Water resources planning and management

In CIV-370 the Kestrel Bay team learned to describe how water moves: in channels, through structures, over the land and through the year. This course asks what engineers do with that knowledge when a whole watershed must be managed. Water resources engineering is the planning and design of the works that store water, convey it, control floods, protect rivers and supply people, and it always involves trade-offs between those purposes. The case for the course is a watershed master plan for Kestrel Creek, commissioned by the City of Duluth with support from the Minnesota Department of Natural Resources (DNR) and the Minnesota Pollution Control Agency (MPCA).

Anika Rao, the firm's water resources lead, sets out the scope at the kick-off meeting. The plan must cover five things that earlier projects treated separately: the small Kestrel Lake dam at the head of the watershed, which supplies a rural water co-op; stormwater from the expanding operations center; the creek crossings at Ridgeway Road and Hillside Road; the flood risk to the lower creek near the lake; and the effect of a changing climate on all of them. Owen Lachance, now an engineer in training, will run most of the calculations, and Sofia Martel, the co-op student, will build the Python tools.

What water resources engineering covers

Water resources engineering grew from irrigation, navigation and water supply, and today it spans four broad purposes. Water supply means finding a reliable source, whether surface water stored in reservoirs, water drawn from rivers and lakes, or groundwater, and delivering it at the quality and pressure users need. Flood management means reducing the damage floods cause, by structures such as dams, dikes and larger crossings, and by non-structural measures such as floodplain regulation, warning and land use planning. Drainage and stormwater management means collecting runoff from developed land, conveying it safely and controlling its quantity and quality before it reaches a receiving water. Environmental protection means keeping enough water in streams for fish and other life, maintaining the shape and habitat of channels, and protecting water quality.

These purposes compete. A reservoir kept full for water supply has little room to absorb a flood; a reservoir kept low for flood control may not refill in a dry year. A culvert sized only for hydraulic capacity may block fish; a channel straightened to pass floods may lose its habitat and erode its banks. The engineer's job is to make the trade-offs explicit, quantify them and help the owner and the public choose.

The planning process

Water resources plans follow a recognizable sequence, whether the subject is a small creek or a large river basin. The team first defines the problem and the objectives with the owner and the agencies: what must the plan achieve, over what period, and how will success be judged? It then gathers data: flows, rainfall, water levels, water use, land use, channel surveys, infrastructure condition and environmental information. Next it forecasts the future, including growth in water demand, changes in land use and changes in climate. It develops alternatives, evaluates them against the objectives, usually with a mix of costs, risks and benefits that cannot all be expressed in money, and recommends a plan with priorities and a schedule. Public and Indigenous engagement runs through every step, not just the end.

In Minnesota, comprehensive watershed management plans are increasingly prepared under the One Watershed, One Plan program, which brings counties, soil and water conservation districts and watershed districts together around a major watershed. A city's surface water management plan sits within that framework. Federal funding or permits can also trigger review under the National Environmental Policy Act, and Minnesota has its own environmental review program for larger projects. The plan for Kestrel Creek is a city plan that the team will align with the regional watershed plan.

The water balance

Every water resources study starts with a water balance: over a chosen period and area, inflow minus outflow equals the change in storage. For a watershed over many years, storage changes little, so precipitation is shared between evapotranspiration, runoff that leaves in the stream and recharge that enters the groundwater. In the Kestrel Creek watershed, the long-term values the team adopts are about 28 in. of precipitation a year and 17 in. of evapotranspiration, leaving about 11 in. to leave the watershed as streamflow or groundwater. About a quarter of that recharges the aquifers, which matters in lesson 11.

The balance also frames demand. A water user taking a small fraction of the mean flow may still cause harm in a dry month, because the mean hides the low flows when fish are most stressed. Water resources planning therefore looks at the whole flow regime: the mean, the seasonal pattern, the low-flow statistics such as the 7-day low flow with a 10-year return period (often written 7Q10), and the floods.

# Kestrel Creek: annual water balance, demand projection and permit thresholds
UNITS = "US"
SI = UNITS == "SI"
A_km2 = 6.4                      # watershed to the Ridgeway Road culvert
P, ET = 712.0, 430.0             # mean annual precipitation and evapotranspiration, mm (illustrative)
R = P - ET                       # long-term runoff + recharge, mm (storage change ~ 0)
recharge = 70.0                  # part of R that goes to groundwater, mm
vol = R / 1000 * A_km2 * 1e6     # m3 per year leaving the watershed
qmean = vol / (365.25 * 86400)   # mean flow, m3/s
if SI:
    print(f"Runoff + recharge = {P:.0f} - {ET:.0f} = {R:.0f} mm/yr  ->  {vol/1e6:.2f} million m3/yr")
    print(f"Mean annual flow at the culvert = {qmean*1000:.0f} L/s; recharge share {recharge/R:.0%}")
else:
    print(f"Runoff + recharge = {P/25.4:.1f} - {ET/25.4:.1f} = {R/25.4:.1f} in./yr  ->  {vol/1233.5:.0f} acre-ft/yr")
    print(f"Mean annual flow at the culvert = {qmean*35.315:.1f} cfs; recharge share {recharge/R:.0%}")

