Expanding Water Supply Intakes and Meeting Minimum Flow Requirements,
Prescription for a Win-Win Solution
Woodrow L. Yonts, P.E. - Division of Water Resources
C.V. Brooks, IV, P.E. - The Wooten Company
Presented at the North Carolina AWWA/WEA Annual Conference, November 1997
A large number of water systems utilize run-of-river intakes as a source of raw water, as opposed to using reservoirs which store water and regulate downstream flows. In response to growth in water supply demand, this type of system may need to increase the amount of water it "skims" from ambient flow. During periods of normal to high flow, the increased withdrawal will usually not significantly affect downstream flows. However, during dry periods an expanded withdrawal has the potential to adversely affect water quality, aquatic life, and other instream uses of flowing water. This is especially problematic for relatively large run-of-river withdrawals from relatively small streams. Also, the periods of highest offstream demand often coincide with the periods of lowest instream flow.
Over the past nine years the North Carolina Division of Water Resources (DWR) has reviewed 31 requests for new or expanded run-of-river intakes. Meeting water supply needs while maintaining adequate instream flows can be quite a challenge for local water systems with growing demands and limited water supply options. In one such recent case, the local water system, their consultant, and state review agencies worked together to develop a plan which protected instream flows and offered the Town flexibility in meeting its water supply needs. This paper will examine this case study to illustrate several key steps which can be taken to help achieve this kind of successful resolution.
Background
The Town of Pilot Mountain is located in the piedmont region of North Carolina and draws its drinking water from Toms Creek - a tributary of the Ararat River in the Yadkin River basin. The Town's existing surface water intake utilizes a low weir located at a drainage area of about 29 square miles; with an average annual discharge of about 35 cubic feet per second (cfs), and a 7Q10 low flow of 4.7 cfs. At 1.6 million gallons per day (mgd), the existing peak capacity of the intake constitutes 53 percent of the 7Q10 flow. This intake is the sole source of water supply for the Town.
At present, Pilot Mountain's average daily demand is about 1.1 mgd, with a peak daily demand of 1.6 mgd. Approximately 85 percent of the water use is industrial. In response to the size of the water demand in relation to the system's capacity, the Town contracted with The Wooten Company to prepare a study of water supply alternatives. This study included an evaluation of interconnection to other water systems, expansion of the existing withdrawal, and construction of a new storage reservoir at different locations. After reviewing the study, the Town elected to expand the existing intake to a peak capacity of 3.2 mgd.
The Department of Environment and Natural Resources (DENR) has adopted rules which define the threshold levels of various activities which trigger preparation of an environmental document under the N.C. Environmental Policy Act. The threshold for improvements to potable water systems is either of the following (15A NCAC 1C .0504(3)(b)(ii)):
The Wooten Company contacted DWR to determine what steps would be necessary to address instream flow issues during preparation of the EA for this project. DWR responded that the existing capacity of 1.6 mgd would not be subject to new instream flow requirements, even though it exceeds the threshold of 20 percent of the 7Q10. However, any amounts above 1.6 mgd would only be permitted to be withdrawn when a minimum flow downstream of the intake weir is being maintained. DWR worked with the North Carolina Wildlife Resources Commission (WRC) to determine this minimum flow requirement using the same procedures developed to determine minimum flows subject to the Dam Safety Law (15A NCAC 2K.0501-.0504). This approach uses a habitat rating system to determine the minimum flow in one of three ways:
Another routine step in evaluating proposals for new or expanded water supply withdrawals is to consult the Division of Water Quality (DWQ). Reducing the stream flow during dry periods has the potential to adversely affect assimilative capacity and the protection of water quality standards if any point source discharges are present downstream from the proposed withdrawal. This will depend on the locations of the intake and outfall(s), the amount of withdrawal, and the treatment technology and effluent limits currently in place for any dischargers. In some cases the flow reduction will significantly reduce assimilative capacity and result in more stringent effluent limits. The difficulty and expense this could create for a downstream discharger will depend on how close the wastewater treatment plant is to the limits of its technological capability.
