Hydrolog-02

Hydrologic Factors of Concern to Conservation Commissioners

Several hydrologic factors can be of key importance to the water balance of wetland resources, and are therefore of particular concern to Conservation Commissioners. These factors are identified in the following discussion, and treated in greater detail in later chapters. The factors include:

Surface Runoff

During a precipitation event, some of the water falling on the earth’s surface is retained on vegetative surfaces and in shallow depressions. Some additional water infiltrates into the ground. The remaining water flows over the ground surface as direct runoff (sometimes referred to as “storm water runoff” or simply, “runoff”).

The volume of runoff that will occur on a site during a given rainfall event depends on a number of factors:

! The area of land from which runoff occurs (known as the watershed);

! amount of precipitation;

! the duration and intensity (volume per unit of time) at which precipitation falls;

! the soils at and near the land surface; and

! the surface cover (combination of exposed earth, vegetation, pavement and roofs).

The rate at which runoff discharges from a given site is known as the runoff rate or discharge rate. The peak rate of runoff from a given site (also referred to as the peak flow rate, peak runoff rate, or peak discharge rate) is the maximum rate of runoff that occurs during a particular storm event. The rate of runoff depends on the following factors in addition to those affecting runoff volumes:

! The roughness of the surface, which is determined by the type of surface cover (type of vegetation, bare ground, pavement);

! The location of the impervious area in the watershed in relation to the point of analysis;

! Slope of the ground surface (flatter slopes result in slower rates of flow over the ground, steeper slopes result in faster rates of flow);

! Total distance the runoff must travel to the point of analysis.

The development of a previously undeveloped site for another use can alter the physical features affecting runoff. The development of an undeveloped site for an urban land use usually involves the creation of impervious surfaces (pavements and roofs) which have particularly significant effects on the volume and rate of runoff. The primary effects of urban development include:

! Increase in volume of runoff: The volume of water available for runoff increases because the impervious area provided by roofs, parking lots, streets, and other impervious surfaces reduces the amount of infiltration that can occur. Note that this increase in runoff volume is directly associated with a decrease in recharge of the groundwater.

! Increase in runoff rates: Urban development involves changes in surface cover, and the introduction of channels, curbing, gutters, and storm drainage collection systems. These changes result in hydraulic efficiencies that increase the velocity of runoff as it flows to the watershed outlet. This results in higher peak rates of storm water discharge.

Thus, if a site is under development, alterations in soils, surface cover, and topography can result in changes in the quantities and rates of runoff entering a wetland. Such alterations can change the water budget of a wetland, with resulting changes in wetland functions. Such changes may include:

! Increase or decrease in the total volume of water reaching a wetland over an extended period of time, affecting the amount of water available to support aquatic and terrestrial habitat;

! Increases in flow volumes during storm events, that result in increased flooding of upland or wetland areas;

! Increases in peak flow rates during storm events. Increases in peak rates may result in increased erosion of upland or wetland resource areas, and subsequent deposition of

sediment within resource areas;

! Changes in patterns of flow, resulting in localized changes in erosion, sedimentation, and surface water storage. Therefore, Conservation Commissioners need to consider the effects of development on the volumes, peak rates, and patterns of runoff entering wetlands.

Groundwater Recharge

When a portion of rainfall infiltrates into the ground surface, some of that water replenishes soil moisture. Some of the water stored or flowing beneath the ground is taken up by vegetation, and returns to the atmosphere by transpiration. Some of the moisture near the ground surface evaporates into the atmosphere. The water that remains after these losses becomes groundwater recharge. Groundwater recharge is important for replenishing of aquifers used as water supplies, and for sustaining “base flow”. Some of the water moving beneath the surface as unsaturated flow (interflow) and saturated flow (groundwater flow), discharges to wetlands, streams, rivers, ponds, lakes and other water bodies, and sustains the base flow of water supplying those resources. Thus, resource areas and drinking water supplies are dependent on groundwater recharge. The amount of groundwater recharge depends on the following factors:

! Quantity of rainfall;

! The characteristics of the soils (some soils have a high capacity to allow the infiltration of water, and other soils have a very low capacity see Table 1.1);

! Surface cover (vegetated surfaces help promote infiltration, whereas impervious surfaces such as pavements and roofs prevent water from penetrating the ground surface);

! The amount of water that may be stored on the surface during a rainfall event (such as depression storage), that remains available for infiltration for a period of time during or after the even

Surface Runoff Water Quality Issues

The activities associated with urban land uses – and in particular with vehicular traffic – result in the generation of pollutants, which accumulate on pavement surfaces, and are carried off by storm water runoff. Land disturbance activities and the increase in peak discharges also result in accelerated erosion of upland areas and stream channels, resulting in greater sediment loads. The earth’s surface has a certain capacity to remove pollutants through natural processes. The hydrologic changes introduced by urban development can alter these processes. For instance, natural infiltration of water into the ground can help remove some pollutants. Reduced infiltration can result in a reduction of this natural function. Changes in the water balance of wetland resource area can result in alterations in vegetation and the time water is in contact with vegetation. These changes can affect the natural processes of sediment removal and pollutant uptake. Reductions in flows to a particular resource area can reduce the water available to dilute contaminants, resulting in potential impacts on living organisms from the more concentrated contaminants.

In areas with cold water fish habitat, runoff from urban development can result in thermal impacts (runoff from pavements and discharges from detention basins can be much warmer than runoff from natural surfaces).

Storm water runoff poses concerns relative to the quality of discharges to resource areas. Because of these potential effects, Conservation Commissioners will be concerned with the provisions of measures to address runoff water quality impacts. The Storm water Management Policy includes standards to address such impacts. The MA DEP has prepared guidance documents for the design and implementation of measures for treating runoff from urban areas.

Conclusion

The resource areas protected under the Wetlands Protection Act and the Storm water Management Policy are assumed toper form certain critical functions in the environment, such as flood control, storm damage prevention, groundwater recharge, pollutant removal, and provision of wildlife habitat. Hydrology plays an important role in each of these functions. Hydrology, which is further described in Chapter2, deals with the movement of water between the atmosphere, the earth’s surface, and it’s subsurface. When considering wetland resource areas, the study of this movement of water focuses on the hydrologic regime of the resource areas. The hydrologic regime (also discussed further in Chapter 2) characterizes the factors that influence the volume of water entering and leaving a resource area. The hydrologic regime also includes the timing, duration, routing, rates, and frequency of flows. Additional site factors governing the hydrologic regime of a resource area include soil conditions, vegetative cover, topography, and groundwater levels. Proposed development and redevelopment projects have the potential to transform these hydrologic conditions, degrade storm water quality, and disrupt the hydrologic regime by changing the rate, timing, and volume of flow contributing to a resource area. As a result, development may impact the wetland function and statutory interests. Although it is virtually impossible to replicate pre-development hydrologic conditions on a site, the Conservation Commission should assess the proposed activities under their jurisdiction to determine whether resource areas are sufficiently protected in the post-development site.

Reference:

            

        Hydrology Handbook for Conservation Commissions

                                                    By----   David Nyman, P.E., ENSR International