APPENDIX B Hydrologic and Hydraulic Analysis
HYDROLOGIC AND HYDRAULIC ANALYSIS PETERSBURG ROAD IMPROVEMENTS DOT&PF Project No. 67879 Prepared for: State of Alaska Department of Transportation and Public Facilities Southeast Region 6860 Glacier Highway Juneau, Alaska 99801 (907) 465-1763 Prepared by: DOWL HKM 4041 B Street Anchorage, Alaska 99503 (907) 562-2000 W.O. D70212 September 2010
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 TABLE OF CONTENTS Page 1.0 INTRODUCTION...1 1.1 Objective...1 2.0 HYDROLOGIC ANALYSIS...1 2.1 Drainage Area Characteristics...1 2.2 Methodology...2 2.3 Soil Conservation Service Graphical Peak Discharge Method...2 2.4 Estimated Runoff...4 3.0 HYDRAULIC ANALYSIS...5 3.1 Existing Conditions...5 3.2 Storm Drain Pipe Material...9 4.0 RECOMMENDATIONS...9 4.1 Storm Drain System Recommendations...9 FIGURES Figure 1: Drainage Basins...3 TABLES Table 1: Summary of Drainage Basin Areas...1 Table 2: Drainage Basin Runoff Summary...4 Table 3: Existing Storm Drain System Capacities...5 APPENDIX Appendix A... Runoff Computations LIST OF ACRONYMS cfs... cubic feet per second CMP... corrugated metal pipe CPEP... corrugated polyethylene pipe DOT&PF... State of Alaska Department of Transportation and Public Facilities GIS... Geographic Information System NRCS... Natural Resources Conservation Service RCN... runoff curve number SCS... Soil Conservation Service T c... time of concentration TR-55... Technical Release 55: Urban Hydrology for Small Watersheds Page i
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 1.0 INTRODUCTION 1.1 Objective The objective of this study is to determine the runoff quantities for 2-, 10-, and 25-year peak discharges draining to the downtown area in Petersburg, Alaska, to facilitate sizing of storm drain systems in support of roadway improvements. Hydrologic and hydraulic computations were performed to evaluate existing drainage infrastructure to determine whether capacity exists to effectively transport the 25-year peak flow and to determine appropriate sizing for locations where existing capacity is exceeded. This report is intended to specify sizing requirements for new storm drain systems to be incorporated into the Petersburg Road Improvements project and is not intended to provide specific locations of new drainage structures. Additional design will be required as utility improvements progress. 2.0 HYDROLOGIC ANALYSIS 2.1 Drainage Area Characteristics Ten drainage basins were defined in order to determine runoff for 2-, 10-, and 25-year storm events. The drainage basins were defined based upon existing topographic maps, Geographic Information System (GIS) data, survey information, and reconnaissance gathered during a site inspection conducted throughout 2009. A map of the drainage basins is presented in Figure 1. The basins are primarily located within developed regions of the City of Petersburg, with the upper boundary of one basin including undeveloped forested hillside. Table 1: Summary of Drainage Basin Areas Drainage Basin Area (acres) 1 4.08 2 7.57 3 5.52 4 11.77 5 0.73 6 4.34 7 1.34 8 1.09 9 0.71 10 43.83 Table 1 lists the contributing size of each basin in acres. Typically, basins drain to the North Harbor in the Wrangell Narrows via several existing piped storm drain systems. Some basins drain to Hammer Slough near Nordic Street via sheet flow or piped storm drain systems. Hammer Slough drains to the Wrangell Narrows. Average slopes of drainage basins at and below Nordic Drive are typically in the range of 1% to 2%, while basins above Nordic Drive are much steeper with slopes as high as 25%. Page 1
