obtained

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 8 from the April 2024 field survey data, summarized in Table 5. Any revised grading or pond sedimentation may alter the findings of this report. The pond is modeled as a wet pond, with a starting water surface elevation set to elevation

1010.74’ based on field survey data.

FIGURE 3. Outlet Control Structure (OCS) for the Northwinds Pond

FIGURE 4. Detailed orifice and weir information for the Northwinds Pond OCS obtained from 4/24/2024 field survey data collected by Terramark.

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 9

Table 5: Depth-Area Curve for Northwinds Detention Pond

Depth (ft)

Elevation (ft)

Area (square feet)

0 1006 980

1 1007 9853

2 1008 30865

3 1009 45416

4 1010 52424

5 1011 56429

6 1012 60223

7 1013 64250

8 1014 68300

9 1015 72508

10 1016 77423

11 1017 83638

12 1018 97545

13 1019 101265

14 1020 105299

15 1021 109857

16 1022 117578

17 1023 124426

19 1025 138975

It is assumed in the existing conditions model that the culverts at Northwinds Parkway are free of sediment and debris. Any decrease in conveyance capacity of these box culverts may decrease their level of service, and it is recommended that routine maintenance be conducted to ensure the culverts are clear and free flowing. This will require a temporary draining of the downstream detention pond to provide access for maintenance.

Existing Conditions Modeling Results

Utilizing the updated PCSWMM model for Tributary 11 to Big Creek, the existing conditions results are summarized in

Table 6 below for the culvert at Northwinds Parkway (assumed fully functional) as well as the Northwinds Detention

Pond. These results indicate a 25-Year and 5-Year existing conditions level of service respectively, which is based on the overtopping elevation of the roadway. The results also indicate that while the existing conditions model shows that Northwinds Parkway culvert has a 25-Year level of service, even during the 2-Year event, the culvert is completely submerged and experiences approximately 16 feet of flooding depth upstream of the culvert.

The peak flow values estimated by the PCSWMM model are considerably larger than the peak flows estimated in the

1996 “Stormwater Management Study & Hydrology Study for Northwinds” Report, likely due to changes in land use and curve number. The 1996 design utilized a post-development curve number of 73, while the subbasins in both the effective HEC-HMS model and the updated PCSWMM model developed for this analysis range from 81 – 89 due to much of the commercial and high-density residential development within the watershed.

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 10

Table 6: Existing Conditions Results Summary for Northwinds Culvert (Fully Functional)

Structure Overtopping Elevation (ft)

Storm Event Peak Flow (cfs) Peak Water Surface

Elevation (ft) Existing Conditions

Level of Service

Northwinds Parkway Culvert (fully functional)

1024.34

2-Year 396.38 1020.15

25-Year

5-Year 476.38 1021.55

10-Year 570.73 1022.79

25-Year 696.36 1024.27

50-Year 825.03 1025.03

100-Year 969.93 1025.36

Northwinds Detention Pond

1021.3

2-Year 411.13 1019.91

5-Year

5-Year 491.83 1021.18

10-Year 586.36 1022.27

25-Year 710.18 1023.42

50-Year 831.93 1024.00

100-Year 1031.96 1024.29

The existing conditions model was also updated to evaluate if the 8’x8 RCB box culverts at Northwinds Parkway were obstructed 50% by debris and sediment. In this modeled scenario, the existing conditions of the culvert decreased from 25-Years to 10-Years as shown in Table 7. This demonstrates that maintaining the culverts to be clear and free flowing, without any structural changes to the culverts or downstream pond can be an effective and cost-effective strategy to improving the culvert level of service. Conducting maintenance to remove any debris or sediment within the box culverts would require a temporary draining of the downstream detention pond to provide access and a dry working area for maintenance.

