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Central Puget Sound

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Hydrologic Modeling for the Miller and Walker Creeks Basin Plan

A hydrologic model was developed as part of the analysis for the Miller and Walker Creeks Basin Plan in the early 2000s. The model allowed stream flows to be predicted given assumptions about land cover and rainfall. It allowed the project team to answer "what if" questions - such questions as "What would stream flows be like if this were all forest?" or "How would the stream behave if the cities and the county required a certain uniform level of detention?"

The first step in hydrologic modeling was to develop the model using HSPF (Hydrologic Simulation Program - Fortran). HSPF is a continuous time-series model that is frequently used to simulate hydrologic conditions in streams. The relationship between rainfall and stream flow was determined by assuming certain land cover and geology characteristics. In other words, assumptions about what happened to the rain when it landed on the ground were made - how much of it runs off the land and enters the stream and how much of it infiltrates (sinks into the ground)? These assumptions were made based on the geology of the basin and the extent and type of development that has occurred. Assumptions about the geology were based on geology mapping done by the United States Geological Survey and other local studies. Assumptions about existing development in the basin were based on 1995 satellite mapping of the basin, and relationships between development type, impervious surface coverage, and runoff based on past modeling efforts. In addition, the comprehensive plans for the cities of Burien, Normandy Park, and SeaTac were examined. The Comprehensive Stormwater Management Plan for the Port of Seattle was also consulted. After initial modeling parameters were developed, the model was calibrated using stream flow data collected at flow gauges in the streams and rainfall data collected at precipitation gauges in the watersheds.

In Miller Creek, the model was calibrated with one precipitation gauge (42U) and two flow gauges, one near the confluence with Walker Creek (42A) and one at the outflow of the Lake Reba regional detention facility (42B). In Walker Creek, records were used from one precipitation gauge (42U) and two flow gauges, one near the confluence with Miller Creek (42E) and one just downstream of the headwater wetland (42C). (View latest stream flow gauge information.)

The table below summarizes the gauge information.

Gauge Records Used for Calibration

 MillerWalker
Precipitation Gauges 42U (10/94 - 9/97) At Lake Reba 42U (10/94 - 9/97) At Lake Reba
Flow Gauges 42A (10/94 - 9/97) Just upstream of confluence with Walker

42B (10/94 - 9/97) Miller Creek Regional Detention Facility outflow
42E (10/94 - 9/97) Just upstream of confluence with Miller

42C (10/94 - 9/97) Just downstream of the headwater wetland at Des Moines Memorial Drive

The calibration procedure consisted of making adjustments to the variables in the model to match the predicted model flows to recorded stream flows. The degree to which the model agreed with the recorded stream flows was a measure of the ability of the model to successfully predict stream flows based on hypothetical rainfall events. This level of agreement was calculated and assigned a numeric value called a correlation coefficient. For Miller Creek, the correlation coefficient for the mean daily flow was 75 percent (0.75) at the Miller Creek Regional Detention Facility (MCRDF). This means that the model can correctly account for 75 percent of the variability observed at that location. While this may not seem as high as desirable, it is a fairly typical degree of a model's predictive power in the natural environment. In the Miller Creek example, it means that the modeled results for mean daily flow may be inaccurate by approximately 25 percent at the MCRDF. The largest source of error in the model is the assumption that rainfall measured at one location, the rainfall gauge, falls uniformly across the basin. The only way to reduce the error associated with this assumption is to install a greater number of precipitation gauges throughout the basin. The degree of variability explained by the models, as represented by their correlation coefficients, is shown in the table below.

Degree of Variability Explained by the Models (based on correlation coefficients)

 MillerWalker
Mean Daily Flow 75% (MCRDF), 86% (Mouth)* 79% (DMMD), 75% (Mouth)*
Daily Peak-Hour Flow 67% (MCRDF), 75% (Mouth)* 70% (DMMD), 78% (Mouth)*

* Two values are presented because there are 2 stream flow gauges used in the calibration. For Miller Creek the flow gauges are located at the Miller Creek Regional Detention Facility (MCRDF) and at the mouth. For Walker Creek, the flow gauges are located at Des Moines Memorial Drive (DMMD) and at the mouth.

