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HomeMy WebLinkAbout8B - Impervious Surface STAFF AGENDA REPORT AGENDA #: PREPARED BY: REVIEWED BY: SUBJECT: DATE: 88 JANE KANSIER, PLANNING COORDINATOR DONALD R. RYE, PLANNING DIRECTOR CONSIDER APPROVAL OF ORDINANCE #96-XX AMENDING THE ZONING ORDINANCE AND CITY CODE PERTAINING TO THE DEFINITION OF IMPERVIOUS SURFACE NOVEMBER 18, 1996 INTRODUCTION: The purpose of this item is to consider approval of an amendment to the Zoning Ordinance and City Code to pertaining to the definition of impervious surface. BACKGROUND: At a recent meeting, the Planning Commission discussed whether or not the current definition of impervious surface included gravel driveways. The Commission concluded the definition only includes paved driveways, even though gravel driveways will become compacted and effectively impervious with frequent use. The Planning Commission then directed the staff to prepare an amendment to the ordinance changing the definition to include gravel driveways. The staff proposed the following definition: IMPERVIOUS SURFACE: The portion of the buildable parcel which has a covering which does not permit water to percolate into the natural soil. Impervious surface shall include, but not be limited to, all driveways and parking areas, whether paved or not, sidewalks greater than 3' in width, patios, tennis and basketball courts, swimming pools, covered decks and other structures. Decks open to the sky and having open joints of at least one-fourth inch (1/4") and sidewalks 3' in width or less shall be exempted from the calculation of impervious surface. The impervious surface of a lot shall be documented by a certificate of survey unless exempted from this requirement by the Zoning Administrator. This amendment is consistent with Council Resolution #95- 54 requiring a survey in the Shoreland District for deck replacements. 96089cc.doc PAGE 1 16200 Eagle Creek Ave. S.E., Prior Lake, Minnesota 55372-1714 / Ph. (612) 447-4230 / Fax (612) 447-4245 AN EQUAL OPPORTUNITY EMPLOYER DISCUSSION: ALTERNATIVES: RECOMMENDATION: ACTION REQUIRED: REPORT A IT ACHMENTS: 96089cc.doc The Planning Commission held a public hearing on this amendment on October 28, 1996. The Commission recommended approval of this amendment. A copy of the Planning Commission minutes are attached to this report. Along with the amendment to the definition, the staff also proposed a change in the allowable amount of impervious surface from 30% to 400k for existing lots of record. The Planning Commission was concerned about the proposed language because they did not believe they had sufficient information to evaluate the proposed change. The Commission tabled this portion of the amendment for further discussion at a later date. The proposed amendment revises the current definition of impervious surface to include "all driveways and parking areas whether paved or nor'. This language recognizes that gravel driveways become compacted with use and that there are no permit required for paving an existing gravel driveway. Normally, zoning ordinances are amended when there has been a significant change in conditions, there has been a Comprehensive Plan amendment or there was a mistake in the original zoning. In this case, the change in the definition is consistent with the intent of the Zoning Ordinance to regulate the amount of impervious surface on a lot. It clarifies exactly what constitutes impervious surface in a practical manner. 1. Adopt the amendment to the Zoning Ordinance as recommended by the Planning Commission. 2. Further discuss this amendment. 3. In the event Alternative #2 is chosen, discussion may be continued in order for staff to provide additional information as directed by the City Council. 4. Deny the proposed amendment. Adopt Alternative #1. A motion and second to adopt the proposed ordinance. Adoption of this amendment requires a 4/5 vote of the Council. 1. Planning Report dated October 28, 1996 2. Proposed Ordinance PAGE 2 3. Minutes of October 28, 1996, Planning Commission Meeting 96089cc.doc PAGE 3 CITY OF PRIOR LAKE ORDINANCE NO. 96- AN ORDINANCE AMENDING SECTION 5-1-7 OF THE PRIOR LAKE CITY CODE AND AMENDING SECTION 8.1 OF THE PRIOR LAKE ZONING ORDINANCE 83-6. The City Council of the City of Prior Lake does hereby ordain: Section 5-1-7 of the Prior Lake City Code and Section 8.1 of the Prior Lake Zoning Ordinance 83-6 are hereby amended to read as follows: Section 5-1-7 and 8.1 are hereby amended by deleting the existing definition of Impervious Surface, and adding the following definition: IMPERVIOUS SURFACE: The portion of the buildable parcel which has a covering which does not permit water to percolate into the natural soil. Impervious surface shall include, but not be limited to, all driveways and parking areas, whether paved or not, sidewalks greater than 3' in width, patios, tennis and basketball courts, swimming pools, covered decks and other structures. Decks open to the sky and having open joints of at least one-fourth inch (J /4 '') and sidewalks 3 ' in width or less shall be exempted from the calculation of impervious surface. The impervious surface of a lot shall be documented by a certificate of survey unless exempted from this requirement by the Zoning Administrator. This ordinance shall become effective from and after its passage and publication. Passed by the City Council of the City of Prior Lake this 18th day of November, 1996. ATTEST: City Manager Mayor Published in the Prior Lake American on the _ day of , 1996. Drafted By: Prior Lake Planning Department 16200 Eagle Creek Avenue Prior Lake, MN 55372 ord96xx.doc 16200 Eagle Creek Ave. S.E.. Prior Lake, Minnesota 55372-1714 / Ph. (612) 447-4230 / Fax (612) 447-4245 AN EQUAL OPPORTUNITY EMPLOYER AGENDA ITEM: SUBJECT: PRESENTER: REVIEWED BY: PUBLIC HEARING: DATE: INTRODUCTION: PLANNING REPORT 4C PUBLIC HEARING TO CONSIDER AN AMENDMENT TO THE ZONING ORDINANCE PERTAINING TO THE DEFINITION AND AMOUNT OF IMPERVIOUS SURFACE (Case File #96-089) ~ JANE KANSIER, PLANNING COORDINATOR DONALD R. RYE, PLANNING DIRECTOR ..x.. YES _ NO OCTOBER 28, 1996 The purpose of this public hearing is to consider an amendment to the Zoning Ordinance to revise the definition of Impervious Surface. The second part of the amendment is to allow an increase in the amount of impervious surface on a lot of record with the use of special techniques. BACKGROUND: The current definition of Impervious Surface is lithe portion of a buildable parcel which does not permit water to percolate into the natural soil.. Subject to the following exemptions, these structures and materials shall constitute impervious surfaces: buildings; paved driveways and walkways of greater than three feet (3? in width; paved patios; covered decks and other structures. The foJ/owing structures and materials shall be exempt from the calculation of impervious surface; decks or patios which are open to the sky and have open joints of at least one-fourth inch (14'7 allowing percolation of water; paved walkways or other structures of three feet (3') in width or less. All such structures and materials shall be documented by a certificate of survey unless exempted from this requirement by the Zoning Administrator. " At a recent meeting, the Planning Commissioners discussed whether or not this definition included gravel driveways. While gravel driveways will become impacted and effectively impervious with frequent use, the definition includes only paved driveways. The Planning Commission then directed staff to prepare an amendment to the Zoning Ordinance which would include gravel driveways 96089pc.doc 16200 Eagle Creek Ave. S.E.. Prior Lake, Minnesota 55372-1714 / Ph. (612) 447-4230 / Fax (612) 447-4245 AN EQUAL OPPORTUNITY EMPLOYER as impervious surface. The proposed amendment is shown on the attached draft ordinance. DISCUSSION: The proposed amendment revises the definition of impervious surface to include "all driveways and parking areas, whether paved or not". This language recognizes that gravel driveways become impacted with frequent use and are effectively impervious. It also takes into consideration the fact that there are no permits required