applicant hearing exhibit #2 - home | dekalb county, ga...shoreline of echo lake at 2267 echo trail...

Applicant Hearing Exhibit #2

Narrative

NARRATIVE:

The purpose of this flood study is to determine the effects on flood water elevations of installing a

gabion basket erosion control measure along the water edge of Echo Lake. A gabion basket erosion

control measure was determined to be the best option to control the erosion from wave action, beaver

activity in the area, and the erosion from ground water entering the dry lake bed when Echo Lake is

periodically drained. The shoreline stabilization options were discussed with Peggy Chambers and

Michael Berry at the Georgia Department of Natural Resources, Environmental Protection Division

prior to the buffer encroachment approval. At the end of the discussion, it was determined that a

structural stabilization method should be implemented as stated in the approval letter from the

D.N.R. dated March 10, 2016. Condition #7 declares “This project must be conducted in accordance

with the Streambank and Shoreline Stabilization Guidance for a Level III Stabilization method”.

The proposed gabion basket erosion control measure shall be located in the rear yard along the

shoreline of Echo Lake at 2267 Echo Trail in the 18th District of DeKalb County, Georgia. Echo

Lake was formed by constructing a dam across the North Fork Peachtree Creek Tributary D-1

approximately 1900 feet west of Briarcliff Road. The August 14, 2017 HEC-RAS project file

produced by Dewberry & Davis was used to establish the existing conditions flood elevations at the

cross section located through the subject property, cross section 29+74. DeKalb County considers

this the 2025 future flood, since the data has not been approved as of the date of this study. To

analyze the effects of the proposed dock and gabion wall, cross section 29+74 was copied and placed

20 feet downstream to create cross section 29+54. The gabion wall profile information was then

added to cross sections 29+54 and 29+74. The difference in elevations between the 2 year storm

event and the dam spillway elevation was used to determine the normal water surface elevation at the

dam cross section. This difference was then used to determine the normal water surface elevation at

the cross section at the subject property. At the normal water surface elevation, the cross section

geometry was raised 6 inches (on the house side of the wall), then at a level elevation 3 feet toward the

lake, and then straight down to meet the existing grade. The new cross section profiles were then

used to re-run the flood study to verify that no change occurred to the flood elevations or flow

velocities. All other cross sections and flow data remained the same as the existing conditions shown

on the Dewberry & Davis HEC-RAS flood data.

A total of 16.9 cu.yd. of soil within the floodplain shall be removed to compensate for the volume of

the proposed gabion wall. This volume is less than the maximum compensation allowed as shown in

Table 1 below. Compensation removal is not needed to demonstrate a no-rise condition.

Table 1 - COMPENSATION VOLUME SUMMARY Area of property in floodplain = 6197 sq.ft. = 0.1423 ac

Volume of compensation limit (150 cu.yd. per acre) = 21.34 cu.yd.

Approx. Volume of gabion baskets above grade = 16.9 cu.yd. (includes 40% voids)

Total Volume to be Compensated = 16.9 cu.yd.

The proposed gabion basket wall does not change the flood elevations or velocities for the 2 – 100

year storm events as shown in Table 2 below.

Table 2 - WATER SURFACE & VELOCITY SUMMARY

GABION BASKET WALL

River River Sta.

Storm Event

Q Total

Existing W.S. Elev.

Proposed W.S. Elev.

W.S. Difference

Existing Velocity

Proposed Velocity

Velocity Difference

(cfs) (ft) (ft) (ft/s) (ft/s)

NFPC_TD-01

22+92

002yr 495.47 858.799 858.799 0.000 5.4 5.4 0.00 NFPC_TD-01 005yr 887.7 859.975 859.975 0.000 5.62 5.62 0.00 NFPC_TD-01 010yr 1326.92 860.985 860.985 0.000 5.42 5.42 0.00 NFPC_TD-01 025yr 1917.07 861.976 861.976 0.000 5.63 5.63 0.00 NFPC_TD-01 050yr 2356.8 862.589 862.589 0.000 5.85 5.85 0.00 NFPC_TD-01 100yr 2832.88 863.322 863.322 0.000 5.91 5.91 0.00

