EXECUTIVE SUMMARY - FINAL REPORT FOR WATERSHED HYDROLOGY PROTECTION AND FLOOD MITIGATION: PHASE I
Environmental Inc., 58 East State Street, Montpelier, VT 05601
To prevent natural resource degradation, the Vermont Department of Environmental Conservation is considering the development of acceptable practices for managing storm water runoff in urbanizing watersheds. Development of strategies to minimize or avoid future flood losses is also contemplated.
Stone Environmental Inc. (SEI) presents the results of a study to review and report on pertinent scientific literature which describes watershed land use, stream hydrology and morphology, aquatic ecosystems and surface water quality. SEI has reviewed flood damage survey reports to characterize damage due to the flood of August 5, 1995. SEI has prepared a recommendation for a technical analysis to determine, for Vermont conditions, thresholds of watershed land use change and watershed development/urbanization which, if exceeded, will result in unacceptable hydrologic reaction or aquatic ecosystem degradation.
Stream runoff and channel morphology reflect the magnitude and frequency of water and sediment inputs to the hydrologic system. Climate, vegetation, geology, and human influences are the dominant controls on water and sediment discharge. Alterations in these factors often initiate changes in the sediment-water relationship if the disruption is sufficiently large.
Geomorphic properties of a stream channel depend upon the hydrologic conditions controlled by the attributes of the watershed. Flooding is a natural phenomenon and has a specific probability of occurring in any given year which reflects the attributes of the watershed and the driving force of climate. Because flooding is a natural part of a stream's hydrologic regime, stream systems have been adjusted to the occurrence of overbank flows over geologic time.
Human-induced land use changes cause various hydrologic and geomorphic adjustments, including alterations in the size and timing of flood peaks and in the magnitude and type of soil erosion. Changes in land use and land cover affect the magnitude-frequency relationship of runoff by reducing the infiltration capacity of the soils in the watershed. These land use changes are subsequently expressed in significant changes in stream channel characteristics. Watershed scale changes such as urbanization, logging, and/or agriculture change the natural rainfall-runoff regime in such a way that large floods begin to occur more frequently and a stream's hydrologic regime becomes more "flashy" -- peak discharges get larger, and baseflow becomes lower. These hydrologic changes generate significant geomorphic adjustments as stream channels tend to get deeper, steeper, and wider and transport a greater volume of sediment as accelerated soil erosion also tends to occur with the development of watersheds.
In general, the changes in stream hydrology and morphology are due to a disequilibrium in the channel caused by increases in sediment and water discharge. Land use changes such as urbanization, logging, changing agricultural practices, and channelization/dredging can cause these equilibrium upsets by increasing runoff. Urbanization creates impervious surfaces within a watershed. The infiltration capacity of the land surface decreases and water is able to run off more quickly, which alters the hydrologic regime. Impervious surfaces and storm sewerage associated with urbanization enhance the peak discharges and therefore the mean annual floods. Channelization can restore low flows and reduce flooding upstream. However, downstream effects of channelization have been shown to include greater flooding, larger stream gradients and increased channel velocity.
High intensity, short duration storms, or micro-storms may have some geomorphic impact in watersheds. However, their impact will be enhanced by alterations in land use. Micro-storms can produce significant increases in erosion and sediment delivery to streams which in turn will cause geomorphic change, but these changes will be more drastic for developed than for undeveloped watersheds.
The literature shows that changes in land use were associated with significant changes in biological communities. However, the nature and magnitude of these changes varied considerably among studies.
Direct effects may result from the introduction of hazardous material into the stream that directly kills fish and/or limits their reproduction. Other more indirect effects occur from watershed development and land use change that secondarily affect the ecological integrity of streams. Changes in the hydrologic regime by agricultural development, logging, and/or urbanization may cause significant geomorphic adjustments that alter and degrade fish habitats -- thus limiting their reproducibility and survival.
A qualitative assessment of flood damage in seven Vermont towns was conducted in this study. The total number of damage incidents in each town was tabulated and compared to the total number of avoidable damage incidents. The result is a range of 29 to 58 percent of the total number of damage incidents that could have been avoided. The total dollar amount for all flood related damage to public property is tabulated along with the total dollar amount of what was determined to be avoidable damage. The avoidable dollar amount ranged from 28 to 61 percent of all money spent on flood repairs.
The literature review demonstrates that measurable impacts to the watershed from development and land use change will occur, but it is difficult to establish a specific minimum threshold for the magnitude of land use change that causes statistically significant hydrologic change -- and more importantly, the minimum specific hydrologic change that will cause statistically significant geomorphic and ecological change. In an effort to quantify these thresholds for Vermont environmental conditions, the Vermont Agency of Natural Resources has requested a scientific investigation (Phase II).
Phase II must be able to establish the current conditions
of Vermont streams and also indicate what they should look like to function
hydrologically and ecologically. Therefore, the approach taken for Phase
II must establish the magnitude of hydrologic, geomorphic, and ecological
conditions of both disturbed and undisturbed streams in Vermont. Phase
II should be a two-pronged approach: the first approach is a field component,
and the second approach is based on already available data. For data
collection, we suggest that a detailed habitat unit classification scheme
be conducted along a selected range of stream reaches: this approach
will reveal the geomorphic characteristics, the ecological characteristics,
and the links between the two.
VT DEC Geology and Mineral Resources Division 1 National Life Drive, Davis 2 Montpelier, VT 05620-3902
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