5-ortloff et al 1982 hydraulic engineering aspects of the chimu chicama-moche intervalley canal

Society for American Archaeology

Hydraulic Engineering Aspects of the Chimu Chicama-Moche Intervalley CanalAuthor(s): Charles R. Ortloff, Michael E. Moseley, Robert A. FeldmanSource: American Antiquity, Vol. 47, No. 3 (Jul., 1982), pp. 572-595Published by: Society for American ArchaeologyStable URL: http://www.jstor.org/stable/280236Accessed: 26/11/2009 01:52

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572 AMERICAN ANTIQUITY [Vol. 47, No. 3,1982]

HYDRAULIC ENGINEERING ASPECTS OF THE CHIMU CHICAMA-MOCHE INTERVALLEY CANAL

Charles R. Ortloff, Michael E. Moseley, and Robert A. Feldman

Of the many canal systems of the Chimu empire the Chicama-Moche Intervalley (La Cumbre) Canal connect- ing the Chicama and Moche valleys represents the highest level of technical achievement. This paper examines the engineering skills of the Chimu as revealed by computer analysis of the open channel flow design techniques they utilized. Analysis of agricultural strategies made possible by this canal and the surveying skills in- herent to its use are examined in detail. The presence of many trial canal paths toward the distal end of the canal Indicate extreme difficulty in overcoming tectonically induced ground-slope changes caused by fault lines near the intervalley divide. The canal was abandoned prior to completion of construction and thus never served to supply the Moche Valley with Chicama water.

Irrigation agriculture requires a sophisticated combination of engineering knowledge, agricultural expertise, and political coordination. As such, it provides a measure of the degree to which a civilization has progressed. The study presented here seeks to analyze the engineering aspects of the Chicama-Moche Intervalley Canal, a massive canal project build about A.D. 1200 on the north coast of Peru. This analysis assesses not only the knowledge of open channel flow design attained by the Chimu engineers, but it also reflects on our current abilities to understand prehistoric engineering.

GENERAL CONTEXT

Coastal Agriculture

Fed by seasonal highland rains in the Pacific watershed of the Cordillera Negra, the lower Moche and Chicama drainages cross the arid Andean coast lands which form part of the world's driest desert (Lettau and Lettau 1978). The climate allows year-around agriculture supported principally by irrigation from large-scale canal systems. Exceptionally high yields are possible and are reflected in the fact that more than 80% of the lower Moche and Chicama irrigated lands are currently under intensive, mechanized cultivation by agroindustrial complexes supplying the international export market.

There has been growing pressure to increase agricultural production by expanding the amount of land under irrigation, but this expansion has been inhibited by topographic irregularities and a marked differential distribution of runoff relative to arable land. Topographic relief is extreme, in part, because the Cordillera Negra fronts the Pacific seafloor, or Nazca Plate, which is actively underthrusting the continental margin at an estimated rate of 10 cm per annum, producing a young, mountainous terrain.

Broken topography and marked differences in the abundance of land and water require large investments in any project designed to move runoff from a drainage where it is abundant across a mountain divide to an area where arable land is abundant. An intervalley canal system designed to connect the two drainages north of the Rio Chicama is currently under construction. Based upon foreign financing and European engineering and technology, it has an investment cost of millions of dollars that will be canceled only after decades of increased agricultural yields. The

Charles R. Ortloff, General Electric Company, Nuclear Energy Division, 175 Curtner Avenue, San Jose, CA 95125

Michael E. Moseley, Robert A. Feldman, Field Museum of Natural History, Chicago, IL 60605

Copyright ( 1982 by the Society for American Archaeology 0002-7316/82/030572-24$2.90/1

REPORTS 573

scope of this project reflects the fact that intervalley systems are of necessity large-scale under- takings. This reclamation project is one of several multinational projects that intrude upon an area formerly united by a single, far larger network of now-abandoned Prehispanic canals.

The five drainages above the Chicama-Moche connection share well preserved prehistoric intervalley canals forming the Lambayeque "megasystem," which Paul Kosok (1940, 1965) identified as a complex of Chimu-phase reclamation works. He argued that the Chicama-Moche link, formed by the so-called "La Cumbre Canal," was sponsored by the Chimor Dynasty, for the purpose of bringing Chicama water to the imperial capital of Chan Chan and its environs. By 1450 A.D., Chimor rule extended from central Peru to near southern Ecuador, and Kosok believed the Lambayeque region was incorporated into the empire as an ongoing megasystem developed by older states in the area. If effectively farmed, the area encompassed by the now-abandoned megasystem would account for approximately one-third of all agricultural land ever reclaimed along the entire Peruvian coast. Kosok overlooked a partially preserved irrigation linkage between the Lambayeque system and the Chicama drainage that would have united what he considered to be two distinct multivalley networks. He also thought that La Cumbre Canal had been a viable under- taking, when in fact it was never put into operation. However, given the geographical scope of Kosok's pioneering research, these are small changes; his effort was not concerned with the details about how much of the multivalley network was operational at any one point in time.