for paving a gravel driveway, so a change in the driveway surface does not change the amount of impervious surface on a lot. The second part of the amendment changes the amount of allowable impervious surface on existing lots of record to forty percent (400k) with the use of specific stormwater management practices. These practices include the use of grass filter strips, dividing impervious surface into smaller areas, and grading and construction techniques which will encourage rapid infiltration of runoff. The staff is proposing this change as a response to many of the variance requests over the last two years. A survey of variance requests to the impervious surface in 1995 and 1996 showed nearly all of the properties were substandard lots. The Planning Commission approved variances allowing from 31.60/0 to 45% impervious surface coverage. The proposed language is consistent with those variances, and sets specific standards for reducing the impact of the increased coverage. This amendment will also help to counter the effect of including gravel driveways in the definition of impervious surface on the smaller existing lots. ALTERNATIVES: 1. Recommend the Council approve the amendments as proposed, or with changes specified by the Planning Commission. 2. Recommend the Council deny the proposed amendments. 3. Table or continue discussion of the item for specific purpose. RECOMMENDA TION: The staff recommends alternative #1. ACTION REQUIRED: A motion and second recommending approval of the proposed amendments. 96089pc.doc Page 2 REPORT ATTACHMENTS: 1. Draft Ordinance Language 2. Summary of Impervious Surface Variance Requests 3. Hearing Notice 4. "The Importance of Imperviousness" Article 96089pc.doc Page 3 ::::::::::::::;:::::::::;:::::::::::::::::::::;:::::;:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::;:::::::;:::;:;:::::::;:::;:::::::::::;:::::::::::::::::::::::::::::::::::::::::::;:;:::::::::::::::::::::::::::::;:::::::::::::::::::::::::::::::::::;:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::;:::;:;::::::::;:;::;::::::;:::::::::::::;::::::::::::::::::::::;::::::::::;::;::::;: CITY OF PRIOR LAKE ORDINANCE NO. 96- AN ORDINANCE AMENDING SECTIONS 5-1-7 AND 5-8-3 (B,1) OF PRIOR LAKE CITY CODE AND AMENDING SECTIONS 8.1 AND 9.3 (B,1) OF PRIOR LAKE ZONING ORDINANCE 83-6. The City Council of the City of Prior Lake does hereby ordain: Sections 5-1-7 and 5-8-3 (B,l) of Prior Lake City Code and Sections 8.1 and 9.3 (B,l) of Prior Lake Zoning Ordinance 83-6 are hereby amended to read as follows: Section 5-1-7 and 8.1 are hereby amended by deleting the existing definition of Impervious Surface, and adding the following definition: IMPERVIOUS SURFACE: The portion of the buildable parcel which has a covering which does not permit water to percolate into the natural soil. Impervious surface shall include, but not be limited to, all driveways and parking areas, whether paved or not, sidewalks greater than 3 ' in width, patios, tennis and basketball courts, swimming pools, covered decks and other structures. Decks open to the sky and having open joints of at least one-fourth inch (1/4") and sidewalks 3' in width or less shall be exemptedfrom the calculation of impervious surface. The impervious surface of a lot shall be documented by a certificate of survey unless exempted from this requirement by the Zoning Administrator. Sections 5-8-3 (B,l) and 9.3 (B,1) are hereby amended by adding (b) (1-4) as follows, and renumbering the succeeding provisions: (b) On lots of record, existing as of the date of this ordinance, impervious surface may be permitted to a maximum of forty percent (40%) providing the following techniques are utilized as applicable: (1) Impervious areas should be drained to vegetated areas or grass filter strips through the use of crowns on driveways, direction downspouts on gutters collecting water from roof areas, or some other method approved by the City. (2) Dividing or separating impervious surface area into smaller areas through the use of grass or vegetated filter strips such as the use of paving blocks separated by grass or sand allowing infiltration. draftord.doc PAGE 1 (3) Use of grading and construction techniques which encourage rapid infiltration such as the installation of sand or gravel sump areas to collect and percolate stormwater. (4) Install berms to temporarily detain stormwater, thereby increasing soil absorption. This ordinance shall become effective from and after its passage and publication. Passed by the City Council of the City of Prior Lake this _ day of , 1996. ATTEST: City Manager Mayor Published in the Prior Lake American on the _ day of , 1996. Drafted By: draftord.doc PAGE 2 95/96 Impervious Variance Requests Case #- Impervious 0/0 I Lot Area Action 95-01 50.000/0 6,686 Denied 39.00% Approved 95-02 36.000ib 4,898 Denied 95-07 33.400ib 7,513 Approved 95-012 32.900ib 16,007 Approved 95-015 35.000ib 10,035 Denied 32.000ib Approved 95-016 35.00% 10,414 Approved 95-020 45.000/0 5,000 Approved 95-022 40.000ib 5,009 Approved 95-025 42.00% 6,516 Withdrawn 33.00% Approved 95-030 30.800ib 9,579 Denied 96-017 37.000ib 5,198 Approved 96-071 34.00% 8,648 Withdrawn 96-086 31.600ib 6,166 Approved Average Lot Area= 7821sq. feet Average Impervious Granted= 35.90/0 770/0 of requests were on lots less than 10,000 sq. feet. 920/0 of requests were on substandard lots (less than 12,000 sq. feet). 620/0 of the requests were for impervious coverage at or less than 350/0. 920/0 of the requests were for impervious coverage at or less than 400/0. Page 1 NOTICE OF PUBLIC HEARING TO CONSIDER AN AMENDMENT TO TITLE 5 (ZONING REGULATIONS) OF THE CITY CODE AND TO THE PRIOR LAKE ZONING ORDINANCE 83-6 REVISING THE DEFINITION AND THE ALLOWABLE AMOUNT OF IMPERVIOUS SURFACE ON A RESIDENTIAL LOT IN THE SHORELAND DISTRICT You are hereby notified that the Prior Lake Planning Commission will hold a public hearing at Prior Lake Fire Station #1, located at 16776 Fish Point Road SE (Southwest of the intersection ofC.R. 21 and Fish Point Road), on Monday, October 28, 1996, at 7:00 p.m. or as soon thereafter as possible. The purpose of the public hearing is to consider the following amendments: 1. An amendment to Section 5-1-7 of the City Code and Section 8.1 of the Zoning Ordinance revising the definition of Impervious Surface to include gravel driveways and other compacted surfaces as an impervious surface; 2. An amendment to Section 5-8-3 (B,l) of the City Code and Section 9.3 (B,I) of the Zoning Ordinance to allow an increase in the amount of impervious surface on a lot of record as of the date of this ordinance with the use of special techniques. The specific language of these amendments is available at the City of Prior Lake Planning Department. If you wish to be heard in reference to this item, you should attend the public hearing. Oral and written comments will be considered by the Planning Commission. If you have questions regarding this matter, please contact the Prior Lake Planning Department at 447-4230 between the hours of 8:00 a.m. and 4:30 p.m. Monday through Friday. Prepared this 9th day of October, 1996 by: Jane Kansier Planning Coordinator City of Prior Lake TO BE PUBLISHED IN THE PRIOR LAKE AMERICAN ON OCTOBER 12, 1996 1:\96zoamnd\imperv\96099pn.doc 1 16200 Eagle Creek Ave. S.E.. Prior Lake, Minnesota 55372-1714 / Ph. (612) 447-4230 / Fax (612) 447-4245 AN EQUAL OPPORTUNITY EMPLOYER . .. . .~.iil__,:JiiiiO~ .~ ~~ .. Impervious cover is a po-werful indicator offuture stream quality The I'mportance of Imperviousness The emerging field of urban watershed protec- tion often lacks a unifying theme to guide the efforts of its many participants-planners. engineers. landscape architects. scientists. and local officials. The lack of a common theme has often made it difficult to achieve a consistent result at either the individual development site or cumulatively, at the watershed scale. In this article a unifying theme is proposed based on a physically defined unit-imperviousness. Impervi- ousness here is defined as the sum of roads. parking lots. sidewalks. rooftops. and other impermeable sur- :r.:rr.m~7-~~~~'S\D faces of the urban landscape. This vari- able can be easily measured at all scales of development. as the percentage of area that is not "green". This research has yielded a surprisingly similar conclusion- stream degradation occurs at relatively low levels of imperviousness (10-20%). ~:;oo.