NFPC_TD-01

24+30


NFPC_TD-01 25+33 Echo Lake Dam Structure

NFPC_TD-01

25+43


NFPC_TD-01

26+37


NFPC_TD-01

29+54


NFPC_TD-01

29+74


NFPC_TD-01

33+25

002yr 549.72 871.835 871.835 0.000 0.14 0.14 0.00

NFPC_TD-01 005yr 938.25 872.372 872.372 0.000 0.22 0.22 0.00

NFPC_TD-01 010yr 1420.39 872.881 872.881 0.000 0.3 0.3 0.00

NFPC_TD-01 025yr 2066.58 873.487 873.488 0.000 0.4 0.4 0.00

NFPC_TD-01 050yr 2537.53 873.906 873.906 0.000 0.47 0.47 0.00

NFPC_TD-01 100yr 3020.4 874.325 874.325 0.000 0.53 0.53 0.00

CONCLUSION:

The roller compacted dam for Echo Lake established the flood elevation within the lake; therefore,

minor fill as proposed has a no-rise effect on the flood elevations and no increase in flow velocities as

demonstrated in this analysis.

Pre Developed Conditions

Main 111261067110296

99379392

90238550814473327002

627453715012

46234402

39113325

29742637

19521611

1214920

484159

NFPC_TD- 01

Main

77487151

68096692

63625735544150774892

4353

3957

32052876

23541980

13931073

831532327

N

FPC_TD-02

Main

431740193605326728422503

2199

20541873

1325

1043706

380145

NFPC_TD-03

Main

397437713581

336931412944

2581

23661723

1417

1060

301

NFPC

_TD

2.1

2267 Echo Trail

3911

3325

2974

26372430

2292

Echo Lake Dam

2267 Echo Trail

0 100 200 300 400 500 600865

870

875

880

885

890

895

900

NFPC_TD_01 Hydraulics Model Plan: EX - Multi Profile 3/13/2019

Station (ft)

Elev

atio

n (ft

)

Legend

EG 500yr

WS 500yr

EG 200yr

WS 200yr

EG FU100YR

WS FU100YR

EG 100yr

WS 100yr

EG 050yr

WS 050yr

EG 025yr

WS 025yr

EG 010yr

WS 010yr

EG 005yr

WS 005yr

EG 002yr

WS 002yr

Ground

Bank Sta

.08 .03 .08

110 120 130 140 150 160

871

872

873

874

875

NFPC_TD_01 Hydraulics Model Plan: EX - Multi Profile 3/13/2019

Station (ft)

Elev

atio

n (ft

)

Legend

EG 500yr

WS 500yr

EG 200yr

WS 200yr

EG FU100YR

WS FU100YR

EG 100yr

WS 100yr

EG 050yr

WS 050yr

EG 025yr

WS 025yr

EG 010yr

WS 010yr

EG 005yr

WS 005yr

EG 002yr

WS 002yr

Ground

Bank Sta

.08 .03

Post Developed Conditions

Main 111261067110296

99379392

90238550814473327002

627453715012

46234402

39113325

29742637

19521611

1214920

484159

NFPC_TD- 01

Main

77487151

68096692

63625735544150774892

4353

3957

32052876

23541980

13931073

831532327

N

FPC_TD-02

Main

431740193605326728422503

2199

20541873

1325

1043706

380145

NFPC_TD-03

Main

397437713581

336931412944

2581

23661723

1417

1060

301

NFPC

_TD

2.1

2267 Echo Trail

0 100 200 300 400 500 600865

870

875

880

885

890

895

900

NFPC_TD_01 Hydraulics Model-PR Wall Plan: Plan 06 3/11/2019

Station (ft)

Elev

atio

n (ft

)

Legend

EG 500yr

WS 500yr

EG 200yr

WS 200yr

EG FU100YR

WS FU100YR

EG 100yr

WS 100yr

EG 050yr

WS 050yr

EG 025yr

WS 025yr

EG 010yr

WS 010yr

EG 005yr

WS 005yr

EG 002yr

WS 002yr

Ground

Bank Sta

.08 .03 .08

120 130 140 150 160

869.5

870.0

870.5

871.0

871.5

872.0

872.5

NFPC_TD_01 Hydraulics Model-PR Wall Plan: Plan 06 3/11/2019

Station (ft)

Elev

atio

n (ft

)

Legend

EG 500yr

WS 500yr

EG 200yr

WS 200yr

EG FU100YR

WS FU100YR

EG 100yr

WS 100yr

EG 050yr

WS 050yr

EG 025yr

WS 025yr

EG 010yr

WS 010yr

EG 005yr

WS 005yr

EG 002yr

WS 002yr

Ground

Bank Sta

.08 .03

Proposed GabionWall

0 100 200 300 400 500 600865

870

875

880

885

890

895

900

NFPC_TD_01 Hydraulics Model-PR Wall Plan: Proposed Wall 3/13/2019

Station (ft)

Elev

atio

n (ft

)

