storm water management and drainage report

8/2/2019 Storm Water Management and Drainage Report

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STORM WATER MANAGEMENT AND DRAINAGE

Hydrology

The site is located in the Pichincha Province northwest of Quito, between the towns of Calacali and Nanegalito.

Quito and the surrounding areas receive approximately 1234 mm of rainfall annually. Available Rainfall Intensity

Duration Frequency (IDF) curves for the Pichincha region indicate the following rainfall intensities for a five (5)

minute storm duration/time of concentration:

Return Period (Yr.) Intensity (mm/hr)

2 85

5 100

10 110

20 120

50 135

These values were obtained from the IDF curves shown below:

Existing Drainage Facilities and Site Conditions

The existing storm water management facilities on-site are generally limited to roadside conveyance swales.

These are located on both sides of the private access roads and consist of vegetated swales and unreinforced

concrete channels. The concrete channels are used predominantly in areas with flatter slopes and smaller

contributing drainage areas. Vegetated channels are utilized in areas with steeper slopes and larger drainage


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areas, and appear to function well due to the ease in establishing vegetative growth. Erosion appears to be

limited to areas that have been recently disturbed to create building pads.

A significant earth slide near the camp entrance is threatening the stability of the private road serving the site.

Therefore, the proposed improvements will include relocating the private road to an area away from the slide,

and directing upland runoff away from the area through grading and the installation of a storm waterconveyance pipe.

Proposed Drainage Improvements

The proposed storm water improvements for this phase of construction will consist of conveyance facilities at

the proposed multi-use Saln building, and a storm drainage pipe to convey runoff through the proposed

parking area near the camp entrance. Concrete channels are currently being utilized to collect and convey roof

runoff from several of the existing buildings. The channels discharge into an inlet box and the flow is dispersed

via a PVC sewer pipe into a heavily vegetated area away from the building. This method appears to be working

well to address localized drainage around the buildings, so a similar system has been proposed for the Saln

building. All proposed storm water facilities have been sized to convey the 50-year storm event. Where the

contributing drainage areas contain future improvements (as indicated on the Master Plan), the runoff

calculations have accounted for the additional impervious surface.

Runoff Calculations:

Proposed Pipe at Parking Area:

Drainage Area = 13,498 m2 (see figure below)


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Calculate CA Value:

Cover Area (m2) Area (hect) C Value CxA

Impervious (Road) 1,410 0.1400 x 0.93 = 0.1311

Grass, >10% 1,300 0.1300 x 0.35 = 0.0455

Grass,


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Calculate CA Value:

Cover Area (m2) Area (hect) C Value CxA

Impervious (Roof) 620 0.0620 x 0.93 = 0.0576

Grass, >10% 130 0.0130 x 0.35 = 0.0046

Total 750 0.0750 0.0622

Calculate Peak Flow:

Q = kCIA = 0.00278 x I x CA = 0.00278 x 135 x 0.0622 = 0.024 m2/sec

(where I = 135mm/hr for 50 yr. storm event, Tc = 5 minutes)

Hydraulic Design of Conveyance Facilities:

Proposed Pipe at Parking Area:

Try 600mm diameter SLPEP at Slope = 8.50%, n=0.012:

QFULL = 1.77 m3/sec > 0.148 m

3/sec (O.K.)

Check Orifice Control at Pipe Entrance, H = 1.00 meter, C = 0.60:

QMAX = 0.751m3/sec > 0.148 m

3/sec (O.K.)

Check Velocity at Discharge based on Actual Flow:

VMAX = 3.89 m/sec = 12.76 fps (Use R-6 Stone Apron at discharge point per chart below)


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Proposed Conveyance Facilities at Multi-Use Building:

Try Rectangular Concrete Channel, Width = 50cm, Depth = 25cm, Slope = 1.00%, n=0.010:

QFULL = 0.310 m3/sec > 0.024 m

3/sec (O.K.)

Check Weir Control at Entrance into Drainage Box, Weir Length = 20 cm, H = 0.25 meters:

QMAX = 0.035m3/sec > 0.024 m

3/sec (O.K.)

Size Pipe from Drainage Box based on Full Flow Capacity, Try 315mm dia. sewer pipe, n=0.012, Slope = 4.50%

QFULL = 0.250 m3/sec > 0.024 m

3/sec (O.K.)

Check Orifice Control at Pipe Entrance, H = 1.00 meter, C = 0.60:

QMAX = 0.207m3/sec > 0.148 m

3/sec (O.K.)

storm water management and drainage report

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