~~~~~~~ Imperviousness is a very useful indicator with which to measure the . impacts of land development on aquatic systems. Reviewed here is the scientific evidence that relates imperviousness to spe- cific changes in the hydrology, habitat structure, water quality and biodiversity of aquatic systems. This re- seuch. conducted in many geographic areas. concen- trating on many different variables. and employing widely different methods. has yielded a surprisingly similar conclusion-stream degradation occurs at rela-I tively low levels of imperviousness (10-20%). Most: importantly, imperviousness is one of the few vari- ables that can be explicitly quantified. managed and controlled at e:lch stage of land development. The remainder of this paper examines in detail the relation- ship between imperviousness and stream quality. The Components of Imperviousness Imperviousness represents the imprint of land de- velopment on the landscape. It is composed of two primary components-the rooftops under which we live. work and shop. and the transport system (roads. driveways. and parking lots) that we use to get from one roof to another. As it happens. the transport com- ponent now often exceeds the rooftop component in terms of total impervious area created. For example, transport-related imperviousness comprised 63 % to 70% of total impervious cover at the site in 11 residen- . tial. multifamily and commercial areas where it had actually been measured." This phenomenon is ob- served most often in suburban areas and reflects the recent ascendancy of the automobile in both our cul- ture and lands~pe. The sharp increases in per capita vehicle ownership. trips taken. and miles travelled have forced local planners to increase the relative size of the transport component over the last two decades. Traditional zoning has strongly emphasized and regulated the first component (rooftops) and largely neglected the transport component While the rooftop component is largely fixed in density zoning, the transport component is not As an example, nearly all zoning codes set the maximum density for an area. based on dwelling linits (rooftops). Thus. in a given area. no more than one single family home can be located on each acre of land. and so forth. Thus a wide range in impervious cover is often seen for the same zoning category. For example. impervi- ous area associated with medium density single family homes can range from 25% to nearly 60%, depending on the layout of streets and parking. This suggests that significant opportUnities exist to reduce the share of imperviousness from the transport component. Imperviousness and runoff The relationship between imperviousness and run- off may be widely understood. but it is not always fully appreciated. Figure 1 illustrates the increase in the site runoff coefficient as a result of site imperviousness. developed from over 40 runoff monitoring sites across the nation. The runoff coefficient ranges from zero to one and expresses the fr.1ction of rainfall volume that is actually converted into storm runoff volume. As can be seen. the runoff coefficient closely tracks percent impervious cover. except at low levels where soils and slope factors also become important. In practical terms this means that the total runoff volume for a one-acre parking lot (R v = 0.95) is about 16 times that produced by an undeveloped meadow (R v = 0.06). To put this in more understandable terms. consider the runoff from a one-inch rainstorm (see Table 1). The total runoff from a one-acre meadow would fill a standard size office to a depth of about two feet (218 cubic feet). By way of comparison. if that same acre was completely paved. a one-inch rainstorm would completely fill your office. as well as the twO next to it. The peale discharge, velocity and time of concentr:ltion r~ of stormwater runoff also exhibit a striking increase after a meadow is replaced by a parking lot (Table 1). Because infiltration is reduced in impervious arc:lS. I one would expect groundwater recharge to be propor- tionately reduced. This. in turn. should translate into lower dry weather stream flows. Actual dat:1.. however. that demonstrate this effect is rare. Indeed. E vetI et :11.. could not find any statistical difference in low stream .~ ~ 100 .W:Ir:r;::l~~.liI~'T'~(;!';.I'lI...~~-D'r.:~-~~a~1 ~i.:~~4 - . . Figure :L; Watershed imperviousness and the storm runoff coefficient Runoff Coefficient (Rv) , 0.9 0.8 0.1 0.6 0.5 0.4 s 8lI 181 0.3 0.2 0.1 o o GlI s 8 181 s s 10 20 30 40 50 60 10 80 Watershed Imperviousness (0/0) 90 100 flow between urban and rural watersheds after analyz- ing 16 North Carolina watersheds. Simmons and Reynolds26 did note that dry weather flows dropped 20 to 85% after development in several urban watersheds in Long Island, New York. It should be noted that transport-related impervi- ousness often exerts a greater hydrological impact than the rooftop-related imperviousness. In residential ar- eas. runoff from rooftops can be spread out over pervious areas, such as backyards. and rooftops are not always directly connected to the storm drain system. This ma.y allow for additional infiltration of runoff. Roads and parking lots, on the other hand. are usually directly connected to the storm drain system. floods, whose frequency can increase by a factor of 10 even at relatively low levels of imperviousness. 14.19.2.3 A major expression of channel instability is the loss of instream habitat strUctures. such as the loss of pool and riffle sequences and overhead cover, a reduction in the wetted perimeter of the stream and the like. A number of methods have been developed to measure the strUcture and quality of instream habitat in recent 1 :. ~ j ~ iI ~ j .. j ., I I I I Imperviousness and the Shape of Streams Confronted by more severe and more frequent floods, stream channels must respond. They typically do so by increasing their cross-sectional area to accom- modate the higher flows. This is done either through widening of the stream banks, downcutting of the stream bed. or frequently. both. This phase of channel instability, in turn, triggers a cycle of streambank erosion and habitat degradation. The critical question is at what level of develop- ment does this cycle begin? Recent research models developed in the Pacific Northwest suggest that a threshold for urban stream stability exists at about 10% imperviousnessJ.4 (Figure 2). Watershed development beyond this threshold consistently resulted in unstable and eroding channels. The, rate and severity of channel instability appears to be a function of sub-bankfull Table :L; Comparison of one acre of parking lot versus one acre of meadow in good condition Parking Runoff or Water Quality Parameter Lot Meadow Curve number (CN) Runoff coefficient lime of concentration (minutes) Peak discharge rate (cfs), 2 yr., 24 hr. storm Peak discharge rate (cfs), 100 yr. storm Runoff volume from one-inch storm (cubic feet) Runoff velocity @ 2 yr. storm (feet/second) Annual phosphorus load (Ibsjac.jyr.). Annual nitrogen load (Ibsjac.jyr.). Annual zinc load (Ibsjac./yr.) 98 0.95 4.8 4.3 12.6 3450 8 2 15.4 0.30 58 0.06 14.4 0.4 3.1 218 1.8 0.50 2.0 NO .~ ;.; ;1 ~ . l:: :". Key Assumptions: Parking lot is 100% impervious with 3% slope, 200 feet flow length, Type 2 Storm, 2 yr. 24 hr. storm = 3.1 inches; 100 yr. storm = 8.9 inches, hydraulic radius = 0.3, concrete channel, and suburban Washington 'C' values. Meadow is 1% impervious with 3% slope, 200 foot flow length, good vegetative condition, 8 soils. and earthen channel. .JIJf.i~~;'ilei'~"'l.lt ::Its:r- :0: Figure 2: Channel stability as a functicrn of imperviousness (Booth and Reinelt, :1..993) ... % :z eX :;) U :lC >- S. :a a:: o "'" eX >- o - "t' ~ o ... ... ... o o i= < -= ... .&: .;: ...:.. !.:~ s:< ~: . i~ 0 .~: $:6& 0; &,go 0 o o. o x . :TAaU!~ U"':T~ OlAHN!LS I..UO!''-''JC! stJ8CA Tar~ 2.n o 1.5 CENI::RALLY STABLE CHANNELS . 