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 2.2 Methodology Hydrologic analyses were performed to determine peak flows for the design recurrence intervals. The 25-year, 24-hour design flows determine the required capacity of the storm drain systems in order to appropriately contain peak flows as typically required by the State of Alaska Department of Transportation and Public Facilities (DOT&PF). The 2-year, 24-hour design flows provide the basis for determining appropriate water treatment measures for new storm drain systems. The 10-year, 24-hour runoff provides an intermediate flow used for evaluating existing storm drain systems and can potentially be used for sizing less critical storm drain systems. These calculated peak flows were used to evaluate existing drainage infrastructure for flood conveyance capacities. The hydrologic data for this study was computed using the Soil Conservation Service (SCS) Graphical Peak Discharge Method. The SCS Method is based upon the United States Department of Agriculture Natural Resources Conservation Service s (NRCS) Technical Release 55: Urban Hydrology for Small Watersheds (TR-55). 2.3 Soil Conservation Service Graphical Peak Discharge Method The SCS Method was applied to the ten basins in order to determine the peak flows for 2-, 10-, and 25-year runoff events. The input data required for the SCS Method includes the drainage areas, runoff curve numbers (RCN), the time of concentration (T c ) and the associated precipitation values. The precipitation values were acquired from 24-hour rainfall maps in SCS s Technical Publication No. 47. TR-55 specifies that Type I storm should be used for all of Alaska. However, the DOT&PF Highway Drainage Manual recommends using a Type IA storm for coastal regions of Alaska. This selection seems logical in this case as the coastal rainforest climate of Petersburg is more comparable to the coastal climate of Oregon and Washington (where Type IA storms are specified for use in TR-55) than to the interior regions of Alaska where Type I storms are specified. The SCS curve numbers are used to describe the surface characteristics of the drainage area and are based upon land cover and hydrologic soil type. Soils are grouped as Type A, B, C, or D based upon rates of hydraulic conductivity, where Group A soils have the most potential for infiltration and Group D soils have the least potential for infiltration. The NRCS does not have soil data readily available for this area. However, soil log data located through NRCS reports lists soils in the Petersburg area as being primarily Type B and C soils. For this analysis, it was Page 2
Drainage Basins Basin Acres 1 4.08 2 7.57 La 3 5.52 ke S 4 11.77 tr e t 6 4.34 7 1.34 8 1.09 7th St reet Fram Street Gauffin Str eet Excel Str eet 7th St reet 5 0.73 6th St reet 9 0.71 10 43.83 Kiseno Street t u e Ira II St reet e a tr Haugen Drive e A l as g S 8th St reet 5th St reet 5th St reet ng ra W 1 «0 4th St reet e nu ve la el 4th St reet 3r d Street 4 «Gjoa Stree t ug lo y 0 P:\Projects\J20212\GIS\Hydro\Drainage Basins.mxd 75 150 Feet 300 Figure 1 Drainage Basins Lumbe r Stree troad Improvements Petersburg February 12, 2010 Pear l F Road Flow Direction Ne ptu ne St ree t Storm Drain ¹ gl ee Topo Lines Lan sin g St 7 «Sin Drainage Ditch le Al Ha rb o r Wa y 5 «8 «Kise n o St 9 «h Marian St Nordic Drive et ers 6 «re mm 3 «St ch Bi r Ha 1st Stre et Dolp hin Stre et Bald er Str eet 1 «2 «2nd Str eet 1222.70212
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 decided to classify all of the soils as Type C in order to be conservative in determining peak runoff flows and to take into account the fact that disturbed soils typically result in higher runoff quantities than undisturbed soils. After determining the hydrologic soil type, the RCN is determined based upon the land cover. For this analysis, an RCN of 70 was selected to represent forested areas (woods, good condition), an RCN of 90 was selected to represent residential (1/8 acres lots) and commercial areas, and an RCN of 98 was selected to represent impervious areas, rooftops, and paved roads. The T c is the total time required for the runoff to flow from the most hydraulically remote point in the drainage basin to the point of investigation. Average basin slopes and flow lengths were determined for all ten basins based upon GIS data and topographical information. Slopes ranged from as low as 1% to 25%. The T c was obtained using procedures described in TR-55 for each basin. The total