Base Flow Determination for Miller Run for Bucknell
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130 Buffalo Road, Suite 103 Lewisburg, PA 17837 570.524.6744 www.hrg-inc.com SEPTEMBER 2010 BASE FLOW DETERMINATION FOR MILLER RUN FOR BUCKNELL UNIVERSITY EAST BUFFALO TOWNSHIP, UNION COUNTY, PENNSYLVANIA HRG Project No. 3117.0429 ©Herbert, Rowland & Grubic, Inc., 2010 TABLE OF CONTENTS PAGE I. EXECUTIVE SUMMARY ............................................................................................... 1 II. INTRODUCTION ............................................................................................................. 3 III. ASSESSMENT OF EXISTING HYDROLOGIC DATA ................................................. 7 IV. BASE FLOW DETERMINATION ................................................................................... 20 V. BASE FLOW SEASONAL PATTERNS .......................................................................... 24 VI. THE EFFECT OF BASE FLOW ON MILLER RUN HYDRAULICS ............................ 25 VII. CONCLUSIONS ............................................................................................................... 28 VIII. REFERENCES ................................................................................................................. 30 APPENDICES APPENDIX A – SUPPORTING CALCULATIONS P:\0031\003117_0429\Admin\Base Flow Study\Table of Contents.doc Herbert, Rowland & Grubic, Inc. 3117.0429 BASE FLOW DETERMINATION FOR MILLER RUN PREPARED FOR BUCKNELL UNIVERSITY I. EXECUTIVE SUMMARY The quality and quantity of flow in Miller Run has been a recent focus of concern for Bucknell University (University). This report discusses the quantity of base flow in Miller Run. Base flow is defined as the sustained or fair weather runoff in a stream that is composed of groundwater effluent and excess soil moisture (Langbein and Iseri, 1995). It is assumed the current level of development in Miller Run has reduced historic base flow and that the best case scenario would be to have a base flow equal to what Miller Run may have had prior to any human impact. The report is broken into four parts: 1. Review of Existing Data: An assessment and summary of existing hydrologic data has been prepared for Miller Run. Various data sources are examined in their relation to the base flow: topography, soils, geology, land use, and stream gage data collected by the Bucknell Geology Department. The data was compiled and was determined to be sufficient to provide an estimate of base flow in a variety of conditions. Please refer to Section III. Assessment of Hydrologic Data. 2. What is a ‘natural’ base flow for Miller Run? The development within the watershed is perceived to have reduced the base flow. A parameter used to represent the lowest base flow is the Q7,10, the lowest 7-day average that occurs, on average, once every 10 years. If Miller Run were substantially less developed, Q7,10 would be 0.13 cfs or 84,000 gpd. Given the specific geologic characteristics of Miller Run discussed in this report, 1.13 cfs or 730,000 gpd would be needed to sustain a measurable flow through the Bucknell Campus throughout the year. Please refer to Section IV. Base Flow Determination. 3. How does the base flow change throughout the year? The minimum base flow of Miller Run varies throughout the year. Figure E1 shows an estimate of ‘Idealized Base Flows’ or the base flows that would likely occur in Miller Run throughout the year if there were no development (or substantially less) in the watershed. This suggests that flow augmentation for Miller Run may only be needed during the driest parts of the year to maintain its natural flow regime. Please refer to Section V. Base Flow Seasonal Patterns. Herbert, Rowland & Grubic, Inc. Page 1 3117.0429 4. What would the Miller Run channel look like if a percentage, or all, of the effluent was delivered from the Lewisburg Area Joint Sewer Authority (LAJSA) Waste Water Treatment Plant (WWTP)? Using simplified hydraulic modeling, it is shown that the proposed flow augmentation from LAJSA WWTP would provide some minimal flow throughout the Miller Run channel (~3 inches of depth in most locations) without adversely affecting the channel capacity to convey storm flows during extreme storm events. Please refer to Section VI. The Effect of Base Flow on Miller Run Hydraulics. Monthly Base Flow Results for Miller Run 1.60 1,000,000 Idealized Base Flow (average year) 1.40 Idealized Base Flow (dry year) 800,000 1.20 cfs or gpd 1.00 Baseflow(gpd) 600,000 0.80 Baseflow(cfs) 0.60 400,000 0.40 200,000 0.20 0.00 - J F M A M J J A S O N D Month Figure E1. Given the uncertainty of the hydrology for this watershed, an overall approach that incorporates future monitoring efforts would best meet the goals of any mitigation strategies the