October 1996

Levels of Service for Comprehensive Watershed Management Planning

by

L. Moris Cabezas, Ph.D., P.E., Parsons Engineering Science

and

Walid M. Hatoum, P.E., Parsons Engineering Science

As our communities have developed a better understanding of the hydrological arid ecological processes taking place in a watershed, the planning goals and objectives established by national, state and local governments have evolved from solving specific short-term problems to developing comprehensive long-term approaches. Those comprehensive approaches take into account the numerous, and often conflicting, aspects of watershed management: flood control, water quality control and ecological protection. Projects considered for implementation need to be evaluated in terms of their potential impacts to those three areas of concern. However, limited resources require that basic criteria be established to determine if a course of action is warranted. Such criteria are referred to as Levels of Service (LOS). This paper discusses approaches currently taken to implement the LOS concept.

Levels of Service for Flood Control

The LOS for flood control are commonly adopted at the local level and can be defined as acceptable flooding criteria for various conditions. The concept, based on expected flood damages, is well established and generally defines flooding conditions associated with several design storms for structures and road access. For example, the LOS may establish that:

· Open conveyance systems should have the capacity to contain within their banks the runoff from a 25-year, 24-hour storm · Closed conveyance systems should have the capacity to convey the runoff from a 10-year, 1-hour storm · Structures should not be flooded during the 100-year, 24-hour storm · Evacuation routes should allow one traffic lane to be passable during the 100- year, 24-hour storm flood stage · Neighborhood streets should not experience over one foot of flood depth during the 100-year, 24-hour storm

There are several other criteria applicable these LOS and they vary among operating agencies. Nevertheless, the flood control LOS can effectively be used to assess the impact and costs of alternative flood control projects.

Levels of Service for Water Quality

LOS for water quality impacts due to stormwater discharges have not been as well defined as those for flood control, but significant progress has been made over the last years toward the adoption of a generalized approach. These LOS are currently established at all levels of government and may take the form of discharge limitations in terms of flow and pollutant concentration, pollutant load reduction goals or even extent of treatment coverage.

The National Pollutant Discharge Elimination System (NPDES) permitting process for stormwater discharges is creating the basis for the establishment of flow and pollutant discharge limitations. This process is aimed at requiring local governments to better understand the spatial layout of their Municipal Separate Storm Sewer Systems (MS4s). One of the ultimate goals of this program is to obtain a clearer understanding of the discharge characteristics from the MS4s to waters of the nation. The NPDES permit requires local governments to formulate management plans for the implementation of Best Management Practices to improve discharge characteristics.

An example of the pollutant load reduction goals approach is that established in Florida through Chapter 62-40.432 FAC, which is administered by the Water Management Districts and has been adopted by local organizations such as the Tampa Bay National Estuary Program (TBNEP). The Comprehensive Plan developed by the TBNEP focuses on implementing projects to control nitrogen loads from stormwater runoff as well as domestic and industrial point sources. Another area of concern is toxic contamination in sediments and the level of risk they pose to marine and human health.

The extent of treatment coverage approach aims to set goals for developing runoff control measures on an area-wide basis. For example, the Sarasota Bay NEP has established the goal of applying stormwater treatment technologies to 50 percent of the contributing watersheds to reduce the contaminant load by 27 percent.

The application of any of the water quality LOS approaches allows for an adequate assessment of impacts versus costs of proposed alternatives.

Levels of Service for Ecological Protection

The least defined criteria for watershed management are those related to ecological protection. An approach that has recently been developed an applied incorporates the concept of Biological Levels of Service (BLOS). The BLOS are defined for each habitat area and reflect the overall habitat value of an ecological community. They constitute the foundation for both establishing potential target conditions and evaluating management practices. Possible criteria for the BLOS include community type, based on plant dominance or aquatic characteristics; plant diversity, density and cover; canopy coverage; level of impact to native vegetation; level of disturbance, such as land-clearing, development, etc.; presence of exotic/nuisance vegetation; effects of fire; availability and quality of wildlife habitat; available contiguous area; proximity to other natural areas; presence of protected species; and hydrologic status (overdrained, excess freshwater inflow, depth to groundwater, length of hydroperiod, channelization of sheetflow, etc.). BLOS criteria may also include an allowable degree of human disturbance for recreational and public use aspects including, but not limited to, fishing, environmental education, boating, camping, hunting, etc. Furthermore, given that wetland community productivity is largely derived by surface ponding and/or water table levels, the hydroperiods necessary to sustain wetland productivity and viability are also identified for each BLOS.

The BLOS criteria are then used to evaluate the alternatives proposed in the comprehensive watershed plan. In addition, once a project is implemented, an ecological assessment of target conditions may be conducted. The ecological assessment could focus on such features as habitat improvement, invasive species removal, buffer zone establishment, wildlife reintroduction, corridor development, improvements to hydrology, wetland creation, and increased signage for environmental education purposes.

Successful application of the LOS and BLOS concepts to watershed management plans permit the systematic analysis of alternatives that include cost estimates for different target conditions. In addition, cost/benefit and risk management analysis can be performed. In summary, when using this approach, it is possible to present (in a structured manner) what systems improvements are being considered; what result the improvements will have in terms of flood control, water quality control, and ecological protection; and, how much the improvements will cost.