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Hydrology Analysis

Hydrology

Stormwater Collection Requirements

What is the First Flush phenomena?

The first flush phenomenon is the initial stormwater runoff that contains the highest pollutants.

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How Does It Relate to GSI?

  • GSI filters the water at the source rather than rerouting it to the local rivers or ocean.

  • GSI is designed to the volume of the first flush stormwater

LA Hydrology Map

The Los Angeles Hydrology Map displays that in the site’s region, a 1 inch, 85th percentile governs over the 0.75 inch.

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All BMP’s were designed according to this criteria, and aligns with the MS4 permits.

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LA Hydrology Map

Rain Gauge Data

  • NOAA

    • Local rainfall monitoring

    • Rain gages placed in various locations measure

      • intensity

      • depth of rain events

Data extraction

The rain gage nearest to our site was chosen as data most relevant to the our site conditions and geographic relevance.

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LA County Hydrology Manual

A 10-year storm event translates to the annual 10% chance of having a storm event of the “10-year storm” magnitude. The precaution of a 10-year storm event was taken to reduce overall flooding on our site and maximize stormwater capture. 

Using Appendix C and D of the Los Angeles Hydrology Manual, my team and I classified our site as having an impervious coefficient of 91, based on site characteristics and used 10-year storm event intensities to get an average runoff coefficient of 0.1. These values were then plugged into the equation shown to determine developed runoff coefficients which were averaged to be 0.83. 

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Rational Method 

Q=CiA

Q, flowrate in cubic feet per second

C, developed runoff coefficient

i, intensity for 1 hour,10-year storm event

A, Area of subbasin

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C = 0.83

i = 0.964 in/hr

A = subarea measurement in acres

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Delineation

Subareas used for the rational method calculation yielded a flowrate for each subarea

  • The highest flowrate calculated was 2.54 cubic feet per second for our proposed onsite parking lot

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LA County Hydrology Manual

Using the same manual, we used our calculated developed runoff coefficient, and 1-hour, 10-year storm event intensity to find flowrates for our delineated site sub basins. Our highest flowrate was calculated to be 2.54cfs for our proposed onsite parking lot represented by our subbasin 7.

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Similarly, volume calculations were done for our subbasins using the same parameters, except for the precipitation depth of 1-inch, corresponding to the total capture an 85th percentile storm event. Subsequently, the summation of our drainage areas results in a total capture volume of about 300 thousand gallons.

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Rational Method

V= C (P/12) A

V, direct runoff volume

C, developed runoff coefficient

P, precipitation depth

A, area of subbasin

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V = volume, acre-feet

C = 0.83

P = 1 inch, simplified 85th percentile

A = subbasin drainage area, acres

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85th Percentile Volumes

Subareas used for the rational method calculation yielded volume for each subarea

  • Sum of the volumes for each subarea

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Total Volume:

40,277 cubic feet (301,279 gallons)

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LID Hydrology Summary Table

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Movement of Onsite Runoff

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Diagram showing the movement of runoff through our site with each of the GSI improvements draining into the CDS unit and eventually the cistern. We ensured that all runoff receives treatment from the CDS unit before entering our cistern. 

Site Pipe Layout

  • Stormwater is captured by LID Infrastructure​

  • Pretreated to store in harvesting system​

  • Significantly reducing both municipal water demand and local runoff pollution.

Hydrology

Stormwater Collection Requirements

Quality

To establish the project’s design criteria, the site’s hydraulic requirements were first determined. This included calculating the Initial Flow Rate (Q) and identifying the applicable Runoff (Collection) Coefficient for the contributing drainage area. After selecting and preliminarily locating the system components, the Developed Flow Rate was computed, enabling component-specific hydraulic calculations to proceed.

Definitions

* Initial Q: amount of rainwater runoff the site produces under existing, undeveloped conditions.

* Runoff Coefficient: how much rainfall becomes runoff based on the surface type and its ability to absorb water.

* Developed Flow Rate: amount of rainwater runoff the site produces under developed conditions.

Quantity

Once a design criteria (a standard to which we need to adhere to) has been established, we can start to select the design elements that we want to incorporate into the property development, which includes:

Planter Boxes, Bioretention, Tree Boxes, Tree Trenches with Permeable Pavers, Vegetated Channel, Rain Garden, CDS, Rainwater Harvesting Cistern.

Standards

What is our minimum water quality standard? what are we designing the capture system for? what codes, manuals, guidelines are we using? What is our bottom line.

Our Best Management Practices (BMP's)

Rainwater Harvesting Cistern

Stormwater Capture

  • Cistern stores captured stormwater underground

  • Designed for 40,280 ft³ of storage

  • Located beneath the parking lot

  • The DUROMAXX system provides a large capacity with low maintenance

  • The CDS unit removes sediment and debris before water enters the cistern

Vegetated Channel

Stormwater Transport

  • Vegetated channel conveys runoff through the site

  • Located at the center of the site

  • Designed for 20,870 ft³ of stormwater

  • Slows the flow and allows infiltration through vegetation and soil

  • Acts as a pretreatment before water enters the storm chamber

  • Pedestrian bridges allow access across the channel

CDS

Stormwater Filtration

Briefly describe your degree and any other highlights about your studies you want to share. Be sure to include relevant skills you gained, accomplishments you achieved or milestones you reached during your education.

Planter Boxes

Stormwater Filtration

  • Captures roof runoff from buildings

  • Four planter boxes per building arranged in an L-shape

  • Placed along building edges and corners to collect runoff from downspouts

  • Designed for 10,900 ft³ of stormwater

  • Engineered soil supports infiltration and pollutant removal

  • Reduces surface runoff and improves water quality

Tree Trenches

Stormwater Filtration

  • Reduces runoff along busy walkways

  • Tree trenches capture runoff between buildings

  • Two trenches with three trees each, spaced by permeable pavers

  • Designed for 3,890 ft³ of stormwater

  • Structural soil and drainage rock support roots and water flow

  • Permeable pavers allow walking and let water enter the system

Tree Boxes

Stormwater Filtration

  • Captures runoff along the surface parking lot

  • Placed between every 5 parking stalls

  • Designed for 3,110 ft³ of stormwater

  • 26 tree boxes used

  • Engineered soil removes automotive pollutants and supports tree growth

  • System is low maintenance

Rain Garden

Stormwater Filtration

  • Captures runoff from surrounding areas

  • Located at the lower part of the site to collect flow

  • Designed for 12,350 ft³ of stormwater

  • Uses layered soil and vegetation for filtration and overflow management

  • Provides primary treatment for stormwater

Bioretention

Stormwater Filtration

  • Captures runoff from parking lot and receive flow from tree boxes

  • Placed along parking lot edges to prevent sheet flow

  • Designed for 11,575 ft³ of stormwater

  • 36-inch engineered soil removes fine particles and automotive pollutants

  • Effective primary treatment for runoff

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