1.0 1 O-yr forested dfscnarae _ 2-yr current discharge 00 ~. . ~ or'~ .~)i.c :x< w ~ )( x : x i..,,)( \ x x . ~ x i CENER,1LL?UNSTAlJLE CHANNELS 0.5 o 0.0 o 1 0 20 .30 <40 50 PERaN"r IMPERVIOUS AREA IN CATCHMENT years. 10.13.21 Where these tools have been applied to urban streams, they have consistently demonstrated that a sharp threshold in habitat quality exists at ap- proximately 10 to 15% imperviousness}.II..;:j Beyond this threshold, urban stream habitat quality is consis- tently classified as poor. Imperviousness and water quality Impervious surfaces collect and accumulate pollut- ants deposited from the atmosphere, leaked from ve- hicles or derived from other sources. During stonns, accumulated POllutants are quickly ;r.:mn....,..:~.;~~~~~ washed off and rapidly delivered to aquatic systems. Monitoring and modeling studjes have consistently indicated that urban POllutant loads are directly related to ~''''~~~'~'Stmtt watershed imperviousness. Indeed, imperviousness is the key predictive variable in most simulation and empirical models used to estimate pollutant loads. For example, the Simple Method assumes that POllutant loads are a direct func- tion of watershed imperviousness2J, as imperviousness is the key independent variable in the equation. Habitat assessment tools have consistently demonstrated that a sharp threshold in habitat Quality exists at approximately 10 to 15% imperviousness. Threshold limits/or maintaining background pollutant loads Suppose that watershed runoff drains into a lake that is phosphorus-limited. Also assume that the present background load of phosphorus from a rural land use amounts to 0..5 Ibslaclyr. The Simple Method predicts that the postdevelopment phosphorus load will exceed background loads once watershed imperviousness (I) exceeds 20 to 25% (Figure 3), th.ereby increasing the risk of nutrient overenrichment in the lake. 102 :.. .':" B11(~:'fiiTi:i~ Urban phosphorus loads can be reduced w urban bestmanagementprnctices (BMPs) are instal: such as stonnwater ponds, wetlands, filters or intil: cion'practices. Perfonnance monitoring data indic::l that BMPs can reduce phosphorus loads by as muc!": 40 to 60%, depending on the practice selected. T impact of this POllutant reduction on c postdevelopment phosphorus loading rate from c. site is shown in Figure 3. The net effect is to raise t. phosphorus threshold to about 35 % - 60% imperviou ness, depending on the perfonnance of the BMP ~ install. Therefore, even when effective practices a: widely applied. we eventually cross a threshold c imperviousness, beyond which we cannot maintai predevelopment water quality. 60 Imperviousness and stream warming Impervious surfaces both absorb and reflect heat. During the sumrnermonths. impervious areas can have local air and ground temperatures that are 10 to 12 degrees warmer than the fields and forests that they . replace. In addition. the trees that could have provided shade to offset the effects of solar radiation are absent Water temperature in headwater streams is strongly influenced by local air temperatures. GalIP' reported that stream temperatures throughout the summer are increased in urban watersheds, and the degree of wann- ing appears to be directly related to the imperviousness of the contributing watershed. He monitored five head- Water streams in the Maryland Piedmont over a six- month period. the streams having differing levels of impervious Cover (Figure 4). Each of the urban streams had mean temperatures that were consistently W,lnner than a forested reference stream, and the size of the increase (referred to as the delta- 1) appeared to be a direct function of watershed imperviousness. Other factors, such as lack of riparian cover and ponds, were also demonstrated to amplify stream warming, but the primary contributing factor appeared to be watershed impervious cover.9 Imperviousness and stream biOdiverSity The health of the aquatic ecosystem is a strong environmental indicator of watershed quality. A num- ber of research studies have recently examined the links between imperviousness and the biological di- versity in streams. Some of the key findings are sum- marized in Table 2. Aquatic insects The diversity, richness and composltJon of the benthic or streambed community has frequently been used to evaluate the quality of urban streams. Noe only are aqU:1tic insects a useful environmental indicator. but they also fonn the base of the stream food ch.un in most regions of the country. .~clh~.:Jiir.l~ Figure 3: The effect of impervious cover on urban phosphorus load under several scenarios, as computed by the Simple Method o CO "'" en .c ::::.. 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 o o ! I I t "'C CO o ~ a. co ~ c: c: < P Load Scenarios - Post-Dev "SMP-HI + BMP-Lo - Background 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 9510 Impervious Cover Figure 4: The effect of impervious cover on stream temperature (Galli, :1.99:1.) 20 - 16 T LL - 'f" C) a- :J - a3 12 'f" a- Q) C. E 'f" Q) 8 'f' .... E a3 Stream Delta- T ! . Q) a- 4 j - ,... Mean CJ) : ~ 'f" Maximum J 0 t 0 10 20 30 40 50 60 70 80 90 10 Impervious Cover Delta-t is the difference in mean or max stream temperature from a developed stream, compared to an undisturbed stream. Watershed Protection Techniques _ Vot.:r. No.3_ Fall 1.994. 103 1~"I(~'r.lr-{~ Figure 5: Impacts 6"( imperviousness on macroinvertebrate communities in the headwater streams of the Anacostia River (Schueler and Galli, :1..992) Metric Values 40 a a 30 a Cl 20 10 o I o GOOD 10 20 30 40 50 % Imperviousness l 0 Metric Values I Metric: VaJues are based \I~ the sum o( scores assigned (or !be rollowinc c:.ale,ories: EPTC balance. EPT Index. A vC'agc Generic Diversiry, Climnomid Abunc:laacc:. Taxonomic Richness (Family and Generic) I Klein 1/\ was one of the first to note that I macroinvertebrate diversity drops sharply in urban streams in Maryland. Diversity consistently became poor when watershed imperviousness ~~::\o..:.."""d1"'''':.=.s~~!,>''.i~~~'~~ exceeded 10 to 15 percent. The same basic threshold has been reported by all other research studies that have looked at macroinvertebrate diversity in urban streams (Table 2). It is clear that few, if any, urban streams can support diverse benthic communities at moderate to high levels of imperviousness (25% or more). In each study, sensitive macro in- ~'~:-"~~'.z~"",.:;~:~ vertebrates were replaced by ones that were more tolerant of pollution and hydrologic stress. Species such as stoneflies, mayflies, and caddis flies largely disappeared and were replaced by chironomids, tubificid wonns, amphipods. and snails. Species that employ specialized feeding strate- gies-shredding leaf litter, grazing rock surfaces. fil- tering organic matter that flows by, and preying on other insects-were lost; A typical example of the relationship between imperviousness and macroinvertebra.te diversity is shown in Figure 5. The graph summarizes diversity trend for 23 sampling stations in headwater streams of the Anacostia watershed.24 While good to fair diversity was noted in all headwater streams with less than 10% imperviousness. nearly all stations with 12% or more impervious cover recorded poor diversity. The same sharp drop in macro invertebrate diversity at around 12 to 15% imperviousness was also observed in streams in the coastal plain and piedmont of Delaware.:5 Other studies have utilized other indicators to mea- sure the impacts of urbanization on stream insect communities. For example, Jones and Clarkl5 moni- tored 22 stations in Northern Virginia and concluded that benthic insect diversity composition changed mark- edly after watershed population density exceeded four or more individuals per acre. The population density roughly translates to half-acre or one acre lot residen- tial use. or perhaps 10 to 20 percent imperviousness. SteedmanZ7 evaluated 208 Ontario stream sites, and concluded that benthic diversity shifted from fair to poor at about 35% urban land use. Since '4urban land" includes both pervious and impervious areas, the ac- tual threshold in the Ontario study may well be closer to 7 to 10% imperviousness.4 Steedman also reported that urban streams with intact riparian forests h61d higher diversity than those that did not, for the same level of urbanization. While the exact point at which stream insect diver- sity shifts from fair to poor is not known with absolute precision. it is clear that few, if any, urban streams C:ln support diverse benthic communities at moderate to high levels of imperviousness (25% or more). FO~~r.. different studies all failed to find stream insect comm u- nities with good or excellent diversity in any highly urban stream.I.:.J.12 .. ."',11 _v-- &1'p.i1;';..., ~"'~,;:;j,~r::r;"'I("'~ .:::F.i'III~.:.fiir;l~ Table 2: Review of key findings of urban stream studies examining the relationship of urbanization on stream quality Ref. Year Location Biological Parameter Key Finding 3 1991 Seattle Fish habitat! channel stability Channel stability and fish habitat quality declined rapidly after 10% imperv. . , . ! . 