Preservation Patterning

Understanding the relative state of preservation of abandoned irrigation structures is central to both the study of their hydraulic engineering, and to issues concerning the time of operation and the degree of efficiency. Agricultural remains are not uniformly preserved in the Moche Valley. Excavation has identified formerly functional canals lacking preserved fields and fur- rows, as well as furrowed fields lacking preserved canals; it is not always clear which features functioned with others. These complications made La Cumbre Canal a useful case study because the project was aborted as it neared completion and thus was not subjected to the modifications associated with long-used systems.

Stretching over more than 70 km, construction went on over the full length of La Cumbre's course. Masonry channel lining of the final phases is present in areas north of Quebrada del Oso (Figure 1), while to the south the course is intermittently trenched but lacking channel leveling, lining, and vertical alignment with completed aqueducts; it could not have carried water to field systems laid out in the vicinity of Chan Chan. There is a general correlation between complete- ness of construction and degree of preservation. South of the Quebrada the trajectory of the canal can be followed, but neither intended slope nor sidewall configuration can be accurately extra- polated on the basis of unfinished trenching (Pozorski and Pozorski 1981). It is the completed masonry channel north of Quebrada del Oso that records Chimu channel design and forms the focus of this study.

Due to their great length, intervalley canals in Peru are subject to differential preservation and selective distortion generated by two environmental forces: El Nifio perturbations and tectonic activity.

El Ninio

A complex set of meteorological and oceanographical interactions, called "El Nino," combine to disturb the normal desert conditions on the Peruvian coast (Wyrtki et al. 1976; Cromie 1980). Besides disruption of marine life, strong Ninos often produce precipitation onshore. These rare but recurrent torrential rains occur in the study area no more frequently than once per 15 years, and often less than once per century (Nials et al. 1979; Moseley 1981b). In severe El Ninlo downpours, runoff charges normally dry drainages or "quebradas," producing violent flash flooding. Where Prehispanic canals cross large quebradas they have often been washed out, whereas between drainages they have survived sheetwash and unchanneled erosion.

574 AMERICAN ANTIQUITY [Vol. 47, No. 3,1982J

_ _ _km 200CX

t <~~~ ~ Chcm Le?scans

N 2 J Hi~~~~tect~ion scaj

420~~~~~~~~~~~A

0t, 6km . odH Sa ur Crcetsa Chicama L 0 ~~~~~~~~~~~~7*50'

e \ of Canals 0C ta5 CnJ o Huanchaco v Pampalet

H Paraleleranzal

La Cha Ca

7 910' 79- |0

Figure 1. The Chicama-Moche Intervalley Canal (adapted from Kus [1972fl.

REPORTS 575

Tectonic Distortion

The Chicama-Moche watershed is experiencing tectonic displacement. Subduction of the seafloor begins less than 200 km to the west and underthrusts the entire northern Cordillera at a dip of c. 100 and a rate of c. 10 cm per annum. Ongoing uplift and subsidence are most easily gauged along the coast, but substantially greater seismic activity occurs inland (Barazangi and Isacks 1976), supporting the contention that relatively greater vertical displacement goes on in the interior of the watershed (James 1971; Myers 1975). Tectonic displacement of the watershed is associated with only weak tensile or compressional stress and most movements are in the form of oscillation of crustal blocks without marked lateral shear (Zeil 1979). Uplift is evident in 2-, 3-, and 8-m marine terraces at the mouth of the Moche Valley (Cossio and Jaen 1967:57), which must have formed after about 3000 B.C. (Richardson 1981). The highest preserved structure is tentatively dated to about 500 B.C. (Nials et al. 1979:8), and the lower terraces are of more recent Moche or Chimu time-phase origin.

Rates of tectonic activity relative to points and planes of crustal movement induce selective stresses on hydraulic structures. The most detailed local record of ongoing tectonic motion during the last two decades comes from the port city of Chimbote, three valleys south of the Rio Moche. A great deal of landscape uplift is gradual and often involves downward oscillation. Between 1960 and 1970 land-to-sea-level relationships at Chimbote registered a total vertical movement of 18 cm, consisting of a 12-cm rise followed by 6 cm of subsidence, resulting in a net uplift of 6 cm for the decade (Wyss 1978). Vertical landscape movements averaging 1.8 cm a year induce very different stresses upon a structure such as La Cumbre Canal than do seismic events which have abruptly raised sections of the Andean coastline from 1 to 3 m (e.g., Darwin 1839:379; Herd et al. 1981). On May 31, 1970, an earthquake measuring 7.7 on the Richter Scale and centered c. 25 km seaward of Chimbote triggered seismic compaction and spreading of surficial sediments that, among other things, caused roadways and walls near the dock to settle by as much as 1 m. Farther inland, movement of dry-slope debris shifted the foundations of a steel mill 30 cm down. Ground shifting under a railway north of Chimbote caused twisting of the rails and slope changes of 1 to 2 ft per hundred, a tilt of more than 10 (Ericksen et al. 1970:21-23).

There is, unfortunately, no information on canal alterations either during the quake or from the 18 cm of gradual vertical movement during the previous decade. In the case of severe seismic shock, liquefaction of earth-bank canals is expectable in channels transporting water at the time. This process lacks, as yet, archaeological verification by excavation. However, seismic impacting has tentatively been identified in one branch of the Lambayeque complex with possible cor- respondence in a Chimu primary canal of the Moche Valley (Shimada 1980). While La Cumbre Canal would not have been subject to liquefaction, the course-especially artificial earth-fill terraces and aqueducts-would have been subject to alteration from sediment compacting and spreading, as well as to tectonic distortion of crustal blocks.