T c is the sum of the overland sheet flow, shallow concentrated flow and channel flow. The TR-55 computations for the SCS Method are included in Appendix A. 2.4 Estimated Runoff Results of the SCS Method for estimating runoff from the ten basins are presented in Table 2. Where assumptions were required regarding soil conditions, land cover and drainage structure conditions, conservative approximations were made to compute flow estimates. This includes using RCN numbers based on Type C soils, determining T c based on practical drainage paths using paved gutter flow, and assuming that existing storm drain systems are in good condition and at moderate slopes. Table 2: Drainage Basin Runoff Summary Drainage Basin Area (acre) Q 2yr,24hr (cfs) Q 10yr,24hr (cfs) Q 25yr,24hr (cfs) 1 4.08 3.1 4.7 5.7 2 7.57 5.8 8.6 10.6 3 5.52 4.2 6.3 7.7 4 11.77 10.2 14.7 17.6 5 0.73 0.7 1.0 1.1 6 4.34 3.3 5.0 6.1 7 1.34 1.3 1.8 2.1 8 1.09 1.0 1.4 1.7 9 0.71 0.7 0.9 1.1 10 43.83 27.9 44.2 55.1 Page 4
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 3.0 HYDRAULIC ANALYSIS 3.1 Existing Conditions The existing storm drain systems in the project area and surrounding vicinity are included in Figure 1. Locations and flow directions of the drainage systems were determined from GIS data, survey information, and field inspections. The full-flow capacity of existing storm drain systems was computed using Manning s equation without headwater or tailwater effects. Where pipe slopes were not known, slopes were assumed based on existing topography and similar piped systems. Slopes were assumed to match road slopes and to have similar slopes to those systems for which survey data was available. The existing storm drain systems analyzed in this study are presented in Table 3. The system capacity is compared to the estimated 25-year, 24-hour runoff from contributing drainage basins. Descriptions of the individual drainage basins follow. Table 3: Existing Storm Drain System Capacities Contributing Basin 1 2 3 4 5 6 7 8 9 10 Pipe Description 24" CMP in Nordic Drive 24" CMP in Dolphin Street South of Nordic Drive 30" CMP in Excel Street 24" CMP in Fram Street 12" CMP in Gjoa Street at Harbor Way 24" CMP in Gjoa Street at Harbor Way 18" CMP in Harbor Way 18" CMP in Nordic Drive and Haugen Drive 12" CMP in Harbor Way 30" CMP in Haugen Drive Assumed Slope 8.0% 1.0% 1.1% 1.3% 1.0% 1.0% 1.5% 2.0% 1.0% 2.7% Discharge Point North Harbor North Harbor North Harbor North Harbor North Harbor North Harbor North Harbor Hammer Slough North Harbor Hammer Slough Q full (cfs) Q 10yr (cfs) Passes Q 10yr Q 25yr (cfs) Passes Q 25yr 34.7 4.7 Yes 5.7 Yes 12.3 8.6 Yes 10.6 Yes 23.3 6.3 Yes 7.7 Yes 14.0 14.7 No 17.6 No 2.2 1.0 Yes 1.1 Yes 12.3 5.0 Yes 6.1 Yes 7.0 1.8 Yes 2.1 Yes 8.1 1.4 Yes 1.7 Yes 2.2 0.9 Yes 1.1 Yes 35.1 44.2 No 55.1 No Page 5
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 Basin 1 is 4.08 acres in size and encompasses the area from Balder Street west to the northern portion of Nordic Drive. Commercial development makes up the majority of the basin area. Runoff from this basin collects in a piped storm drain system located in Balder Street and Nordic Drive. The existing storm drain system consists of 18-inch corrugated metal pipe (CMP) in Balder Street and 24-inch CMP in Nordic Drive and drains to an outfall at the North Harbor west of Nordic Drive. The 2-year, 24-hour runoff for this basin is 3.1 cubic feet per second (cfs); the 10-year, 24-hour runoff is 4.7 cfs; and the 25-year, 24-hour runoff is 5.7 cfs. The 24-inch CMP in Nordic Drive has an estimated full-flow capacity of 34.7 cfs (at a slope of 8.0%) and is adequately sized for all analyzed storm recurrence intervals. The pipe slope and subsequent hydraulic capacity may decrease near the system outfall. This will need to be confirmed during the design process. Basin 2 is 7.57 acres in size and encompasses the area between Dolphin Street and Balder Street, including portions of Third Street, Second Street, First Street, and Nordic Drive. This basin consists primarily of commercial and