105 1 1 \ J I Ii r i 1 .'IIF-i~~::leii:ToO~r:r:~1 ,l!.l'~ ..*"'If=--'''ir.f~ Fish Surveys The abundance and diversity of the fish community can also serve as an excellent en vironmental indicator. Surprisingly, relatively few studies have examined the influence of imperviousness on fish communities in headwater streams. The results of one study is illus- trated in Figure 6. Four similar sub watersheds in the Maryland Piedmont were sampled for the number of fish species present. As the level of watershed imper- viousness increased, the number of fish species col- lected dropped. Two sensitive species (trout~d sculpin) were lost as imperviousness increased from 10 to 12% and four mo~ were lost when impervious cover in- creased to 25%. Significantly, only two species re- mained in the fISh conununity at 55% imperviousness. Sensitive species, defined as those with a strong depen- dence on the substrate for feeding and/or spawning, showed a more precipitous decline. Klein1t\ found a similar relationship between fish diversity and water- shed imperviousness in several dozen headwater streams in the Maryland Piedmont Salmonid fISh species (trout and salmon) and anadromous fish species appear to be most negatively impacted by imperviousness. Trout have stringent temperature and habitat requirements, and seldom are present in mid-Atlantic watersheds where impervious- ness exceeds 15%.1/ Declines in trout spawning suc- cess are evident above lO% imperviousness. II In the Pacific Northwest, Luchetti and Feurstenburgll sel- dom found sensitive coho salmon in watersheds yond 10 or 15% imperviousness. Booth and Reine noted that most urban stream reaches had pOor quai fish "habitat when imperviOusness exceeded 8 to 12 Fish species that migrate from the ocean to Sp<l '. in freshwater creeks are also very Susceptible to i~ pacts of urbanization such as fish barriers, polIutic flow changes, and other factors. For example. Limbu and Schmidtl7 discovered that the density of anadr: mous fish eggs and larvae declined sharply after a 10' imperviousness threshold was surpassed in 1 sub watersheds draining into the Hudson Ri vel'. The influence of imperviousness on other urbar water resources Several other studies point to the strong influenc~ of imperviousness on other important aquatic systems such as shellfish beds and wetlands. Even relatively low levels of urban developmenc yield high levels of bacteria, derived from urban runoff or failing septic systems. These consistently high bac- terial counts often result in the closure of shellfish beds in coastal Waters and it is not surprising, th~t most closed shellfish beds are in close proximity. to urban areas. Indeed, it may be difficult to prevent shellfish closure when more than one septic drain field is present per seven acres-a very low urban density.' Although it is widely believed that urban runoff accounts for Figure 6: FISh diversity as a function of watershed imperviousness in four subwatersheds in the Maryland Piedmont (Schueler and Galli, 1.992) Number of Species I1.;2%1 14 12 10 8 6 4 2 o Good Hope Trib. Nursery Run (Paint Branch) (NW aranc'''' f25%1 --- ---- I Hollywood Sr. Wheaton Branch (PaInt BrancII) (SJigo Creelt) - Sensitive Species - Total No. of Species e Humbera in brackets repreaent level. of water.lI.d imD.'vlou.n.... Sourc:ea: t) MdDNR, 1983 21 PAdDHR. 1986 3) lePRa. 1989 106 ..i(i.~~"'"@r.r.:I,I,I.~":~ . :::F-! ~ 6'1 :.1".:.1". U;;~ many shellfish bed closures (now that most point sources have been controlled). no systematic attempt has yet been made to relate watershed imperviousness to the extent of shellfish bed closures. Taylor9 examined the effect ofwatershed develop- ment on 19 freshwater wetlands in King County. Washington. and concluded that the additional storm- water contributed to greater annual water level fluctua- tions (VILF). \Vhen the annual WLF exceeded about 8 inches, the richness of both the wetland plant and amphibian community dropped sharply. This increase in WLF began to occur consistently when upstream watersheds exceeded 10 to 15% imperviousness. Implications at the Watershed Level The many independent lines of research reviewed here converge toward a common conclusion- that it is extremely difficult to maintain predevelopment stream quality when watershed development exceeds 10 to 15% impervious cover. What implications might this apparent threshold have for watershed planning? Should low density or high density development be ~ncourag~d? At first glance. it would seem appropriate to limit watershed development to no more than 10% total impervious cover. While this approach may be wise for an individual "sensitive" watershed, it is probably not practical as a uniform standard. Only low density development would be feasible under a ten percent zoning scenario, perhaps one-acre lot residential zon- ing. with a few widely scattered commercial clusters. At the regional scale, development would thus be spread over a much wider geographic area than it would otherwise have been. At the same time, addi- tional impervious area (in the form of roads) would be needed to link the community together. Paradoxically. the best way to minimize the cre- ation of additional impervious area at the regional scale is to concentrate it in high density clusters or centers. The corresponding impervious cover in these clusters is expected to be very high (25% to 100%). making it virtUally impossible to maintain predevelopmentstream quality. A watershed manager must then confront the fact that to save one stream's quality it may be neces- sary to degrade another. A second troubling implication of the impervious! stre:mt quality relationships involves the large ex- ;Janses of urban areas that have already been densely jeveloped. Will it be possible to fully restore stream =iuality in watersheds with high impervious cover? Some early watershed restoration work does suggests :hat biological diversity in urban streams can be par- ially restored, but only after extensive stormwater "errotit and habitat structures are installed. For ex- ample. tish and macroinvertebrate diversity has been panially restored in one tributary of SliC70 Creek, M I dill 0 ary an. n other urban watersheds. however, com- prehensive watershed restoration may not be feasible, due to a lack of space, ~~::..i~'~"~a~:I'!"'V".;..~N~ feasible sites, or funding. Paradoxically, the best way to minimize the creation of addi- tional impervious area at the regional scale is to concentrate it in high density clusters. ~J~.:::i~~~':..