In the case of gradual tectonic movement, unlined, earth-bank canals are probably self-adjust- ing to slow landscape slope changes when transporting sufficient water for channel erosion to induce the shape changes necessary for maintenance of subcritical flow (Ortloff et al. 1981). A better engineering record of flow design is preserved by lined channels where erosional modification during use is controlled. Still better is an unused, lined channel which should reflect purely theoretical design intent, unmodified by use-testing.

Lined versus unlined channels should also provide somewhat different records of cumulative tectonic displacement. For example, on the south side of the Moche Valley excavation has identified formerly functional canal sections, which theodolite survey shows have zero slope or run slightly uphill. It is not clear what percentage of slope alteration is the product of ongoing gradual displacement or what was induced by individual seismic events. However, the cumulative slope change represented by unlined channels should date after use, when erosional self-correction ceased. Alternatively, the cumulative alteration of lined channels should begin after their construction and include time of use as well as abandonment. Thus, La Cumbre Canal should reflect the effects of approximately 800 years of tectonic movement and seismic activity.

576 AMERICAN ANTIQUITY [Vol. 47, No. 3,19821

The points and planes of crustal movement induce selective slope distortion in a structure the length of La Cumbre Canal because it crosses over and between oscillating bedrock blocks, some exposed and others sediment-covered. Most of the blocks are graphically defined by the drainage pattern in terms of quebradas following faults and interquebrada plains riding on block surfaces. There is selectively greater stress where the canal crosses between blocks than where it rides atop a block of interdrainage surface. Instrument sightings north of the intervalley divide show that different stretches of the completed canal, from c. 0.5 to 8 km or more in length, have different gradients. Most sections have shallow declinations with slopes falling below 0.013 (13 m/km; downhill slopes are given as positive, uphill as negative). In other sections, the preserved course has shallow uphill gradients with slopes falling below - 0.006. Although total cumulative slope alteration over 800 years falls below the 1 to 2% warping (0.01-0.02) of the Chimbote rail line during the 1970 seismic event, it is significant that the greatest uphill slope changes in La Cumbre Canal occur not on interquebrada plains, but where the course contours up quebrada sides in the process of crossing between blocks. This means that the greatest vertical distortion is registered in crossing from the side of one block to the side of another. Horizontal displacement is more difficult to gauge because of the washout of aqueducts and channels in quebrada midpoints. Presumably, many washouts occur at points of greatest stress, which would be at fault lines.

CANAL DESCRIPTION

The Chicama-Moche Intervalley Canal has impressed various authors (Larco Hoyle 1945, 1946; Lumbreras 1974; Von Hagen 1965) and has received study by Kosok (1965) and Farrington (1980; Farrington and Park 1978), with Kus (1972) reporting in greatest detail. According to Kus, the intake and initial section of La Cumbre Canal are probably represented by the modern Sausal Canal which feeds off the south side of the Rio Chicama at elevations between 300 and 350 m some 44 km inland from the river mouth (but see Pozorski and Pozorski [1981] for an alternative interpretation). The preserved La Cumbre channel begins about 17 km down the Sausal course at an elevation of about 250 m (Kus 1972; Pozorski and Pozorski 1981) (Figure 1). From this point the length of the channel is about 54 km to its juncture with the Moche Valley Vichansao Canal at an elevation of c. 125 m, 3.1 km north of Chan Chan. From the canal junction the Chicama flow was then to be channeled northwest to irrigate the plains of Esperanza, Rio Seco, and Huanchaco. Chicama waters would have traversed upwards of 84 km to reach fields on the latter plain. From the Sausal inlet to the outlet junction with the Vichansao the straight line distance is 42 km, but the distance traveled by the intervalley course is twice this figure due to the extreme degree of contouring necessary to negotiate broken topography. If the purpose of La Cumbre Canal was simply to irrigate the plains west of Chan Chan it could have followed a shorter, more direct course from the intervalley divide. However, Chimu strategy called for a substantially larger labor investment and the creation of a switchback in the flow direction of the Chicama waters at the head of Pampa Esperanza. This apparently was done to insure adequate irrigation on Pampa Esperanza and thereby charge the aquifers that fed groundwater wells within Chan Chan and supplied the city with potable water (Day 1974).

The pass between the Chicama and Moche valleys has an elevation of c. 230 m, and from the canal intake up to this point, the critical engineering concern centered upon maintaining the channel at sufficient height to cross the divide. Conserving channel altitude required contouring the course through high, very broken terrain. Once across the divide, maintaining high elevation was not a paramount design factor, and the course was cut through lower, relatively unbroken terrain. Formed by wide alluvial fans, the intervalley pass is an area of low relief. The actual change from broken to unbroken terrain occurred 7.5 km to the north at Quebrada del Oso, where the canal leaves a long section of difficult topography. This point is also where completed construction of the La Cumbre ends.