The Town of Pilot Mountain has recently re-located its wastewater discharge to the Ararat River downstream of the confluence with Toms Creek. Because of the large increase in drainage area between the withdrawal and discharge sites, and the relatively small size of the withdrawal, DWQ did not think the proposed increase in withdrawal would affect water quality below the wastewater treatment plant outfall.
DWR communicated the instream flow constraints to the Town of Pilot Mountain and The Wooten Company so that they could evaluate how water supply needs would be met during low flow periods when only the existing 1.6 mgd withdrawal would be available. Analysis of costs and benefits, determination of safe yield and acceptable risk, and evaluation of how frequently water conservation might be invoked, are left up to water systems and their consultants, although the review agencies may offer input. The potential alternatives evaluated by The Wooten Company included:
Evolution of the Instream Flow Agreement
Representatives from the Town of Pilot Mountain, The Wooten Company, and state agencies met several times and considered different proposals in arriving at the agreement which all parties found satisfactory. The evolution of this agreement is outlined below.
DWR determined that a minimum flow of 10.3 cfs needed to be maintained in order for the expanded withdrawal to occur. A meeting was held with the Town and consultant to explain the requirement and offer general suggestions of how water supply demands might be met during low flows. At this meeting the agencies were informed that interconnection was a possibility, but would take time to achieve. The use of an existing offstream storage reservoir was also discussed, but this would need time for upgrading to restore its full capacity. The Town was also interested in conducting an instream flow study of aquatic habitat to see if the minimum flow might be reduced. A study might show that a minimum flow as low as the 7Q10 of 4.7 cfs might be adequate, or the requirement might remain at 10.3 cfs. In any case, it would not be increased. Such a study requires measurements at three distinctly different flows. Without regulation by a dam to provide a given flow, the data collection process can stretch out over a year or more.
It became clear from this initial discussion that the Town needed to expand its water supply capabilities since peak demands were approaching plant capacity. However, taking steps to address instream flow concerns might take some time. DWR therefore proposed a five year interim agreement during which the instream flow study and interconnection arrangements could be completed. During this five year interim, the intake would be reduced to 1.6 mgd when flows below the weir are at or below the 10.3 cfs minimum. However, when offstream storage is reduced by 50 percent, the withdrawal could be increased up to 2.0 mgd. When offstream storage is fully depleted, the full withdrawal of up to 2.4 mgd could be made directly from Toms Creek.
The agencies found this interim agreement acceptable for several reasons.
After further deliberation, Pilot Mountain decided that a calendar-based deadline offered too little flexibility in responding to changes in their water supply demand. The prospects for a successful interconnection agreement were unclear. They were also concerned about the uncertainties of the changing global economy and its potential to affect their large industrial customers. Therefore, the Town wanted to be cautious about acquiring too much debt for water supply improvements. On a positive note, these industries continue to become more efficient users of water as old equipment is gradually replaced - further slowing the growth rate of water demand.
Pilot Mountain therefore proposed to go with a "hard and fast" minimum flow requirement right from the start. Withdrawals would not exceed 1.6 mgd unless a flow of 10.3 cfs was present downstream of the intake. Additional needs above 1.6 mgd would be met by using offstream storage coupled with water conservation. From a strict regulatory standpoint, this proposal was acceptable to the agencies. Minimum flows would be maintained, and the permitted capacity would not exceed the available water. However, the agencies wanted to be sure that the Town realized how frequently water conservation might be necessary and were prepared to bear that burden. Analyses by The Wooten Company indicated that the upgraded offstream storage reservoir would hold about 51 days of "make up" water to add to the 1.6 mgd from Toms Creek. Wooten's stream flow analyses indicated that withdrawals would have to be reduced to 1.6 mgd for up to 45 consecutive days every other year. Water conservation measures would likely be an annual event, and the offstream reservoir would be nearly drained every other year.