After development and calibration, the program was used to assess the response of the stream to varying amounts of precipitation under various land covers. For each model run, 50 years of rainfall was allowed to "rain" on the basins. The rainfall assumed during model runs was based on precipitation measured at Seattle-Tacoma International Airport from 1949 to 1998. The resulting data were used to generate curves showing peak flows (high flows that occur during storms) and low flows [minimum flows that occur during times of no precipitation when streams rely solely on base flow (i.e., ground water)]. In addition, calculations of erosive work were performed for the Miller Creek basin, as it has experienced historically high degrees of erosion. The same historic rainfall was used for all model runs so that a consistent, long-term basin response to precipitation could be determined.

To assess the impact of future development on the basin, assumptions were made concerning its likelihood of occurrence. Initially, it was assumed that full development would occur consistent with existing zoning. This assumption, however, lead to an enormous increase in impervious surface area that appeared inconsistent with existing development patterns and rates. Instead, an analysis of development potential was done by examining the relationship between land values and improvement values. Parcels for which the improvement value was less than or equal to the land value were identified as being likely to develop in the near future. Those parcels, the "red parcels," were assumed to be developed in subsequent model runs (see Figure MW1). Although there is no way to accurately predict exactly which parcels will be developed and when, this designation of red parcels seems to be a much more realistic approach than assuming that complete re-development of the basin under existing zoning will occur in the next 20 years.

Fig MW1 - Prediction of Parcel Development and Redevelopment in the Miller and Walker Creek Basins

MW1 - 
Prediction of Parcel Development and Redevelopment in Miller and Walker Basins

Consultants hired by the Airport Communities Coalition reviewed the development, calibration, and early use of the model. They provided a favorable review and saw no barriers to using the model to more fully assess basin responses to potential future scenarios.

A number of model runs for each of the basins are presented below. Each figure is accompanied by a summary of what the modeling scenario shows. A key describing the assumptions of each run is also included

Miller Creek

Analysis of Peak Flows

M2 - Peak Flows near Miller Creek Mouth

Figure M2 shows peak flows near the mouth of Miller Creek under fully-forested and existing conditions. In addition, a goal flow is shown that represents a basin-wide land cover of 75% forest, 15% grass, and 10% impervious surface. The figure shows that peak flows in the basin are much higher now (Current) than they were prior to development (Forest). The increase has ranged from approximately 70 to 2000 percent, with the largest increases occurring with more frequent storms (those with smaller return periods).

Under forested conditions, the range of storm flows expected would be approximately 6 cubic feet per second (cfs) for a 1-year return period (a storm flow likely to occur on average once every year) to 140 cfs for a 100-year return period (a storm flow likely to occur on average once every 100 years). Under current conditions, storm flows range from approximately 95 cfs to 240 cfs for the 1-year to 100-year return periods. The goal flows for the 1-year to 100-year return periods of 40 cfs to 150 cfs represent an approximate reduction of 40 to 60 percent relative to current flows.

M3 - Peak Flows at Miller Creek RDF

Figure M3 shows the same peak flow information for Miller Creek at the Miller Creek Regional Detention Facility (MCRDF). Under forested conditions peak flows would vary from approximately 2.5 cfs to 60 cfs, while under current conditions the peak flows vary from about 30 cfs to 75 cfs. The goal flows represent a 12 to 50 percent reduction relative to current flows.

M4 - Impacts of 
Standards on Flows at the Mouth of Miller Creek

Figure M4 depicts the effect of two different storm water regulations on peak flows near the mouth of Miller Creek. If a Level 1 detention standard is used for the red parcels [except for the Port of Seattle that is required to use Level 2 (75/15/10)], it does little to reduce the peak flows below current levels. A Level 2 (Forest) detention standard for the red parcels [except for the Port of Seattle that is required to use Level 2 (75/15/10)] does measurably reduce peak flows, but still does not attain the goal flow. This modeling run suggested that a Level 2 detention standard should be used instead of a Level 1 standard.

M5 - Impacts of 
Standards on Flows at MCRDF

Figure M5 compares Level 1 and Level 2 detention standards for Miller Creek at the MCRDF. Again, this modeling run suggests that Level 2 detention standards are preferable, although unable to achieve the goal flow.

M6 - Impacts of 
Different Standards on Flows at the Mouth of Miller Creek

M7 - Impacts of 
Different Standards on Flows in Miller Creek at MCRDF

Figures M6 and M7 compare two different Level 2 detention standards, the typical Department of Ecology Level 2 (Forest) standard versus the Level 2 (75/15/10) standard approved by Ecology for use by the Port of Seattle. The different standards produce nearly identical results. This was an important finding as the Level 2 (75/15/10) standard will be much more affordable for red parcel developers to meet than the Level 2 (Forest) standard. Also, it did not seem equitable to hold differing developments in the basin to differing detention standards. Because Ecology has already approved such a standard for the Port of Seattle, there is precedent to apply it to the basin as a whole, although Ecology would need to approve of such an application.