Table 7: Existing Conditions Results Summary for Northwinds Culvert (50% Blocked)

Overtopping Elevation (ft)

Storm Event Peak Flow (cfs) Peak Water Surface

Elevation (ft) Existing Conditions

Level of Service

Northwinds Parkway Culvert (50% blocked)

1024.34

2-Year 380.56 1020.66

10-Year

5-Year 454.63 1022.31

10-Year 535.25 1023.73

25-Year 699.69 1025.15

50-Year 874.06 1025.50

100-Year 1019.28 1025.71

Northwinds Detention Pond

1021.3

2-Year 391.74 1019.81

5-Year

5-Year 466.80 1021.00

10-Year 543.95 1021.96

25-Year 715.21 1023.22

50-Year 906.95 1024.03

100-Year 1071.68 1024.33

Water Quality Volume Calculations

Based on the topographic survey data of the pond storage collected by Terramark, there is approximately 3.2 acre-feet of storage below the low-level orifice at elevation 1010 feet. This value is smaller than the 1996 “Stormwater

Management Study & Hydrology Study for Northwinds” Report which indicated that the water quality volume required by the standards at the time of development would be 3.9 acre-feet. This may indicate that further sedimentation of the pond may be occurring and decreasing the available water quality volume storage that was originally permitted.

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 11

Proposed Conditions Alternatives

Since the existing conditions model results indicate that a fully functional double barrel 8’x8’ RCB culvert achieves a

25-Year level of service, proposed conceptual options focused on approaches to reduce the ponding of water within the culverts at Northwinds Parkway, rather than improve the level of service at Northwinds Parkway.

Option 1: Non-Structural Alternative

Option 1 summarizes the non-structural alternative, which includes actions the City can implement that do not involve any physical changes to the pond or culverts at Northwinds Parkway. In this option, the culverts would maintain a 25-

Year level of service assuming they are fully functional, or a 10-Year level of service if they are 50% blocked with sediment as summarized in Table 6 and Table 7. Ponding depths up to 17.4 feet at the culverts during peak storm events would remain unchanged from the current conditions, and it is anticipated that maintenance of the culverts would be limited due to extended inundation of the culverts up to 6.7 feet during non-storm events. If no structural solutions are implemented, it is recommended to coordinate with Planning & Zoning Department to ensure that any upstream or adjacent future development consider the current conditions of the pond, the flooded culverts at

Northwinds Parkway, and that utilizing the existing pond for additional stormwater management measures may result in increased flooding of the Northwinds Parkway culverts.

Option 2: Clear and Maintain Culverts as Fully Functional

Option 2 involves ensuring that the two 8’x8’ RCB culverts at Northwinds Parkway are fully functional. By clearing and maintaining these two culverts to provide their full conveyance capacity, the City can maintain a 25-Year level of service at Northwinds Parkway, compared to a 10-Year level of service assuming that the culverts are 50% obstructed with sediment or debris. This option does not involve any additional dredging of the downstream stormwater pond, and conducting maintenance to remove any debris or sediment within the box culverts would require a temporary draining of the downstream detention pond to provide access and a dry working area for maintenance. This option does not provide any significant decrease in downstream water surface elevation, and even as fully functional, the culverts will remain inundated up to 17.4 feet during peak storm events and up to 6.7 feet during non-storm events. By maintaining the culverts as fully function, the City can maintain a higher level of service, but this option does not decrease the ponding of the culverts since the water surface elevation at the culverts is controlled by the downstream detention pond pool elevation.

Table 8: Option 2 Results Summary

Overtopping Elevation (ft)