residential properties. This basin drains through an 18- and 24-inch CMP system located in Dolphin Street and Nordic Drive before reaching an outfall at the North Harbor. The 2-year, 24-hour runoff for this basin is 5.8 cfs; the 10-year, 24-hour runoff is 8.6 cfs; and the 25-year, 24-hour runoff is 10.6 cfs. The 24-inch CMP in Dolphin Street has an estimated full-flow capacity of 12.3 cfs (at a slope of 1.0%) and is adequately sized for all analyzed storm recurrence intervals. Basin 3 is 5.52 acres in size and includes the area between Excel Street and Dolphin Street from approximately Third Street west to Nordic Drive and the north end of Harbor Way. The basin consists of commercial and residential developments. Runoff from Basin 3 drains via gutter and ditch flow to a piped storm drain system located in Excel Street. The system consists primarily of 30-inch CMP pipes and drains to an outfall at the North Harbor at the intersection of Excel Street and Harbor Way. The 2-year, 24-hour runoff for this basin is 4.2 cfs; the 10-year, 24-hour runoff is 6.3 cfs; and the 25-year, 24-hour runoff is 7.7 cfs. The 30-inch CMP in Excel Street has an estimated full-flow capacity of 23.3 cfs (at a slope of 1.1%) and is adequately sized for all analyzed storm recurrence intervals. Page 6
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 Basin 4 is 11.77 acres in size and encompasses the area between Fram Street and Excel Street from approximately Fourth Street to Harbor Way, including a portion north of Excel Street from Third Street to Fifth Street. This is a developed area consisting of residential and commercial properties. This basin drains via ditch flow to Fram Street, where runoff collects in a piped storm drain system consisting of 18- and 24-inch CMP. Runoff flows through the 24-inch CMP to an outfall at the intersection of Fram Street and Harbor Way. The 2-year, 24-hour runoff for this basin is 10.2 cfs; the 10-year, 24-hour runoff is 14.7 cfs; and the 25-year, 24-hour runoff is 17.6 cfs. The 24-inch CMP in Fram Street has an estimated full-flow capacity of 14.0 cfs (at a slope of 1.3%) and is undersized for the 10- and 25-year storm events. Basin 5 is 0.73 acres in size and includes the area encompassing Gjoa Street between Nordic Drive and Harbor Way. This basin consists of impervious areas (roadways, roofs, etc) and drains to a piped storm drain system in Gjoa Street and Harbor Way. The pipes in this system are 12-inch CMP and drain to an outfall at the North Harbor. The 2-year, 24-hour runoff for this basin is 0.7 cfs; the 10-year, 24-hour runoff is 1.0 cfs; and the 25-year, 24-hour runoff is 1.1 cfs. The 12-inch CMP system in Basin 5 has an estimated full-flow capacity of 2.2 cfs (at a slope of 1.0%) and has sufficient capacity for all analyzed storm recurrence intervals. Basin 6 is 4.34 acres in size and is encompasses an area around Gjoa Street from Second Street to Sing Lee Alley. The basin includes residential and commercial development. The basin drains to a piped storm drain system comprised of 18- and 24-inch CMP in Gjoa Street draining to an outfall at the North Harbor. The piped storm drain system from Basin 6 drains though Basin 5 to an outfall adjacent to the outfall from Basin 5. The 2-year, 24-hour runoff for this basin is 3.3 cfs; the 10-year, 24-hour runoff is 5.0 cfs; and the 25-year, 24-hour runoff is 6.1 cfs. The 24-inch CMP in Gjoa Street has an estimated full-flow capacity of 12.3 cfs (at a slope of 1.0%) and has sufficient capacity for all analyzed storm recurrence intervals. Basin 7 is 1.34 acres in size and is located west of Nordic Drive at the harbor, including a portion of Sing Lee Alley and Harbor Way. This basin consists almost entirely of impervious area. Runoff from this basin collects in a piped storm drain system in Harbor Way consisting of 18-inch CMP draining to an outfall at the North Harbor. The 2-year, 24-hour runoff for this basin is 1.3 cfs; the 10-year, 24-hour runoff is 1.8 cfs; and the 25-year, 24-hour runoff is 2.1 cfs. Page 7