M8 - Proposed 3rd Runway 
Flow Impacts  in Miller Creek at the Mouth

M9 - Proposed 3rd Runway Flow Impacts in Miller Creek at MCRDF

Figures M8 and M9 explore the impacts of the proposed 3rd runway and its association mitigation on the peak flows in Miller Creek. What is found is that the peak flows will actually decrease in Miller Creek due to construction of the 3rd runway and its association mitigation. This is because the detention standard required by Ecology for the Port is a Level 2 (75/15/10) standard. This is a restorative standard that improves hydrologic conditions over those caused by current land use that is more developed than 75 percent forest, 15 percent grass, and 10 percent impervious surface. Because the proposed runway is a large red parcel, mitigation of flows from it cause a relatively large positive effect on peak flows. This analysis underscores the importance of the Port fully and effectively implementing its required mitigation.

M10 - Impact of 
Additional Detention on Peak Flows at the Mouth

Figures M10 and M11 examine the effects of additional storm water detention facilities on peak flows at the mouth and at the MCRDF, respectively. The detention option not only meets the goal flow, but provides more forest-like flows.

M11 - Impact of 
Additional Detention on Peak Flows at the MCRDF

Figure M11 shows that the proposed detention facility expansion at the MCRDF will need to be evaluated over time to ensure that its greatly reduced flows relative to forested conditions do not cause excessive accumulations of fine materials. If necessary, operation of the facility could be modified to more closely match forested flows. While there are other potential detention options in addition to those considered in this modeling run, the important point is that approximately 65 acre-feet of additional storm water detention in the basin can achieve, and surpass, the goal flow.

Analysis of Erosive Work

Field investigations revealed that Miller Creek has experienced severe erosion in many areas. Part of the flow control strategy in this basin plan is to reduce erosion in the stream. To analyze the ability of the various peak flow reduction strategies (discussed above) to do this, a measure of mitigation of erosive work was developed. First, the erosive work on the stream was calculated for the range of flows expected under each scenario presented above. Then, the percent of mitigation of erosive work for any particular flow reduction option was expressed as the following ratio:

(erosion due to development with no mitigation - erosion due to development with mitigation provided by scenario x)
/ (erosion due to development with no mitigation - erosion under forested conditions)
* 100

where scenario x is one of the peak flow reduction scenarios described above.

M12 - Erosive Work Analysis Results

Figure M12 shows the results of the mitigation of erosive work analysis. Under forest conditions there is 100% mitigation of erosive flows. Conversely, with no mitigation of development there is 0% mitigation of erosive flows. Under current conditions erosive flows are mitigated only about 32% at the mouth and about 33% at the MCRDF. Under the goal flow regime, mitigation of approximately 87% occurs at the mouth and 83% at the MCRDF. None of the regulatory measures alone will achieve erosive mitigation equivalent to the goal flow. The Level 1 and Level 2 detention regulations will achieve less than half of the erosion mitigation of the goal flow regime. Note that the Level 2 (Forest) and Level 2 (75/15/10) detention standards achieve essentially identical degrees of erosion mitigation, about 35%. The detention option, however, which includes Level 2 (75/15/10) detention standards and 65 ac-ft of additional detention in the basin, does a good job of approximating the goal. The goal is exceeded for erosive work at the MCRDF by about 20% (i.e., there is less erosion in the detention option than in the goal). In fact, the detention option has more erosion mitigation than even the forested option (the bar for the MCRDF detention option actually extends beyond the graph and shows about 7% more erosion mitigation than the forest option). Erosive work at the mouth is about 9% greater under the detention option than the goal. While not a perfect match to the goal, the detention option does provide enormous benefits relative to the current degree of erosion.

Analysis of Low Flows

M13 - Low Flow Analysis Results

M14 - Low Flow 
Analysis Results

Figures M13 and M14 show the results of the low flow analysis. Although no problems relative to low flows have been reported in Miller Creek, there are reductions in low flows due to development. Figure M13 shows that low flows at the mouth have been reduced by up to about 15% relative to fully-forested conditions. There is essentially no difference in the low flows expected under various regulatory scenarios (current, Level 1, and Level 2) - the multiple curves representing these scenarios have been combined. Figure M14 shows a similar situation. Low flows at the MCRDF have been reduced by up to about 40%. There is essentially no difference in low flows due to regulatory differences; therefore, only a single line is shown to represent current, Level 1, and Level 2.