In April of 1997, one last meeting was held in Winston-Salem to make sure all parties understood the most recent proposal, and to see if any other options were available. This was attended by representatives from Pilot Mountain, Surry County, DWR, the Public Water Supply Section, The Wooten Company, Hydro Management Services (operators of the water system), the Division of Pollution Prevention and Environmental Assistance (offering technical assistance to industries), and the two major industrial users in the Town. Everyone had reservations about the frequency of water conservation steps under the most recent proposal. The Town had serious misgivings about an interim agreement with calendar deadlines, but the agencies did not want an open-ended interim agreement. Without some assurance that another source of water would become available through interconnection or some other means, Toms Creek could be subject to more and more frequent withdrawals in excess of 1.6 mgd during low flow periods as water usage grows.
It was at this meeting that the concept of an interim period delineated by growth in demand was first suggested, rather than a calendar timetable. Such an approach is not without precedent, in that the Division of Water Quality requires the development of an expansion plan when the wastewater treatment plant load reaches 80 percent of its capacity. A demand-driven requirement offered Pilot Mountain the flexibility it had been seeking to add infrastructure only as it is needed and to be less vulnerable to unforeseen reductions in industrial use. By linking the end of the interim agreement to increases in water use, the agencies were assured that instream flows would not be sacrificed if the Town could not develop another water source in a timely manner.
The agreement developed at the April, 1997 meeting was acceptable to all of the stakeholders. Its features are outlined below:
1. Water Supply Plan: The Town will prepare and adopt a specific plan for a new water supply source to supplement the existing supply to meet the Town's future water supply needs. The new water source must be online before either:
a. the average daily water use exceeds the lesser of 1.9 mgd or 80 percent of the upgraded treatment capacity. (the average daily water use will be based on a calendar year of total water use).
If the new water source is not available as required in 1(a) or 1(b), the maximum daily withdrawal from Toms Creek will be 1.6 mgd when the flow below the intake is less than the required minimum flow of 10.3 cfs and the North Carolina Division of Environmental Health will place a moratorium on new water connections.
Prior to beginning construction on the expanded water supply treatment facility, the Town will amend its Local Water Supply Plan on file with the Division of Water Resources to include this agreement and the amended water shortage response ordinance (see item 3). This amendment to the Water Supply Plan will need to be adopted by the Town after review and approval by the Division of Water Resources.
The Town will follow this agreement and the implementation plan as outlined in the approved water supply plan to develop the alternate water supply source.
2. Off stream Water Supply Storage: The Town will retrofit the off stream impoundment to prevent excess seepage and restore the storage volume to about 41 million gallons.
3. Water Conservation: The Town will amend its water shortage response ordinance to include more stringent water conservation measures. Mandatory water conservation measures will be triggered by the level of water available for public supply in the off stream impoundment. The amended water shortage response ordinance must be acceptable to the Division of Water Resources.
The Town will conduct water audits at least annually to ensure that the amount of leaks and unaccounted for water remains at an acceptable level.
4. Criteria for Water Withdrawal and Implementation of Water Shortage Response Measures: The first trigger point is when flows below the intake drop below 10.3 cfs. When this occurs, the withdrawal from Toms Creek will be reduced to 1.6 mgd. During periods when the withdrawal from Toms Creek is reduced, additional water, if required, would be provided by the 41 mg off stream storage reservoir to meet the total water needs not to exceed 2.4 mgd. Level 1 water conservation measures will be enacted.
The second trigger point is when off stream storage is reduced to 50 percent of usable water supply storage remaining, as defined in the amended water shortage response ordinance. When this occurs, the withdrawal from Toms Creek may be increased to 2.0 mgd with any additional water needs coming from off stream storage. Level 2 water conservation measures will be enacted.
The third trigger point is when the off stream water supply storage is fully depleted. At this point, the withdrawal from Toms Creek may be increased not to exceed 2.4 mgd. Level 3 water conservation measures will be enacted.
5. Stream Gage and Monitoring: The Town will install and maintain a calibrated staff gage downstream of the weir and water supply intake. The calibration and rating table of "elevation versus stream flow" for the gage will be sent to the Division of Water Resources within 30 days of installation. Also, the rating table will be checked every two years for accuracy with the results reported to the Division of Water Resources.
