Impacts of Urbanization on Waterways

Transcription

1 Impacts of Urbanization on Waterways Cooperative Extension Bringing the University to You Audiovisual S. Donaldson Impacts of Urbanization on Waterways Susan Donaldson Melody Hefner University of Nevada Cooperative Extension Audiovisual Healthy streams are essential for protecting water quality. They also add to the quality of life in our area, providing recreational opportunities such as fishing and swimming, and an attractive element of green in the parched desert. As our community grows, it becomes increasingly important to safeguard our streams by planning ahead to avoid impacts. Portions of this presentation were adapted from The Impacts of Urbanization, Center for Watershed Protection, Impacts of Urbanization on Waterways 1

2 Nonpoint Education for Municipal Officials NEMO is an educational program for land use decision makers addressing the relationship between land use and water resource protection. NEMO stands for Nonpoint Education for Municipal Officials. Impacts of Urbanization on Waterways 2

3 The effects of urbanization on water resources can be organized into four categories: Water movement (hydrology) Stream channel shape and function (fluvial geomorphology) Water quality Habitat This presentation will examine four categories of impacts to our water resources. Impacts of Urbanization on Waterways 3

4 How does urbanization affect stream hydrology? Disrupts natural water balance Increases flood peaks,, storm water runoff, and bankfull flows More frequent flooding Lower baseflow to streams (less water in the stream) One of the major impacts of urbanization on streams is the effect on stream hydrology. Stream hydrology is defined as the study of the movement or flow of water in streams. Understanding water movement is essential to understanding the impact of development on urban streams. Impacts of Urbanization on Waterways 4

5 Greater infiltration Greater uptake by plants Reduced infiltration Reduced uptake by plants Increased surface runoff Center for Watershed Protection Water balance is a measure of the amount of water entering and leaving a system. As rain falls to earth, some of it is infiltrated, absorbed, and evapotranspired, and some becomes runoff. As you learned earlier, in a pre-developed setting, much of the rainfall is absorbed by the surrounding vegetation, soil and ground cover. This diagram shows how development and its corresponding increase in impervious cover disrupts the natural water balance. In the post-development setting, the amount of water running off the site is dramatically increased and the amount of water infiltration is decreased. The changes in the water balance in urban streams are exemplified by changes in the volume of runoff, increased flood peaks, increased and earlier peak flows and frequency of bankfull flows, floodplain widening, and decreased dry weather flows. Impacts of Urbanization on Waterways 5

6 Curbs and gutters are designed to deliver storm water away from the road surface in an efficient and timely manner. Catch basins or inlets collect storm water and direct it through pipes to a downstream storm water detention/ retention facility or to the receiving waters. Photos Copyright 1999, Center for Watershed Protection Flooding occurs when there are excessive runoff volumes. These excessive volumes are caused by both the total amount of impervious cover as well as the rate at which the runoff is delivered to the stream. Curbs and gutters, storm drains, storm drain pipes, ditches, catch basins and other drainage systems quickly speed the runoff to a storm water detention/retention facility or directly into the nearest water body. Curbs and gutters are designed to deliver storm water away from the road surface in an efficient and timely manner. Catch basins or inlets collect storm water and direct it through pipes to a downstream storm water detention/retention facility or to the nearest water body. Impacts of Urbanization on Waterways 6

7 Often, the runoff is directly discharged into nearby water bodies without any treatment. Photo Copyright 1999, Center for Watershed Protection And as you now know, most of our local storm drains deliver storm water to the nearest water body without treatment. Impacts of Urbanization on Waterways 7

8 The large amount of runoff entering an urban stream can frequently result in flooding, such as this moderate overbank flooding event. S. Donaldson This picture was taken one week after the January 1997 flood peak. Note that Steamboat Creek in Pleasant Valley is still flowing over its banks due to releases of water from Little Washoe Lake. Impacts of Urbanization on Waterways 8

9 Center for Watershed Protection This slide illustrates the effect of urbanization on hydrograph peak discharge. A hydrograph is a graph showing the changes in streamflow with respect to time. During storms, in pre-developed natural conditions (the solid line), the stream flow gradually increases to a relatively flat, prolonged peak that is about twice the pre-storm flow rate, and gradually descends to a low-flow condition ( gradual recession ). During storms, in the urbanized condition (dashed line), the flow rapidly increases to a peak that occurs earlier in time due to the rapid delivery of water from storm drains and pavement. You ll notice that the peak flows are much higher than occurred prior to development (more than double) which means that flows in the stream will be much higher than occurred previously, and flooding may increase. The flow then sharply decreases, often to a low-flow condition that is lower than occurred prior to development. This means that during dry periods, the flow in streams is decreased and impacts to water users or aquatic habitat may occur. Impacts of Urbanization on Waterways 9