Legend

EG 500yr

WS 500yr

EG 200yr

WS 200yr

EG FU100YR

WS FU100YR

EG 100yr

WS 100yr

EG 050yr

WS 050yr

EG 025yr

WS 025yr

EG 010yr

WS 010yr

EG 005yr

WS 005yr

EG 002yr

WS 002yr

Ground

Bank Sta

.08 .03 .08

100 120 140 160

870

871

872

873

874

875

876

NFPC_TD_01 Hydraulics Model-PR Wall Plan: Proposed Wall 3/13/2019

Station (ft)

Elev

atio

n (ft

)

Legend

EG 500yr

WS 500yr

EG 200yr

WS 200yr

EG FU100YR

WS FU100YR

EG 100yr

WS 100yr

EG 050yr

WS 050yr

EG 025yr

WS 025yr

EG 010yr

WS 010yr

EG 005yr

WS 005yr

EG 002yr

WS 002yr

Ground

Bank Sta

.08 .03

Appendix A: Dock & Gabion Basket Analysis

APPENDIX A:

Although a boat dock is not proposed at this time, it is included in the study to illustrate that it does

not increase the water surface elevation or flow velocit of North Fork Peachtree Creek. A bridge

section with the dimensions of the proposed dock was added at station 29+64. The new cross section

profiles, along with the dock were then used to re-run the flood study to verify that no change

occurred to the flood elevations or flow velocities as shown in Table 4 below. The volume of the

dock was also calculated and shown in Table 3 below. The volume of the gabion baskets and dock is

less than the compensation limit, therefore an increase to the compensatory grading allowance is not

required.

Table 3 - COMPENSATION VOLUME SUMMARY Area of property in floodplain = 6197 sq.ft. = 0.1423 ac

Volume of compensation limit (150 cu.yd. per acre) = 21.34 cu.yd.

Approx. Volume of gabion baskets above grade = 16.9cu.yd. (includes 40% voids)

Approx. Volume of dock above grade = 2.8 cu.yd. Total Volume to be Compensated = 19.7 cu.yd.

Table 4 - WATER SURFACE & VELOCITY SUMMARY

GABION BASKET WALL & DOCK

River River Sta.

Storm Event

Q Total

Existing W.S. Elev.

Proposed W.S. Elev.

W.S. Difference

Existing Velocity

Proposed Velocity

Velocity Difference

(cfs) (ft) (ft) (ft/s) (ft/s)

NFPC_TD-01

22+92


NFPC_TD-01

24+30


NFPC_TD-01 25+33 Echo Lake Dam Structure

NFPC_TD-01

25+43


NFPC_TD-01

26+37


NFPC_TD-01

29+54


NFPC_TD-01 29+64 Dock

NFPC_TD-01

29+74


NFPC_TD-01

33+25

002yr 549.72 871.835 871.835 0.000 0.14 0.14 0.00

NFPC_TD-01 005yr 938.25 872.372 872.372 0.000 0.22 0.22 0.00

NFPC_TD-01 010yr 1420.39 872.881 872.881 0.000 0.3 0.3 0.00

NFPC_TD-01 025yr 2066.58 873.487 873.488 0.000 0.4 0.4 0.00

NFPC_TD-01 050yr 2537.53 873.906 873.906 0.000 0.47 0.47 0.00

NFPC_TD-01 100yr 3020.4 874.325 874.326 0.000 0.53 0.53 0.00

CONCLUSION:

The roller compacted dam for Echo Lake established the flood elevation within the lake; therefore,

proposed dock and gabion basket wall has a no-rise effect on the flood elevations and no increase in

flow velocities as demonstrated in this analysis.

3325

2974

26372430

2292

1952

Sta. 2964 -Proposed Dock

2267 Echo Trail

0 100 200 300 400 500 600865

870

875

880

885

890

895

900RS=2964 Upstream (Bridge)

Elev

atio

n (ft

)Legend

Ground

Bank Sta

0 100 200 300 400 500 600865

870

875

880

885

890

895

900RS=2964 Downstream (Bridge)

Station (ft)

Elev

atio

n (ft

)

120 140 160 180

866

868

870

872

874

876

RS=2964 Upstream (Bridge)El

evat

ion

(ft)

Legend

Ground

Bank Sta

120 140 160 180

866

868

870

872

874

876

RS=2964 Downstream (Bridge)

Station (ft)

Elev

atio

n (ft

)

Proposed Dock

Proposed GabionWall

applicant hearing exhibit #2 - home | dekalb county, ga...shoreline of echo lake at 2267 echo trail...

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