4._Corrected_Final_Civil_Narrative.pdf

CIVIL ENGINEERING DESIGN

FGWB 10-0030

REPAIR STORM DRAINAGE

FINAL DESIGN

Existing Conditions:

Area A:

Both the north and south side of Gym road is severely eroded due to lack of stabilization and storm water conveyance. The north side has a drainage swale between the roadway and sidewalk which fills up with sediment. The sidewalk separates two swales which are linked together at the bottom of the slope with a culvert which is not functioning.

Area B:

A stream adjacent to Bldg 600 is currently being piped under the parking lot. On the downstream side of the pipe the discharge has caused erosion around and under a retaining wall. Also the velocity of the discharge has caused a separate channel to be created diverting the flow directing into Little Lake. Originally the stream bypassed Little Lake, as per our delineation. The original stream needs to be reestablished and future diversion eliminated.

Area C:

The existing drainage channel alongside Spill Pond 3 has large amounts of sediment limiting the required flow during certain storm events. The upstream flow goes directly into Spill Pond 3 by being blocked, from the drainage channel, by a weir wall. This weir wall was constructed in the

1970s and reconstructed in 2011 to allow the larger storm events to bypass the spill pond and flow directly through the drainage channel.

Area D:

The site adjacent to the running track is used for USAF Civil Engineering training. Due to the heavy equipment movement the area stays disturbed. Rills have developed and sediment is being discharged into the downstream wetland.

1065223550C Typewritten Text 13 December 2013

Proposed Improvements:

Area A:

The proposed improvements include the construction of 24” concrete curb and gutter along both sides of Gym road from the second entrance to the parking lot for Building 401 down to the

Rottenwood Creek crossing. On the northern side of Gym Road the existing sidewalk will be removed along the new curb & gutter. The existing drainage channel will be regraded to provide a positive slope for Stormwater conveyance to the Rottenwood Creek. This drainage channel will be armored with stone to not only eliminate the erosion but to also slow the velocity of the drainage conveying down the steep slope towards Rottenwood Creek. At the end of the curb and gutter on the northern side the existing inlet and piping will be removed and replaced with a new curb inlet and drainage piping to direct the road drainage back to the armored swale allowing the drainage to flow to Rottenwood Creek. Intermediate drainage flumes with sidewalk scuppers will be installed down the road to aid in the interception of drainage. On the southern side of

Gym Road the existing drainage channel behind the new curb and gutter will be regraded to provide a positive slope for Stormwater conveyance to the Rottenwood Creek. This drainage channel will be armored with stone to not only eliminate the erosion but to also slow the velocity of the drainage conveying down the steep slope towards Rottenwood Creek.

Area B:

The proposed improvements include the removal of existing debris that is causing the drainage flow to be diverted into Little Lake. The existing drainage channel adjacent to the 60” outfall pipe will be regraded in order to install the rip rap outlet protection to not only eliminate the erosion issues and slow the velocity of the drainage flow.

Area C:

The proposed improvements include the removal of all sediment and debris in the existing drainage channel alongside Spill Pond 3 from the existing weir structure to the outfall pipes of

Spill Pond #3 and re-grade to provide a positive constant slope. The new channel shall have a 5’ wide bottom and 5:1 side slopes for maintenance availability.

Area D:

The proposed improvements include the removal of a portion of the existing earth mound providing a clear area between the toe of the earth mound and the existing tree line. A terrace will be constructed approximately halfway up the mound to reduce the velocity of any runoff.

Also to prevent further erosion encroachment into the wetland area, jersey barriers shall be placed along the top of the bank at the existing tree line. To handle the drainage a filter ring will be installed at the low point in front of the jersey barriers. An 8” diameter HDPE pipe will be placed from the filter ring down the slope towards the wetlands to allow the drainage an outfall but to prevent sediment from entering the wetlands.

Calculations:

Gutter Flow Calculations

Uniform Cross Slope Procedure:

Concrete gutter-Asphalt Pavement – Manning’s n value (n) = 0.015

Cross Slope (Sx) = 0.023

Longitudinal Slope (S) = 0.066

Spread (T) = 1.5 ft

Q = 0.56 x Sx 1.67 x S 0.5 x T 2.67 n

Q = 37.33 x .002 x .257 x 2.95

Q= .057 cfs

Capacity of Inlet

Q = rate of discharge into inlet (cfs)

C = 0.67, orifice coefficient

A = clear opening area of inlet (28.08 ft g = 32.2 ft/s d = depth of water above grate (ft)

CFS = factor of safety for clogging, 1.0

Q = CA(2gd) 0.5

CFS

Solving for d:

d = (QCFS/CA)

2g d = (0.057/18.81)

64.4 d = 1.15

64.4 d = .018 ft, or 0.21 in

Length of Curb Inlet Required

Lr = length of curb inlet required (ft) z = 0.6, for English

Q = Flow Rate in gutter (cfs)

S = Longitudinal slope (ft/ft)

Sx = Cross Slope (ft/ft) n = Manning’s n value (n)

Lr = zQ 0.42

S 0.3

(1/nSx) 0.6

Lr = 0.6 x 0.057 0.42 x 0.066 0.3 x (1/(0.015 x 0.023)) 0.6

Lr = 0.6 x 0.300 x 0.442 x 119.48

Lr = 9.51 ft

Outlet Protection Calculations

Hydraulic Capacity:

Where:

n = 0.012

A = 19.63 sf

P = 15.71 ft

R = 1.25 ft

S = 0.0043

Manning’s Equation – Full Flow Velocity

Vfull = (1.486) x R 2/3 x S 1/2

= 123.83 x 1.16 x .064855

= 9.32 fps

Manning’s Equation – Full Flow Discharge

Qfull = (1.486) x A x R 2/3 x S 1/2

= 123.83 x 19.63 x 1.16 x .064855

= 182.95 cfs

Rock Riprap Apron Calculations

Where:

Q = 182.95 cfs d = 60 inches d50 = 1.2 ft

From attached graph:

Apron Length (La) = 34 ft

Upper Width = 3d = 15 ft

Lower Width = d + La = 49 ft