~~ A proposed scheme for classifying urban stream quality potential The thresholds provide a reason- able foundation for classifying the potential stream quality in a watershed based on the ultimate amount of impervious cover. One such scheme is outlined in Table 3. It divides urban streams into three management categories based on the general relationships between impervious cover and stream quality: 1. .. " .; ; $ j, Stressed streams (l to 10% impervious cover) 2. Impacted streams (11 to 25% impervious cover) 3. Degraded streams (26 to 100% impervious cover) H it tf if ;1 I ~ The resource objective and management strategies in each stream category differ to reflect the potential stream quality that can be achieved. The most protec- tive category are "stressed streams" in which strict zoning, site impervious restrictions. stream buffers and BMPs are applied to maintain predevelopment stream quality. "Impacted streams" are above the threshold and can be expected to experience some degradation after development (i.e., less stable channels and some loss of diversity). The key resource objective for these streams is to mitigate these impacts to the greatest extent possible, using effective BMPs. The last category, degraded streams, recognizes that predevelopment channel stability and biodiversity cannot be fully maintained, even when BMPs orretro- fits are fully applied. The primary resource objective shifts to protect downstream water quality by remov- ing urban pollutants. Efforts to protect or restore bio- logical diversity in degraded streams are not aban- doned; in some priority sub watersheds intensi ve stream restoration techniques are employed to attempt to partially restore some aspects of stream quality. In other subwatersheds, however, new development (and impervious cover) is encouraged to take place so as to protect stressed and impacted streams. ~ f ~ f f :- : f ;- , ~ ; ; . ;. ~ t ~ j . I J Watersh~d-based zoning Watershed-based zoning is based on the premise that impervious cover is a superior measure to gauge the impacts of growth, compared to population den- sity. dwelling units or other factors. The key steps in . Watershed Protection Tedmiques a Vat..LNcL 3:. a Falt1.994-:::.! :07 11~llilf:.4:.'i1t.:i~' Table 3: A possible scheme for classifying and managing for headwater urban streams based on ultimate imperviousness Urban Stream Classification Stressed (0-10% Imperv.) Channel stability Stable watershed-based zoning are as follows. First, a com- munity undertakes a comprehensive physical. chemi- cal and biological monitoring program to asses the currenc quality of its encire inventory of screams. The d<lt<l <lI'e used co identify the most sensitive stream systems <lnd to refine impervious/scream qualicy rel<l- tionships. Next. ex.isting imperviousness is measured and m:lpped at the subwatershed level. Projections of future impervious cover due to forecasced growth <lI'e also made at this time. The third step invol ves designating the future scream quality for each sub watershed based on some adapta- tion of the urban stream classification scheme pre- sented earlier. The existing land use master plan is then modified to ensure that future growth (and impervious cover) is consistent with the desigmued scream classi- fication for each subwatershed. The final step in the w<ltershed-based zoning pro- cess involves the adoption of specific resource objec- tives for each stream and subwatershed. Specific poli- cies and pr.1ctices on impervious cover limits, BMPs, and buffers are then instituted to meet the stream resource objective. and these pr.1ctices directly applied to future development projects. Watershed-b<lSed zoning should provide managers wi th gre:lter confidence that resource protection objec- tives C:ln be met in future development. It also forces local governments to make hard choices about which stre:lms wilJ be fully protected and which will become Impacted (11-25% Imperv.) Degraded (26-100% Imperv.) Unstable Highly Unstable at least panially degraded. Some environment:. and regul41tors will be justifi<lbly concerned abou streams whose quality is explicitly sacrificed U~ this scheme. The explicit stream quality decis: which are at the heart of watershed-based zor. however. are preferable to the uninformed and ranc "non-decisions" that are made every d:lY under present zoning system. A cautionary note While the research on impervious cover and stre: quality is compelling, it is doubtful whether ic can se~ as the sole foundation for leg<llIy defensible zoning:: regulatory actions at the current time. One key re:l.$ is that the research has not been standardized. D i ffere investigators, for example, have used different me~ ods to define and me:1Sure imperviousness. Secor. researchers have employed a wide number of te: niques to me<lSure stream quality characteristics :r- are not always comparable with each other. Thir most of the studies have been confined to few ecoregio: in the country. Little research h<lS been conducted the Northe:1St, Southeast, Midwest, and semi-ar Western regions. Lastly, none of the studies has 'f ex.amined the effect of widespread application ofBMI on impervious Cover/stream quality relationships. Un: studies determine how much BMPs can "che:lt" tr impervious cover/stream quality relationship. it C:ln t argued that stnJctUr:ll pr.lctices alone can compensa for imperviousness effects. 108 .'l/.f.i~~.f:iiiil::;;".,,~r::;;iifjr"i!:-~'(."l.~l''''~ . a;:r.,.. i l;.ar' fi ir.l ~ On the positive side. it may be possible for -a-- community to define the impervious cover/stream qual- ity relationship in a short time and at relatively low cost. A suggested protocol for conducting a watershed monitoring study is presented in Table 4. The protocol emphasizes comparative sampling of a large popula- tion of urban subwatersheds of different increments of imperviousness (perhaps 20 to 50). A rapid sampling program collects consistent data on hydrologic, morphologic, water quality, habitat and biodiversity variables within each sub watershed. For comparison purposes, series of undeveloped and un- . disturbed reference streams are also monitored. The sampling data are then statistically and graphically analyzed to determine the presence ofimperviousnessl stream quality relationships. The protocol can be readily adapted to examine I how BMPs can shift the stream quality/impervious- ness relationship. This is done by adju~ting the sam- pling protocol to select two groups of ~-"":"'~;;:!'~"".;~~~:E"Jlil\ study sub watersheds-those that are effectively served by BMPs and those that are not. Site designers can use a wide range of techniques to minimize impervious cover by 1.0 to 50%. Minimizing impervious caller ~.......~~.:..~~ Reducing impervious cover can be an effective element of the overall BMP system for a development site. As noted earlier, imperviousness need not be a fixed quantity. A site designer can utilize a wide range of techniques to minimize impervious cover at devel- opment site (Table 5) that collectively can reduce imperviousness by 10 to 50%. (See Technical Nares 38 and 39 in this issue.) if I Table 4: Proposed protocol for defining functional relationships between watershed imperviousness and stream quality ~ I ~ . Genera' study design A systematic evaluation of stream quality for a population of 20 to 50 small subwatersheds that have different levels of watershed imperviousness. Selected field measurements' are collected to represent key hydrological, morphological, water quality, habitat and biodiversity variables within each defined subwatershed. The population of subwatershed data is then statistically analyzed to define functional relationships between stream quality and imperviousness. . Defining reference streams Up to 5 non-urban streams in same geo-hydrological region, preferably fully forested, or at least full riparian forest coverage along same length. Free of confounding NPS sources, imperviousness less than 5%, natural channel and good habitat structure. . Basic Subwatershed Variables Watershed area, standard definition and method to calculate imperviousness, presence/absence of BMPs. f 1 _ f f ~ i ., f i I i I . Selecting subwatersheds Drainage areas from 100 to 500 acres, known level of imperviousness and age, free of confounding sources (active construction, mining, agriculture, or point sources). Select three random non-overlapping reaches (100 feet) for summer and winter sampling of selected variables in each of five key variables groups: 1. Hydrology variables: summerdryweatherflow, wetted perimeter, cross-sectional area of stream, peak annual stonn flow (If gaged). 2. Channel morphology variables: channel alteration, height. angle and extent of bank erosion, substrate ,embeddedness, sediment deposition, substrate quality. 3. Water quality variables: summer water temperature, turbidity, total dissolved solids, substrate fouling index, E? toxicity test, wet weather bacteria, wet weather hydrocarbon. 