Municipal Separate Storm Sewer System (MS4) Philpott.pdf

Memo

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To: USACE, Wilmington District From: Tanner Goff, EIT Dan Back, PE

Recipient's Office File: 1784132019 Date: April 20, 2023

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

PURPOSE AND APPROACH

The USACE, Wilmington District is in the process of repairing the steep slope downstream of the right abutment of Philpott Dam, midway upslope following a landslide in May 2020. The purpose of this project is to provide a slope stability design that will reduce the risk of personnel injury and damage to infrastructure.

Drainage improvements are also proposed including a drop inlet for capturing runoff above project limits along the upper access road, and improvements to the existing storm system that discharges to the tailrace. An analysis of this existing storm system was performed along with an analysis of the pre- and post-development conditions for the watersheds that drain to it. This analysis was performed to determine what actions are needed to be taken to fulfill the water quality and quantity compliance requirements of the Virginia Department of Environmental Quality (VDEQ) regulations, specifically 9VAC25-870 for stormwater runoff. This memorandum has been prepared to describe the following design calculations.

1. Calculate stormwater runoff areas draining to proposed drainage improvements at the project site.

2. Evaluate capacity of existing stormwater system adjacent to the powerhouse.

3. Size and specify four drop inlets (two new & two replacements).

4. Confirm the dimensions & size the stone for the proposed rock-flume on the re-graded slope.

5. Calculate the required post-development run-off rate for the lower watershed (outfall to tailrace) and determine the volume of detention required for the post-development condition.

See Figure 1 for an overview of the project and the calculations referenced above. Calculations were performed with guidance from the Virginia Administrative Code Section 9VAC25-870.

abq64963 Approved abq64963 mking

April 20, 2023

USACE, Wilmington District

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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The analysis to determine inlet/pipe capacity for the upper access road was performed in 2021 utilizing Autodesk Storm and Sanitary Analysis (2019). Another analysis was performed in 2022-2023 which built upon the original analysis to determine the existing storm system capacity and to size the required detention for the lower watersheds. Computational Hydraulics International (CHI) PCSWMM software was used for this updated analysis. See Attachment E1 for the results of the Autodesk Storm and Sanitary Analysis for the 100-year, 24-hour and 50-year, 24-hour storm events as well as an overview figure of the delineated watershed to the upper access road inlet.

DROP INLET DETAILS AND PIPE CAPACITY

Upper Access Road Drainage Ditch

Stantec recommends a VDOT DI-12C drop inlet to capture runoff upstream of the project boundary from the upper access road drainage ditch that drains through the existing culvert running underneath the road. The multi-grate drop inlet can be specified to the desired length/grate segments required up to 13 ft – 8.75 inches.

Analysis of the inlet for the 100-year, 24-hour storm event showed that a 48 inch (L) x 24 inch (W) grate opening (double grate frame) captured the runoff. The inlet was modeled with a 30% clogging factor. The reason behind this is due to the inlet being on grade in a ditch where leaf litter could easily clog it up and produce additional bypass for the proposed ditch as litter accumulates. The pipe capacity was calculated and compared utilizing gravity flow (Manning’s Equation). Stantec recommends the use of a 36-inch HDPE smooth walled pipe. A standard VDOT winged headwall should be utilized on the adjacent slope. The analysis summary for the drop inlet and conveyance pipe are below:

VDOT DI-12C Drop Inlet Actual Grate Dimensions: VDOT DI-12C drop inlet & utilizing 2 grates gives us 48”x24” grate opening.

Rim Elevation: 1025.3 ft Invert Elevation: 1019.78 ft. (This was set by the tabulation chart from VDOT Inlet standards for 36” pipe) Inlet Location: ON Grade

100yr-24hour Storm Results:

Grate Cogging Factor: 0% Peak Flow during Analysis: 55.82 cfs Peak Flow Intercepted by Inlet:33.96 cfs Peak Flow Bypassing Inlet: 12.98 cfs Inlet Efficiency during Peak Flow: 76.75%

Grate Cogging Factor: 30% Peak Flow during Analysis: 55.82 cfs Peak Flow Intercepted by Inlet:42.85 cfs Peak Flow Bypassing Inlet: 21.86 cfs Inlet Efficiency during Peak Flow: 60.8

36 inch HDPE Smooth Wall Pipe:

Length: 75ft (from centerline of ditch to 1019 ft contour on site plan) Inlet Elevation: 1019.78 ft Outlet Elevation: 1019.00 ft Mannings n: 0.013 Slope: 1.04%

WITH 30% Clogging Factor:

Design Flow Capacity: 68.02 cfs Peak Flow During Analysis: 33.94 cfs

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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Max Velocity Attained: 7.43 ft/sec

WITHOUT 30% Clogging Factor:

Design Flow Capacity: 68.02 cfs Peak Flow During Analysis: 42.83 cfs Max Velocity Attained: 8.43 ft/sec

Existing Storm System

The above analysis for the drop inlet and pipe for the upper access road drainage ditch was performed in 2021 using Autodesk Storm and Sanitary Analysis. An additional analysis was performed in 2022-2023 to determine the size of drop inlet necessary to replace two of the existing drop inlets within the existing storm system draining to the tailrace, as well as a new inlet that will capture runoff from a portion of the proposed grading. This analysis was carried out with guidance from Chapter 9 of the VDOT Drainage Manual. Here, the recommended design procedures for sizing grate inlets in a sag (typically in depressed medians or ditches) are as follows:

• Rational Method to determine design peak runoff rate.

• Assumed efficiency of grate will be reduced by 50% due to clogging.

• Capacity of inlet up to a depth of 5” is calculated as a weir, and capacity of inlet for depths greater than 17” is calculated as an orifice.

Nomographic solutions of the equations for weir and orifice flow are provided in Appendices 9C-13 through 9C-16 of the VDOT Drainage Manual for standard VDOT grate type inlets. These nomographic solutions were used along with calculated peak flow rates to determine the recommended drop inlets.

The pre- and post-development watershed analysis can be found in the following sections of this memo. The post development drainage areas that drain to inlets being sized were delineated as part of that analysis and were used here to calculate the Rational Method peak runoff.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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Rational Method formula

Q = CiA

Where:

C = Area weighted runoff coefficient (VDOT Drainage Manual Appendix 6E-1)

• C = 0.3 for forested areas

• C = 0.9 for paved and roofed areas i =4.0 inches/hour (VDOT Drainage Manual Table 9-1).

A = Drainage Area (in acres)

Drainage areas were delineated from the proposed surface contours as part of the pre- and post-watershed analysis. Stormwater discharge results are summarized in Table 1.

