The Final Project Design page includes the Site Investigation, the Hydrologic Analysis, the Proposed Storm Drainage Design, the final Hydraulic Analysis, the Final Proposed Design, and the Cost Estimate to Construct.
JAMM Engineering conducted an initial site investigation on January 26th, 2022. No fieldwork, such as surveying, was necessary for this investigation. Based on the initial site investigation and analysis of previous studies, it was determined that Grandview Drive currently cannot convey the 25-year storm runoff.
The current drainage system on Grandview Drive is a paved road, a roll curb and gutter, and two catch basin inlets at the end of the road near Cedar Avenue. Click on the button below for a detailed description of the current road and drainage system conditions.
The Coconino County Parcel Viewer was used to hand delineate the watershed. In Figure 1, the red line is the watershed boundary. The blue line in the middle of the watershed is the longest flow path at 3820.5 feet. The watershed point of concentration was chosen to be the intersection of Grandview Drive and Cedar Avenue because this is where the flow leaves Grandview Drive. The time of concentration for the longest flow path for the 25 year storm event was determined to be 0.13 hours or 7.8 minutes. This information was also used to determine the flow on the road by using the Rational Method Tool. The flow was determined to be 211 cubic feet per second. A more detailed description of the hydrologic analysis is contained in the Final Report on the Documents page.
Figure 1: Watershed Delineation
Design Alternatives
Two decision matrices were created to determine the final design. The first one is Table 1 below, which compares the corrugated metal pipe (CMP) types that could be used for the drainage inlet design. The chosen type is the Spec Dual Wall CMP - 18 Guage, Steel-Lined, Polymer-Coated with a diameter of 24 inches. This type of CMP has a high strength and it is designed to allow the water to flow with minimum resistance.
Table 1: CMP Decision Matrix
The second decision matrix was created to compare the drainage design alternatives. The first proposed design consists of a trunk line, or main pipe line, with three laterals to carry water from the road to Spruce Avenue Wash. In this design, the trunk line would have an inlet at the top of Grandview Drive and an outlet at the bottom of the road, connecting to the existing catch basins. The second design includes three lateral pipes and raising the curb and gutter by two inches along the entire road. The third design includes a shorter trunk line connecting the three lateral pipes. Instead of having an inlet at the top of the road and outlet at the bottom, this design will only have inlets for each lateral pipe. Figures of these three designs can be seen in the Final Report on the Documents page.
The criteria being considered for the decision matrix are the Conservation of Property, Affordability, Limited Maintenance Upkeep, Limited Environmental and Social Impacts, and Conveyance. Each criterion is weighed equally because they are all important parameters that need to be considered in the design. If the criteria were to be weighed differently, one might receive less attention than the others and result in unforeseen impacts. Each criterion will be scored 1-5, 1 being the worst, 3 being moderate, and 5 being the best. The score for each category will be summed for each design and the one with the highest score will be chosen as the final design
Table 2: Drainage Design Decision Matrix.
Based on the results in the decision matrix, it was determined that Design 2 is the best option because it received the highest score. This design uses the least amount of property, it is the most affordable option, it has the least amount of time and money required for its maintenance upkeep, has more positive societal and environmental impacts than negative ones, and can effectively convey stormwater flow. This was also the only design to propose changes to the curb and gutter. This design will prevent severe flooding on the road and enable the highest conveyance of water from the road to Spruce Avenue Wash.
Hydraulic Analysis
A hydraulic analysis was conducted using HEC-RAS to determine the surface flow on Grandview Drive after implementing the design changes. The changes to the road include increasing the curb height by two inches along the entire road and placing three proposed laterals that convey water from the road to Spruce Avenue Wash. The information this analysis provides includes water surface elevation, velocity, flow area, Froude numbers, and other water and flow characteristics
The cross sections at the steepest and flattest areas on Grandview drive were analyzed to determine if they are compliant with City of Flagstaff stormwater standards following the design changes. These standards include that the maximum allowable velocity for a road cannot exceed 18 ft/s and that Froude numbers for concrete lined roads should not exceed 2
The cross section at the steepest area on Grandview Drive is a Station 17+00 with a slope of 2.38%. The velocity at this cross section is 9.45ft/s and it has a Froude number of 2.24. The velocity is compliant with City of Flagstaff standards but the Froude number exceeds the standard. However, as seen in Figure 2 below, the road is capable of conveying the 25-year storm flow at this cross section after increasing the curb height and placing the three lateral pipes.
Figure 2: Cross section at 17+00 modeled in HEC-RAS.
The cross section at the flattest area on Grandview Drive is a Station 28+00 with a slope of 1.02%. The velocity at this cross section is 7.98 ft/s and it has a Froude number of 1.6. Both these parameters are compliant with City of Flagstaff standards. As seen in Figure 3 below, the increase in curb height is still below the 25-year storm flow level. However, this cross is located above any of the lateral pipes, so the road does not experience a decrease in flow at this location.
Figure 3: Cross section at 28+00 modeled in HEC-RAS.
Final Proposed Design
The final proposed design includes raising the curb height by two inches and placing three lateral pipes to convey water from the road to Spruce Avenue Wash. Click the button below to view the details of the proposed design, including materials, dimensions, and Maricopa Association of Government (MAG) standards used. The Final Design Plans are on the Documents page.
The cost to prepare and form a rolled 6 inch concrete curb and gutter, including the material and labor costs, is estimated to be $17.17 per foot. The total cost to construct the rolled curb and gutter along the entire road is $61,572.31.
A Spec Dual Wall Corrugated Metal Pipe (CMP) that is 18 Guage, Steel-Lined, Polymer-Coated and 24 inches in diameter costs $20.16 per foot. The estimated cost of purchasing and putting the lateral pipes into the ground is $10,886.20.
The cost to construct each concrete casing is $1,800 for a total of $5,400 for all three concrete casings.
The total cost of construction is estimated to be $77,858.51. The break down of the cost is recorded in the table below.
Road and Drainage Conditions