Storm Event

Peak Flow (cfs) Peak Water Surface

Elevation (ft) Option 2

Level of Service

50% Blocked

Fully Functional

50% Blocked

Fully Functional

50% Blocked

Fully Functional

Northwinds Parkway Culvert

1024.34

2-Year 380.56 396.38 1020.66 1020.15

10-Year 25-Year

5-Year 454.63 476.38 1022.31 1021.55

10-Year 535.25 570.73 1023.73 1022.79

25-Year 699.69 696.36 1025.15 1024.27

50-Year 874.06 825.03 1025.50 1025.03

100-Year 1019.28 969.93 1025.71 1025.36

Northwinds Detention

Pond 1021.3

2-Year 391.74 411.13 1019.81 1019.91

5-Year 5-Year

5-Year 466.80 491.83 1021.00 1021.18

10-Year 543.95 586.36 1021.96 1022.27

25-Year 715.21 710.18 1023.22 1023.42

50-Year 906.95 831.93 1024.03 1024.00

100-Year 1071.68 1031.96 1024.33 1024.29

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 12

Table 9: Concept Level Cost Estimate for Option 2

Option 3: Increased Orifice Sizing at Northwinds Detention Pond OCS

Option 3 involves adding an additional 18” orifice to the existing OCS structure at elevation 1012’ as shown in Figure

5 to allow additional flow through the OCS to lower the peak water surface elevation, and assumes the culverts at

Northwinds Parkway are fully functional. A summary of the results for this proposed option are included in Table 10.

This option does not improve the level of service at either the Northwinds Parkway Culvert or the Northwinds

Detention Pond, however slight decreases in water surface elevation are seen up to 0.2 feet. Ponding depths up to

17.4 feet at the culverts during peak storm events would remain similar to the current conditions, and ponding depths at the culverts during non-storm events would decrease from 6.7 feet to 6.6 feet. This option results in downstream flow increases up to 9 cfs as summarized in Table 18. The anticipated cost for this option is $55,800 as detailed in

Table 11.

Item Amount Unit Unit Cost Total Cost

Control of Water 1 LS $10,000.00 $10,000.00

Clearing and Grubbing 1 LS $5,000.00 $10,000.00

Erosion Control 1 LS $5,000.00 $5,000.00

Construction Cost $25,000.00

20% Contingency $5,000.00

Total Estimated Cost $30,000.00

FIGURE 5. Proposed OCS modifications for Option 3

New 18” orifice at elevation

1012’

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 13

Table 10: Option 3 Results Summary

Overtopping Elevation (ft)

Storm Event

Peak Flow (cfs) Peak Water Surface

Elevation (ft) Option 3

Level of Service

Existing Proposed Existing Proposed Existing Proposed

Northwinds Parkway Culvert

1024.34

2-Year 396.38 401.76 1020.15 1019.92

25-Year 25-Year

5-Year 476.38 482.66 1021.55 1021.32

10-Year 570.73 577.51 1022.79 1022.57

25-Year 696.36 725.15 1024.27 1024.21

50-Year 825.03 839.89 1025.03 1024.95

100-Year 969.93 974.73 1025.36 1025.36

Northwinds Detention

Pond 1021.3

2-Year 411.13 417.01 1019.91 1019.65

5-Year 5-Year

5-Year 491.83 498.02 1021.18 1020.91

10-Year 586.36 591.81 1022.27 1022.00

25-Year 710.18 712.06 1023.42 1023.26

50-Year 831.93 815.8 1024.00 1023.95

100-Year 1031.96 1018.46 1024.29 1024.25

Table 11: Concept Level Cost Estimate for Option 3

Option 4: Lowered Orifice at Northwinds Detention Pond OCS

Option 4 involves adding an additional 18” orifice to the existing OCS structure a lower elevation of 1010’ as shown in

Figure 6 to allow additional flow through the OCS to lower the peak water surface elevation, and assumes the culverts at Northwinds Parkway are fully functional. A summary of the results for this proposed option are included in

Table 12. This option does not improve the level of service at either the Northwinds Parkway Culvert or the

Northwinds Detention Pond. Ponding depths up to 17.4 feet at the culverts during peak storm events would remain similar to the current conditions, and ponding depths at the culverts during non-storm events would decrease from 6.7 feet to 5.9 feet. It is recommended that the City confirm that with these proposed modifications still meet the required water quality volume for the pond and associated developments. This option results in downstream flow increases up to 7 cfs as summarized in Table 18. The anticipated cost for this option is $55,800 as detailed in Table 13.