Table 1: Summary of Design Approach/Standards

Drainage Area Area (acres) Weighted C Intensity (in/hr) Peak Runoff (cfs) Slope to DI-2 0.3 0.86 4 1.03

Slope to DI-3 0.4 0.86 4 1.38

Slope to New Inlet 1.98 0.33 4 2.61

From the analysis, it was determined that a dual-grated VDOT DI-12C would be sufficient to intercept the calculated peak flow rate for each drainage area.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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EXISTING STORM SYSTEM REVIEW

As part of the hydrologic and hydraulic modeling for the project, analysis was performed on the existing storm system at the bottom of the slope adjacent to the powerhouse. The inlets were evaluated with the following design storms (See Attachment E2 for NOAA Rain Data):

• 25-year, 24-hour Total Rainfall (NOAA Atlas 14)

• 10-year, 24-hour Total Rainfall (NOAA Atlas 14)

• 5-year, 24-hour Total Rainfall (NOAA Atlas 14)

• 1-year, 24-hour Total Rainfall (NOAA Atlas 14)

During the 1-year, 24-hour storm event, flooding occurred at inlet DI-2, and during the 5-year, 24-hour storm event, flooding occurred at both inlets DI-2 and DI-3. The 8-inch RCP pipe linking DI-2 and DI-3 surcharged as well as the 8-inch RCP linking DI-3 to STMH-2 during both storm events. The same result was obtained during the 10-year, 24-hour and 25-year, 24-hour storm events. No other inlets or conveyance pipes experienced surcharging/flooding during the analysis. See Figure 2 for visual aid. It was determined that the two existing 8-inch RCP pipes connecting DI-2 to DI-3 and connecting DI-3 to STMH-2 are undersized in their existing condition, and it is recommended by Stantec that both existing 8-inch RCP pipes be replaced in place by two 18-inch corrugated-plastic-pipes (CPP), respectively, and that both DI-2 and DI-3 be replaced with VDOT DI-12C dual grated inlet. Neither pipe or inlet flooded or surcharged under this proposed condition during the analysis. Results of this analyses can be seen in Attachment E3.

Figure 2: Existing Storm System Aerial Overlay

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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Two unknown variables existed and may affect runoff calculations.

1. From previously provided survey information, an 18 inch RCP pipe ties into STMH-1 that flows from the Dam. The catchment area, slope of pipe and approximate length of pipe are all unknown and were not included in the analysis of the existing system. Based on site observations and discussion with USACE, it is believed that that the 18 inch RCP pipe does not receive flow and does not contribute drainage to the stormwater system.

2. A 6-inch PVC pipe found during survey exploration appears to link to another inlet but a catch basin was not located during field explorations. The pipe is in the vicinity of a portion of the landslide and the assumption is that the inlet was covered by debris. For the analysis, the watershed for this pipe was directed downstream to DI-4.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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PROPOSED ROCK FLUME & DITCH EVALUATION

Three hydraulic analyses were performed to evaluate the proposed rock-lined flume that is to be constructed on the proposed benched hillside (displayed in Figure 3 below), the ditch draining to the proposed inlet at the bottom of this hillside, & the groin ditch between the existing slope & dam that drains to the existing DI-1.

Figure 3: Proposed Benching & Flume

The intent of the hydraulic analysis was to accomplish the following:

• Confirm the size & geometry of the proposed flume & ditch that drains to the new road inlet.

• Size the channel lining to be used in the proposed flume & ditch as well as the groin ditch.

It is worth noting that the proposed rock-flume is a grouted riprap flume; a conservative assumption was made to perform the channel lining sizing calculation as if the flume were a flexible-lining system (calculation doesn’t account for grout). Also, the existing groin ditch at Philpott Dam is not quite a “ditch” per say, it’s more of a grouted connector between the slope and the front face of the dam to limit seepage. Modifications made to the groin during construction are only intended to stabilize this area, not necessarily for this area to act as a conveyance channel for stormwater. Therefore, the intent of the calculation below is to confirm the stability of the groin ditch in consideration of the flow that will be applied over it & not to size & specify a new conveyance channel. The analytical approach for these calculations is summarized in Table 2 below.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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Table 2: Analytical Approach

Analytical Input Approach

Design Storm Event • Proposed Rock Flume & Ditch to New Road Inlet o 100-year, 24-hour (9.11”)

• Groin Ditch o 2-year, 24-hour (3.68”) o 10-year, 24-hour (5.56”)

Peak Stormwater Discharges Taken from the Post-Development Conditions analysis found in the following sections of this memo & in Attachment E4 - PCSWMM Output for Post-Development Conditions & Proposed Detention Structures:

• The flume receives drainage from 1.5 acres of contributing drainage area, resulting in a peak discharge of 14.91 cfs during the 100-year, 24-hour event.

• The ditch to new road inlet receives drainage from 1.98 acres of contributing drainage area, resulting in a peak discharge of 18.94 cfs during the 100-year, 24-hour event.

• The groin ditch receives drainage from

0.2811 acres of contributing drainage area, resulting in a peak discharge of 1.38cfs during the 2-year, 24-hour event & 2.12 cfs during the 10-year, 24-hour event.

Calculations • Groin Ditch o Per USACE “Hydraulic Design of Flood Control Channels” to confirm groin stability

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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• Proposed Flume & Ditch o Per USACE “Hydraulic Design of Flood Control Channels”, & Manning’s Equation to confirm channel geometry.

• Calculations Included in Attachment E5

Based on the analysis performed, the following recommendations are made:

• Proposed Flume o 10’ bottom width & 3H:1V side slopes o Channel Depth of 1.0’ o Minimum size 12” D50 channel lining

• Proposed Ditch Draining to New Road Inlet o 4’ Bottom width & 3H:1V side slopes o Channel Depth of 1.5’ o Minimum size 12” D50 channel lining

• Groin Ditch o Maintain groin ditch geometry & stabilize with a minimum size 6” D50 channel lining.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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PRE AND POST DEVELOPMENT HYDROLOGIC AND HYDRAULIC EVALUATION

The USACE, Wilmington District is currently working with the VDEQ regarding pre- and post-development runoff conditions for the Philpott Dam Landside Remediation site. Stantec was asked to compile pre-development and post-development runoff calculations. The governing water quantity standard that guided our analysis was channel protection for natural stormwater conveyance systems, as stated in VDEQ regulation 9VAC25-870-66 Part B-3. Here, it is stated that the maximum peak flow rate from the 1-year, 24-hour storm following a land-disturbing activity shall be determined by the following improvement factor (I.F.)

equation:

𝑄𝑄𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 ≤ 𝐼𝐼.𝐹𝐹.∗

�𝑄𝑄𝑃𝑃𝑃𝑃𝐷𝐷−𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 ∗ 𝑅𝑅𝑅𝑅𝑃𝑃𝑃𝑃𝐷𝐷−𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷�

𝑅𝑅𝑅𝑅𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷

Where:

I.F.= Improvement Factor = 0.8 for sites larger than 1 acre in disturbance (our case)

R.V.=Runoff Volume (Cu. Feet)

Q=Peak Flow Rate (Cu. Feet/Second)

The general approach of this analysis was performed in CHI PCSWMM. The NRCS – SCS Curve Number Method was utilized in calculating runoff for the Philpott Dam Site as stated in VDEQ regulation 9VAC25-870- 72 - Design Storms and Hydrologic Methods. Per VDEQ, the 1-year, 24-hour storm event was utilized to obtain the peak flow rates and total runoff for the right abutment below upper access road watersheds. The NOAA Precipitation Data Server shows that the total rainfall for the 1-year, 24-hour storm event in Henry Co, Virginia is 3.04 inches. The pre- and post-development watersheds & outfall locations for the project site can be seen in Attachment E6 and E7, respectively. Attachment E6 and E7 also contains a summary table for the hydrology inputs for the project (i.e. curve numbers (CN) and times of concentration (TOC)), the flows from each drainage area and the total peak flow of the combined watershed at the outfall of the drainage system.

