Hydraulics Memorandum_ TX USACE STR ACCESS (1).pdf

Subject: TX USACE STR ACCESS (1) - Final Hydraulics Memo

From: Megan Frye Hydraulic Engineer

FHWA-Central Federal Lands Lakewood, CO

Jenna Bergstrom Career Development Program FHWA-Central Federal Lands Lakewood, CO

To: Thomas Sohn Project Manager Lakewood, CO

This memorandum presents the field site observations, design analyses, and hydraulic recommendations for the proposed project along B.A. Steinhagen Lake on the Naches River, located near Town Bluff, TX and just upstream of the Big Thicket National Preserve. The proposed work will reconstruct Corps of Engineers Road, a 0.2-mile asphalt paved road used by Corp of Engineers staff to access the impoundment dam that creates the lake. Figure 1 provides a regional project location map for reference.

Figure 1: Regional Map Proposed Project Location

Memorandum

Date: August 13, 2020

The existing road width varies between 35’ and 42’ of asphalt pavement. At the edge of pavement on both sides corrugated steel half culverts channel rainfall runoff from the roadway and the adjacent grassed margins. The culverts ineffectively catch but to do contain the very localized surface runoff.

Excess runoff his causing undermining of the roadway edge of pavement despite the efforts of local partners to slow the water with in-culvert obstructions (Figure 3 and Figure 4). The proposed changes are to replace the half culverts with a ditch along both sides of the roadway. Given the steep grade of the alignment, ditch lining will be necessary to mitigate erosions.

This memo addresses the hydraulic recommendations associated with the proposed project and address the following items.

Hydrologic and hydraulic criteria and computational methods

Hydraulic analysis and recommendations for ditch size and lining

Figure 2: Proposed Project Location Along the Corp of Engineers Road.

Figure 3: Pavement Failure Figure 4: Energy Dissipation

DESIGN CRITERIA

The current and projected average daily traffic counts (ADT) of the access road are unknown but given that it has gate controlled access to an employee only area, ADT is not a factor in the hydraulics design criteria. The design criteria are based on the criteria defined for a low standard road by the Project Development and Delivery Manual. As such, the following design criteria apply:

Ditch Design

Capacity: 10-year flood

Stability: 10-year flood (Permanent Lining)

Depth of Flow: water surface elevation ≤ bottom of aggregate base layer

Slope: 0.5% min.

Stability: Permissible shear stress

HYDROLOGIC ANALYSIS

To complete a detailed analysis of the hydrology throughout the project, the contributing drainage areas on the left and right side of the roadway were delineated using Google Earth and DEM elevation maps.

Figure 5 shows the two sub-basins.

Figure 5: Corps of Engineers Road Drainage Basin

The Texas Department of Transportation Drainage Manual states that basins less than 200 acres should use the Rational Method to calculate runoff flow rates. The Rational Method (Q=CIA) calculations were completed using the FHWA Hydraulics Toolbox.

The NRCS Soil Maps identifies Type D soils in the project area. The land use in the two sub-basins is a mix of grassland/herbaceous (roadside grass), low density development (roadway), and woody wetlands (adjacent forest/scrub/shrub). Soil Type and Land use were used to develop the Runoff Curve Numbers (CN) and Time of Concentration (Tc).

The runoff coefficients were determined using the equations presented in Table 6-4 of the Urban Drainage and Flood Control Criteria Manual with soil data from the NRCS Soil Classification Map and using a 3.2% impervious area. All of the soils on the project are Type D. The runoff coefficients for the 2- , 5-, 10-, and 25-year return intervals are shown below in Table 3.

Runoff Coefficient

2-yr 5-yr 10-yr 25-yr

0.14 0.29 0.37 0.50

Table 3. Runoff Coefficients

Rainfall Intensity (I) was developed from precipitation data taken from NOAA Atlas 14 data collected near the City of Town Bluff, TX. The parameters used for each basin are presented in Table 4 for the 2-, 5-, 10-, and 25-year return intervals.

Basin Number

Drainage Area

(acres)

Time of Concentration (min.) Precipitation (in/hr.)

2-yr 5-yr 10-yr 25-yr 2-yr 5-yr 10-yr 25-yr

Left 1.53 5 5 5 5 6.7 8.4 9.6 11.4

Right 1.83 5 5 5 5 6.7 8.4 9.6 11.4

Table 4. Rational Method Variables

The peak discharges for these basin are shown in Table 5.

Basin Number

Peak Discharge (cfs)

2-yr 5-yr 10-yr 25-yr

Left 2.5 4.0 5.5 9.0

Right 2.0 4.5 7.0 10.5

Table 5. Peak Discharges for Basins

HYDRAULICS ANALYSIS

PROPOSED DITCH

The goal of this hydraulic analysis was to select the most appropriate shape and lining combination for a roadside ditch along the full length of the project. A total of 16 combinations were analyzed based on their capacity and stability for a given peak discharge. Specifically, for each a v-ditch and flat-bottom ditch, all of the following linings were evaluated: bare soil, grass, open-weave textile (permanent), erosion control blanket (permanent), turf-reinforcement mat (permanent), and riprap classes 1, 2, and 3.