On the southern flank of the Chicama Valley the canal passes through alternate areas of desert pavement, stabilized sand drifts, and scattered foothills in the vicinity of Cerro Gasnape (Figure 1). Between La Cumbre and the modern Sausal Canal there are remnants of poorly preserved

REPORTS 577

Prehispanic canals which have received neither systematic survey nor excavation. Several can be followed intermittently up to the Oso drainage and represent either precursors or abortive channels of La Cumbre, while other channel remnants are probably early canals that irrigated the south side of the Chicama. Along the flanks of the Chicama Valley where La Cumbre crosses plains of porous, unconsolidated sediments there is often no masonry lining of the earthbank channel. Judging from similar Chimu channels that functioned in the Moche Valley, the design philosophy called for silt to accumulate during use to form nested layers of low porosity material that would seal the canal bed. Continuous masonry channel lining begins along the western flanks of Cerro Tres Cruces, where the canal overrides long stretches of bedrock as it turns out of the Chicama drainage and heads for Quebrada del Oso. The massive aqueduct crossing this dry river is now destroyed. Incomplete construction begins on the south bank, and a number of partially trenched courses appear near the intervalley divide, reflecting alternative strategies of canal placement.

Where masonry lining was employed there are often numerous pits downslope from the canal where mining went on for suitably sized cobbles to face the channel. At several such stations, piles of stone sorted for size are present, reflecting specialized construction that required certain dimensional limits of materials, as will be discussed.

No fields are present along the uncompleted segment of the course. However, some field systems were laid out adjacent to the completed section (Pozorski and Pozorski 1981). The largest complex occurs on the plains immediately north of Quebrada del Oso and is associated with a Chimu administrative center (Kus 1972; Keatinge 1974). Other field areas are located in the regions of Sausal, Cerro Lescano, and on the western flank of the Tres Cruces hill chain. In overview, it seems that systematic layout of fields did not occur until the canal was near completion.

Quebrada Del Oso

The two largest drainages crossed by La Cumbre Canal north of the intervalley divide are Quebrada del Oso and Quebrada Huascar (Figure 2). They bracket the Cerro Tres Cruces bedrock hill chain, which is the most rugged topography negotiated by the canal. The Huascar drainage is the older of the two quebradas and formerly incorporated the Oso Basin. Quebrada del Oso was formed by stream capture along a major fault transecting the Tres Cruces block and the Huascar Basin. A narrow, rock wall canyon breaches the hill chain. Along its course a 40-m "waterfall," representing either a bedrock structure or a secondary, perpendicular fault, divides the stream between an upper-pirated basin and a lower outwash surface. For the stream capture to be sustained, any significant vertical displacement along the Oso fault requires relatively greater rise along the north side of the canyon and/or proportionately greater subsidence along the south side. The opposite pattern of relative movement would act to turn the stream back to its original Huascar drainage. Stream capture began after formation of the highest two erosional terraces in the upper Huascar Basin, but before formation of the lowest terrace and active floodplain. In the outwash section of the Oso drainage, lithic stage occupation has not been found, but there are scattered remains dating back to the first millennium B.C.

Approximately 2 km upstream from the Oso canyon, La Cumbre changes from a canal cut through the sedimentary flanks of the Chicama Valley to an artificially elevated channel running across bedrock hills with slopes often exceeding 600. Crossing the fractured stone face of the Tres Cruces range ranks as one of the most prodigious large-scale construction projects-entail- ing precise survey and engineering-yet identified in the aboriginal New World. The channel is supported by terrace-aqueduct structures standing upwards of 10 stories high (30 + m) that are banked against the mountain slopes. The core of these structures is comprised of soil and cobbles transported from the plains below, as well as angular fire-quarried rock from the mountain face along the course. Wider at the base than summit, the structure exteriors are built up in masonry- faced terraces 1 to 2 m high.

Where the channel shifts from sediment to bedrock there is a shift from a downhill slope to a neutral or slightly uphill inclination, which continues for about 0.5 km up to the largest bedrock

578 AMERICAN ANTIQUITY [Vol. 47, No. 3, 1982]

quebrada negotiated by the canal. Crossing the quebrada, the slope changes uphill to c. - 0.005 for the last 1.5 km up to the Oso quebrada. This length of uphill slope channel is clearly bracketed by bedrock faults. It is possible to profile both the last masonry channel and two underlying prior

*...1..e,,....,..t.' i l l l l l ', :. . . 4. .:. 4~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~... . .........

Figure 2. Vertical aerial photograph of the zone around Quebrada del Oso. (1) Limits of modern cultivation, (2) the Intervalley Canal (path marked by black diamonds); (3) lower Quebrada del Oso, (4) Oso canyon; (5) upper Oso basin, (6) Cerro Tres Cruces, (7) Quebrada Huascar. (Instituto Geografico Militar, Lima: Project AF 60-17, negative 1726).

REPORTS 579

courses over the length of this interquebrada span: essentially, the three superimposed channels are parallel. The most economical explanation of the trichannel profile is that it records the effects of gradual, increasing north-to-south vertical displacement of the Tres Cruces bedrock block which generated the Quebrada del Oso stream capture. In other words, there is a coincidence of fault boundaries, direction of block movement, drainage pattern, and canal-slope distortion that are mutually self-predicating in the context of ongoing geodynamics.