6. Instream Flow Study: The Division of Water Resources will work with the Town's consultant to complete an instream flow study to determine if the minimum flow requirement below the intake can be reduced. The 10.3 cfs minimum flow target below the intake may be reduced as the result of the instream study, but the modified minimum flow will be no less than the 7Q10 low-flow of 4.7 cfs (3.0 mgd) and no greater than 10.3 cfs.
Keys to Arriving at this Solution
Not every situation involving water supply expansion and instream flow issues has been resolved to this level of satisfaction for all parties. Our main purpose in describing this case study is to identify the key factors which led to this successful agreement. The "perfect" resolution may not always be possible, but the level of satisfaction and chances for success can be enhanced by the following items.
Prior Planning - Pilot Mountain did not wait until their water supply situation had reached a state of crisis. This allowed time to consider various alternatives and develop a workable solution. The study of water supply alternatives prepared for the Town by The Wooten Company provided useful information about what options were available, how much water they could provide, and their costs.
Communication & Cooperation - The Town and consultant focused on finding a solution rather than denial of the problem or objecting to requirements. They asked for and received a clear explanation of how the minimum flow was determined. The consultant prepared analyses of safe yields and frequency/duration of water conservation being invoked, and then made this information understandable for the Town and its major water users. Overall, the final agreement would not have been possible without a coordinated effort by the various agencies, Pilot Mountain, The Wooten Company, and the Town's major water customers.
Alternate Sources of Water - Having an offstream storage reservoir and being located where interconnection with another system is feasible were keys to making the final agreement acceptable to the agencies. Offstream storage offers a degree of buffering for instream flows and aquatic habitat in Toms Creek during the interim period. The potential for water from another source through interconnection offers the prospect of a long-term solution to the Town's water supply needs during low flow periods. Offstream storage and interconnection also help address the Public Water Supply Section's concerns regarding reliability of yield and emergency back-up sources.
Local Water Conservation Ordinance - Having a water shortage ordinance already in place will make implementing the final agreement easier. With a few adjustments, this ordinance can be modified such that any reductions in minimum flow coincide with water conservation steps being taken. By doing so, instream flows and aquatic habitat do not bear the entire burden during periods of low water availability.
Applications to Other Local Water Supply Plans
The North Carolina General Assembly enacted legislation requiring all local governments that supply or plan to supply water to prepare local water supply plans (LWSP's) and submit them to DWR by January 1, 1995 (G.S. 143-355(l) and (m)). Each LWSP is an assessment of water needs through the year 2020. The plans must be updated every five years, or more frequently as information changes. A strategy for meeting any potential water supply deficit should be proposed in the plan. The LWSP must be current and approved before state funds can be granted to assist in most water supply improvements.
As they revise and update their LWSP's, local water systems should look for opportunities for offstream storage and interconnection. Any existing storage or interconnections should be preserved through regular maintenance and land use ordinances which prevent point and non-point source degradation. Regional cooperation on new projects or interconnections should be explored before water shortages reach a crisis point. This allows sufficient lead time for successful negotiation of agreements.
If a water system is planning a new or expanded source they should consult early with DWR to see if instream flow concerns will need to be addressed during preparation of the Environmental Assessment. This will allow adequate time for completion of the work if a field study will be required, or if the local system elects to do a study. Data collection may take more than a year unless there is flow regulation by an upstream dam. If an instream flow study is being conducted to see if a "desktop" recommendation can be reduced, there is no guarantee that the field study result will be lower.
The most broadly applicable outcome of the Pilot Mountain agreement has been the development of a screening tool to identify water systems which need to have a definitive plan adopted to address their future water needs. DWR has begun using the ratio of daily demand to water supply capacity to identify systems which need to start planning to augment their supply. All LWSP's are being screened in this manner as their five year renewals are reviewed prior to approval. The two threshold criteria being used are: