Conducting an upstream and downstream analysis as part of a Type A or B or specialty report identifies, evaluates, and documents the impacts and risks, if any, that a project will have on the drainage conveyance system, properties, and sensitive areas. All projects that propose to discharge stormwater from WSDOT ROW and meet the requirements below are required to provide an analysis as part of the hydraulic report; see the hydraulic report outline for more information. For projects that require a flood risk assessment see additional guidance in Chapter 7.
•Projects that add 5,000 square feet or more of new, impervious surface area
•Projects where known drainage or erosion problems indicate there may be impactson either the upstream or downstream conveyance system, properties, or sensitiveareas
•Projects that add less than 5,000 square feet of new, impervious surface and wherethe project is within 300 feet of a stream or if the project’s stormwater dischargesinto a stream within 0.25 mile upstream or downstream of WSDOT’s ROW
•Projects that alter existing hydrology or drainage
1-3.7.1 Upstream and Downstream Analysis for Type A and B Reports
At a minimum, the analysis must include the area of the project site to a point 0.25 miledownstream of the site and upstream to a point where any backwater conditions cease.The results of the analysis must be documented in the project hydraulic report. Potentialimpacts to be assessed in the report also include but are not limited to changes in flowsfor extreme events, changes in flood duration, water surface elevations (WSELs), bankerosion, channel erosion, and nutrient loading changes from the project site. The analysisis divided into three steps that follow sequentially:
1.Review of resources
2.Inspection of drainage conveyance systems in the site area
3.Analysis of upstream and downstream effects
1-3.7.2 Review of Resources
The PEO reviews available resources to assess the existing conditions of the drainageconveyance systems in the project vicinity. Resource data commonly include aerialphotographs, area maps, floodplain maps, wetland inventories, stream surveys, habitatsurveys, engineering reports concerning the entire drainage basin, the Climate ImpactsVulnerability Assessment statewide map, geographic information system (GIS) and lightdetecting and ranging (LiDAR) information, and any previously completed upstream ordownstream analyses. All of this information shall encompass an area 0.25 miledownstream of the project site’s discharge point from WSDOT’s ROW and upstream toa point where any backwater conditions cease.
The background information is used to review and establish the existing conditions of the drainage conveyance system. This baseline information is used to determine whether the project will improve upon existing conditions, have no impact, or degrade existing conditions if no mitigating measures are implemented. The RHE and HQ Environmental Services Office staff will be able to provide most of this information. Other resource information sources include the Washington State Department of Ecology (Ecology), the Washington Department of Fish and Wildlife (WDFW), and local agencies.
1-3.7.3 Inspection of Drainage Conveyance System
The PEO must inspect the conveyance system and identify any existing problems that might relate to stormwater runoff. The PEO will physically inspect (if possible) the drainage conveyance system at the project site and downstream from the WSDOT ROW for a distance of at least 0.25 mile and upstream to a point where any backwater conditions cease. The inspection shall include any problems or areas of concern that were noted during the resource review process or in conversations with local residents and the WSDOT Maintenance Office. The PEO shall also identify existing or potential conveyance capacity problems in the drainage system, existing or potential areas where flooding may occur, existing or potential areas of extensive channel destruction or erosion, and existing or potential areas of significant destruction of aquatic habitat (runoff treatment or flow control) that can be related to stormwater runoff. If areas of potential and existing impacts related to project site runoff are established, actions must be taken to minimize impacts to upstream and downstream resources.
1-3.7.4 Analysis of Upstream and Downstream Effects
This final step analyzes information gathered in the first two steps of the analysis. It is necessary to determine if the project will create any drainage conveyance problems downstream or make any existing problems worse. The PEO must analyze upstream and downstream effects to determine corrective or preventive actions that may be necessary. If the project is within a medium- or high-vulnerability location according to the Climate Impacts Vulnerability Assessment statewide map, the PEO must run extreme events (for example, the 100-year storm event) and evaluate the impacts and stability of the conveyance system. The PEO will perform a risk assessment based on the extreme events showing impacts to the conveyance system and to downstream properties and sensitive areas.
PEOs will consult the Highway Runoff Manual for further guidance on the design flow for runoff treatment and flow control BMP design. In some cases, analysis of effects may indicate that no corrective or preventive actions are necessary. If corrective or preventive actions are necessary, the following options must be considered:
• Design the on-site treatment and/or flow control facilities to provide a greater level of runoff control than stipulated in the minimum requirements in Chapter 3 of the Highway Runoff Manual.
• Take a protective action separate from meeting Minimum Requirements 5 and 6 in the Highway Runoff Manual for runoff treatment and flow control. In some situations, a project will have negative impacts even when the minimum requirements are met. Below are two examples:
• Roadway runoff in a project’s threshold discharge area (TDA) was sheet-flowing to the roadway side slopes in the pre-developed condition but is now being collected and conveyed to a stormwater detention pond in the post-developed condition. The detention pond’s emergency overflow usually discharges to the same location as the riser structure and overflow structure but sometimes discharges to a different location. In both scenarios, even though the detention pond will provide flow control for more frequent storm events (up to the 25-year for eastern Washington or 50-year for western Washington), the larger, less frequent storm events (100-year) may not have flow control. These scenarios need to be analyzed as part of the downstream analysis. Because the stormwater is now collected and conveyed to one or two discharge locations, there may be more flow at those discharge locations than in the pre-developed condition. If a situation is encountered where downstream impacts will result from the project, the corrective action must be applied to the project based on a practicability analysis.
• If a project is flow control exempt, the conveyance system downstream of the project site shall be inspected to ensure adequate capacity. The PEO shall also analyze and document any changes to the downstream conveyance system, properties, and sensitive areas. If there are any negative impacts, the PEO shall perform a risk analysis showing what would happen if no actions were taken to minimize the negative impacts.