ENGINEERING ANALYSIS

Preservation patterning has important implications for interpretation of the archaeological record, especially as it applies to engineering analyses. Anyone failing to take into consideration the effects of preservation patterning can arrive at erroneous conclusions. We know that sections of the Intervalley Canal now run uphill-the question is why. Were they built that way or has their slope been altered by factors such as tectonic tilt, seismic compaction, and movement of the unconsolidated sediments under the channel?

We have evidence, both prehistoric and modern, of tectonically induced slope changes equal to or greater than those postulated for the Intervalley Canal, so tectonic distortion is a plausible explanation for the apparent slope errors. This explanation receives additional support when we look at the channel itself. The clear evidence presented below of manipulation of the channel geometry (also seen in the Pampa Huanchaco system [Ortloff et al. 1981]) shows sophisticated knowledge of open channel flow, knowledge which is at odds with an interpretation that the apparent slope errors were due to sloppy engineering.

On the basis of both internal and external evidence, we feel entirely justified in extrapolating the overall canal slope to that of the study section north of Quebrada del Oso, and then proceeding with a detailed and sophisticated analysis. It should be remembered that the discussion that follows is based on this estimated slope, but it also should be noted that the specific conclusions are valid over a wide range of channel slopes and that the method of flow simulation has univer- sal applicability.

Field Procedures and Methodology

Hydraulic engineering aspects of the Intervalley channel were studied for the zone immediately north of Quebrada del Oso in order to determine the level of open channel flow design sophistication employed by Chimu engineers. The present analysis focuses exclusively on the hydraulic characteristics of the uppermost late channel as it existed just prior to abandonment. In the Quebrada del Oso section the masonry channel is well-preserved, and considerable data can be collected without excavation. During survey, however, loose sand was removed to permit measurement of canal surface geometry down to the most recent bottom. Profiles were subsequently excavated to determine earlier channel geometries, but these were not used for the present analysis.

Survey and recording began at the northern extant terminus of the now-destroyed aqueduct (denoted as L = 0, Figure 1) crossing the quebrada. Proceeding in an upstream direction, we recorded readings of channel bottom width (B), wall slope (Z), and Manning Roughness Factor (n) at approximately 50-m intervals (Table 1; see Glossary for definitions of terms). Where special features occurred (overflow chutes, wall roughness changes, channel geometry changes, etc.), the 50-m measurement interval was reduced and details of the feature recorded. With these same procedures, a 1,606-m length of the Intervalley channel was surveyed upstream of the reference station.

Since channel slope in the 1,606-m section has been altered by tectonic effects, an estimate of slope at the time of construction is necessary in order to proceed with the analysis. Such an estimate, when combined with Table 1 data, provides data for hydraulic calculations necessary to assess the function of the channel cross section shaping associated with the 1,606-m canal length. To this end, the present-day local slope is recorded at 50-m intervals and at feature locations. Us-

580 AMERICAN ANTIQUITY [Vol. 47, No. 3,19821

Table 1. Sample Intervalley Canal Parameters.

Station (m) ih* Z n B (m) Dn (m) DC (m) 0 to 20 m 0.026 2 0.027 3.603 1.253 1.861

33 0.009 2 0.028 2.201 40 0.009 2 0.027 3.658

120 0.017 1.82 0.036 2.896 147 0.017 1.82 0.027 3.962 170 0.015 0 0.022 3.962 220 0.015 0 0.023 4.267 226 0.015 0 0.025 1.524 258 0.012 2.014 0.032 2.643 0.610 0.689 276 0.009 2.014 0.027 3.048 0.541 0.630 306 0.009 2.014 0.025 3.048 0.412 0.389 356 0.009 0.58 0.026 5.791 0.413 0.389 406 0.015 0.58 0.025 5.791 456 0.015 0.58 0.025 5.791 0.344 0.389 506 0.015 0.29 0.022 7.925 556 0.015 0.29 0.025 7.925 606 0.015 0 0.025 7.925 656 0.015 0 0.025 7.925 706 0.015 0 0.025 7.925 0.288 0.316 756 0.015 0 0.026 7.925 0.459 0.466 792 0.015 0.5 0.031 4.572 0.930 0.755 806 0.015 2 0.032 0.506 856 0.015 2 0.032 0.506 0.896 1.303 906 0.015 2 0.032 0.506 956 0.017 1 0.032 1.219 0.875 0.893

1006 0.009 1 0.032 0.914 1106 0.009 1 0.032 0.457 11.143 0.981 1156 0.017 1 0.025 1.372 I0.856 0.981 1206 0.017 1 0.031 0.914 0.954 0.981 1256 0.017 1 0.022 2.438 1306 0.017 0 0.028 2.134 1356 0.017 1 0.028 2.438 0.498 0.774 1406 0.017 0 0.024 2.438 (0.707 0.702 1456 0.017 0 0.026 4.267 10.707 0.702 1606 0.012 1 0.029 2.438 0.629 0.656

*Based on assumptions about design intent explained in "Field Procedures" section.

