1 Using Annual Hydrographs to Determine Effective Impervious Area

Reducing the ammmt of directly connected impervious areas improves watercourse health and increases the potential for sustainable fish communities in streams. It also reduces the impacts of frequently-occurring rainfall-mnoff events, and shifts watercourse hydrology closer to pre-development conditions. There is increasing focus on low impact development (LID) strategies as a means of improving watercourse health. Accurately determining the reduction of effective impervious area (EIA) has become cmcial for assessing the effectiveness ofthese strategies. Quantifying EIA is also essential in developing and calibrating hydrologic models. In addition, it is important to test the effectiveness of LID strategies over the entire year, to take into account the large variability in antecedent conditions. Continuous simulation models focus on all events throughout the year, and not just a large design event. Although several methods exist for measuring EIA, they tend to overestimate the effectiveness of LID strategies in wet climates when soils are saturated for a significant portion of the year. An annual hydro graph method is proposed to determine EIA. This involves using existing gauged creek systems and rainfall records from rainfall gauging networks. This enables the use of available data to determine a watershed's response to rainfall throughout the year, and thereby compute a year -round EIA. The results of this type of analysis will help to assess the effect of implementing LID strategies for existing or proposed developments, particularly for areas with wet climates.


Impervious Area-Why Do
We Care?
There are three primary reasons why an accurate estimate of impervious area is important.These three reasons arc: • to quantifY watershed health; • to develop more accurate continuous simulation hydrologic models; and to measure the effectiveness of strategies that are designed to infiltrate as much rainfall as possible before runoff occms.

Environmental Impacts of Increasing Impervious Area
The relationship between imperviousness and environmental degradation has been well documented by Schueler (1995).With increasing imperviousness, it has been fmmd that runoff peaks and volumes, bank erosion, and water temperature increase while water quality, fish populations, and macro-invetiebrate populations decrease.Therefore, quantifYing imperviousness is important to establish a relative ranking of watershed health.

Hydrologic Modeling
Hydrologic models typically require imperviousness as an input parameter, whether it is directly or indirectly required.In particular, an estimate of the directly connected impervious area on an annual basis is critical for an accurate continuous simulation of a watershed's response to rainfall.

Effectiveness of Low Impact Development Strategies in a Wet Climate
To reduce the environmental impacts of directly connected impervious areas to the drainage system and downstream watercomses, low impact development (LID) strategies such as green roofs, porous surfaces, grassy swales, and disconnected roofleaders are increasingly recommended.LIDs are stormwater management best management practices that are employed at the site level.These methods typically involve capturing frequently occurring rainfall events and preventing runoff through infiltration, evaporation, and transpiration.This increasing focus on LID strategies makes assessment of their effectiveness and suitability very important.In particular, for areas with high annual rainfall volumes such as the coastal areas of western BC and western Washington, the effectiveness of these strategies where soils are saturated for a significant portion of the winter is of great interest.The effectiveness of these strategies on an annual basis is more significant than their effectiveness during an individual event or a specific design storm.Therefore, measuring the effectiveness of these strategies will provide information that will estimate the reduction in directly co1111ected impervious area that can be expected for each type ofLID strategy.

Total and Effective Impervious Area
Total impervious area (TIA) is typically expressed as the sum of all impermeable surfaces within a defined drainage area, and is expressed as a percentage of the entire drainage area.EIA. is typically expressed as the portion of the TIA that is directly co1111ected to a piped drainage system or a watercourse, and is also expressed as a percentage of the entire drainage area.Some typical and alternative methods for determining both TIA and EIA are described below.

Typical methods for determining TIA
The hvo most commonly used methods for detennining TIA use the following: • land-use or zoning mapping; and • aerial or satellite photography.Both of these methods are discussed in more detail in this section.

Land-use/zoning mapping combined v\'ith typical imperviousfactors
The use of land-use or zoning mapping that is typically available from most municipal governments is a commonly used method to determine TIA.A watershed boundary is delineated on the land-use or zoning map and typical impervious percentages are applied to each land-use or zoning designation.Typical impervious percentages are available from a number of sources, but local knowledge of developments and verification of the impervious percentages must be used to increase the accuracy of this method.

Aerial/satellite photography
Visual interpretation or pixel identification techniques can be used on aerial and satellite photographs to determine TIA (Zandbergen and Schreier, 2000).Visual interpretation :involves analyzing an aerial photograph ofthe watershed area and measuring the impervious surfaces.Pixel identification techniques can also be used to digitally analyze aerial or satellite photographs to determine the total impervious area.Calibrating or training the pixel identification routines on known sub-basins within the watershed must be completed to increase the accuracy of the TIA estimates.

Alternative Methods for Determining TIA
Ground surveys and stereo-photogrammetly methods are also available for determining TIA, but are not commonly used because they are labour intensive and expensive (Zandbergen and Schreier, 2000).However, where existing information exists, these two methods will provide more accurate estimates of the TIA.

Ground surveys
Ground surveys to determine impervious areas are not commonly used primarily due to the expense of completing the survey.A ground survey is labour intensive, yet it provides the most accurate measure of TIA.At the site level, this is an effective way to determine TIA.