10 Bankfull flooding (or the condition of the flow that "fills up" the channel) occurs much more frequently in highly urbanized areas and has the potential to be extremely erosive and damaging to the natural form and function of the stream. Photo Copyright 1999, Center for Watershed Protection In addition to the increase in flood peaks and peak discharge, the frequency of bankfull flows also increases with increased urbanization. Bankfull flows are simply runoff events that fill the normal channel of a stream to the top of the banks. Bankfull flows are significant because they are the channel-forming flows in streams, and they are highly erosive, turbid, and damaging to the natural morphology of the stream. Impacts of Urbanization on Waterways 10

11 Floodplain After Filling ASFPM Development within the floodplain area also changes the fluvial geomorphology, or stream form and function. Not only is the volume of runoff increased due to less infiltration resulting from increases in impervious surfaces, space in the floodplain is now occupied by fill and buildings. This causes the floodplain height to rise, potentially damaging adjacent buildings that once were considered safe during floods. This effect was seen during the 1997 flood in the Sparks industrial area. Impacts of Urbanization on Waterways 11

12 Decline in streamflow due to diminished groundwater recharge An increase in impervious surface often decreases the amount of rainfall available for infiltration. Without infiltration, the groundwater will not be recharged and the stream will lose this potential source of water. Thus low flows tend to be lower in urbanized watersheds. Center for Watershed Protection The hydrologic impact of urbanization is not limited to storm events. During dry weather periods, urban streams tend to have less flow because groundwater recharge from storm water infiltration has been diminished. While streams that have never been developed retain their flow during dry weather conditions, many urban streams lack the baseflow (flow contributed by groundwater) necessary to sustain healthy habitat conditions during extended periods of dry weather. Impacts of Urbanization on Waterways 12

13 Summary: Hydrologic Effect Disrupted natural water balance Increased flood peaks,, storm water runoff, and bankfull flows More frequent flooding Lower baseflow to streams (less water in the stream) The changes in hydrology may have unexpected impacts to the surrounding land. As water tables drop, plants that depend on certain levels of soil moisture may die. This changes the plant community, dries the soils, and has the potential for increasing soil erosion and wind-borne dust. Impacts of Urbanization on Waterways 13

14 How does urbanization affect the stream form and function? Stream widening & erosion Reduced fish passage Degradation of habitat structure Decreased channel stability Loss of pool-riffle structure needed by fish Fluvial geomorphologic changes, or changes in the shape, function, and composition of streams, occur along with hydrologic changes as urbanization increases. Impacts of Urbanization on Waterways 14

15 Functioning channel Downcutting and bank erosion have occurred. This diagram shows the progression of stream channel degradation. Stream is very wide and shallow. This stream is not functioning. UNCE Many functioning channels are narrow and deep, with overhanging banks. Vegetation shades the water surface, keeping temperatures cool. Stored ground water sustains grass, shrub, and/or tree communities. The riparian resources of vegetation, water, and soil are working together. Once downcutting and bank erosion have occurred, the stream becomes wider and the water table drops. As the stream becomes wider and shallower, the water gets hotter in the summer. In cold climates it may begin to freeze solid in the winter. Both are problems for cool-water fish and other species. The water no longer spills out onto the floodplain on a regular basis. Because water is no longer freely available on the old floodplain, water-loving vegetation on the new stream terrace dies. This further weakens the streambanks. As the riparian area loses its vegetation, erosion increases, and the channel becomes very wide and very shallow before it creates a new floodplain down along the sides of the channel. Water-loving vegetation may begin to grow on the new floodplain in the bottom of the channel. The stream, if left alone, will slowly begin to heal itself in a new form with a new floodplain in the bottom of the incised channel. Too many streams have already experienced some level of degradation. Fortunately, many that incised long ago are beginning the process of healing via increased width, floodplain development, and riparian vegetation. Impacts of Urbanization on Waterways 15