ing a range of assumed slopes, computer calculations of the hydraulic flows through the 1,606-m canal length reveal the basic design intent of canal cross-sectional shaping. By next subtracting the most representative theoretical slope from the present-day slope, we achieve an estimate of the total angular distortion of this region over an 800-year time span. The net estimated local slopes at the time of construction are then listed in Table 1. For an ancient theoretical slope of -0.013 and for an average present-day slope of - -0.006 (Pozorski and Pozorski 1981) we see

that an approximate net slope change of - -0.019 (1.09?) has occurred over the 800-year interval. The -0.013 ancient slope value is close to that for critical flow in the channel; this value represents the maximum possible flow rate through the channel, and as such forms an upper bound to its flow rate. Additionally, the supply flow rate computed for the Intervalley Canal is approximately that of its intended destination canal system-the Esperanza and Huanchaco canals. The computed theoretical Intervalley Canal flow rate is 4.67 m3/sec at the Oso station; the independently computed total flow rate to the coeval Esperanza and Huanchaco canals is 4.45 m3/sec (Ortloff 1981: Ortloff et al. 1981). The theoretical supply flow rate from the Intervalley Canal under the assumption of critical flow is then very close to that of the canals intended to be fed by the Intervalley Canal.

Two aspects of the engineering work behind the Intervalley Canal must be considered in the analysis: first, the theoretical design intent in initial route survey and channel configuration and

REPORTS 581

second, the end product characterized by adjustments, reappraisals, and compromises of the ideal that were necessary in order to fit the constraints of unexpected field problems. In the sections that follow, the design intent of the Intervalley is first given according to the signs and signals in these works, which are recognizable to hydraulics engineers. A section is then presented on the proximity to original intent realized in the final construction. The design intent is gauged by use of Table 1 results; these results contain several extrapolations and interpolations of parameters across destroyed/eroded canal sections. Solutions to the equations for fluid flow through the channel given by Table 1 then reveals the design intent of the channel shape throughout the 1,606-m stretch of the canal.

DESIGN INTENT

Basic Hydraulic Concepts

Fluid flow through a canal is governed by a large number of parameters (ib, n, B, and Z). Addi- tionally, the flow classification (subcritical, critical, and supercritical) tells whether a decrease in canal cross-sectional area has a large upstream influence (subcritical flow) or a localized distur- bance effect (usually in the form of a hydraulic jump for a supercritical flow). For example, a subcritical flow entering a local channel constriction (choke) would result in a water height change that may extend for many kilometers upstream. For purposes of determining whether supercritical or subcritical flow exists in a channel, it is necessary to compute the theoretical normal depth (Dn) and the critical depth (Dc) based on channel cross-section geometry, n and ib (Hender- son 1966). If Dn> Dc, then subcritical flow exists; if Dn < Dc. then supercritical flow exists. For subcritical flow the Froude number (Fr) is less than unity; for supercritical flow, Fr> 1. The special case of Fr = 1 is denoted as critical flow (for which Dn = Dc also). The consequences of maintaining Fr close to unity result in a maximum flow rate for a given inlet-to-outlet height difference for a fixed channel area. The degree to which Chimu engineers consciously manipulated all of the above-mentioned parameters and streamwise effects is then the basis for a large part of the following discussion.

Calculation of Channel Flow

In technical terms, subcritical chokes are said to exist when the channel constriction has a critical (Fr = 1) flow at the throat and an upstream subcritical (Fr < 1) flow. Calculations, based on channel geometry field data (Table 1), of the effects of a choke must include upstream adjustment of the estimated water level by standard hydraulics calculation methods (Chow 1959; Henderson 1966). Chokes with upstream supercritical flows result in hydraulic jumps standing in front of the choke. Calculation of theoretical water heights in this case was accomplished by momentum and energy matching of the upstream and downstream flow from the hydraulic jump. From ib, B, and Z then, an averaging-marching calculation procedure (Chow 1959; Henderson 1966; Morris and Wiggert 1972) based on simultaneous computer solution of continuity, momentum, energy, and Manning equations was initiated for streamwise water height and mean velocity solutions. Beginning with an initial estimate of flow rate (Q), we calculated the water height in the 1,606-m section of the canal. At sections where calculated water height exceeded canal wall height, another lower Q estimate was made and the calculations restarted until the water height was contained by the canal. In this manner the correct flow rate through the Intervalley Canal was determined. From this theoretical calculation the fluid mechanics of water flowing through the ancient canal can once again be visualized. Further, by examination of the details of the nonuniform flow through the many changing canal cross sections, the original design intent of the Chimu engineers is revealed.

Due to large reaches of supercritical flow, upstream influence from these sections is nonexis- tent, so that selected portions of the canal flow may be calculated independently. Results of these calculations are shown in Figure 3. In Figure 3a the bottom width (B) of the idealized trapezoidal cross section is shown as a function of upstream length from the L = 0 reference point. Wide


1524- Mean slope 50 km length

1371

D Mv Hydraulic jump Suprc r it

choke

1220 - ~~~~~~~~~~~~~~~~~~~~local 'b ,c

point Dn>Dc Dn<Dc xz

Subcritical Supercritical

1067 - (overf low *

c h u t e )~ E ^eD

z 914 0 C ri ti calI ?- seCtionl De c Critical flow

9 ~~~~~~~~~~Dn Dc

I-

z 762 LU

iU Flow Direction

UJ 610

457 z -I . B Z

305~~~~~~~~~~~~~~~~~ 305 F Hydraulic

jump D

152 Subcritical .

choke

L ~ ~,0 I .I

3 1.5 0 1.5 3m 0 1 2 3 0 0.01 Canal Width Froude ib

Number

(a) (b) (c)

Figure 3. Intervalley Canal parameters in the study section north of Quebrada del Oso.