Item Amount Unit Unit Cost Total Cost

Outlet Control Structure Modifications 1 EA $6,500.00 $6,500.00

Control of Water 1 LS $10,000.00 $10,000.00

Clearing and Grubbing 1 LS $15,000.00 $15,000.00

Erosion Control 1 LS $15,000.00 $15,000.00

Construction Cost $46,500.00

20% Contingency $9,300.00

Total Estimated Cost $55,800.00

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 14

Table 12: Option 4 Results Summary

Overtopping Elevation (ft)

Storm Event

Peak Flow (cfs) Peak Water Surface

Elevation (ft) Option 4

Level of Service

Existing Proposed Existing Proposed Existing Proposed

Northwinds Parkway Culvert

1024.34

2-Year 396.38 402.57 1020.15 1019.85

25-Year 25-Year

5-Year 476.38 482.16 1021.55 1021.25

10-Year 570.73 576.21 1022.79 1022.50

25-Year 696.36 693.25 1024.27 1024.06

50-Year 825.03 809.33 1025.03 1024.94

100-Year 969.93 973.51 1025.36 1025.36

Northwinds Detention

Pond 1021.3

2-Year 411.13 417.01 1019.91 1019.57

5-Year 5-Year

5-Year 491.83 497.49 1021.18 1020.85

10-Year 586.36 591.19 1022.27 1021.96

25-Year 710.18 708.03 1023.42 1023.23

50-Year 831.93 813.5 1024.00 1023.94

100-Year 1031.96 1014.37 1024.29 1024.24

FIGURE 6. Proposed OCS modifications for Option 4

New 18” orifice at elevation

1010’

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 15

Table 13: Concept Level Cost Estimate for Option 4

Option 5: Dry Extended Detention Facility

Option 5 involves converting the pond downstream of Northwinds Parkway from a wet detention pond into a dry extended detention (ED) facility, and assumes the culverts at Northwinds Parkway are fully functional. By converting to a dry pond and regrading the channel through the pond, more storage volume will be available during storm events. According to the Georgia Stormwater Managmement Manual (GSWMM) Volume 2, if installed as per the recommended design criteria and properly maintained, dry extended detention basins may provide partial water quality benefits and are presumed to remove 60% of the total suspended solids (TSS) load. Another BMP should be used in a treatment train with dry detention basins to provide the additional required water quality treatment.

Outlet Control Structure Modifications 1 EA $6,500.00 $6,500.00

Control of Water 1 LS $10,000.00 $10,000.00

Clearing and Grubbing 1 LS $15,000.00 $15,000.00

Erosion Control 1 LS $15,000.00 $15,000.00

Construction Cost $46,500.00

20% Contingency $9,300.00

Total Estimated Cost $55,800.00

FIGURE 7. Schematic of Dry Extended Detention Basin from GSWMM Vol 2

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 16

This option assumes that retrofitting the pond to a dry ED basin would require the same water quality volume as permitted for the original design, rather than implementing current design standards for the pre-existing pond. The proposed retrofits to the outlet control structure include adding an additional 5” low level orifice at elevation 1006 feet, sized to provide a total water quality volume of 3.9 acre-feet as noted in the 1996 “Stormwater Management Study &

Hydrology Study for Northwinds” Report. Current GSWMM standards would require 9.4 acre-feet of water quality volume storage.

Since the original design report does not reference any channel protection volume, overbank flood control volume, or extreme flood protection volume values, additional orifices of the outlet control structure were not modified from their existing condition. For the existing conditions model and all proposed options, the detention pond overtops during the

10-Year event, and therefore does not meet the overbank flood control and extreme flood protection GSWMM standards. If determined by the City that converting this existing pond to a dry ED basin if the preferred option, it is recommended that detailed outlet control structure sizing be conducted to confirm that the retrofitted OCS can safely pass the channel protection, overbank flood control, and extreme flood protection events.