16 In watersheds with less than 5 percent impervious cover, streams are typically stable and pristine. Photo Copyright 1999, Center for Watershed Protection Following are some examples of streams in watersheds with varying amounts of impervious cover. An undeveloped watershed with less than 5 percent impervious cover is characterized by excellent streams conditions - good riparian cover, high quality substrate, and wetted perimeter during low flow conditions. Impacts of Urbanization on Waterways 16

17 While this stream at 8-10 percent impervious cover is still relatively stable, signs of stream erosion are more apparent. Photo Copyright 1999, Center for Watershed Protection While this stream at 8-10 percent impervious cover is still relatively stable, signs of stream erosion are more apparent and include loss of the wetted perimeter, more eroded material in the banks, and debris. Generally, most urban streams begin to enlarge as impervious cover exceeds 10 percent in the watershed. This finding has been discovered in various places like Puget Sound, Washington; Austin, Texas; and Vermont. The enlargement process may take up to 50 years to fully occur, but urban streams with more than 10 percent impervious cover are characterized by various degrees of stream enlargement and widening, erosion, downcutting, decreased channel stability, and embeddedness. Impacts of Urbanization on Waterways 17

18 The surrounding area of this stream is approximately 20 percent impervious cover and the stream exhibits much more bank erosion. Photo Copyright 1999, Center for Watershed Protection Note the vertical banks with eroded material at their base. The absence of a vigorous root structure provides little protection from erosion as flows increase. Impacts of Urbanization on Waterways 18

19 This stream has a surrounding area of approximately 30 percent impervious cover. Photo Copyright 1999, Center for Watershed Protection This stream has a surrounding area of approximately 30 percent impervious cover. The large amount of impervious cover has increased the size of the stream by a factor of five to ten. The manhole in the middle of the picture was originally in the floodplain and is an indicator of the degree to which channel erosion has occurred. Impacts of Urbanization on Waterways 19

20 The stream pictured here has downcut several feet in elevation because of the increased storm water flow. In this case, the forested wetland in the floodplain is now hydraulically disconnected from the stream that sustained it. Photo Copyright 1999, Center for Watershed Protection Not only can a stream widen, but given the right soil conditions, it can cut down (or incise) as well. In this slide, the stream has dropped several feet in elevation because of increased storm water flows. The forested wetland in the floodplain is now hydraulically disconnected from the stream that sustained it. These trees will likely die due to lack of water. Impacts of Urbanization on Waterways 20

21 In many highly urbanized areas, natural streams have been channelized to speed runoff along, but these fail to provide any habitat value. Photo Copyright 1999, Center for Watershed Protection When impervious cover exceeds 40 percent, engineers are often called in to channelize or stabilize stream channels. In the past, concrete channels were used to speed runoff further along, but there is no habitat value to these engineered channels. Impacts of Urbanization on Waterways 21

22 How does urbanization affect water quality? Increased stream temperature (thermal stress) Increased pollutants Increased risk of fishing advisories/beach closures Increase in costs! In addition to hydrologic and geomorphologic changes to the stream, urbanization directly impacts the quality of the receiving water. Some of the indicators of the impact of urbanization on water quality include increased stream temperature and pollutants. Impacts of Urbanization on Waterways 22

23 Urbanization causes the temperature of streams to rise, both during low flow and storm events. Photo Copyright 1999, Center for Watershed Protection Stream temperature is a very important habitat parameter for salmon, trout, and other fish and insects. Temperature variability can dictate the growth of aquatic insects and timing of migration and molts. Impervious cover increases air and soil temperatures and can create an increase of 5 to 10 degrees Fahrenheit in urban streams. Impacts of Urbanization on Waterways 23

24 S. Donaldson An increase in pollutants can adversely affect aquatic organisms and can result in beach closure, safety concerns, and/or activity limitations for people. High levels of bacteria can create unsafe conditions, leading to warnings and use restrictions. Impacts of Urbanization on Waterways 24

25 C. Conway Another common pollutant in urban storm water is sediment. Sediment can smother bottom organisms or it can clog gills of fish and aquatic insects when it is in the water column. Sources of sediment include streambank erosion, construction sites, and the runoff from paved surfaces. This picture shows sediment entering the Truckee River from the North Truckee Drain on an average day. Greater amounts enter the Truckee River after rainstorms. Impacts of Urbanization on Waterways 25