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 The 18-inch CMP has an estimated full-flow capacity of 7.0 cfs (at a slope of 1.5%) and is adequately sized for all analyzed storm recurrence intervals. Basin 8 is 1.09 acres in size and encompasses an around surrounding the intersection of Haugen Drive and Nordic Drive. The area is impervious and drains through a piped storm drain system consisting of 18-inch CMP to an outfall at Hammer Slough, located west of downtown Petersburg. The 2-year, 24-hour runoff for this basin is 1.0 cfs; the 10-year, 24-hour runoff is 1.4 cfs; and the 25-year, 24-hour runoff is 1.7 cfs. The 18-inch CMP has an estimated full-flow capacity of 8.1 cfs (at a slope of 2.0%) and is adequately sized for all analyzed storm recurrence intervals. Basin 9 is 0.71 acres in size and includes the area encompassing the north portion of Harbor Way. This area is impervious and drains through a piped storm drain system consisting of 12-inch CMP to an outfall at the North Harbor, located west of downtown Petersburg. The 2-year, 24-hour runoff for this is 0.7 cfs; the 10-year, 24-hour runoff is 0.9 cfs; and the 25-year, 24-hour runoff is 1.1 cfs. The 12-inch CMP system in Basin 9 has an estimated full-flow capacity of 2.2 cfs (at a slope of 1.0%) and has sufficient capacity for all analyzed storm recurrence intervals. Basin 10 is 43.83 acres in size and encompasses a large portion of the east side of downtown Petersburg, including the area spanning from approximately Kiseno Street to Fram Street between Second and Eighth Streets. Basin 10 consists primarily of developed residential areas, with some undeveloped forested areas present in the northeast portion of the basin. The basin drains via a network of road-side drainage ditches to a piped storm drain system in Haugen Drive. The piped storm drain system includes segments of 18-, 24-, and 30-inch CMP. From the intersection of Haugen Drive and First Street, the system drains south through 30-inch CMP to an outfall at Hammer Slough. The 2-year, 24-hour runoff for this basin is 27.0 cfs; the 10-year, 24-hour runoff is 42.7 cfs; and the 25-year, 24-hour runoff is 53.3 cfs. The 30-inch CMP has an estimated full-flow capacity of 35.1 cfs (at a slope of 2.5%) and is undersized for the 10- and 25-year storm events. Page 8
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 3.2 Storm Drain Pipe Material All existing storm drain systems within the project area are believed to be CMP. The use of smooth-walled corrugated polyethylene pipe (CPEP) is recommended for upgrading storm drain systems in the project area during utility improvements. CPEP has decreased sidewall roughness compared to CMP, resulting in approximately twice the flow capacity of equal-diameter CMP. This often allows using smaller sized pipes than would be required for CMP, facilitating ease of construction and fewer utility conflicts. Additionally, CPEP is not adversely affected by corrosive soils or marine environments, increasing the life of drainage infrastructure. Based upon topographic information and existing storm drain information, providing adequate slope for new pipes does not appear to be a significant constraint. In many cases, pipe slopes may be at or above 1%, allowing for increased pipe capacity. Where providing adequate cover is a concern, the use of smaller diameter CPEP may help to alleviate some of these cover constraints. 4.0 RECOMMENDATIONS 4.1 Storm Drain System Recommendations The following recommendations are based upon the hydraulic capacities of pipes compared to the estimated peak discharges as determined by the SCS Method. These recommendations are for planning purposes and do not take into account all design limitations, including cover requirements, structure placement, and potential utility conflicts. Additional design will be required during later phases of the project to determine pipe slopes, cover availability, and structure locations and elevations. Where sufficient data was not available to precisely determine the slopes of existing pipes, slopes were assumed to match roadway slopes