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Within view of the San Francisco Peaks, Spring Valley Road at Red Lake Wash commonly experiences runoff from large storm events. The current drainage facility has been proven to be inadequate to handle these large storm events. In order to rectify the situation, the Coconino County Public Works Department has requested that a new drainage facility, a bridge-type box culvert, be designed to better accommodate larger storm events. The four existing corrugated metal pipes will be replaced by new, pre-cast box culverts. The exact size and efficiency of this culvert was found by using software designed for both hydrologic and hydraulic analysis. This new drainage facility will be more aesthetically pleasing than the existing culverts, improve drainage conditions, and provide the much needed scour protection for the wash bed and roadway embankment.

Existing Conditions

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Northern Arizona University
Flagstaff, AZ 86011

Phone: 520-955-3941
Email: grg28@nau.edu

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Spring Valley Road will need to be closed during construction at Red Lake Wash, so to ensure that residents can still access the area, a traffic control plan was designed. This traffic control plan includes all of the signage necessary as specified by the Manual on Uniform Traffic Control Devices (MUTCD). Traffic on the east side of the project will be routed up Pine Street to Hoctor Road, where drivers can easily access State Route 64. Drivers on the west side of the project will still be able to access State Route 64 by using Spring Valley Road.

Traffic Control Plan

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Bentley PondPack was used in this analysis to determine the runoff flow rate. A design storm was created within the PondPack software. The method used in PondPack was the Soil Conservation Service (SCS) Type-II 24-hour rainfall curve. To create this storm, the rainfall depth during a 50-year 24-hour storm was needed. This information was found on the National Oceanic and Atmospheric Association’s (NOAA) website. NOAA Atlas 14 was used to determine the design storm. Because of the close proximity of Red Lake Wash to the area of Williams, Arizona, the data for Williams was used. Each sub-basin was inputted into PondPack. For each sub-basin, the area in acres was determined, the length of the flow path that the runoff flows along, and the average slope of this flow path. Due to the area and vegetation described earlier, a runoff curve number of 63 was used in accordance with the Soil Conservation Service.

PondPack Schematic

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Once the runoff flow rate was determined from PondPack, a culvert analysis was performed. Using Bentley CulvertMaster, the runoff flow rate was used to design new box culverts. The necessary length of the culverts was determined from the provided survey data, as well as the inlet and outlet invert elevations. The wash intersects the roadway at about a 25o angle. The skew of the wingwalls was determined based on the 25o skew. The analysis was performed several times before it was determined that the roadway would be overtopped during large storm events. To prevent overtopping, the entire road elevation would need to be elevated. Upon discussion with client, it was decided that the roadway would be allowed to overtop. This reduces the need to raise the elevation of the current roadway.

Pre-Cast Box Culvert—Contech Bridge Solutions

It was decided that three 10’x4’x60’ pre-cast box culverts from Contech Bridge Solutions, Inc. be used for this project. A rip rap apron consisting of rocks with an average diameter of 12 inches be used on the downstream portion of the drainage structure to prevent scouring. Along with this apron, a geo-textile fabric will be used to prevent any excess settlement of the drainage structure.

Scour Protection

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GHI Consulting Engineers performed a geotechnical evaluation to verify that the underlying soil can withstand the load of the proposed box culverts. A geotechnical report, prepared by Ninyo & Moore Geotechnical and Environmental Sciences Consultants, was provided by the client. Using the information within the geotechnical report, it was determined that the underlying soil can in fact support the load of the proposed culverts.
Bearing capacity and settlement were the two main aspects of the underlying soil that needed to be verified. AASHTO LRFD Bridge Design 2007 was used to determine if the soil in the project area is adequate for the loading. According to the geotechnical report, the allowable bearing capacity for the soil is 2 kips per square foot, or 4,080 kips for the proposed culverts. The net load is approximately 1,795 kips, which includes the box culverts, rip rap apron, soil overburden, and traffic loads. The maximum allowable settlement, according to the geotechnical report, is 1”. From the calculations, it was determined that the maximum allowable settlement is expected to be about 0.4”.

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The Engineers Construction Cost Estimate for this project was based on current historical bid data and the R.S. Means 2008 Estimating Software. Using these two methods together allowed GHI Engineers to determine the line item unit prices. Some of the items that were not included on this construction estimate were permits, utility relocation, the acquisition of additional right of way, temporary construction easements, and a storm water pollution prevention plan. The final design of this project incorporated ease of implementation and the best available practices which in turn produced an economical and effective solution.

	BID SCHEDULE
ITEM NO.	ITEM DESCRIPTION	UNIT	QTY.	UNIT PRICE	EXTENDED AMOUNT
1	MOBILIZATION	L. SUM	1	10%	$17,951
2	EARTHWORK (EXCAVATION)	CY	430	15.00	$6,450
3	AC REMOVAL	SY	200	4.75	$950
4	CMP REMOVAL	LF	200	11.25	$2,250
5	SAW CUTTING	LF	120	2.25	$270
6	BOX CULVERT **	LF	60	1858	$141,468
7	FENCE REMOVE/REPLACE	LF	200	15.65	$3,130
8	TRAFFIC CONTROL	L. SUM	1	3000.00	$3,000
9	RIP RAP	CY	50	95.00	$4,750
10	STRUCTURAL BACKFILL	CY	70	82.00	$5,740
11	ABC ROADWAY	TON	70	27.00	$1,890
12	ASPHALTIC CONCRETE	TON	110	61.50	$6,765
13	EROSION CONTROL	L. SUM	1	800.00	$800
14	SURVEYING	L. SUM	1	2050	$2,050
15	QUALITY CONTROL	L. SUM	1	2000	$2,000
	SUBTOTAL				$199,464
	CONTINGENCY	15%			$29,620
			BID TOTAL		$229,084

Construction Cost Estimate—Pre-Cast Estimate

There are many variables that can have an effect on the total cost of the final design of a project. Some of these variables include the engineering design cost, and the actual construction cost. The estimated engineering design hours was based on all three group members working the same amount of time on each task. Once the design actually begun, it was determined that all three group members did not need to be present at each meeting to successfully design the project. During the design of the project, we decided that two group members working on each task would ensure the same quality of project at a more efficient rate. The estimated cost was calculated using a Project Manager, Senior Design Engineer and a Junior Design Engineer. The actual cost was based on the use of a Senior Design Engineer and a Junior Engineer. The Project Manager fees were added only on the Project Management task of the project.

Engineer Cost Estimate

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Hydrologic Analysis

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Hydraulic Analysis

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Traffic Control Plan

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