As shown in Figure 8, by converting to a dry ED pond, the depth of flooding after the storm event has past decreases from 6.7 feet of ponding at the Northwinds Parkway culverts down to only 2.4 feet of ponding. With additional regrading of the pond, it is possible to reduce the ponding depth even further.

A summary of the results for this proposed option are included in Table 14. This option does not improve the level of service at Northwinds Parkway, however by allowing the pond to fully drain, access for inspection and maintenance to the culverts will be improved between periods of rain and wet weather. This option results in no downstream flow increases as summarized in Table 18. The anticipated cost for this option is $97,800 as detailed in Table 15.

FIGURE 8. Depth results for the 100-Year storm event show a decrease of 4.3 feet of ponding at Northwinds

Parkway culverts between existing (top) and Option 5 (bottom) results for non-peak flows.

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 17

Table 14: Option 5 Results Summary

Overtopping Elevation (ft)

Storm Event

Peak Flow (cfs) Peak Water Surface

Elevation (ft) Option 5

Level of Service

Existing Proposed Existing Proposed Existing Proposed

Northwinds Parkway Culvert

1024.34

2-Year 396.38 397.74 1020.15 1019.98

25-Year 25-Year

5-Year 476.38 476.62 1021.55 1021.42

10-Year 570.73 569.98 1022.79 1022.68

25-Year 696.36 694.31 1024.27 1024.20

50-Year 825.03 818.80 1025.03 1025.00

100-Year 969.93 987.33 1025.36 1025.39

Northwinds Detention

Pond 1021.3

2-Year 411.13 410.65 1019.91 1019.72

5-Year 5-Year

5-Year 491.83 491.34 1021.18 1021.05

10-Year 586.36 585.80 1022.27 1022.17

25-Year 710.18 709.14 1023.42 1023.37

50-Year 831.93 825.74 1024.00 1023.99

100-Year 1031.96 1030.22 1024.29 1024.28

Table 15: Concept Level Cost Estimate for Option 5

Option 6: Replace Existing Box Culverts with Span Bridge

At the request of the City of Alpharetta, an alternative option was developed which involved removing the existing double barrel 8’x8’ RCB culverts with a 150’ span bridge, with a 12’ opening height. A summary of the results for this proposed option are included in Table 16. This option improves the level of service at Northwinds Parkway from 25-

Years to 50-Years and does not change the level of service of the detention pond. Depths up to 16.7 feet at the bridge location during peak storm events are slightly lower than the current conditions, and depths at the culverts during non-storm events would decrease from 6.7 feet to 2.8 feet, assuming the channel is regraded to tie into the existing grade of the pond. Maintenance of the culverts would no longer be required after their removal. This option results in no downstream flow increases as summarized in Table 18. The anticipated cost for this option is

$3,109,650 as detailed in Table 17.

Outlet Control Structure Modifications 1 EA $6,500.00 $6,500.00

Control of Water 1 LS $40,000.00 $40,000.00

Clearing and Grubbing 1 LS $20,000.00 $20,000.00

Erosion Control 1 LS $15,000.00 $15,000.00

Grading Complete 1 LS $50,000.00 $50,000.00

Construction Cost $81,500.00

20% Contingency $16,300.00

Total Estimated Cost $97,800.00

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 18

Table 16: Option 6 Results Summary

Overtopping Elevation (ft)