4. Habitat Variables: pool- riffle ratio, pool frequency, depth and substrate, habitat complexity, instream cover, riffle substrate quality, riparian vegetative cover, riffle embeddeness 5. Ecological Variables: fish diversity, macroinvertebrate diversity, index of biological integrity, EPA Rapid Bioasessment Protocol, fish barriers, leaf pack processing rate. ~ Watershed Protection Techniques . Vol. !-No. 3. . Fall 1994. - 109 . ' ~ U' f~'!'i iTMr::JI Table 5: Twenty-(our strategies to minimize impervious area at the site level (adapted City of Olympia, 1.994; Schueler. 1.995; and PZC, 1.992) 1. Narrower residential road widths 2. Reduced road lengths 3. Hourglass streets 4. Cluster development 5. Shared driveways 6. Angled Parking with one way traffic flow 7. Smaller parking stalls 8. Reduced parking space ratios for some uses 9. Shared parking facilities in commercial areas 10. Shorter residential driveways 11.. Reduced cul-de-sac radii 12. Cul-de-sac donuts 13. Vertical parking structures 14. Two and three story buildings 15. Stream buffers 16. Grass swales rather than cUrb/gutters 17. Open space requirements (residential) 18. Open space landscaping requirements (co; 19. Sidewalks only on one-side of street 20. Reduced side and rear yard setbacks 21.. Decrease distance between lots (fror'/tage) 22. Hammerhead-shaped tumarounds 23. Rear yard grading to buffer 24. Permeable SPillover parking areas Conclusion Research has revealed that imperviousness is a powerful and important indicator of future stream quality and that significant degradation occurs ar rela- tively low levels of development The strong relation- ship between imperviousness and stream quality pre. sents a serious challenge for urban Watershed rr.anag- e:-s. It underscores the difficulty in maintaining urban stre~ quality in the face of development. At the same time. imperviousness represents a common currency that can be measured and managed by planners, engineers and landscape architects alike. It links activities of the individual development site with its cumulative impact at the watershed scale. With further researc~ impervious cover can Serve as an important foundation for more effective land use plan- ning decisions. References 1. Benke. A. E. Wi1Ieke, F. Parrish and D. Stites. 1981. Effects of urbanization on stream ecosys- tems. Completion report Project No. A-OSS-GA. Office of Water Research and Technology. US Dept of Interior. 2. Black and Veatch. 1994. Longwell Branch Restoration-feasibility study. Vol 1. Carrol County. MD Office of Environmental Services. 220 pp. 3. Booth. D. 1991. Urbanization and the natural drainage system-impacts. solutions and prog- noses. Northwest Environmental Journal. 7(1): 93-118. 4: Booth. D. and L. Reinelt 1993. Consequence Urbanization on Aquatic Systems.- measL effects, degradation thresholds. and corree: strategies.pp. 545-550 in Proceedings Wa shed '93 A National conference on ~.acers:- Management March 21-24. 1993. Alexand:- Virginia. S. City of Olympia. 1994(a). Impervious Surf", Reduction Study: Technical and Policy Anal sis-Final Report. Public Works Departme:- Olympia. Washington. 83 pp. 6. City of Olympia. 1994(b), Impervious SUrfOlc Reduction Study. Draft Final Report. Publi Works Department. City of Olympia. Washing ton. 183 pp. 7. Du~ A and K. Cromartie. 1982. CoascOll poIlu tion from septic tank drainfields. Journal of tht Environmental Engineering Division (ASCE 108 (EE6). 8. Even et al. 1994. Effects of urbanization and lane use changes on low stream flow. North Carolim Water Resources Research Institute, Repon No. 284. 66 pp. 9. Galli. J. 1991. Thermal impacts associated with urbanization and stonnwater management beSt management practices. Metropolitan Washing- ton Council of Govemments. Maryland Depan- ment of Environment. Washington. D.C. 188 pp. 10. Galli, J. 1993. Rapid Stream Assessment Tech- nique. Metropolitan Washington Council of Go v- ernments. Washington, D.C. 110 .."iil....=_ . . ..,*I"I(~':'r.~ 11. Galli. 1. 1994. Personal communication. Depart- ment of Environmental Programs. Metropolitan Washington Council of Governments. Wash- in gton. DC. 12. Garie. H and A. McIntosh. 1986. Distribution of benthic macroinvertebrates in streams exposed to urban runoff. Water Resources Bulletin 22:447-458. 13. Gibson. G.,M. Barbour. J. Stribling and J. Karr. 1993. Biological Criteria: Technical Guidance for Streams and Small Rivers. US EP A Assess- ment and Watershed Protection Division. Wash- ington. D.C. 14. Hollis, G. 1975. The effect of urbanization on floods of different recurrence intervals. Water Resources Res. 11(3): 431-435. 15. Jones, R. and C. Clark. 1987. Impact of Watershed Urbanization on Stream Insect Communities. American Water Resources Association. Water Resources Bulletin. 15(4) 16. Klein. R. 1979. Urbanization and stream quality impairment. American Water Resources Asso- ciation. Water Resources Bulletin. 15(4). 17. Limburg. K and R. Schimdt. 1990. Patterns of fish spawning in Hudson river tributaries-response to an urban gradient? Ecology 71 (4): 1231-1245. 18. Luchetti. G and R. Fuersteburg. 1993. Relative fish use in urban and non-urban streams. pro- ceedings. Conference on Wild Salmon. Vancouver. British Columbia. 19. Macrae. C and J. Marsalek. 1992. The role of storm water in sustainable urban development. Proceedings Canadian Hydrology Symposium: 1992-hydrology and its contribution to sustain- able development,1une 1992. Winnipeg, Canada. 20. Pedersen. E and M. Perkins. 1986. The use of benthic invertebrate data for evaluating impacts of urban runoff. Hydrobiologia. 139: 13-22. 21. Plafkin. J. M. Barbour, K. Porter. S. Gross and R. Hughes. 1989. Rapid Bioassessment Protocols for use in streams in rivers: benthic macroinver- tebrates and fish. US EP A Office of Water. EPA-444(44Q)/4-3901. Washington, D.C. 22. Planning & Zoning Center. Inc. 1992. Grand Traverse Bay Region Development Guidebook, Lansing Michigan. 125 pp. 23. Schueler, T. 1987. Controlling urban runoff-a practical manual for planning and designing urban best management practices. Metropolitan Washington Council of Governments. Wash- ington. DC 240 pp. 24. Schueler, T. and John Galli. 1992. Environmental Impacts of Stormwater Ponds. in Watershed Restoration SourceBook. Anacostia Restoration Te:un. Metropolitan Washington Council of Governments. Washington. DC. 242 pp. 25. Shaver. E.. J. Maxted. G. Curtis and D. Caner. 1995. Watershed Protection Using an Integr.lted Approach. in Stormwater NPDES .Related Moni- toring Needs. Engineering Foundation. Ameri- can Society of Civil Engineers. Crested Butte. CO. August 7-12, 1994. 26. Simmons, D and R. Reynolds. 1982. Effects of urbanization on baseflow of selected south-shore streams, Long Island. NY. Water Resources Bulletin. 18(5): 797-805. 27. Steedman. R. J. 1988. Modification and assess- ment of an index of biotic integrity to quantify 'stream quality in Southern Ontario. Canadian Journal of Fisheries and Aquatic Sciences. 45:492-501. 28. Steward, C. 1983. Salmonid populations in an urban environment-Kelsey Creek.. Washing- ton. Masters thesis. University of Washington. 