REPORTS 583

variations in Z ib, and n occur concurrent with large B changes. For example, at the 762-m marker a steep-sided, narrow base (B = 0.42 m), deep trapezoidal section opens out to a shallow trapezoid of wide base (B = 7.32 m). A cycle of trapezoidal base expansions and contractions appears evident from Figure 3a.

Calculations of water depths, mean velocity, normal and critical depth, local critical slope

(ib,c)' and Froude number reveal a conscious design objective of maintaining canal Froude number close to unity (Figure 3b), thus producing an average critical flow in the canal. For a given initial potential energy head represented by the absolute inlet height, the maximum flow rate through the canal is achieved by maintaining a Froude number of 1 throughout. Thus, it appears that the Intervalley system is carefully tailored to transport a known flow rate in such a manner as to make that flow rate the maximum for the available potential energy. Further, by a series of subcritical and supercritical chokes upstream of the Quebrada del Oso aqueduct, the Froude number is continually varied about unity by changes in n, ib, Z, and the cross section so as to avoid the flow instabilities associated with small disturbances at Fr = 1. For supercritical Fr > 1 flows, a combination of n, ib, and cross-section geometry changes modulate the Froude number by producing weak hydraulic jumps at stations 1219, 1036, and 306 to reduce the flow to subcritical. In these locations the upstream Froude number never exceeds 2, so that total head loss through the jump is small. For subcritical flows, the Froude number is increased by means of slope and channel geometry changes (stations 1189 and 1036, for example). In the approach to the now-destroyed aqueduct crossing Quebrada del Oso, a supercritical choke is employed to create a large hydraulic jump at station 309 so that the flow crossing the large aqueduct is subcritical. From a design point of view, the presence of a slow velocity flow across an aqueduct is prefer- able, as the aqueduct is made from unconsolidated fill material subject to wall erosion. The low- speed flow then limits erosion of the canal walls and permits a narrow sidewall embankment to contain the flow, conserving valuable labor by keeping the aqueduct width small. This type of structural optimization by sophisticated open channel flow controls is entirely consistent with the basic strategy of channel design for a maximum transport rate and points to a level of technical sophistication previously unknown in the history of hydraulics among Precolumbian cultures.

It is interesting to note the effects of the canal cross-sectional shaping for slopes yielding non- critical flows. If the canal slope was higher than that assumed, then the choke systems at the aqueduct work even more effectively. That is, a stronger hydraulic jump is formed and the flow velocity downstream of the jump is further reduced, thus limiting flow-induced erosion even more. Throughout the system the flow will tend more toward supercritical values, but with approximately the same flow rate as for critical flow. For shallower slopes, the choke section before the main aqueduct again works to produce the same effect, but in a somewhat different manner. Sub- critical flow passes through the choke opening, is accelerated through the throat, then increases velocity until a weak hydraulic jump is formed in the expansion region. A low-speed flow is then produced for passage over the aqueduct. Therefore, for both off-design cases the preaqueduct choke system functions to produce low-speed flow through the large aqueduct. Overall, though, the observed design is more consistent with critical flows, as determined by the computer simula- tions.

Computation of normal and critical depths along the canal indicates that Dn and Dc continually interchange as maximum values despite wide variations in channel geometry (Table 1). This indicates a design strategy of achieving critical or near-critical flow throughout the channel length. For the calculated flow rate (Q) value, the local critical slope ib, c is close to both the total length mean slope and the local bed slope (Figure 3c), indicating that the design optimization toward unit Froude number probably prevails throughout the entire canal length. At canal locations where hydraulic jumps occur, the upstream Froude number is always less than 2, indicating a relatively weak jump. From a design point of view this limits the energy lost due to turbulence and viscous mixing, thus conserving the total head of the flow. In many places near-critical flow is achieved by adjustments in ib, n, Z, and B, indicating that the designers understood the complex interrelation- ships of these parameters in controlling flow velocity.


Located at stations 1006 and 945 are overflow chutes near the top rim of the canal bank that would limit flow to the maximum Q value of the canal. Any excess above the design Q is drained off by these chutes. The chute at station 945 leads away from a partially destroyed section of the canal with enough structure remaining to indicate that the chute ran from near the top rim of a basin. This again indicates that only when the canal was over the design flow rate Q did the chutes function.

It may be concluded that the Intervalley was purposefully designed to carry a specific Q value; further, the system was designed to carry this Q optimally. Local variations in canal slope due to unavoidable topographic features (which affect flow velocity) were countered in a manner indicating cognizance of the complex interrelationship between local channel geometry, roughness, and bed and sidewall slope. The streamwise effects of subcritical and supercritical choking and of velocity modulation by channel width changes appear to be well understood. Survey during the 1978 field season has led to the discovery of yet another choked-flow canal geometry preceding an aqueduct (Figure 4). This system is located at point M in Figure 1, where an overflow chute is also present.

