Preliminary Drainage Study: Town of Hillsboro Pedestrian & Traffic Safety Project Traffic Calming Project UPC# 70587

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1 Preliminary Drainage Study: Town of Hillsboro Pedestrian & Traffic Safety Project Traffic Calming Project UPC# 70587

2 Introduction: The intent of this study was to perform a preliminary drainage study of the Town of Hillsboro within the proposed project limits identified in the Master Plan To Take Back Hillsboro s Main Street (February 2006). The level of study conducted was a broad overview of the existing drainage facilities, identification of any potential problems with the existing system to be dealt with through the proposed drainage plan, and an outline of what will be required for the proposed drainage design associated with the proposed project. Existing Drainage Conditions: The Town of Hillsboro is located at the foot of Short Hill Mountains, where most of the town s runoff originates. The profile of Route 9 (Charles Town Pike) through Hillsboro can be characterized as a rolling profile, with a high point located at the junction of Rte T-1812 or at Station of the VDOT survey conducted for this project. Drainage from north of Route 9 is either collected by one of 7 culverts or grate inlets located along the corridor, or simply is collected in the roadway along various types of curbs, asphalt mounds, or masonry walls until a point where the drainage seeks a low point and continues to the North Fork of Catoctin Creek. These low points are natural drainage swales, driveway entrances, connected roadways, or small lots between dwellings. Existing Drainage Structures: This study identified 7 hydraulic structures located along the corridor that conveys drainage either through cross culverts, or through grate interception and conveyed with a storm drain pipe. The Department analyzed these structures to determine their efficiency and effectiveness to either be utilized in the proposed design or upgraded as part of the project. The results of this preliminary analysis indicate that most of these structures would need to be upgraded to meet Departmental policies of providing roadway protection against the 10-year storm.

3 Drainage Area (Acres) Year Design Q (cfs) Shoulder Elevation FT " RCP " CMP 6.0 Structure ID EX 1 Station Description 24" CMP EX 2 EX " CMP 18" CMP EX EX 5 EX EX " CMP (w/ Grate tops (2)) 30" RCP (w/ Grate top) HW Elevation (FT) HW/D 0.94 Upgrade Required (Y/N) N Y N N/A N < N/A 2.22 Y Y Y Reason Roadway and HW/D > 1.5 OK OK Roadway HW/D > 1.5 Roadway and HW/D > 1.5 Proposed Condition Drainage: The Master Plan to Take Back Hillsboro s Main Street report indicated 3-typical sections to be utilized throughout the corridor. Each of the typical sections identified the use of an undivided roadway with 11.0 feet lanes, with curb and gutter sections, including a valley gutter for drainage. Also identified in the plan were the use of 2-roundabouts, one located at the western gateway to the town, and one located at the eastern gateway to the town. At various locations along the corridor parking is proposed to be achieved by either back-in parking, or parallel parking. Additionally, the proposed typical sections show sidewalks to be placed on both sides of the street. Below are illustrations referenced in the Master Plan, which were used for consideration in the proposed drainage requirements.

4 The proposed typical sections indicated above will produce additional impervious area along the project corridor. With additional impervious area, stormwater management may need to be considered, but based on a preliminary additional impervious area estimate; the entire corridor will only add an additional 0.5 acres (see Appendix for summary of estimate). Since the new impervious area is less than 1 acre, no stormwater management facilities are required to be incorporated as part of this project. FEMA Flood Plain Boundaries: The image above is a FEMA Firmette for Loudoun County, and more particularly the Town of Hillsboro. This indicates that the major stream (North Fork Catoctin Creek) that runs along the Southern edge of the Town of Hillsboro is delineated by FEMA as a Zone AE. Upon further examination of the image, the FEMA 100-year floodplain seems to

5 inundate a portion of Route 9 along the Western portion of town, in the proximity of Stony Point Road (Route 719). A Zone AE floodplain is an indication that the stream is a previously studied reach, which has identified BFEs (Base Flood Elevations) to be used for floodplain management purposes. The Master Plan for the Hillsboro Pedestrian & Traffic Safety Project indicates that a roundabout and widening of the existing roadway is proposed. Widening of the road, and changes to the existing road profile to accommodate these new features changes the grading and floodplain characteristics of the North Fork of Catoctin Creek. These changes could produce increases to the established floodplain elevations and boundaries, and consequently impact adjacent property owners during the 100-year storm event. To quantify any impacts incurred by the proposed design layout, a Hydrologic and Hydraulic Analysis needs to be conducted utilizing the same FEMA study that was used to establish the Base Flood Elevations indicated on the FEMA FIRM (Flood Insurance Rate Map). VDOT drainage policies indicate that if a floodplain is identified as a ZONE AE, that any road improvements proposed must not increase the 100-year flood elevations. Valley Gutters: Also noted in the typical sections was the use of valley gutters to convey the drainage along the roadway. Valley gutters are typically shallow depth concrete ditches that have very little drainage capacity. The natural drainage patterns in the Town of Hillsboro are such that a significant amount of the runoff through the Town is from offsite areas. Considering this fact, valley gutters would not be the most effective way to convey the drainage along the Route 9. If valley gutters are to be utilized, they should be used only to convey runoff from the roadway for short distances. Recommendations for proposed storm drain system: Ideally, the proposed drainage system should be designed to intercept the offsite drainage before it gets to the sidewalks to keep water from seeping under the sidewalks that would eventually break up the sidewalks. Additionally by picking up the drainage prior to the sidewalks would help preclude the development of ice on the sidewalks during freezing and melting periods of winter. With a proposed curb and gutter system being proposed, a closed drainage system will need to be developed to run throughout the corridor, and utilize the current drainage outfall locations with upgraded cross pipes. The most efficient and effective type of inlet that can be proposed along this corridor would be a slot inlet located along the curb sections to maintain spread in the travel lanes to a minimum. Unfortunately, these types of inlets will require the inlet structure tops to be utilized as part of the sidewalk to keep the typical section width to a minimum. On a positive note, if these structures are strategically placed at points where off site run-off would enter into the sidewalk and roadway sections, a connection on the back side of these inlets can be made to intercept the off site runoff into the storm drain system and conveyed properly.

6 Another drainage structures that can be considered for design are grate inlets located along the curb sections. Although these structures can be located such that they would not conflict with the sidewalk sections; they would require many more inlet structures to be placed due to their less effectiveness in conveying drainage. Grate inlets also have a significantly higher probability of clogging. Considering that the proposed corridor will be utilizing a significant amount of landscaping close to the roadway, the probability of these grates to be come clogged due to leaves and debris would be great. With either of the proposed closed storm drain systems, conflicts with the water distribution line located along the North side of Route 9 will need to be carefully considered or relocated. Considerations for Faux Humped Bridge Treatment: Existing grate inlet for 30 culvert located where proposed Faux Bridge to be located The Master plan indicates the use of a Faux humped bridge treatment to be used at approximately station , where a wet weather stream current enters into a combination grate inlet on the North side of Route 9 and outfalls via a 30 culvert into a well defined swale. Based on preliminary drainage area delineation, this structure receives 15.6 acres of offsite runoff. Based on the Department s preliminary hydraulic performance analysis of this structure, when a 10-year storm occurs much of the flow would build up and actually overtop the roadway rather than be intercepted by this inlet. Considering this, if the roadway profile is raised in this immediate area to accommodate the Faux Humped Bridge, the drainage conveyed to this inlet would back up onto the adjacent property located upstream, rather than overtop the roadway as being experienced now. The Department would suggest incorporating a new inlet structure that would be able to utilize the existing 30 culvert with maximum efficiency. By replacing the inlet section with structure that can convey significantly more flow than is currently being achieved.

7 Conclusions: With the incorporation of a curb and gutter type of drainage system along the Town of Hillsboro corridor will require that a storm drain system to be incorporated into plans. A summary of the estimated runoff to the curb and gutter sections along the proposed project corridor is included in the appendix (exhibit 2). The proposed drainage design should focus on intercepting off-site runoff to the fullest extent possible to limit the size and quantity of inlets located in the roadway section. Special care should be taken in areas where the proposed typical section indicates parallel parking with landscaping peninsulas extending toward the travel-way. These areas could very easily trap runoff in the corner of the parking spaces. More than likely, most of the exiting outfall culverts and storm drains will need to be upgraded to accommodate both the additional impervious areas and intercepted off-site drainage areas. The proposed drainage design should include the following areas of analysis: Delineated drainage area maps for all proposed structures Supporting computations for any culvert utilized in the proposed condition (LD- 269) Supporting computations for any storm drain inlets and pipe system sizing (LD- 204) Supporting HGL computations for the proposed storm drain systems (LD-347) Supporting computations for any ditch sections proposed (LD-268) Adequate outfall computations for all outfalls associated with the project Complete hydraulic and hydrologic analysis (HHA) if grading will encroach into the FEMA floodplain of North Fork of Catoctin Creek Phased E&S plans for construction of proposed improvements Confirmation that SWM is not required based on final design layout

8 Appendix: Town of Hillsboro- Traffic Calming Project- Preliminary Hydraulic Study SWM Estimate Considering there is a major drainage divide located within the Town of Hillsboro Limits The SWM requirements were based on two major outfalls being located within the Project Limits. The major drainage divided considered within the project limits is located at approximately West of STA Area (Sq Ft) Area (Acres) Existing Impervious Area= Proposed Impervious Area= Difference= <1.0 Acre additional Impervious Area, No SWM required East of STA Area (Sq Ft) Area (Acres) Existing Impervious Area= Proposed Impervious Area= Difference= <1.0 Acre additional Impervious Area, No SWM required Exhibit 1 Exhibit 2: Estimated Gutter Flows for Proposed Condition: Summary of Proposed Gutter Flows From Sta To Sta Ex Outfall Q(10) RT Gutter (cfs) Q(10) LT Gutter (cfs) EX EX EX EX EX EX EX Exhibit 1

9 Existing Drainage Summary Structure ID Station Description Drainage Area (Acres) 10-Year Design Q (cfs) Shoulder Elevation HW Elevation (FT) HW/D Upgrade Required (Y/N) EX " CMP FT N Reason Roadway and HW/D > 1.5 EX " CMP Y EX " CMP N OK EX " CMP (w/ Grate tops (2)) N/A N OK 30" RCP (w/ Grate EX top) < N/A Y Roadway EX " RCP Y HW/D > 1.5 Roadway and HW/D > EX " CMP Y 1.5 Exhibit 3

10 Drainage Area Map of Hillsboro Exhibit 4

11 Stage-Discharge Working Curve EX Combinatin Slot inlet with Grate Inlet: Assumptions: Slot height: Slot width 4 in (0.33 FT) 2.5 ft Height of Inlet top above slot invert: 1.0 ft Slot open area: 0.333x ft^2 Slot Wetted P 5.66 Ft Grate WxL 2.25 ft x 2.25ft Grate area (minus bar spacing) 2.33 ft^2 (per DI-1 in sump chart) Grate Wet P 9.00 ft Ref: Kings Handbook of Hydraulics Length of Slot opening 2.5 Clear area of opening of Slot 0.83 Weir coefficient used 3.1 Orifice coefficient used 0.6 Perimeter of Grate Opening 9 Clear area of Grate Opening 2.33 Note: Does not include water quality orifice flow. Head (ft) Inlet Orifice Discharge Inlet Weir Discharge Inlet System Head on Grate Orifice Discharge for Grate Inlet Weir Discharge for Grate Grate System Total System X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Head >1.0 feet overtopping roadway Head >1.0 feet overtopping roadway Head >1.0 feet overtopping roadway Head >1.0 feet overtopping roadway Head >1.0 feet overtopping roadway Head >1.0 feet overtopping roadway Head >1.0 feet overtopping roadway Head >1.0 feet overtopping roadway Head >1.0 feet overtopping roadway Performance Curve Q (cfs) Head (ft)