Watershed Planning – Water Quality or Flood Control? The Planning Process
USEPA (2013) notes six basic steps to develop and implement a watershed master plan. The first step is to build partnerships with surrounding communities. Few communities can go alone to resolve such impacts. For example, water may enter a community watershed from upstream and leave to impact another community downstream, overwhelming their system.
The second step is to characterize the watershed. The process to accomplish this involves:
- Identify land uses including vacant land, wetlands, etc.
- Acquire soils data
- Acquire topographic data (Lidar)
- Identify relevant waterways
- Identify basins for flood routing
- Acquire FEMA flood maps
- Identify storm of interest (25 year and 100 year for category 4 in the CRS manual)
- Model flood response (Cascade)
- Develop flood risk/hazard mapping
- Identify areas of concern (repetitive loss)
Groundwater is relevant when the ground and surface waters are directly connected, and the soil lacks capacity for holding much infiltration. In addition, many plans are completed with the intent of improving water quality, thereby crossing paths with TMDL plans Note that USEPA recognizes the difficulty in obtaining watershed-related information with precision and acknowledges that a balanced approach is needed to address this concern.
There is a connection between WMPs and CRS. To earn CRS credit, communities must adopt regulatory standards that, at least, are creditable under element 452a Stormwater Management Regulations, require that runoff from all storms up to and including the 25-year event be managed, and ensure that future peak flows do not increase over current rates. In addition, coastal communities are required to evaluate the effects of sea level rise. Therefore, one strategy for reducing the effort associated with implementing this CRS element is working with neighboring communities that share watersheds. Working with other communities can help to maximize credits for the CRS element because of the way the impact adjustment is calculated. In addition, by working with neighboring communities, the cost associated with completing the hydrologic modeling necessary to earn credit for this element could be split.
Step 3 involves identifying measures to reduce impacts (watershed, regional, and local). At the watershed level, this is difficult to do, but the ability to use collected data to drill down is useful. Note that addressing local issues requires the ability to take the current data and drill down to details. An example process that USEPA (2013) suggests for capital plans is:
- “Inventory existing management efforts in the watershed, taking into account local priorities and institutional drivers.
- Quantify the effectiveness of current management measures.
- Identify new management opportunities.
- Identify critical areas in the watershed where additional management efforts are needed.
- Identify possible management practices.
- Identify relative pollutant reduction efficiencies.
- Develop screening criteria to identify opportunities and constraints.
- Rank alternatives and develop candidate management opportunities.”
Item 1 above is generally completed through the following measures:
- Review and evaluation of existing watershed data, including identification of features requiring immediate maintenance
- Development of preliminary watershed model diagram
- Establishment of GIS database for watershed resource features and parameter inventory through desktop and field reconnaissance
Floodplain analysis includes developing a watershed model and identifying associated inundation polygons. It builds upon information generated from the watershed evaluation so that planning and management decisions can be formulated. Floodplain analysis may include the following tasks:
- Completion of the watershed resource feature and parameter inventory GIS database for the watershed using the acquired information
- Assemblage of GIS database information into a specific format for a selected computer program which predicts the watershed’s response to the hydrologic cycle
- Watershed model development, calibration, and verification
- Floodplain delineation
Step 4 involves implementation, which means local communities participate in defining projects and solutions as well as the timing and means to fund them. This is where many watershed plans fail – the ability to fund outside a jurisdiction is fraught with many difficulties. Capital plans, bond issues, etc. are all part of the plan.
The final two steps are implementation and monitoring progress so that updates can be made. USEPA recognizes that the processes involved in watershed assessment, planning, and management are iterative and that targeted actions might not result in complete success during the first or second cycle.
The reason this is relevant to water utilities was noted earlier – funds are needed, sourcewater protection is relevant to watershed plans and the results may impact your water supply. Watershed planning occurs at a variety of levels of government – sometimes with consultants, other times via universities or regional agencies. But in any case it is important to be involved in the process.