as determined from topographic data. A minimum slope of 1% was assumed for estimating pipe capacity near system outfalls. A more detailed investigation of pipe slopes in the project area will be necessary during the design process to confirm hydraulic capacities. Overall, the majority of existing storm drain systems (excluding those in Basin 4 and 10) has adequate capacity to convey the 25-year, 24-hour flood flows. During the design process, it is recommended that the condition of the existing systems be inspected for signs of corrosion (rust), sedimentation, and failing joints and concrete structures. If the existing systems appear to be in good condition, they can be retained and incorporated into the roadway design if this is practical in meeting design objectives. If the existing Page 9
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 storm drain systems appear to be in poor condition or cannot readily be incorporated into the roadway improvements, it is recommended the systems be replaced with new CPEP storm drains systems sized to convey the 25-year, 24-hour peak flows. Typically 18- or 24-inch CPEP systems would provide adequate hydraulic capacity. The existing 24-inch CMP storm drain system in Fram Street receiving runoff from Basin 4 is undersized for 10- and 25-year storm events. It is recommended that this system be replaced with a new CPEP storm drain system to improve hydraulic capacity. Replacing the existing system with 24-inch CPEP (full flow capacity of 19.0 cfs at a slope of 0.6% or greater) or 30-inch CPEP (full flow capacity of 24.3 cfs at a slope of 0.3% or greater) would provide adequate capacity to convey the runoff for a 25-year, 24-hour storm event (17.6 cfs). To successfully upgrade the storm drain system in Basin 4, the pipe would need to be replaced all the way to the outfall at the North Harbor. It is recommended that the existing 12-inch CMP storm drain systems located in Basins 5 and 9 (in Harbor Way) be replaced with new 18-inch CPEP systems to decrease future maintenance demands. The use of 18-inch pipe would help reduce the effects of sedimentation within the system and would facilitate easier cleaning and inspection as necessary. The storm drain systems in Basins 5 and 9 could also be connected to the larger systems draining Basins 4 and 6 (after ensuring that overall system capacity is adequate) to reduce the number of outfalls and allow for more efficient treatment for water quality before discharging stormwater to the North Harbor. The existing 30-inch CMP storm drain system in Basin 10, located in the west end of Haugen Drive, is undersized for 10- and 25-year storm events. It is recommended that no new storm drain pipes or systems be connected to the existing system in Basin 10. Routing any additional runoff to the storm drain system in Basin 10 would require upgrading the existing system. Increasing the system capacity would requiring replacing the existing 30-inch CMP with 30-inch CPEP (full flow capacity of 54.4 cfs at a slope of 1.5% or greater) or 36-inch CPEP (full flow capacity of 56.0 cfs at a slope of 0.6% or greater) to convey the runoff for a 25-year, 24-hour storm event (55.1 cfs). Upgrading the storm drain system draining Basin 10 would require installing new pipe to the outfall located at Hammer Slough. As the proposed roadway improvements will not be Page 10
Hydrologic and Hydraulic Analysis Petersburg, Alaska Petersburg Road Improvements DOT&PF Project No. 67879 significantly increasing the impervious area, peak flows draining to the existing 30-inch CMP system are not expected to increase. None of the existing storm water systems contain measures to improve the water quality of collected runoff. It is recommended that new or partial replacement storm drain systems be designed with a structural treatment device to improve stormwater quality before runoff is discharged to receiving water bodies. An oil-water separator or similar device should be installed upstream of the system outfall to improve water quality. Page 11
APPENDIX A RUNOFF COMPUTATIONS