The outputs of the analysis that was performed in PCSWMM for the pre- and post-development conditions for the 1-year, 24-hour storm event can be found in Attachment E8.

Pre-Development Overview

The existing site is steep and covered in a mixture of forested areas, moderate grass cover, and rock outcrops scattered throughout the downstream right abutment slope. Per the NRCS Web Soil Survey, the site is composed entirely of hydrologic soil group C. The right abutment slope was determined to have a curve number of 79 (fair grass cover, 50-75%) due to the moderate grass cover and scattered rock outcrops.

Impervious areas were given a curve number of 98; this includes the road to the tailrace and powerhouse. A summary of pre-development conditions is compiled in Table 3 below.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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Table 3: Pre - Development Conditions for Right Abutment Slope

Post-Development Overview

To stabilize the slope, the intent is to remove unstable material along the slope to get down to stable rock.

This will increase the curve number and runoff for each watershed within the area of disturbance. The generic curve number for gravel, bedrock, or compacted rock is listed in TR-55 as 89 for hydrologic soil group C. This number was then utilized to calculate new weighted averages for each watershed. Due to the proposed grading & benching on the hillside, the catchment areas changed slightly from the pre-development condition.

The total contributing drainage area to the stormwater system increased by roughly an acre, resulting in the addition of 4 new watersheds on the proposed benched area of the graded hillside. Each of these watersheds will drain directly to the proposed rock-flume that runs down the benched hillside. This flume will outlet to a proposed inlet and pipe connecting drainage from the new watersheds to the stormwater system. See Table 4 below for a summary of post-development conditions.

Table 4: Post - Development Conditions for Right Abutment Slope

The VDEQ requires that a land disturbance/project discharging to a “natural stormwater conveyance”, adhere to the methodology stated in section B.3.a of Channel Protection Regulation 9VAC25-870-66. The pre- and post-development flowrates were taken as the routed flowrate at the outlet of the total watershed. The following calculations were performed for pre-development and post-development conditions of the right abutment slope below the upper access road.

Watershed Area (acres) Weighted CN Total Runoff (in) Total Runoff (ft) Area (ft^2) Runoff Volume (ft^3) Dam to DI-1 0.3130 98.00 2.96 0.2467 13634.28 3363.12 Slope to DI-1 0.4120 79.00 1.42 0.1183 17946.72 2123.70 Slope to DI-2 0.3083 84.98 2.04 0.1700 13429.55 2283.02 Slope to DI-3 0.7280 81.28 1.70 0.1417 31711.68 4492.49 Slope to DI-4 0.6495 80.15 1.56 0.1300 28292.22 3677.99

15940.32

Pre - Development Watershed Conditions 1-Year, 24-Hour Runoff Calculations

Summation

Watershed Area (acres) Weighted CN Total Runoff (in) Total Runoff (ft) Area (ft^2) Runoff Volume (ft^3) Dam to DI-1 0.2811 98.00 2.79 0.2325 12244.716 2846.90 Slope to DI-1 0.4276 89.00 2.10 0.1750 18626.256 3259.59 Slope to DI-2 0.3006 90.08 2.22 0.1850 13094.14 2422.42 Slope to DI-3 0.4013 90.79 2.22 0.1850 17480.628 3233.92 Slope to DI-4 0.0847 93.25 2.40 0.2000 3689.532 737.91 Slope_to_Flume1 0.1056 73 1.31 0.1092 4599.936 502.16 Slope_to_Flume2_1 0.609 74.18 1.35 0.1125 26528.04 2984.40 Slope_to_Flume2_2 0.8039 77.12 1.47 0.1225 35017.884 4289.69 Slope to New Inlet 0.4626 75.31 1.39 0.1158 20150.856 2334.14

22611.12Summation

Post - Development Watershed Conditions 1-Year, 24-Hour Runoff Calculations

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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Where:

I.F.= Improvement Factor R.V.=Runoff Volume

Discharge at Tailrace:

𝑄𝑄𝑃𝑃𝑃𝑃𝐷𝐷−𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 = 7.09 𝑐𝑐𝑐𝑐𝑐𝑐 (see Attachment E8)

Total Runoff:

𝑅𝑅𝑅𝑅𝑃𝑃𝑃𝑃𝐷𝐷−𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 = 15,940.32 𝑐𝑐𝑓𝑓3 (𝑐𝑐𝑠𝑠𝑠𝑠 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑠𝑠 3)

𝑅𝑅𝑅𝑅𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 = 22,611.12 𝑐𝑐𝑓𝑓3 (𝑐𝑐𝑠𝑠𝑠𝑠 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑠𝑠 4)

𝐼𝐼.𝐹𝐹.∗

(7.09 𝑐𝑐𝑐𝑐𝑐𝑐 ∗ 15,940.32 𝑐𝑐𝑓𝑓3)

22,611.12 𝑐𝑐𝑓𝑓3

= 𝐼𝐼.𝐹𝐹.∗ (4.99 𝑐𝑐𝑐𝑐𝑐𝑐)

For our site I.F.=0.8; Site > 1 acre in disturbance

(0.8) ∗ (4.99𝑐𝑐𝑐𝑐𝑐𝑐) = 4.0 𝑐𝑐𝑐𝑐𝑐𝑐

STORMWATER DETENTION REQUIREMENT RESULTS

To fulfill the requirements of VDEQ regulation 9VAC25-870-66 Part B-3 for channel protection of natural stormwater conveyance systems and meet the allowable post-development peak flow rate, it was determined that stormwater detention would be required for the lower watershed (outfall to tailrace). The required storage volume for the design storm was determined using PCSWMM. The outputs of the analysis that was performed in PCSWMM for the post-development conditions for the 1-, 5-, 10-, 25-, 50- and 100-year, 24-hour storm events can be found in Attachment E4. Design and modeling considerations were as follows:

• Size the detention vault and outlet system to confine the post-development peak flow rate to the calculated allowable peak flow rate for the 1-year, 24-hour storm event.

• Size the detention vault and outlet system to pass the 5-, 10-, 25-, 50-, and 100-year, 24-hour storm events without flooding the vault or stormwater system.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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PCSWMM modeling resulted in a required minimum detention storage for the lower watershed of 3,625 cubic feet to detain the post-development peak flow rate for the 1-year, 24-hour rainfall event to 4.0 cfs. A 5,000 cubic feet underground storage vault was chosen for the design to allow the larger storm events (up to the 100-year, 24-hour event) to pass through the system without causing flooding and provide 1 foot of freeboard inside the structure.

Features of the stormwater detention vault that were used in modeling are as follows:

• Outlet structure with 7.5” bottom orifice to limit the release from the 1-year, 24-hour storm event and riser structure with overflow weir to allow larger storm events to pass without causing flooding.

• Vault which has 3,625 cubic feet of storage volume to the overflow, to detain runoff from the 1-year, 24-hour storm event and pass up to the 100-year, 24-hour event without flooding.

• 4 new stormwater conveyance pipes, each sized at 24” in diameter, and 2 new manholes to convey stormwater from STMH-2 to the new detention structure as well as 1 new inlet to capture runoff from the additional post-development watershed.

Results of modeling within PCSWMM convey that the above detention vault configuration can detain the 1-year, 24-hour event with an outflow limited to the allowable post-development peak flow rate of 4.0 cfs.

Modeling results also convey that the structure can pass the 100-year, 24-hour storm event without flooding the stormwater system and leaving 1 foot of freeboard within the structure. The modeled output for energy balance compliance can be found below in Table 5.

Table 5: Energy Balance Compliance

Outfall Storm Event I.F.

Storm Flow Summary Q Allowable per EB Eq. (cfs)

Q Allowable

Utilized (cfs)

Proposed Q (cfs)

Meets E.B.

(Yes/No)

Ex.

Peak

(CFS)

RV Pre (ac-ft)

RV Post (ac-ft)

Proposed Outfall

(Prop_OF) 1-Yr 0.8 7.09 0.366 0.519 4 4 3.94 Yes

Detailed drawings of the detention vault can be found in Attachment E9.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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FLOOD PROTECTION

Flood protection for the project has been assessed for compliance in accordance with 9VAC25-870-66.C. The site has one discharge point that flows directly to the Smith River, as displayed in Figure 4 below. The location of the regulatory floodway in relation to the Proposed Outfall can also be seen in Figure 4. As shown, the runoff from the outfall drains directly to a mapped FEMA floodway; which is a the limit of analysis for the assessment of downstream flood protection stated in 9VAC25-870-66.C.3. Table 6 below lists the pre- & post-development volume, flow rates, & depths for the 10-year, 24-hour storm at the outfall for reference.

Figure 4: Proposed Outfall Location

Table 6: Flood Protection Summary Table

Volume

(MG)

Peak (cfs)

Depth (ft)

Volume

(MG)

Peak (cfs)

Depth (ft)

Proposed Outfall

(Prop_OF) 10-yr 0.267 13.02 0.93 0.376 19.52 1.59

Pre-Development Post-Development Outfall

Storm Event

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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WATER QUALITY COMPLIANCE

Water quality has been assessed for the entirety of the project site in accordance with 9VAC25-870-63 and reduction requirements have been evaluated using the Virginia Runoff Reduction Method (VRRM). Table 7 below shows the site landcover characteristics for post-development conditions and Table 8 shows the required phosphorous reduction for the site. This information is also provided in Attachment E10, which shows the full VRRM calculation spreadsheet for new development and Attachment E11, which shows post-development land use maps and soils information.

Table 7: Land Cover Documentation (VRRM New Development)

Table 8: Phosphorus Reduction Requirement (VRRM New Development)

The post-development land-cover accounts for 14.00 acres of conserved forest / open space. The areas being conserved have been included in Attachment E11 of this report. These areas will be recorded under easement prior to permit termination.

Reference: Hydrologic and Hydraulic Analysis for Inlet, Existing Storm System, Lower Watershed Detention Requirements, Philpott Dam Landslide Remediation

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CLOSURE

The following are noted from these calculations:

1. 4 drop inlets were sized as part of this evaluation, 2 new and 2 replacements. A dual grated VDOT DI-12C drop inlet is recommended for each.

2. It is recommended that the pipes connecting DI-2 to DI-3 and DI-3 to STMH-2 each be replaced with 18-inch diameter CPP.

3. Pre and post development runoff calculations indicate that stormwater detention is necessary, and it is recommended that a 5,000 cubic feet underground detention vault be constructed.

4. Calculations discussed in this memorandum only considered stormwater runoff from the right abutment hillside.

If you have questions or comments regarding the contents within this memorandum, please do not hesitate to contact us.

Stantec Consulting Services Inc Stantec Consulting Services Inc

Dan Back, PE credentials Senior Principal Phone: 859-422-3034 Dan.Back@stantec.com

Tanner Goff, EIT credentials Water Resources Engineer In Training Phone: 859-422-3109 Tanner.goff@stantec.com

Attachment: Attachment E1: Autodesk Storm and Sanitary Analysis Summary Attachment E2: NOAA Rainfall Data Attachment E3: PCSWMM Output for Existing Storm System Review Attachment E4: PCSWMM Output for Post-Development Conditions & Proposed Detention Structure Attachment E5: Proposed Flume & Ditch Calculations Attachment E6: Pre-Dev Watershed Map & Summary Table Attachment E7: Post-Dev Watershed Map & Summary Table Attachment E8: PCSWMM Output for Pre- and Post-Development H&H Evaluation Attachment E9: Detention Vault Plan Sheets Attachment E10: Water Quality Compliance Attachment E11: Land Use Maps & Soils Information

c. C.C.

Attachment E1: Autodesk Storm and Sanitary Analysis Summary

Watershed to Upper Access Road Inlet

A

USACE Wilmington District Philpott Dam

"/ Proposed Inlet Proposed Culvert Drainage Area

C

Us er s\ tg of f\

D es kt op

\P hi lp ot t\

_2

2_ Up d at ed _M od el

\0 2_

_U p da te

\u p p er

W S.

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Re vi se d

: 2

3-

-2

By : t go ff

Project Location

Client/Project

Attachment No.