29. Taylor. B.L. 1993. the influences of wetland and watershed morphological characteristics and re- lationships to wetland vegetation communities. Master's thesis. Dept. of Civil Engineering. University of Washington. Seattle. W A. 30. Yoder C.. 1991. The integrated biosurvey as a tool for evaluation of aquatic life use attainment and impairment in Ohio surface waters. in Biologi- cal Criteria: Research and Regulation; 1991. .'llm r:r.:;; <<!I:i'ii"i i:r.o 11:1. PLANNING COMMISSION MINUTES Planner Jenni Tovar distributed a Ie r ftom-th~licant requesting to withdraw the application. -----.... ~ C. CASE #96-089 CONSIDER AN AMENDMENT TO TITLE 5 (ZONING REGULATIONS) AND TO THE PRIOR LAKE ZONING ORDINANCE 83-6 REVISING THE DEFINITION AND THE ALLOWABLE AMOUNT OF IMPERVIOUS SURF ACE ON A RESIDENTIAL LOT IN THE SHORELAND DISTRICT. Commissioner Criego read the Opening Statement for public hearings. A sign-up sheet was circulated to the public in attendance. Planning Coordinator Jane Kansier presented the staff report. The amendment to the Zoning Ordinance is to revise the definition of Impervious Surface. The second part of the amendment is to allow an increase in the amount of impervious surface on a lot of record with the use of special techniques. At a recent meeting, the Planning Commissioners discussed whether or not this definition included gravel driveways. While gravel driveways will become impacted and effectively impervious with frequent use, the definition includes only paved driveways. The Planning Commission directed staff to prepare an amendment to the Zoning Ordinance which would include gravel driveways as impervious surface. The proposed amendment revises the definition of impervious surface to include "all driveways and parking areas, whether paved or not". This language recognizes gravel driveways become impacted with frequent use and are effectively impervious. It also takes into consideration the fact there are no permits required for paving a gravel driveway, so a change in the driveway surface does not change the amount of impervious surface on a lot. The second part of the amendment changes the amount of allowable impervious surface on existing lots of record to forty percent (40%) with the use of specific storm water management practices. These practices include the use of grass filter strips, dividing impervious surface into smaller areas, and grading and construction techniques which will encourage rapid infiltration of runoff. The staff is proposing this change as a response to many of the variance requests over the last two years. A survey of variance requests to the impervious surface in 1995 and 1996 showed nearly all of the properties were substandard lots. The proposed language is consistent with those variances, and sets specific standards for reducing the impact of the increased coverage. This amendment will also help to counter the effect of including gravel driveways in the definition of impervious surface on the smaller existing lots. Staff recommended approval of the amendments. MNI02896.DOC PAGE 3 Comments from the public: Dave Smith, 2590 Spring Lake Road, does not feel it is fair to set a certain percentage on impervious surface. Jim Albers, 14992 Storms Circle, questioned the storm water management. Kansier and Rye explained the procedures and techniques. Commissioner Criego closed the public hearing. Comments from the Commissioners: Kuykendall: . Rye spoke on sand packing as impervious surface. . It would be helpful and useful for someone to come up with designs or examples to help or show how these treatments work for the public. . Supportive. V onhof: . This is the most significant change the Commissioners have looked at with the most far reaching implications. Our city sits on top of a watershed and a large lake. This will impact almost every lot. It is a significant change to go from 30% to 40% impervious surface. . Has not seen any correlation between the figures or impact with the runoff. No supporting data in the reports. . What is the standard going to be? Are they going to have to use these practices? . Kansier explained the procedures. Anything after 30%, up to 40% must fall within the storm water management requirements. . Rye said there is a wealth of information on storm water management and how to deal with problems. . Examine ordinances. Concerned with the impact of changing to 40%. Wuellner: . Is it possible with the best possible management practices to control runoff to any great extent on a lot having a very significant slope to it? . Many lake lots are long and narrow with steep grades. Can the property owner control the runoff as the City intends it to be? . Should this 40% apply to only substandard lots? What is the real intent? . Should this apply to just shoreland lots? . There are a lot of unanswered questions. . Not comfortable making a significant change to the status quo because of those concerns. MNI02896.DOC PAGE 4 Stamson: . Same concerns as other commissioners. . The best management practices were not clear. Be more specific. . Concern for 40% on larger lots. . More open to applying to substandard lots. Criego: . Same concerns. . The original question was gravel roads and definition of impervious surface. . Should not change the ordinance because of substandard lots. . How does this apply to new lots? Kansier explained the new lots will be larger, wider and easier to fit by design on the lots. This is aimed at the shoreland substandard lots. . Prefer to continue the process as in the past. . Criteria is too open in the ordinance. There has to be a lot more thought and discussion. Rye suggested to look at the purpose of what impervious surface coverage regulation really is. The staff view is, if someone can accommodate the situation in the ordinance address through the rule rather than try to address it as an exception through the v~ance process. Preferably address it through the rule. The DNR reviewed it and said from their point of view it was fine. It accomplished what they were trying to accomplish in setting the maximum impervious surface coverage for properties located in the shoreland district. It was not done to get rid of a couple of variances a year. The intent was to have the ordinance address the situation rather than have people come in and go through proceedings. Open Discussion and Comments: Wuellner: The concept is reasonable use of property. That is the problem the City is facing as well as setbacks and design standards. Concern for thermal pollution. V onhof: Talk about result and runoff problems. Engineers will be able to give you runoff rates. What is the quality of our ground water? What will the impact be in 10 years? What if we have a terrible lake? There needs to be additional protections within the shoreland district that do not apply to other districts. Concern is anything above 30%. Wuellner: Could go along at a higher impervious surface for a flatter property as opposed to a steep slope property. The main concern for Prior Lake is the water quality. MNI02896.DOC PAGES Criego: This will be hard to implement. Agrees with Wuellner in taking too big of a step too quickly. The initial intent was to decide if a gravel road was impervious. Not ready to increase to 40%. The issue is runoff. Why 40%? Not enough study done to act. Kuykendall: The performance criteria are important. Engineering design may be required. It may cost, but it will control and manage. Give people the alternative to design. There should be practical ways to show the public. Stamson: The benefit would be to get the practices up front. In the future the Commissioners will be approving variances for hardships and not get the perfonnance criteria. The ordinance is a pro-active way to get it up front. Rye felt the performance approach is a combination of specifying what you are trying to achieve and tell how to achieve it. The DNR applied shoreland rules for the entire State. They established the 25% impervious standards to make sure it covered all the bases. The DNR is concerned for water quality. MOTION BY WUELLNER, SECOND BY VONHOF, TO RECOMMEND APPROVAL OF DRAFT ORDINANCE 96-XXX CHANGING THE IMPERVIOUS SURFACE DEFINITION TO INCLUDE GRA VEL DRIVEW A YS. TABLE THE REMAINDER OF THE ORDINANCE FOR FURTHER DISCUSSION REGARDING THE 40% FOR IMPERVIOUS SURFACE. MOTION FOR AN AMENDMENT BY KUYKENDALL, SECOND BY VONHOF, RECOMMENDING STAFF TO COME BACK AT A REASONABLE TIME WITH A DEFINITION FOR PERFORMANCE CRITERIA. Vote taken signified ayes by Wuellner, Vonhof, Kuykendall, Stamson and Criego. MOTION CARRIED. The Commissioners decided to delay "D" of the public hearings to the end of the meeting. 6. / /,P A. Case #96-1 - Appeal of Hillcrest Homes, Inc., for a decision of the Zoning Officer relating to setba~v~~aging. ., Jenni Tovar presented,the staff'report. On October 11, 1996 the Building Department received an applicat1~n for a buildtttgpermit from Hillcrest Homes for the construction of a single famil welling with attached"garage on property located at 5600 Fairlawn Shores Tra' . The proposed structure has ~setback from the Ordinary High Water Level ......... '''" MNI02896.DOC PAGE 6