26 Effects of urbanization on habitat Loss of buffer zones Loss of large woody debris Creation of fish barriers Decline in habitat value of streams Consequences of habitat decline Decline in habitat quality for all wildlife Decline in plant and animal diversity Decline in reproduction and loss of plants and animals species Along with changes in hydrology, geomorphology, and water quality, the habitat value of urban streams diminishes with increased impervious cover. There are numerous impacts to the aquatic habitat as well as the streamside (riparian) zone. Impacts of Urbanization on Waterways 26

27 Here the banks of the stream are lined with grass, which provides some protection for the streambank but lacks the stability, shading, and habitat benefits that trees can provide. Photo Copyright 1999, Center for Watershed S. Donaldson Protection All vegetation is not equal! Trees are useful for providing stability, shade, and habitat benefits. This stream, while it is well-vegetated with grass and has good floodplain access, could have been planted with a variety of types of vegetation to improve habitat. Also, the intensive use of turf grass is likely associated with fertilizer and pesticide inputs. Impacts of Urbanization on Waterways 27

28 worldpoto.com Culverts and dams such as this act as barriers to fish migration and cut off spawning areas. The creation of fish barriers is another impact of urban development. Culverts and dams like this one at Marble Bluff act as barriers to fish migration unless they are carefully designed. Barriers can prevent the movement of both anadramous (migrating) and resident fish. In some cases, the fish barriers are created by culverts that are put in stream crossings for roads and other urban infrastructure. As the stream erodes down, vertical barriers to fish movement are created that cut off spawning areas. In many areas of the West Coast, where salmon spawn, fish barriers are a major biological problem associated with urban development. Impacts of Urbanization on Waterways 28

29 In this urban stream, habitat structure is lost and flows are more shallow, slow-moving and indistinct. S. Donaldson Photo Copyright 1999, Center for Watershed Protection In an enlarged urban stream, habitat structure is lost and flows become more shallow, slow moving and indistinct. The channel enlargement process sharply degrades stream habitat structure and flows become more shallow, slow-moving and indistinct. High-quality streams have a series of pools, riffles, and glides that provide unique and stable habitat areas for different fish and aquatic insects. Urban streams are characterized by highly scattered and poor-quality stream beds. Impacts of Urbanization on Waterways 29

30 This wide stream with huge sediment deposits, no forest canopy, and likely poor aquatic community is classified as non-supporting. Photo Copyright 1999, Center for Watershed Protection Much of the biological diversity and many of the human uses of impacted streams can be protected and restored through proper watershed management. If degradation proceeds to the non-supporting classification, restoration becomes much more difficult. Non-supporting streams can no longer support a diverse stream community and water contact recreation is no longer possible due to high bacteria levels. Impacts of Urbanization on Waterways 30

31 S. Donaldson What makes a healthy stream? A well-functioning or healthy stream: Has clean, clear water Has vegetation on its banks to knit soil together and reduce erosion Aids in flood retention and recharge Can handle typical sediment loads Provides habitat and supports greater biodiversity The official definition is provided below: Proper functioning condition (PFC): Riparian-wetland areas are functioning properly when adequate vegetation, landform, or large woody debris is present to dissipate stream energy associated with high waterflows, thereby reducing erosion and improving water quality; filter sediment, capture bedload, and aid floodplain development; improve flood-water retention and groundwater recharge; develop root masses that stabilize streambanks against cutting action; develop diverse ponding, and channel characteristics to provide the habitat and the water depth, duration, and temperature necessary for fish production, waterfowl breeding, and other uses; and support greater biodiversity. The functioning condition of riparian-wetland areas is a result of interaction among geology, soil, water, and vegetation. Impacts of Urbanization on Waterways 31

32 S. Donaldson This local creek (Galena) has some of the attributes of a healthy stream. Note the variety of riparian vegetation shading the stream. The stream itself is relatively narrow, but has room to spread out onto the floodplain. There are no obvious signs of major erosion. The water is clear and cool, and supports trout. Impacts of Urbanization on Waterways 32

33 S. Donaldson Steamboat Creek near the Truckee River is very different from Galena Creek. Notice the steep, vertical, eroded banks; the widened channel; the lack of vegetation other than noxious weeds (tall whitetop); the murky water; and the lack of a floodplain. This creek is the largest source of NPS pollution to the Truckee River. Impacts of Urbanization on Waterways 33

34 Photo Copyright 1999, Center for Watershed Protection S. Donaldson Efforts are underway to restore a number of local creeks. Here, Dry Creek has been routed into a low-flow channel with good floodplain access. However, work remains to be done to establish a diverse, desirable plant community. Currently, many of the plants are noxious weeds or undesirable species such as stinging nettles. There is no shade to cool the water. Impacts of Urbanization on Waterways 34