Stereo-photogrammetry
Stereo-photogrammetry is an accurate method that uses aerial photos.A photogrammetrist synthesizes the aerial photos to remove distortion and delineate buildings, roads, and other impermeable surfaces.This delineated digital layer can then be analyzed to detennine the TIA.This method is labour intensive and requires specialized equipment.As a result, it is expensive and not commonly used (Zandbergen and Schreier, 2000).

Determining EIA
This section focuses on three methods most commonly used to determine EIA.These are : field measurement, empirical equations, and calibrated computer models.

Field measurement
EIA can be determined through a field survey.The field survey includes cataloguing each impervious surface to determine whether or not it is directly connected to a pipe, ditch or other impervious surface that is itself connected.
The areas that are not directly connected are then subtracted from the TIA to calculate the EIA.This method relies on the judgment of the surveyor to accurately determine the direct and indirect connections and may only be valid at the time of the survey.Soil saturation may cause some impervious areas to become "connected" during larger rainfall events and these areas are difficult to predict.

Empirical equations
Empirical equations for detennining EIA have been developed as part of several different studies.One relationship was proposed by Alley and V eenhuis ( 1983) based on work completed for highly urbanized drainage areas in Denver, Colorado.They proposed the equation: 41  (14.1)Laenen (1983) developed another relationship based on work completed in Oregon on more than 40 watersheds.The equation proposed was: The data used by Laenen to develop Equation 14.2 was re-analyzed by Sutherland and a series of equations were developed to provide estimates ofEIA values for input into hydrologic models.The equations varied by using distinctions between the physical characteristics of the development within each of the basins analyzed.The Sutherland EIA equations are summarized as follows: Extremely disconnected basins, with either extensive infiltration measures or basin serviced predominantly with ditches/swales: Somewhat disconnected basins, either 50% of urban areas serviced by ditches or swales and roofs disconnected or an average basin with some infiltration measures: A1A = 0.04 X TMI.? (14.4)Average basins, no infiltration measures, roofs disconnected: EIA = 0.1 X TIAI. 5  (14.5) Highly connected basins, no infiltration measures, roofs connected: Totally connected basins, no infiltration measures, roofs connected: EIA=TL4 (14.7)Dinicola (1989) compiled the work of Alley and Veenhuis, Laenen, and Prysch and Ebbert ( 1986) and values for EIA were proposed for a range ofland uses.However, they were essentially based on Equation 14.1 and not a development of new relationships.

Calibrated computer models
EIA can be detetmined by calibrating a physically based computernmoffmodel such as the US EPA Stormwater Management Model (SWMM) where the many input parameters can be predicted confidently.This method requires extended flow monitoring data to calibrate and validate the EIA values, preferably for a number of events with the full spectrum of expected antecedent conditions for the region.

Annual Hydrograph Method to Determine Effective Impervious Area
To better assess the effectiveness of LID strategies, a fourth method for determining EIA is proposed.This method uses rainfall and flow monitoring data to develop a measured value ofEIA for the basin or site, on an mmual basis.
It is important in many areas to understand what the rainfall-mnoff response will be on an annual basis because of the large variability in antecedent conditions that can be expected.The analysis of a single storm event may significantly over or under estimate perfonnance, depending on the antecedent conditions.This is especially true in the coastal areas of western BC and western Washington where satuTated soil conditions exist for a significant portion ofthe year.
The following sections describe the methodology of the annual hydrograph method and thTee case studies.

Annual Hydrograph Methodology
The annual hydrograph method to determine EIA is based on the following relationships: Rail  14.8 states that on a volume basis, rainfall must equal runoff plus any losses.This equation can be used to calculate losses from a system due to depression storage, evaporation, transpiration, and infiltration to deep groundwater.Equation 14.9 isolates the portion of the cumulative measured flows in Equation 14.8 that come from the EIA by subtracting the interflow and shallow groundwater from the measured flows.The interflow and shallow groundwater flows can be estimated by one of two methods.The simplest and most cost-effective method is to use an established baseflow separation of the measured flow response, and verify results by inspection using local knowledge of nmoff response.The more complex method is to build, calibrate and validate a model such as SWMM that includes groundwater routines.The computer modeling approach will be a more accurate method to remove interflow and shallow groundwater, but at a significantly higher cost.
Rainfall and flow data are plotted as cumulative volumes for the period of record available.It is preferable to analyze one wet and one dry year to obtain an average EIA.The annual hydro graph method depends on the availability of a continuous record of both rainfall and stTeamflow data.The method should only be used on smaller watersheds where the rainfall is recorded in the watershed boundaries by one or more gauges to reduce the impact of spatial variability.Alternatively, rain gauge data could be corrected with Doppler radar information.This would allow the method to be used for larger watersheds.

Annual Hydrograph Case Studies
Three case studies are presented, for thTee types of drainage areas in the Greater Vancouver area ofBritish Columbia.This area receives high annual volumes of rainfall (ranging from 800 to more than 2000 mm) and soil conditions are saturated for a significant portion of the winter (3 months).The first case study, Chantrell Creek, is a suburban watershed in Surrey, BC.The second, Upper Serpentine River, is a highly developed mixed use watershed in Surrey, BC.The third, the Kerr Wood Leidal Associates (KWL) office roof, is a typical flat roof office building in North Vancouver, BC.