# demand of the rural water co-op served from Kestrel Lake, plus the operations site wash bay
pop0, growth, years = 1200, 0.015, 25
lpcd = 300.0                     # average day, L per person per day
wash = 40.0                      # operations site vehicle wash, m3/d
maxday = 2.0                     # maximum day / average day
print()
for yr in (0, 10, 25):
    pop = pop0 * (1 + growth) ** yr
    avg = pop * lpcd / 1000 + wash          # m3/d
    mx = pop * lpcd / 1000 * maxday + wash
    if SI:
        print(f"Year {yr:2d}: population {pop:5.0f}  average day {avg:5.0f} m3/d ({avg/86.4:4.1f} L/s)  maximum day {mx:5.0f} m3/d")
    else:
        print(f"Year {yr:2d}: population {pop:5.0f}  average day {avg*264.17/1e3:5.0f} thousand gal/d ({avg/86.4*15.85:4.0f} gpm)  maximum day {mx*264.17/1e3:5.0f} thousand gal/d")
avg25 = pop0 * (1 + growth) ** years * lpcd / 1000 + wash
print(f"\nYear-25 average demand is {avg25/86.4/(qmean*1000):.0%} of the mean flow at the culvert")
if SI:
    limit = 50.0                 # Ontario: permit to take water above 50,000 L/d
    print(f"Permit to take water needed? {avg25:.0f} m3/d against the 50 m3/d threshold: {'yes' if avg25 > limit else 'no'}")
else:
    limit = 10000 / 264.17       # Minnesota: appropriation permit above 10,000 gal/d (or 1 million gal/yr)
    print(f"Water appropriation permit needed? {avg25*264.17:,.0f} gal/d against 10,000 gal/d: {'yes' if avg25 > limit else 'no'}")

Runoff + recharge = 28.0 - 16.9 = 11.1 in./yr  ->  1463 acre-ft/yr
Mean annual flow at the culvert = 2.0 cfs; recharge share 25%

Year  0: population  1200  average day   106 thousand gal/d (  73 gpm)  maximum day   201 thousand gal/d
Year 10: population  1393  average day   121 thousand gal/d (  84 gpm)  maximum day   231 thousand gal/d
Year 25: population  1741  average day   149 thousand gal/d ( 103 gpm)  maximum day   287 thousand gal/d

Year-25 average demand is 11% of the mean flow at the culvert
Water appropriation permit needed? 148,553 gal/d against 10,000 gal/d: yes

Who decides: water law and agencies

Engineers design within a legal framework that allocates water, regulates works in and near water and protects fish and water quality. The details differ by jurisdiction, but the questions are the same: who may take water and how much, who may build in a stream or floodplain, and who is responsible if a dam or channel fails.

In the United States, water rights are mostly a matter of state law. Minnesota follows a regulated riparian system: landowners along a water body have rights to reasonable use, but anyone appropriating more than 10,000 gallons a day or one million gallons a year needs a water appropriation permit from the DNR. Work in public waters, including dams, culverts and channel changes, needs a DNR public waters work permit, and the DNR's dam safety program regulates dams. Federal law adds Section 404 of the Clean Water Act, under which the U.S. Army Corps of Engineers permits the placement of fill in waters of the United States, the National Flood Insurance Program, and the Great Lakes–St. Lawrence River Basin Water Resources Compact, which governs diversions from the basin.

Indigenous rights add another layer. Treaties and the rights of Indigenous nations to water, fish and traditional uses must be respected, and the duty to consult falls on governments but shapes how engineers plan engagement and schedule work. On Kestrel Creek, the team includes the nearby First Nation's lands and resources office in its engagement list from the start.

Conservation authorities and watershed organizations

Minnesota has several kinds of watershed organization. Watershed districts, formed under state law around a watershed, can regulate, levy and build projects; soil and water conservation districts work with landowners on conservation; and in the metropolitan area, watershed management organizations have defined roles. Around Duluth, the city, the county, the soil and water conservation district and state agencies share the work. For Kestrel Creek, the DNR holds the public waters, dam safety and floodplain programs, the MPCA the stormwater permits, and FEMA's flood insurance study the regulatory floodplain.

Demand forecasting

Water demand is usually forecast as population times a per capita rate, plus non-residential users, with peaking factors for the maximum day and peak hour. Per capita rates fell in many North American cities after about 2000 as low-flow fixtures and water metering spread, so using an old rate overstates demand and the cost of new works. The forecast should test a range of growth and a range of rates, and it should show how conservation programs and leak reduction change the result. Climate change enters through higher summer demand for lawns and gardens in longer, warmer summers, and through changes in supply, the subject of lesson 2.

For the co-op, the team adopts 79 gallons per person per day, based on two years of metered data, and a maximum day factor of 2.0, typical of small rural systems where lawn watering dominates summer peaks. The 25-year average demand of about 103 gpm becomes the draft on Kestrel Lake in lesson 2, together with a minimum release to the creek for fish.

Reading the results

The annual water balance gives a mean flow at the culvert of about 2.0 cfs, and the co-op's demand grows from about 73 to 103 gpm over 25 years. That is only 11 % of the mean flow at the culvert, but the dam controls only 1.2 square miles of the 2.5 square mile watershed, so the draft is close to half of the mean inflow to Kestrel Lake, and far more than the dry-summer inflow. The co-op's appropriation permit will need to be amended for the higher rate, and the DNR will ask how the protected flow in the creek is maintained in drought.

Practical points

• Start every study with a water balance; it catches errors of an order of magnitude early.
• Look beyond the mean flow to the low flows and the floods; the extremes drive the design.
• List the approvals at the start; they set the schedule and often the design criteria.
• Forecast demand as a range, and show the effect of conservation and leak reduction.
• Engage agencies, the public and Indigenous nations early, and keep a record of what was heard and how it changed the plan.