Title

Philpott Dam Henry CO, VA

Drainage Area (4.95 Acres)

Proposed Inlet

Phillpott Dam

Project Description Rev2_Philpott_Dam_65%.SPF

Project Options

CFS

Elevation

EPA SWMM

SCS Curve Number Hydrodynamic

YES

NO

Analysis Options Jun 27, 2021 00:00:00 Jun 28, 2021 00:00:00 Jun 27, 2021 00:00:00 0 days 0 01:00:00 days hh:mm:ss 0 00:05:00 days hh:mm:ss 0 00:05:00 days hh:mm:ss 30 seconds

Number of Elements Qty

Rainfall Details SN Rain Gage Data Data Source Rainfall Rain State County Return Rainfall Rainfall

ID Source ID Type Units Period Depth Distribution (years) (inches)

1 Time Series 50yr-24hr Henry Co. Virginia Cumulative inches Virginia Henry 50 7.92 SCS Type II 24-hr

Outlets Pollutants Land Uses

Links Channels Pipes Pumps Orifices Weirs

Nodes Junctions Outfalls Flow Diversions Inlets Storage Nodes

Runoff (Dry Weather) Time Step Runoff (Wet Weather) Time Step Reporting Time Step Routing Time Step

Rain Gages Subbasins

Enable Overflow Ponding at Nodes Skip Steady State Analysis Time Periods

Start Analysis On End Analysis On Start Reporting On Antecedent Dry Days

File Name

Flow Units Elevation Type Hydrology Method EPA SWMM Infiltration Method Link Routing Method

Subbasin : Inlet_watershed

Input Data

Area (ac) ..................................................... 4.95 Impervious Area (%) ................................... 24.00 Weighted Curve Number ............................ 82.65 Conductivity (in/hr) ...................................... 0.1500 Drying Time (days) ...................................... 7.00 Average Slope (%) ...................................... 20.0000 Equivalent Width (ft) .................................... 500.00 Impervious Area Manning's Roughness ........................... 0.0150 Pervious Area Manning's Roughness ........................... 0.0350 Curb & Gutter Length (ft) ............................. 0.00 Rain Gage ID

Composite Curve Number Area Soil Curve Soil/Surface Description (acres) Group Number 50 - 75% grass cover, Fair 4.00 C 79.00 Paved parking & roofs 0.95 C 98.00 Composite Area & Weighted CN 4.95 82.65

Subbasin Runoff Results

Total Rainfall (in) ......................................... 7.92 Total Runon (in) .......................................... 0.00 Total Evaporation (in) .................................. 0.0000 Total Infiltration (in) ..................................... 1.2610 Total Runoff (in) .......................................... 6.49 Peak Runoff (cfs) ........................................ 48.93 Weighted Curve Number ............................ 82.65 Time of Concentration (days hh:mm:ss) ..... 0 00:10:15

Project Description Rev2_Philpott_Dam_65%.SPF

Project Options

CFS

Elevation

EPA SWMM

SCS Curve Number Hydrodynamic

YES

NO

Analysis Options Jun 27, 2021 00:00:00 Jun 28, 2021 00:00:00 Jun 27, 2021 00:00:00 0 days 0 01:00:00 days hh:mm:ss 0 00:05:00 days hh:mm:ss 0 00:05:00 days hh:mm:ss 30 seconds

Number of Elements Qty

Rainfall Details SN Rain Gage Data Data Source Rainfall Rain State County Return Rainfall Rainfall

ID Source ID Type Units Period Depth Distribution (years) (inches)

1 Time Series 100yr-24hr Henry Co. Virginia Cumulative inches User Defined

Outlets Pollutants Land Uses

Links Channels Pipes Pumps Orifices Weirs

Nodes Junctions Outfalls Flow Diversions Inlets Storage Nodes

Runoff (Dry Weather) Time Step Runoff (Wet Weather) Time Step Reporting Time Step Routing Time Step

Rain Gages Subbasins

Enable Overflow Ponding at Nodes Skip Steady State Analysis Time Periods

Start Analysis On End Analysis On Start Reporting On Antecedent Dry Days

File Name

Flow Units Elevation Type Hydrology Method EPA SWMM Infiltration Method Link Routing Method

Input Data

Area (ac) ..................................................... 4.95 Impervious Area (%) ................................... 24.00 Weighted Curve Number ............................ 82.65 Conductivity (in/hr) ...................................... 0.1500 Drying Time (days) ...................................... 7.00 Average Slope (%) ...................................... 20.0000 Equivalent Width (ft) .................................... 500.00 Impervious Area Manning's Roughness ........................... 0.0150 Pervious Area Manning's Roughness ........................... 0.0350 Curb & Gutter Length (ft) ............................. 0.00 Rain Gage ID ............................................... *

Composite Curve Number Area Soil Curve Soil/Surface Description (acres) Group Number 50 - 75% grass cover, Fair 4.00 C 79.00 Paved parking & roofs 0.95 C 98.00 Composite Area & Weighted CN 4.95 82.65

Subbasin Runoff Results

Total Rainfall (in) ......................................... 9.11 Total Runon (in) .......................................... 0.00 Total Evaporation (in) .................................. 0.0000 Total Infiltration (in) ..................................... 1.2970 Total Runoff (in) .......................................... 7.64 Peak Runoff (cfs) ........................................ 57.37 Weighted Curve Number ............................ 82.65 Time of Concentration (days hh:mm:ss) ..... 0 00:09:42

Attachment E2: NOAA Rainfall Data

6/27/2021 Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat=36.7806&lon=-80.0271&data=depth&units=english&series=pds 1/4

NOAA Atlas 14, Volume 2, Version 3 Location name: Bassett, Virginia, USA* Latitude: 36.7806°, Longitude: -80.0271°

Elevation: 875.01 ft**

* source: ESRI Maps

** source: USGS

POINT PRECIPITATION FREQUENCY ESTIMATES

G.M. Bonnin, D. Martin, B. Lin, T. Parzybok, M.Yekta, and D. Riley

NOAA, National Weather Service, Silver Spring, Maryland

PF_tabular | PF_graphical | Maps_&_aerials

PF tabular PDS-based point precipitation frequency estimates with 90% confidence intervals (in inches)1

Duration Average recurrence interval (years)

1 2 5 10 25 50 100 200 500 1000

5-min 0.370 (0.340‑0.403)

0.443 (0.406‑0.484)

0.526 (0.481‑0.575)

0.585 (0.534‑0.637)

0.657 (0.597‑0.714)

0.705 (0.638‑0.768)

0.754 (0.678‑0.822)

0.798 (0.713‑0.872)

0.851 (0.752‑0.934)

0.890 (0.780‑0.979)

10-min 0.591 (0.542‑0.643)

0.708 (0.650‑0.773)

0.842 (0.770‑0.920)

0.935 (0.854‑1.02)

1.05 (0.951‑1.14)

1.12 (1.02‑1.22)

1.20 (1.08‑1.31)

1.26 (1.13‑1.38)

1.35 (1.19‑1.48)

1.40 (1.23‑1.54)

15-min 0.739 (0.678‑0.804)

0.890 (0.817‑0.972)

1.07 (0.974‑1.16)

1.18 (1.08‑1.29)

1.33 (1.21‑1.44)

1.42 (1.29‑1.55)

1.52 (1.36‑1.65)

1.60 (1.43‑1.75)

1.70 (1.50‑1.86)

1.76 (1.54‑1.94)

30-min 1.01 (0.930‑1.10)

1.23 (1.13‑1.34)

1.51 (1.39‑1.65)

1.71 (1.56‑1.87)

1.97 (1.79‑2.14)

2.14 (1.94‑2.33)

2.32 (2.08‑2.53)

2.48 (2.22‑2.72)

2.70 (2.38‑2.96)

2.85 (2.50‑3.14)

60-min 1.26 (1.16‑1.37)