Walker Creek

Analysis of Peak Flows

W2 - Peak Flows Near Walker Creek Mouth

Figure W2 shows peak flows near the mouth of Walker Creek under fully-forested and existing conditions. In addition, a goal flow is shown that represents a basin-wide land cover of 75% forest, 15% grass, and 10% impervious surface. The figure shows that peak flows in the basin are 3 to 8 times higher now (Current) than they were prior to development (Forest), with the largest increases occurring with more frequent storms (those with smaller return periods).

Under forested conditions, the range of storm flows expected would be approximately 3 cubic feet per second (cfs) for a 1-year return period (a storm flow likely to occur on average once every year) to 22 cfs for a 100-year return period (a storm flow likely to occur on average once every 100 years). Under current conditions, storm flows range from approximately 23 cfs to 73 cfs for the 1-year to 100-year return periods. In order to meet the goal flows ranging from 11 to 36 cfs for the 1-year to 100-year return periods, respectively, a reduction of approximately 50 percent is needed relative to current flows.

W3 - Peak Flows On Walker Creek at DMMD

Figure W3 shows the same peak flow information for Walker Creek at Des Moines Memorial Drive (DMMD). Under forested conditions peak flows would vary from approximately 2 cfs to 12 cfs, while under current conditions the peak flows vary from about 8 cfs to 26 cfs. The goal flows represent a 40 to 50 percent reduction relative to current flows.

W4 - Impact of 
Different Standards on Peak Flows Near the Walker Creek Mouth

Figure W4 depicts the effect of two different storm water regulations on peak flows near the mouth of Walker Creek. If a Level 1 detention standard is used for the red parcels [except for the Port of Seattle that is required to use Level 2 (75/15/10)], it reduces the peak flows below current levels by several cfs (0 to 5). A Level 2 (Forest) detention standard for the red parcels [except for the Port of Seattle that is required to use Level 2 (75/15/10)] achieves nearly identical reductions in peak flows, but still does not attain the goal flow. This modeling run showed that there was little difference between the Level 2 (Forest) and Level 1 standard. In order to protect the higher-quality habitat generally found in Walker Creek, a Level 2 (75/15/10) standard is proposed for the basin. It will be a restorative standard and yet much more affordable for developers than Level 2 (Forest). In addition, it will be consistent with proposed Miller Creek detention standards and those approved for the Port.

W5 - Impact of 
Different Standards on Peak Flows On Walker Creek Near DMMD

Figure W5 compares Level 1 and Level 2 detention standards for Walker Creek at DMMD. The detention standards achieve identical results for peak flows, a reduction in a few cfs, although neither is able to achieve the goal flow. Again, a Level 2 (75/15/10) detention standard is proposed in order to be conservative.

At this time no additional flow control projects are being suggested for Walker Creek. This decision is made based on field surveys that indicate that, in general, Walker Creek is stable and is in much better condition than Miller Creek. A flow, water quality, and habitat monitoring program is proposed for both Miller and Walker Creek so that any changes, either good or bad, can be observed. If conditions in Walker Creek worsen over time, then additional flow control measures, such as detention or infiltration facilities, can be proposed.

W6 - Impact of Proposed 3rd Runway on Walker Creek Peak Flows

W7 - Impact of 
Proposed 3rd Runway on Walker Creek Peak Flows

Figures W6 and W7 explore the impacts of the proposed 3rd runway and its association mitigation on the peak flows in Walker Creek. Peak flows will actually decrease in Walker Creek due to construction of the 3rd runway and its association mitigation. This is because the detention standard required by Ecology for the Port is a Level 2 (75/15/10) standard. This is a restorative standard that improves hydrologic conditions over those caused by current land use that is more developed than 75 percent forest, 15 percent grass, and 10 percent impervious surface. Because the proposed runway is a large red parcel, mitigation of flows from it cause a relatively large positive effect on peak flows. This analysis underscores the importance of the Port fully and effectively implementing its required mitigation.