Storm Event

Peak Flow (cfs) Peak Water Surface

Elevation (ft) Option 6

Level of Service

Existing Proposed Existing Proposed Existing Proposed

Northwinds Parkway Culvert

1024.34

2-Year 396.38 409.08 1020.15 1019.65

25-Year 50-Year

5-Year 476.38 484.16 1021.55 1020.98

10-Year 570.73 578.62 1022.79 1022.12

25-Year 696.36 705.66 1024.27 1023.46

50-Year 825.03 815.69 1025.03 1024.23

100-Year 969.93 962.80 1025.36 1024.65

Northwinds Detention

Pond 1021.3

2-Year 411.13 388.65 1019.91 1019.58

5-Year 5-Year

5-Year 491.83 476.71 1021.18 1020.89

10-Year 586.36 570.96 1022.27 1022.01

25-Year 710.18 704.68 1023.42 1023.3

50-Year 831.93 825.12 1024.00 1023.98

100-Year 1031.96 996.07 1024.29 1024.25

Table 17: Concept Level Cost Estimate for Option 6

Traffic Control 1 LS $100,000.00 $100,000.00

Clearing and Grubbing 1 LS $25,000.00 $25,000.00

Grading Complete 1 LS $50,000.00 $50,000.00

Control of Water 1 LS $100,000.00 $100,000.00

Removal of Existing Culvert 1 EA $10,000.00 $10,000.00

Precast Catch Basin Complete 20 VF $750.00 $15,000.00

150' Span Bridge Complete 12000 SF $175.00 $2,100,000.00

Graded Aggregate Base, 8-inch, Incl Matl 280 SY $50.00 $14,000.00

Asphalt Concrete, 9.5mm Superpave, Incl Bitum Matl & H Lime 20 TON $225.00 $4,500.00

Asphalt Concrete, 19mm Superpave, Incl Bitum Matl & H Lime 65 TON

175.00 $11,375.00

Mill Asphalt Conc Pavement, 1 1/2in Depth 350 SY $15.00 $5,250.00

Traffic Striping, Skip 5in White 300 LF $5.00 $1,500.00

Sidewalk Removal and Replacement 100 SY $100.00 $10,000.00

Curb and Gutter 500 LF $70.00 $35,000.00

Temporary Seeding 1300 SY $5.00 $6,500.00

Permanent Seeding Complete 750 SY $5.00 $3,750.00

Permanent Sodding Complete 550 SY $20.00 $11,000.00

Silt Fence, Complete, Type C 500 LF $5.00 $2,500.00

Curb Inlet Filter, Complete 2 EA $500.00 $1,000.00

Construction Exit 2 EA $5,000.00 $10,000.00

Reconstruct Crosswalk and Pedestrian Light 1 LS $75,000.00 $75,000.00

Construction Cost $2,591,375.00

20% Contingency $518,275.00

Total Estimated Cost $3,109,650.00

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 19

Downstream Impacts at Highway 400

As noted above, the proposed options may result in some increases in flow downstream of the private Northwinds

Parkway Pond. Table 18 summarizes the impacts at Highway 400, showing increases in flow and elevation for the more frequent storm events only. The greatest increase in water surface elevation is 0.2 feet for the 10-Year event for the Option 2: Clearing and Maintaining Culverts as Fully Functional. No increases in downstream flow or water surface elevation are seen for the 50-Year or 100-Year events for any of the proposed options.

Table 18: Impacts at Highway 400

Option Storm Event

Downstream Peak Flow (cfs) Peak Water Surface Elevation (ft)

Existing Proposed Difference Existing Proposed Difference

Option 1 (Non-Structural

Alternative) N/A N/A N/A N/A N/A N/A N/A

Option 2 (50% Blocked vs.

Fully Functional)

2-Year 336.29 342.64 6.35 1003.31 1003.37 0.06

5-Year 417.39 430.25 12.86 1004.01 1004.12 0.11

10-Year 483.11 509.85 26.74 1004.55 1004.75 0.20

25-Year 625.97 645.52 19.55 1005.62 1005.77 0.15

50-Year 832.85 809.53 -23.32 1007.17 1007.00 -0.17

100-Year 1018.75 999.74 -19.01 1008.86 1008.67 -0.19

Option 3 (Increased Orifice

Sizing)

2-Year 342.64 350.78 8.14 1003.37 1003.44 0.07

5-Year 430.25 438.57 8.32 1004.12 1004.19 0.07

10-Year 509.85 514.84 4.99 1004.75 1004.79 0.04

25-Year 645.52 644.65 -0.87 1005.77 1005.76 -0.01

50-Year 809.53 772.45 -37.08 1007.00 1006.72 -0.28

100-Year 999.74 984.22 -15.52 1008.67 1008.52 -0.15

Option 4 (Lower Orifice)