1.54 (1.42‑1.69)

1.94 (1.78‑2.12)

2.23 (2.04‑2.43)

2.62 (2.38‑2.85)

2.90 (2.63‑3.16)

3.20 (2.87‑3.48)

3.48 (3.11‑3.81)

3.87 (3.42‑4.25)

4.16 (3.65‑4.58)

2-hr 1.54 (1.41‑1.69)

1.89 (1.72‑2.07)

2.39 (2.18‑2.62)

2.78 (2.53‑3.04)

3.30 (2.98‑3.61)

3.71 (3.34‑4.06)

4.14 (3.70‑4.52)

4.58 (4.05‑5.01)

5.19 (4.53‑5.69)

5.67 (4.90‑6.23)

3-hr 1.68 (1.54‑1.84)

2.06 (1.89‑2.26)

2.61 (2.39‑2.86)

3.04 (2.77‑3.31)

3.61 (3.26‑3.93)

4.06 (3.64‑4.42)

4.53 (4.04‑4.93)

5.00 (4.43‑5.47)

5.67 (4.96‑6.21)

6.20 (5.36‑6.82)

6-hr 2.11 (1.93‑2.32)

2.57 (2.36‑2.84)

3.24 (2.97‑3.58)

3.79 (3.44‑4.16)

4.54 (4.10‑4.99)

5.17 (4.63‑5.67)

5.83 (5.17‑6.40)

6.54 (5.72‑7.17)

7.55 (6.49‑8.29)

8.38 (7.10‑9.23)

12-hr 2.60 (2.37‑2.88)

3.17 (2.89‑3.51)

4.02 (3.66‑4.45)

4.72 (4.27‑5.21)

5.75 (5.16‑6.32)

6.60 (5.87‑7.24)

7.55 (6.63‑8.26)

8.57 (7.41‑9.37)

10.1 (8.55‑11.1)

11.4 (9.46‑12.5)

24-hr 3.04 (2.79‑3.33)

3.68 (3.38‑4.04)

4.70 (4.32‑5.15)

5.56 (5.09‑6.07)

6.84 (6.21‑7.44)

7.92 (7.15‑8.60)

9.11 (8.15‑9.88)

10.4 (9.24‑11.3)

12.4 (10.8‑13.4)

14.0 (12.1‑15.3)

2-day 3.60 (3.31‑3.92)

4.36 (4.00‑4.76)

5.54 (5.08‑6.04)

6.50 (5.95‑7.08)

7.90 (7.20‑8.60)

9.08 (8.22‑9.87)

10.3 (9.31‑11.2)

11.7 (10.5‑12.7)

13.7 (12.1‑15.0)

15.4 (13.4‑16.8)

3-day 3.80 (3.50‑4.15)

4.61 (4.23‑5.03)

5.85 (5.37‑6.38)

6.86 (6.28‑7.49)

8.33 (7.60‑9.08)

9.56 (8.67‑10.4)

10.9 (9.81‑11.9)

12.3 (11.0‑13.4)

14.4 (12.7‑15.7)

16.1 (14.1‑17.7)

4-day 4.01 (3.69‑4.38)

4.85 (4.47‑5.31)

6.16 (5.66‑6.73)

7.22 (6.62‑7.89)

8.76 (7.99‑9.56)

10.0 (9.12‑11.0)

11.4 (10.3‑12.5)

12.9 (11.6‑14.1)

15.1 (13.3‑16.5)

16.9 (14.8‑18.5)

7-day 4.56 (4.25‑4.93)

5.49 (5.12‑5.94)

6.86 (6.38‑7.40)

7.98 (7.40‑8.60)

9.58 (8.84‑10.3)

10.9 (10.0‑11.7)

12.3 (11.2‑13.2)

13.8 (12.5‑14.9)

16.0 (14.3‑17.2)

17.8 (15.7‑19.2)

10-day 5.20 (4.85‑5.60)

6.23 (5.82‑6.71)

7.69 (7.17‑8.27)

8.86 (8.24‑9.52)

10.5 (9.72‑11.3)

11.8 (10.9‑12.7)

13.2 (12.1‑14.2)

14.7 (13.4‑15.8)

16.8 (15.2‑18.1)

18.5 (16.5‑20.0)

20-day 7.03 (6.58‑7.53)

8.38 (7.84‑8.97)

10.1 (9.45‑10.8)

11.5 (10.7‑12.3)

13.3 (12.4‑14.3)

14.8 (13.7‑15.8)

16.3 (15.0‑17.4)

17.7 (16.3‑19.0)

19.8 (18.0‑21.3)

21.4 (19.3‑23.0)

30-day 8.69 (8.17‑9.25)

10.3 (9.68‑11.0)

12.2 (11.4‑13.0)

13.6 (12.7‑14.5)

15.4 (14.4‑16.4)

16.9 (15.7‑17.9)

18.2 (16.9‑19.4)

19.6 (18.1‑20.9)

21.3 (19.7‑22.8)

22.7 (20.8‑24.3)

45-day 11.0 (10.3‑11.7)

13.0 (12.2‑13.8)

15.1 (14.2‑16.1)

16.7 (15.7‑17.8)

18.8 (17.7‑20.0)

20.4 (19.1‑21.7)

21.8 (20.4‑23.2)

23.3 (21.7‑24.8)

25.1 (23.3‑26.8)

26.4 (24.4‑28.3)

60-day 13.0 (12.3‑13.8)

15.3 (14.5‑16.2)

17.7 (16.7‑18.7)

19.4 (18.3‑20.5)

21.6 (20.4‑22.9)

23.3 (21.9‑24.6)

24.8 (23.3‑26.2)

26.3 (24.6‑27.8)

28.1 (26.2‑29.9)

29.4 (27.4‑31.4)

1 Precipitation frequency (PF) estimates in this table are based on frequency analysis of partial duration series (PDS).

Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% confidence interval. The probability that precipitation frequency estimates (for a given duration and average recurrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper bounds are not checked against probable maximum precipitation (PMP) estimates and may be higher than currently valid PMP values.

Please refer to NOAA Atlas 14 document for more information.