35 Knowledge of the amount of impervious cover in a watershed can help managers assess the most effective techniques for managing streams. Photo Copyright 1999, Center for Watershed Protection Why is there so much emphasis on stream condition? Stream condition directly affects water quality. It s not always easy to predict whether there will be impacts to waterways from development, however. Impervious cover estimates can help us determine the impacts we should expect to experience, and some strategies for coping. Impacts of Urbanization on Waterways 351

36 Impervious surfaces vs. runoff Increased: Runoff speed Bankfull flows Peak flows Volume Flood heights Decreased: Infiltration Baseflow Groundwater recharge Pollutant filtration Runoff quality This slide summarizes what we ve learned about the effects of impervious surfaces on runoff quantity and quality. Impacts of Urbanization on Waterways 36

37 Relationship between impervious cover and stream quality WATERSHED IMPERVIOUSNESS ( percent) ADAPTED FROM SCHUELER, ET. AL., 1992 STREAM DEGRADATION DEGRADED IMPACTED PROTECTED A simple model to classify and manage urban streams can be constructed based on impervious cover. The model helps determine the impacts of impervious cover based on three stream categories: sensitive, impacted, and non-supporting. When impervious cover is less than 10 percent, we still can maintain a high quality stream system that maintains stable banks, has high fish diversity and good water quality. Once impervious cover exceeds 10 percent, the pristine conditions of the sensitive stream cannot be maintained. Impacted streams characterized by 10 percent and 25 percent impervious cover begin to show varying degrees of influence from land development, including unstable channels, declining water quality, and diminished biological communities. Non-supporting or degraded streams exceed 25 to 30 percent impervious cover and are characterized as highly unstable, have poor fish and aquatic insect diversity, and very poor water quality. Certain measures can be taken to reduce impacts, but water quality is still going to suffer some level of degradation in these developed watershed. Urban Stream Classification Model Impervious cover <10 percent = sensitive or protected Impervious cover percent = impacted Impervious cover >25 percent = degraded to non-supporting Impacts of Urbanization on Waterways 37

38 Las Vegas, 1973 USGS We hear a lot about incredible growth rates in the Las Vegas Area. These images are from a USGS circular. This first one shows development in Las Vegas in The images used in this circular were generated from land cover data developed by the USGS. The data sources include the Geographic Information Retrieval and Analysis System (GIRAS) for the 1970s images and the National Land Cover Dataset (NLCD) for the 1990s images. The images were developed using a geographic information system (GIS). In the final images, all urban areas are shown in red. A shaded-relief map of each region was used to display the topographic context of the red polygon coverage. For all of these images, urban land is defined as areas transformed into a built-up environment for human use. It includes residential areas, commercial and industrial developments, transportation features, and institutions. This image is from USGS circular 1282, Urban Growth in American Cities Glimpses of U.S. Urbanization, Roger Auch, Janis Taylor, and William Acevedo, January Impacts of Urbanization on Waterways 38

39 Las Vegas, 1992 USGS This second Las Vegas image shows substantially increased amounts of development in Las Vegas as of This image is from USGS circular 1282, Urban Growth in American Cities Glimpses of U.S. Urbanization, Roger Auch, Janis Taylor, and William Acevedo, January Impacts of Urbanization on Waterways 39

40 Reno, 1973 USGS But Las Vegas is not the only area in Nevada experiencing significant growth. This map shows the Truckee Meadows area in This image is from USGS circular 1282, Urban Growth in American Cities Glimpses of U.S. Urbanization, Roger Auch, Janis Taylor, and William Acevedo, January Impacts of Urbanization on Waterways 40

41 Reno 1992 USGS This graphic shows the spread of development in the Truckee Meadows as of This image is from USGS circular 1282, Urban Growth in American Cities Glimpses of U.S. Urbanization, Roger Auch, Janis Taylor, and William Acevedo, January Impacts of Urbanization on Waterways 41

42 Where do we go from here? NRCS photo gallery We re faced with evidence daily that growth has steadily increased since 1992 in the Truckee Meadows. Growth is a fact of life, but we must ensure that our development plans and methods work towards decreasing runoff, promoting infiltration, and protecting waterways. This will help us meet our ultimate goal - protecting water quality. Impacts of Urbanization on Waterways 42