Chantrell Creek
Chantrell Creek is a 381 ha watershed located within Suney, BC as shown in Figure 14.1.The watershed is composed primarily of single family residential development with roofleaders typically disconnected and the TIA is estimated to be 30%.The rainfall data used for the annual hydrograph analysis is from a rain gauge located in the middle of the watershed at the ChantreU Creek Elementary SchooL The streamflow station used for the analysis is located on ChantreU Creek at 32nd A venue.The City of Surrey has had a policy of roof leader disconnection within the watershed.Roof area in single-family lot typically comprises 30% to 50% of the lot area.Disconnection of the roof leaders should significantly reduce the EIA.Plotting this data on a cumulative volume basis results in the plot shown in Figure 14.3.The difference between rainfall and runoff amounts to approximately 55%.As described in Equation 14.8, this 55% is attributable to evaporation, transpiration, and deep groundwater.The interflow and shallow groundwater is then subtracted from the streamflow using the constant-slope method for baseflow separation.This assumes that interflow and shallow groundwater are equal to the streamflow after a rainfall event has passed, and rises linearly during a rainfall event until reaching an inflection point on the recession limb of the storm hydro graph.Figure 14.4 shows the estimate of the interflow and shallow ground\vater in comparison to the measured creek flows for some small events.After removing the interflow and shallow groundwater volume from the streamflow vohune, Figure 14.5 shows the estimated flows from the EIA.

Date
The EIA for Chantrell Creek is therefore estimated to be 20% using the annual hydrograph method, which is significantly less than the 30% TIA estimated from the air photo.This indicates that a reduction in runoff does occur, despite the presence of saturated soil conditions for a significant portion of the year.The LID strategy of disconnecting roof leaders to front lawns does appear to be effective.

Upper Serpentine River
The Upper Serpentine River is a 240 ha watershed located in Surrey, BC as shown in Figure 14.6.The watershed is composed of a mix of single family residential, commercial, and light industtial with no known LID strategies within the watershed.The TIA for the Upper Serpentine River is estimated to be 66%.The rainfall data used for the annual hydro graph analysis is from the GVRD Whalley rainfall station.The streamflow station used in the analysis is located on the Serpentine River at 104th Street.
Two years of data, 1999 and 2000, were analyzed to detennine an EIA for the Upper Serpentine River basin, and the results are shown in Figure 14.7.a green roof.As a result, the response to rainfall has been monitored as the pregreen roof condition.The TIA for the KWL Office Roof is 100%.The rainfall data used for the annual hydro graph analysis is from a rain gauge located on the middle of the roof.The flow monitoring data is collected using a weir installed in the roof drainage sump.Figure 14.9 shows lower volumes running off the roof than would be computed using the TIA of 100%.This reduction is attributable to the storage in the gravel and subsequent evaporation.Therefore the EIA for this roof is 82% and the tar and 30 mm gravel roof behaves like a partial LID strategy, even without implementing the green roof.The annual hydro graph method provides an effective way to assess the performance of sites such a<: the KWL Office Roof.

Comparison of results
The three case studies were examined using the empirical equations for detem1ining EIA and the results then compared to the results from the annual hydrograph method.From this comparison, it can be seen that the EIA equations provide similar results to the annual hydrograph method, with the exception of the KWL Office Roof.The empirical equations appear to significantly over-estimate the EIA when the TIA is high at this particular site.This result is not surprising, in part because the other methods were developed to estimate EIA for watersheds and not for small, isolated sites.

Conclusions
The benefits of using the annual hydro graph method are: it can be used without developing a complex watershed computer model; it can be used to assess the performance of LID strategies at both the site and the watershed level; and it provides a reasonable estimate of the year-round EIA for the range of antecedent soil moisture levels found throughout the year.
\\'here a year of streamflow and rainfall data are available, we believe that the ammal hydrograph method is more accurate because it uses site or watershed specific data that truly represent existing conditions.In addition, this method can be used for watersheds where the type of development and the type of LID strategies that may have been included during development are unknown.
Furthennore, the ammal hydrograph method of EIA determination could be used to develop additional relationships for all LID strategies.This should provide stormwatermanagementplanningprofessionals with additional tools to assist in making the decisions that will improve watershed health.

Summary
The following is a summary of the main points of this chapter: EIA is an important parameter for computing watershed health, understanding the rainfall-mnoff response, and calibrating continuous simulations of rainfall-runoff.• The annual hydrograph method is a valuable tool to determine EIA more accurately and on an annual basis.EIA exists in coastal areas, even with saturated soils for a significant portion of the winter.• The annual hydrograph method is an effective way to assess the performance of LID strategies, especially in regions where antecedent conditions vary throughout the year.• Tar and gravel roofs with poor drainage can perform like a partial LID strategy.The annual hydrograph method can be used to develop performance relationships for all LID strategies.
The following table summmizes the results: 82~:,B