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PF graphical https://www.commerce.gov/ http://www.noaa.gov/ https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat=36.7806&lon=-80.0271&data=depth&units=english&series=pds 2/4

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Maps & aerials

Small scale terrain https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat=36.7806&lon=-80.0271&data=depth&units=english&series=pds 3/4

Large scale terrain

Large scale map

Large scale aerial

3km

2mi

100km

60mi

100km

60mi https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat=36.7806&lon=-80.0271&data=depth&units=english&series=pds 4/4

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US Department of Commerce National Oceanic and Atmospheric Administration

National Weather Service National Water Center

1325 East West Highway Silver Spring, MD 20910

Questions?: HDSC.Questions@noaa.gov

Disclaimer

100km

60mi https://www.commerce.gov/ http://www.noaa.gov/ http://www.nws.noaa.gov/ https://www.weather.gov/owp/oh mailto:HDSC.Questions@noaa.gov http://www.nws.noaa.gov/disclaimer.html

Attachment E3: PCSWMM Output For Existing Storm System Review

Existing Conditions SWMM Output for 1-year-24-hr Event

EPA STORM WATER MANAGEMENT MODEL - VERSION 5.1 (Build 5.1.015)

Element Count Number of rain gages Number of subcatchments ... 5 Number of nodes Number of links Number of pollutants Number of land uses

Raingage Summary Data Recording Name Data Source Type Interval 100yr-24hr 100yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

10yr-24hr 10yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

1yr-24hr 1yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

25yr-24hr 25yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

2yr-24hr 2yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

50yr-24hr 50yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

5yr-24hr 10yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

Subcatchment Summary Name Area Width %Imperv %Slope Rain Gage Outlet

DAM_to_DI-1 0.31 49.45 100.00 146.3100 1yr-24hr

DI-1

Slope_to_DI-1 0.41 56.33 0.00 79.5000 1yr-24hr

DI-1

Slope_to_DI-2 0.31 50.17 31.00 78.1200 1yr-24hr

DI-2

Slope_to_DI-3 0.73 119.76 12.00 77.8400 1yr-24hr

DI-3

Slope_to_DI-4 0.65 81.44 6.00 68.9000 1yr-24hr

DI-4

Node Summary Invert Max. Ponded External Name Type Elev. Depth Area Inflow

DI-1 JUNCTION 833.77 4.14 100.0

DI-2 JUNCTION 831.38 3.26 100.0

DI-3 JUNCTION 830.26 4.37 100.0

DI-4 JUNCTION 827.85 4.54 100.0

STMH-1 JUNCTION 829.83 5.30 100.0

STMH-2 JUNCTION 826.07 9.04 100.0

STMH-3 JUNCTION 827.29 7.98 100.0

Ex_OF OUTFALL 825.38 2.00 0.0

Link Summary Name From Node To Node Type Length %Slope Roughness

DI-1_to_STMH-1 DI-1 STMH-1 CONDUIT 17.5

17.2213 0.0150 DI-2_to_DI-3 DI-2 DI-3 CONDUIT 51.0

2.1966 0.0150 DI-3_to_STMH-2 DI-3 STMH-2 CONDUIT 29.5

1.6273 0.0150 DI-4_to_STMH-3 DI-4 STMH-3 CONDUIT 27.5

1.8549 0.0150 STMH-1_to_STMH-2 STMH-1 STMH-2 CONDUIT 99.0

1.8589 0.0150 STMH-2_to_Spillway STMH-2 Ex_OF CONDUIT 30.5

2.2629 0.0150 STMH-3_to_STMH-2 STMH-3 STMH-2 CONDUIT 64.5

1.7987 0.0150

Cross Section Summary Full Full Hyd. Max. No. of Full Conduit Shape Depth Area Rad. Width Barrels Flow

DI-1_to_STMH-1 CIRCULAR 1.50 1.77 0.38 1.50 1 37.78 DI-2_to_DI-3 CIRCULAR 0.67 0.35 0.17 0.67 1 1.55 DI-3_to_STMH-2 CIRCULAR 0.67 0.35 0.17 0.67 1 1.34 DI-4_to_STMH-3 CIRCULAR 1.25 1.23 0.31 1.25 1 7.62 STMH-1_to_STMH-2 CIRCULAR 2.00 3.14 0.50 2.00 1 26.73 STMH-2_to_Spillway CIRCULAR 2.00 3.14 0.50 2.00 1 29.49 STMH-3_to_STMH-2 CIRCULAR 1.25 1.23 0.31 1.25 1 7.51

NOTE: The summary statistics displayed in this report are based on results found at every computational time step, not just on results from each reporting time step.

Analysis Options Flow Units ............... CFS Process Models:

Rainfall/Runoff ........ YES

RDII ................... NO

Snowmelt ............... NO Groundwater ............ NO Flow Routing ........... YES Ponding Allowed ........ YES Water Quality .......... NO Infiltration Method ...... CURVE_NUMBER Flow Routing Method ...... DYNWAVE Surcharge Method ......... EXTRAN Starting Date ............ 06/27/2021 00:00:00 Ending Date .............. 06/28/2021 00:00:00 Antecedent Dry Days ...... 0.0 Report Time Step ......... 00:05:00 Wet Time Step ............ 00:05:00 Dry Time Step ............ 01:00:00 Routing Time Step ........ 5.00 sec Variable Time Step ....... YES Maximum Trials Number of Threads Head Tolerance ........... 0.005000 ft

Control Actions Taken

************************** Volume Depth Runoff Quantity Continuity acre-feet inches Total Precipitation ...... 0.610 3.037 Evaporation Loss ......... 0.000 0.000 Infiltration Loss ........ 0.208 1.034 Surface Runoff ........... 0.366 1.820 Final Storage ............ 0.038 0.188 Continuity Error (%) ..... -0.162

************************** Volume Volume Flow Routing Continuity acre-feet 10^6 gal Dry Weather Inflow ....... 0.000 0.000 Wet Weather Inflow ....... 0.366 0.119 Groundwater Inflow ....... 0.000 0.000 RDII Inflow .............. 0.000 0.000 External Inflow .......... 0.000 0.000 External Outflow ......... 0.366 0.119 Flooding Loss ............ 0.000 0.000 Evaporation Loss ......... 0.000 0.000 Exfiltration Loss ........ 0.000 0.000 Initial Stored Volume .... 0.000 0.000 Final Stored Volume ...... 0.000 0.000 Continuity Error (%) ..... 0.000

Time-Step Critical Elements

Link DI-1_to_STMH-1 (93.21%)

Highest Flow Instability Indexes

All links are stable.

Routing Time Step Summary

Minimum Time Step : 1.03 sec Average Time Step : 3.20 sec Maximum Time Step : 5.00 sec Percent in Steady State : -0.00 Average Iterations per Step : 2.01 Percent Not Converging : 0.04 Time Step Frequencies :

5.000 - 3.155 sec : 55.88 %

3.155 - 1.991 sec : 35.03 %

1.991 - 1.256 sec : 5.63 %

1.256 - 0.792 sec : 3.47 %

0.792 - 0.500 sec : 0.00 %

Subcatchment Runoff Summary

Total Total Total Total Imperv Perv Total Total Peak Runoff Precip Runon Evap Infil Runoff Runoff Runoff Runoff Runoff Coeff Subcatchment in in in in in in in 10^6 gal CFS

DAM_to_DI-1 3.04 0.00 0.00 0.00 2.96

0.00 2.96 0.03 1.32 0.973 Slope_to_DI-1 3.04 0.00 0.00 1.42 0.00

1.42 1.42 0.02 1.02 0.467 Slope_to_DI-2 3.04 0.00 0.00 0.77 0.92

1.12 2.04 0.02 1.05 0.672 Slope_to_DI-3 3.04 0.00 0.00 1.15 0.35

1.34 1.70 0.03 2.09 0.559 Slope_to_DI-4 3.04 0.00 0.00 1.28 0.18

1.38 1.56 0.03 1.72 0.514

Node Depth Summary

Average Maximum Maximum Time of Max Reported Depth Depth HGL Occurrence Max Depth Node Type Feet Feet Feet days hr:min Feet