Analysis of Low Flows

W8 - Low Flow Analysis for Walker Creek

W9 - Low Flow 
Analysis for Walker Creek

Figures W8 and W9 show the results of the low flow analysis. Although no problems relative to low flows have been reported in Walker Creek, there are reductions in low flows due to development. Low flows at the mouth have been reduced by up to about 15% relative to fully-forested conditions; at DMMD the reduction is up to about 25%. There is essentially no difference in the low flows expected under various regulatory scenarios (current, Level 1, and Level 2) - the multiple curves representing these scenarios have been combined. As part of the Port of Seattle's environmental mitigation for the proposed 3rd runway, the Department of Ecology has required low flow augmentation. The Port is required to release water to Walker Creek at a rate of 0.11 cfs continuously between August 1 and October 31 each year. The figures show the effect of this release on low flows.

Key to Modeling Runs

Forest
The model assumes that the only land cover in the basin is forest. The amount of runoff vs. infiltration for the forest cover is dependent on the underlying soil type, either till or outwash.

Goal
The land cover is assumed to exhibit runoff characteristic of a 75% forest, 15% grass, and 10% effective impervious area land cover. This goal flow should not be interpreted to be an absolute value. It is representative of a flow regime that is likely to provide a stable stream with desirable habitat. The goal flow alone will not solve water quality or habitat problems, but will work in concert with improved water quality and habitat improvements to enhance the stream.

Current
This scenario represents the hydrology in the basin given the development present during 1995. That year was used because land cover information, and a corresponding relationship to effective impervious area, was readily available. Although some changes have obviously occurred since 1995, the amount of conversion of land from forest or grass to impervious is believed to be relatively small (i.e., in a basin that is already nearly built out, not much change occurs over time).

Level 1
The model was run with the assumption that all new development (the red parcels) would apply Level 1 flow control to new impervious surfaces. Level 1 flow control is intended to reduce flooding by controlling the peak flow rates of storm water released from developed areas during frequent storm events (the 2-year and 10-year flow events). The 2-year and 10-year post-development flow rates are to be equal to the pre-development flow rates. This is the current standard applied per the King County Surface Water Design Manual. The Port of Seattle's proposed airport expansion was assigned the mitigation approved by Department of Ecology in its Comprehensive Stormwater Management Plan, which is Level 2 (75/15/10).

Level 2 (Forest)
The model assumed that all new development (the red parcels) would use a Level 2 (Forest) flow rate and duration control for new and replaced impervious surfaces. The Level 2 (Forest) standard would require that both flow rates and flow durations after development be equal to those occurring under a 100% forested land cover for flows ranging from one-half of the 2-year flow up to the 50-year flow. The Port of Seattle's proposed airport expansion was assigned the mitigation approved by Department of Ecology in its Comprehensive Stormwater Management Plan, which is Level 2 (75/15/10).

Level 2 (75/15/10)
The assumption was that all new development (the red parcels) would use a Level 2 (75/15/10) flow rate and duration control for new and replaced impervious surfaces. The Level 2 (75/15/10) standard would require that both flow rates and flow durations after development be equal to those occurring under a land cover of 75% forest, 15% grass, and 10% impervious surface for flows ranging from one-half of the 2-year flow up to the 50-year flow. The Port of Seattle's proposed airport expansion was assigned the mitigation approved by Department of Ecology in its Comprehensive Stormwater Management Plan, which is Level 2 (75/15/10).

No 3rd Runway
In this modeling run it was assumed that the Port's proposed third runway and all of its associated mitigation will not be constructed. All other potential development, the red parcels, was assumed to develop with a Level 2 (75/15/10) detention standard.

Detention
This assumed an additional 40 acre-feet (ac-ft) of storage at the Miller Creek Regional Detention Facility (MCRDF) for a total of 130 ac-ft of storage, plus 12 ac-ft of storage at the confluence of Arbor Lake and Hermes Depression, plus 12.5 ac-ft of additional storage at Ambaum Pond (for a total of 15 ac-ft of storage). It also assumed Level 2 (75/15/10) for all of the red parcels. For the MCRDF expansion, the following assumptions were made: Changed gate setting from 2 ft. to 1.5 ft. Changed overflow from 10 ft. to 12 ft. Decreased outflow by about 60% from original would require orifice control structure.

Stewardship of the Miller/Walker Creeks basin is jointly funded by the City of Burien, City of Normandy Park, City of SeaTac, King County, and the Port of Seattle. On behalf of the partners, this page is proudly hosted by King County Department of Natural Resources and Parks - Water and Land Resources Division.