2-Year 342.64 349.00 6.36 1003.37 1003.42 0.05

5-Year 430.25 437.09 6.84 1004.12 1004.17 0.05

10-Year 509.85 512.07 2.22 1004.75 1004.77 0.02

25-Year 645.52 642.35 -3.17 1005.77 1005.75 -0.02

50-Year 809.53 767.54 -41.99 1007.00 1006.68 -0.32

100-Year 999.74 981.87 -17.87 1008.67 1008.5 -0.17

Option 5 (Dry Extended

Detention)

2-Year 342.64 337.94 -4.7 1003.37 1003.33 -0.04

5-Year 430.25 428.04 -2.21 1004.12 1004.1 -0.02

10-Year 509.85 505.87 -3.98 1004.75 1004.72 -0.03

25-Year 645.52 643.69 -1.83 1005.77 1005.76 -0.01

50-Year 809.53 798.36 -11.17 1007.00 1006.91 -0.09

100-Year 999.74 998.47 -1.27 1008.67 1008.66 -0.01

Option 6 (Bridge

Replacement)

2-Year 342.64 321.96 -20.68 1003.37 1003.18 -0.19

5-Year 430.25 410.41 -19.84 1004.12 1003.95 -0.17

10-Year 509.85 487.09 -22.76 1004.75 1004.58 -0.17

25-Year 645.52 634.53 -10.99 1005.77 1005.69 -0.08

50-Year 809.53 789.98 -19.55 1007.00 1006.85 -0.15

100-Year 999.74 980.58 -19.16 1008.67 1008.49 -0.18

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 20

Permitting Considerations

Several environmental permits may be required for proposed options discussed above, which are listed in Table 19.

Additional or different permits may be required based on the actual project design.

Table 19: Potential Permit Requirements

Permit Option 1 Option 2 Option 3 Option 4 Option 5 Option 6

City of Alpharetta Land Disturbance Permit:

The Planning and Development Department issues Land Disturbance Permits for projects that disturb land in the City.

NPDES Construction Permit:

A NPDES Construction Permit is required for sites with greater than 1-acre of disturbed area with associated fees.

Georgia EPD Stream Buffer Variance:

A stream buffer variance is needed for encroachment into the first 25-foot of stream bank on either side from the point of wrested vegetation.

Alpharetta Stream Buffer Variance:

A stream buffer variance is needed for the encroachment from 25-feet to 100-feet in the stream buffer.

U.S. Army Corps of Engineers Section 404 permit:

A Section 404 permit may be required for this project and will likely fall under a Nationwide

Permit 3a, which covers maintenance of existing facilities.

FEMA Letter of Map Revision

(CLOMR/LOMR):

A CLOMR/LOMR may be required to show the changes to the floodplain resulting from the culvert improvements if no-rise conditions are not supported by hydraulic modeling.

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 21

Recommendations

By comparing the improvements to level of service, ponding depth of the Northwinds culverts, and total project cost, Option 5: Extended Dry Detention Facility is the most effective option to address the concerns of frequent ponding of the existing culverts at Northwinds Parkway. This option does not improve the level of service of the culverts, however it provides the greatest decrease in ponding during non-storm events to allow for better inspection and maintenance of the culverts. Any redesign efforts to implement Option 5 should include an analysis of the water quality volume requirements and ensure that the GSWMM standards are still met for any pond retrofits, and it is recommended that detailed outlet control structure sizing be conducted to confirm that the retrofitted OCS can safely pass the channel protection, overbank flood control, and extreme flood protection events. Since this pond is privately owned, coordination with the City and the property owners is required for this option. The City may also consider requiring the implementation of Option 5 prior to any private developments using the facility for any future stormwater management measures.

N o r t h w i n d s P a r k w a y P o n d A n a l y s i s 22

Attachment A:

4/24/2024 Survey Data