DI-1 JUNCTION 0.05 0.31 834.08 0 12:00 0.31

DI-2 JUNCTION 0.11 3.31 834.69 0 12:00 3.29

DI-3 JUNCTION 0.17 3.88 834.14 0 12:00 3.88

DI-4 JUNCTION 0.06 0.48 828.33 0 12:00 0.48

STMH-1 JUNCTION 0.07 0.42 830.25 0 12:00 0.42

STMH-2 JUNCTION 0.12 0.86 826.93 0 12:00 0.86

STMH-3 JUNCTION 0.06 0.41 827.70 0 12:00 0.41

Ex_OF OUTFALL 0.11 0.67 826.05 0 12:00 0.67

Node Inflow Summary

Maximum Maximum Lateral Total Flow Lateral Total Time of Max Inflow Inflow Balance Inflow Inflow Occurrence Volume Volume Error Node Type CFS CFS days hr:min 10^6 gal 10^6 gal Percent

DI-1 JUNCTION 2.34 2.34 0 12:00 0.041

0.041 0.006

DI-2 JUNCTION 1.05 1.05 0 12:00 0.0171

0.0171 -0.001

DI-3 JUNCTION 2.09 3.05 0 12:00 0.0336

0.0507 0.014

DI-4 JUNCTION 1.72 1.72 0 12:00 0.0276

0.0276 0.016

STMH-1 JUNCTION 0.00 2.34 0 12:00 0

0.041 0.026

STMH-2 JUNCTION 0.00 7.09 0 12:00 0

0.119 0.016

STMH-3 JUNCTION 0.00 1.72 0 12:00 0

0.0276 0.000 Ex_OF OUTFALL 0.00 7.09 0 12:00 0

0.119 0.000

Node Surcharge Summary

Surcharging occurs when water rises above the top of the highest conduit.

Max. Height Min. Depth Hours Above Crown Below Rim Node Type Surcharged Feet Feet

DI-2 JUNCTION 0.20 2.646 0.000

DI-3 JUNCTION 0.31 3.211 0.492

Node Flooding Summary

Flooding refers to all water that overflows a node, whether it ponds or not.

Total Maximum Maximum Time of Max Flood Ponded Hours Rate Occurrence Volume Depth Node Flooded CFS days hr:min 10^6 gal Feet

DI-2 0.05 0.09 0 12:00 0.000 0.052

Outfall Loading Summary

Flow Avg Max Total Freq Flow Flow Volume Outfall Node Pcnt CFS CFS 10^6 gal Ex_OF 98.88 0.36 7.09 0.119 System 98.88 0.36 7.09 0.119

Link Flow Summary

Maximum Time of Max Maximum Max/ Max/ |Flow| Occurrence |Veloc| Full Full Link Type CFS days hr:min ft/sec Flow Depth DI-1_to_STMH-1 CONDUIT 2.34 0 12:00 10.27 0.06 0.19 DI-2_to_DI-3 CONDUIT 1.08 0 12:01 3.09 0.70 1.00

DI-3_to_STMH-2 CONDUIT 3.05 0 12:00 8.74 2.28 1.00 DI-4_to_STMH-3 CONDUIT 1.72 0 12:00 4.46 0.23 0.35 STMH-1_to_STMH-2 CONDUIT 2.33 0 12:00 5.00 0.09 0.21 STMH-2_to_Spillway CONDUIT 7.09 0 12:00 6.44 0.24 0.38 STMH-3_to_STMH-2 CONDUIT 1.72 0 12:00 2.93 0.23 0.48

Flow Classification Summary

Adjusted ---------- Fraction of Time in Flow Class /Actual Up Down Sub Sup Up Down Norm Inlet Conduit Length Dry Dry Dry Crit Crit Crit Crit Ltd Ctrl

DI-1_to_STMH-1 1.00 0.06 0.00 0.00 0.00 0.00 0.00 0.94 0.00 0.00 DI-2_to_DI-3 1.00 0.00 0.00 0.00 0.99 0.00 0.00 0.00 0.98 0.00 DI-3_to_STMH-2 1.00 0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.00 DI-4_to_STMH-3 1.00 0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.00 STMH-1_to_STMH-2 1.00 0.06 0.00 0.00 0.00 0.00 0.00 0.94 0.00 0.00 STMH-2_to_Spillway 1.00 0.01 0.00 0.00 0.06 0.93 0.00 0.00 0.10 0.00 STMH-3_to_STMH-2 1.00 0.00 0.00 0.00 0.07 0.15 0.00 0.78 0.09 0.00

Conduit Surcharge Summary

Hours Hours --------- Hours Full -------- Above Full Capacity Conduit Both Ends Upstream Dnstream Normal Flow Limited DI-2_to_DI-3 0.20 0.20 0.31 0.01 0.01 DI-3_to_STMH-2 0.17 0.31 0.17 0.29 0.17

Analysis begun on: Thu Mar 16 19:09:07 2023 Analysis ended on: Thu Mar 16 19:09:07 2023 Total elapsed time: < 1 sec

Existing Conditions SWMM Output for 5-year-24-hr Event

Number of rain gages Number of subcatchments ... 5 Number of nodes Number of links

Data Recording Name Data Source Type Interval 100yr-24hr 100yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

10yr-24hr 10yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

1yr-24hr 1yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

25yr-24hr 25yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

2yr-24hr 2yr-24hr Henry Co. Virginia CUMULATIVE 6 min.

Name Area Width %Imperv %Slope Rain Gage Outlet

DAM_to_DI-1 0.31 49.45 100.00 146.3100 5yr-24hr

DI-1

Slope_to_DI-1 0.41 56.33 0.00 79.5000 5yr-24hr

DI-1

Slope_to_DI-2 0.31 50.17 31.00 78.1200 5yr-24hr

DI-2

Slope_to_DI-3 0.73 119.76 12.00 77.8400 5yr-24hr

DI-3

Slope_to_DI-4 0.65 81.44 6.00 68.9000 5yr-24hr

Invert Max. Ponded External Name Type Elev. Depth Area Inflow

DI-1 JUNCTION 833.77 4.14 100.0

DI-2 JUNCTION 831.38 3.26 100.0

DI-3 JUNCTION 830.26 4.37 100.0

DI-4 JUNCTION 827.85 4.54 100.0

STMH-1 JUNCTION 829.83 5.30 100.0

STMH-2 JUNCTION 826.07 9.04 100.0

Name From Node To Node Type Length %Slope Roughness

DI-1_to_STMH-1 DI-1 STMH-1 CONDUIT 17.5

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