University might choose to employ. Future strategies to include the base flow of Miller Run should include: stream restoration techniques targeted to sustain or increase base flow; stormwater mitigation that focuses on detention and the slow release of stormwater runoff; and flow augmentation from the LAJSA WWTP. Herbert, Rowland & Grubic, Inc. Page 2 3117.0429 II. INTRODUCTION Miller Run is a headwater stream that runs through the campus of Bucknell University (University) before it joins Limestone Run, and shortly thereafter joins the West Branch Susquehanna River. The quality and quantity of flow in Miller Run has been a recent focus of concern for the University. The Bucknell University Environmental Center has identified excessive nutrient and sediment loadings, stream degradation, and an unstable flow regime in Miller Run (El-Mogazi, 2009). Sections of the 0.90 square mile Miller Run watershed have been observed to run completely dry during prolonged drought periods while some sections have been observed to maintain enough water to sustain perennial aquatics species such as sunfish (Lepomis marchirus and Lepomis gibbosus) and minnows (Exoglossum maxillingua) (Kochel et al., 2009). The focus of this report is to provide an estimate of the low-flow regime in Miller Run. Figure 1 shows an overall map of the Miller Run watershed and two stream gage locations currently operated by the Bucknell Geology Department. The two stream gages are MR2, located at a footbridge near Hunt Hall, and MR1, located at an access bridge near the Art Barn. For the purpose of this study, and to describe specific characteristics of different area of Miller Run, the watershed was divided into four subwatersheds: 1) Miller Run, located just above the confluence with Limestone Run; 2) North Miller Run, located to the north upstream of a discharge pipe near Gerard Field House; 3) South Miller Run downstream (bl) MR1 and 4) South Miller Run upstream (ab) MR1. An assessment and summary of existing hydrologic data has been prepared for Miller Run. Using this data, three specific questions were explored: 1. “What is a ‘natural’ base flow for Miller Run?” 2. “How does the base flow change throughout the year?” 3. “What would the Miller Run channel look like if a percentage or all of the effluent was delivered from the Lewisburg Area Joint Sewer Authority Wastewater Treatment Plant (LAJSA WWTP)?” This report is meant to supplement the “Water Reclamation Feasibility Study/Corridor Report for Bucknell University & The Lewisburg Area Joint Sewer Authority” (referred hereafter as Corridor Report; HRG, 2009). The Corridor Report was prepared to develop and analyze the costs and benefits of a water reclamation project, part of which would divert treated effluent from the LAJSA WWTP to the adjacent Miller Run watershed, with the intent of exploring any possible economic advantages for the community associated with the overall cost of water treatment. Herbert, Rowland & Grubic, Inc. Page 3 3117.0429 Figure 1. Overall Watershed of Miller Run and Flow Gage MR2 and MR1 Herbert, Rowland & Grubic, Inc. Page 4 3117.0429 Base Flow and the Hydrologic Cycle The water cycle is sustained by a continuous cycle of precipitation, runoff, evapotranspiration, infiltration, groundwater recharge, and base flow. Figures 2 and 3 provide a simplified representation of the hydrologic cycle on Miller Run for undeveloped and impervious acres, respectively. In an undeveloped acre of Miller Run (depicted in Figure 2), about 39% of the annual precipitation within the watershed leaves the watershed as storm runoff (i.e., seen in the channel only during and slightly after a rainfall event). Around 27% of the annual precipitation in an undeveloped acre in Miller Run becomes base flow (i.e., flow in a channel that seeps into a channel due to excessive soil moisture or groundwater). The typical characteristics of this base flow are important to the ecological integrity of the watershed: the flows are filtered through the soil where stormwater pollutants (e.g., hydrocarbons or nutrients) are reduced or removed and the flow is typically detained so that the watershed maintains some flow during dry periods. In an impervious acre of Miller Run, 95% of precipitation may become runoff while little or no precipitation contributes to base flow. With the implementation of prudent stormwater management measures, the effect of new impervious land use can be minimized, but much of the development that has occurred within the watershed was prior to the enactment of any stormwater regulations. The precise effect of development depends on its relative location, the distance from the bed and banks of the stream, the soils of a particular location, and many other factors. However, with an estimated 82 acres of impervious land use (14.6% of the watershed;