Assumptions were made for the soil, grass, and ditch slopes that would be used for this project. Using the USDA/NRCS Soil Survey of Tyler County, TX, it was found that the existing soil on the project is a Woodville fine sandy loam (NRSC soil Group D). This soil is assumed to be non-cohesive with a D75 < 0.75” and a plasticity index of 0.75. A void ratio of 0.5 was assumed for this type of soil. These soil assumptions allowed for the calculation of the soil grain roughness. For the ditches lined with grass, it was assumed there would be “good” grass cover, “mixed” growth forms of sod and bunch, and an average grass stem height of 0.25’. These grass assumptions allowed for the calculations of the grass roughness and grass cover coefficients. Using Google Earth, a profile was created to estimate the maximum ditch slope, minimum ditch slope, and average ditch slopes (along the length of the project) which were 16%, 11%, and 13.5%, respectively. An assumed 1:3 slope was used for the sides of both the v-ditch and flat-bottom ditch.

The first qualification for each ditch shape and lining combination is its capability to convey the design flow. After selecting a ditch shape and lining, a ditch depth was estimated for every combination. Using the ditch depth, the hydraulic radius could be calculated. Finally, the ditch depth, hydraulic radius, and manning’s n-value (dependent on the lining) were used to calculate a discharge. All ditch shape and lining combinations yielding a discharge within 5% of 7.0 cfs were deemed acceptable for this qualification. If the discharge was outside of this range, a new depth was chosen and the discharge computation restarted with the new depth value.

In order to complete the discharge analysis, a design flow of 7.0 cfs was found using the Rational Method. The manning’s n-values for bare soil and RECP linings were found using the “typical” values found in Table 2.1 of HEC-15 (2005). The manning’s n-value for grass was computed using the average ditch slope, grass roughness coefficient, unit weight of water, and hydraulic radius. The manning’s n-values for Classes 1, 2, and 3 of riprap depended on the D50 median values (found in Table 705-1 of the FP-14) and Da value (dependent on ditch cross-sectional area and ditch top width). Different n-value equations were used if the ratio of Da to D50 was greater or less than 1.5. All equations for the manning’s n-values can be found in HEC-15 (2005).

All ditch shape and lining combinations that met the capacity requirement were analyzed for stability using the maximum calculated shear stress and the permissible shear stress. Using the same depth input that was used in the discharge calculation, the average ditch slope, and unit weight of water, the maximum shear stress was calculated. If the permissible shear stress value was larger than the maximum shear stress multiplied by the safety factor, the ditch shape and lining combination was considered acceptable for both capacity and stability requirements.

For the shear stress analysis, a safety factor of 1.0 was used as the maximum shear stress is already a conservative value The permissible shear stress value for bare soil was found using the soil assumptions listed above and Table 2.3 of HEC-15 (2005). The permissible shear stress values for grass were found using the grass assumptions listed above, the bare soil permissible shear stress, soil roughness coefficient, grass cover coefficient, and the manning’s n-value calculated for that particular ditch shape.

All equations needed to calculate the grass permissible shear stress value can be found in HEC #15. The permissible shear stress values for erosion control blanket and open-weave textile linings were found using Table 713-3 of the FP-14. Finally, the riprap Class 1, 2, and 3 permissible shear stresses were interpolated using values from Table 2.3 of HEC-15 (2005).

The following table represents all ditch and lining combinations that passed both the discharge and shear stress analysis qualifications:

Table 6. Acceptable Ditch Size and Lining Combinations

Any of the ditch type and lining combinations in the above table will be able to sustain the design flow of this region. However, we recommend using class 3 riprap with a base width of 1.5 feet. This design closely matches a small segment of the existing roadside ditch that seems to be adequate for the area.

The minimal base width of 1.5 feet is recommended as there are space restrictions to the left and right of the roadway shoulder.

Ditch Shape Lining Type Base Width Top Width Depth of

Flow

Permissible

Shear Stress

Max Shear

Stress

2.5 4.06 0.26 2.5 2.19

3 4.44 0.24 2.5 2.02

1 3.4 0.4 6 3.37

1.5 3.66 0.36 6 3.03

2 3.92 0.32 6 2.70

2.5 4.24 0.29 6 2.44

3 4.56 0.26 6 2.19

2 5.16 0.45 3.84 3.79

2.5 5.54 0.4 3.84 3.37

3 4.5 0.36 3.84 3.03

3.5 4.82 0.34 3.84 2.86

1.5 3.4 0.5 4.8 4.21

2 3.66 0.47 4.8 3.96

2.5 3.92 0.45 4.8 3.79

3 4.24 0.41 4.8 3.45

3.5 4.56 0.38 4.8 3.20

N/A 3.18 0.53 6 4.46

N/A 3.24 0.54 6 4.55 V-Ditch

Turf Reinforcement

Mat

Open-Weave

Textile

Class 2 Riprap

Class 3 Riprap

Flat-Bottom

Ditch

Turf Reinforcement

Mat

REFERENCES

FHWA (2007) “Project Development and Design Manual for Central Federal Lands Highway Division” Chp. 7. Federal Highway Administration (FHWA).

FHWA (2005) “HEC-15, Design of Roadside Ditches with Flexible Linings, Third Edition.” Federal Highway Administration (FHWA)

FHWA Hydraulic Toolbox v. 4.3 (October 2017).

Mile High Flood District (2017) “Urban Storm Drainage Criteria Manual, Vol. 2.” Mile High Flood District.

Texas Department of Transportation. “Hydraulic Design Manual” Revised July 2016.