Hydrologic

The purpose of performing the hydraulic analysis was to gather information on the existing conditions and compare them to the developed site. The information needed was to delineate the watershed, determine the surface coefficients, analysis rainfall data, determine the time of concentration, and determine runoff. All this data is considered to ensure the conditions after development of the site are not harmful to the surrounding area.

Watershed Delineation

The watershed of project site was determined by first using StreamStats to get a general idea of where the watershed was. Watershed was then hand-delineated using an earth point overlay in google earth to get the topography in the area. The exit of the watershed is on the west side of the current parking lot. Once a pdf was made of the watershed area it was plugged in to Bluebeam to calculate the area of the watershed which came out to be 15.66 acres and the area of the current parking lot was determined to be 0.17 acre. The new parking lot area will have an area of 1.43 acres.

Surface Type/Land Use

With the site being much smaller than the overall area of the watershed the analysis of the flow through the area and the time of concentration was split into two different analyses. One analysis was done just considering the area of the site and the other was considered with the watershed.

To start to determine the flow of water over the watershed delineated there are a few values that need to be determined. One of these factors being the Resistant Coefficient of the material that is going to be on the site. The other is the Runoff Coefficient for that same material. The coefficient can be determined based on the depth of water during the storm event. For the materials considered the coefficients for the existing ranged from 0.33 to 0.59, 0.43 to 0.69 for gravel, and 0.94 to 0.95 for asphalt.

The runoff coefficient shown in the table below represents the resistance a liquid will experience when in certain land usages. The coefficients chosen were 0.125 for existing, 0.825 for asphalt, and 0.08 for gravel. The existing was found in “Lawns: Sandy soil, avg., 2-7%” using a value between the range given. The asphalt value was found in “Streets; Asphaltic”, again using a value in between the range. Finally, the gravel was found using the “Drives and walks” using a value in between the range of C values. These values are used in the rational method.

Rainfall Data

The rainfall data was taken from NOAA Atlas 14. This was performed by selecting the region where the project site is located and getting the rainfall data for that location. This data was then placed in the ADOT Rational Tool and used in the calculation of time of concentration hand calculations. The data collected was the depth and intensity of precipitation. The precipitation depth was needed to plug into the rational tool and to find the coefficients for the hand calculations of the flow rate for each storm. The intensity data was used to check the rational tool time of concentration calculations.

Time of Concentration

The time of concentration was determined using ADOT’s method in the drainage manual. First, the time of concentration values were determined from the rational tool. Then, the rainfall intensities were used for each storm year and duration. The time of concertation that was closest to the storm duration for the intensity used was selected for the rainfall intensity for that storm event. First considering the values of just the site the Kb factor will be constant, depending on the material, along with the flow path, and the intensity. The constant values thought the materials were the length being 0.07 miles and slope being 342.86 ft/mile. the Time of Concentration is smaller as the intensity of the storm grows larger. With the material with less infiltration the time of concentration will be smaller not allowing the water to be disturbed by any friction due to materials or infiltration of water into the material that it is flowing over.

When considering the entire watershed, the length of flow path for the watershed from the watershed delineation was calculated at 0.303 miles. The flow path is the longest path in the watershed that precipitation would have to travel. The slope of the longest flow path is 231.02 ft/mile. To determine this value the elevation, change between the lowest and highest part of the flow path was divided by the longest flow path. For the storm intensity used the values ranged from 10 min to 1 hour. The minimum of 10 minutes was used because the drainage manual used stated to use 10 as the minimum even if a lower time of concertation was determined. The coefficient for the watershed to be used in the calculation was determined from the table below. The coefficient was selected to be 0.2 foothills because from the site visit the team determined that area was hilly but, with slope between 10-50 percent. The Time of Concentration is smaller as the intensity of the storm grows larger. With the material with less infiltration the time of concentration will be smaller not allowing the water to be disturbed by any friction due to materials or infiltration of water into the material that it is flowing over.

Runoff Flow Determination

The flow of a material is very important to determine if any adjustment needs to be made so that farther down the watershed there is no disturbance in the area. The flow of the material can be determined through the rational method. The data for the watershed was first placed into the ADOT rational tool to calculate the runoff generated by development. The results of the tool estimated that the runoff from the pre and post development were the same. After the rational tool was used the calculations were checked by hand. The parking lot area for pre-development conditions is 0.17 acre and 1.43 acre for the post development. The values that will be used for the hydraulic analysis and compliance with codes will be the hand calculated values for the flow rate. All of the storm events are within 1 cubic feet per second of each other. The material used in the post development is the asphalt because it has a higher coefficient of runoff.

Hydraulic

From the site visit and analysis, the team found that there isn’t a drainage feature in the project site rather the stormwater runs off overland with a minimal amount of water runoff going into a small channel on the western border of the site. That channel begins on the western border of the project site and runoffs southwest which resides on private land. With the project requirements and constraints being that the engineering team cannot design on private land, a retention or detention basin cannot be included in a final design as the area would take up the parking lot location.

In section 1.2 of Design Standards for Stormwater Detention and Retention for Pima County a detention is not necessary if a waiver is granted for the parking lot being because site has less than two residences per acre and maintains natural drainage patterns. With there being two different road surface designs, the team will be changing the runoff coefficient, which increases the amount of runoff from the pre-existing ground. The gravel runoff coefficient used is 0.3 from the table of runoff coefficients for Rational Method.

To deal with the increase of stormwater runoff, the team has designed a riprap that essentially increases roughness on the surface to slow down the velocity of the draining water. With the impervious surfaces being implemented, the velocities needed to be slowed.

Using D50 6' Diameter riprap helps the team achieve that goal.