It is interesting to note that destroyed canal sections (labeled D in Figure 3a) occur at locations of the calculated hydraulic jumps (except for station 762). In the event of a massive rain, channel flow could exceed the capacity of the overflow chutes to regulate water level, thereby creating large Fr increases; the possibility of local overflow at hydraulic jump locations then is a distinctly possible mode of canal bank failure at D locations.

Examination of the Manning Roughness Coefficient (n) as a function of canal length (Figure 5) reveals local adjustments. Large variations appear in the vicinity of hydraulic jumps associated with choke locations. Since n ranges over nearly a 100% change, it can be argued that wall roughness may have been used as an adjustment control on local Fr values, thus implying a knowledge of this effect on flow velocity regulation.

In many places a lined trapezoidal cross-section channel is found with wide top walls, upon which secondary walls are constructed. Calculations reveal that the topmost set of walls are not interactive with the flow at any station. The use of such constructions is undoubtedly for the purpose of walkways alongside the canal-especially on steep-sided aqueduct and terrace structures.

The total inventory of canal geometries available to Chimu engineers appear to range from stone-lined, variable B trapezoids, with flush-mounted stones set in plaster, to unlined versions with large Z and B variations. Sidewall erosion of the unlined shapes gradually leads to near- parabolic equilibrium cross sections.

Local theoretical flow velocities fall, for the most part, in the range of V = 1.67 to 3.05 m/sec. After long-term stabilization of the canal bed in unlined sections, and given that no input material is eroded from lined sections, most of the silt carried in the initial canal length is in the form of suspended load. After transit of a sufficiently long distance it is likely that most of the suspended load has settled onto the canal bed, whereupon bed load transport is the dominant means of sediment movement. That silt would not remain in suspension can be seen by a simple calculation. To maintain sediment in suspension, a flow with a particle Reynolds number (Henderson 1966:412) equal to -400, i.e., Re* = V* D80/v 400, is necessary. A calculation of Re* for the Intervalley Canal reveals a value an order of magnitude less than 400 (based on a measured silt effective particle diameter D80 of 0.15 mm) indicating that a stable bed existed and no scouring occurred. Thus, once settling had taken place the silt remained in the form of bed load. In the above expres- sion, V* is the friction velocity and v the water kinematic viscosity.

Canal Use Strategies

The computed maximum flow rate (Q) for the Intervalley Canal is 4.67 m3/sec. Figure 6 shows the hydrographs of the Moche and Chicama rivers, i.e., the seasonal river flow rate as a function of month. The stippled area on the hydrograph represents the Intervalley flow rate subtracted from the Chicama and added to the Moche Valley water supply. The first noticeable effect of

REPORTS 585

design intent upon the delivery of Intervalley waters to the Moche Valley is an extension of the watering season in the vicinity of Chan Chan by about 3 months' time. If, for example, in mid- December, when the Moche flow rate is on the order of 7 m3/sec, adding the full Intervalley flow

60 to aquaduct

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2 30 Section Properties

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(/) BB 22.7 24.7 Z CC overflow weir 0.38m from bottom 3:r. DD 45 60

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B B OL = Left wall slope

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Supercritical Choke Ahead of Aquaduct At Station M (fig 1)

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B Base width (im)

Figure 4. Supercritical choke at Station M.


1524

1371

Hydraulic Jump Location

1220

- Hydraulic Jump Location

914

E

z 0 I 762

z

w cr

457

Hydraulic Jump 305 Location

152

.01 .02 .03 .04 .02 .025 .03 .035 LOCAL BED SLOPE ib MANNING ROUGHNESS FACTOR n

Figure 5. Canal bed slope (estimated) and Manning Roughness Factor.

REPORTS 587

effectively gives a water yield of 11.67 m3/sec to the Moche Valley. This flow rate would only be available to the Moche Valley from its waters alone in mid-January, so any agricultural planting done at that time could be advanced 1 month ahead with the inclusion of Intervalley waters. Similarly, the water available from the Moche River alone in mid-May is nearly doubled by addi-

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tion of Intervalley waters, to that equal to the maximum available from the Moche River alone at the end of the watering season in mid-April. If all the Chicama waters are taken in June, then at least the water flow rate obtained from the Moche River alone in mid-May can be sustained. Therefore the agricultural production rate in mid-May from Moche waters alone can be sustained at the same rate from Intervalley waters for 2 months longer (during this time the Moche River flow rate drops off to levels insufficient for valley agriculture). Similarly, the mid-January Moche River flow rate can be matched by the mid-December rate with the addition of Intervalley waters.

At the height of the Moche watering season in March there is a 15% flow rate increase available to the Moche Valley from Intervalley waters; this undoubtedly was to be used in ambitious plans for field expansion to the Rio Seco, Esperanza, and Pampa Huanchaco areas. Calculations based on Huanchaco Canal excavations have indicated that the theoretical flow-rate from the In- tervalley is equal to the Huanchaco Canal input flow rate, thereby indicating a possible destination for Intervalley waters (Ortloff 1981).

The hydrographs shown in Figure 6 represent recent 40-year averages of maximum and minimum flow rate values; over this time period three standard deviation departures from the mean occur in 2.5% of the years represented, as a result of El Ninio coastal rains. For these years each valley was subject to flooding conditions, and the Intervalley subject to severe erosion from active quebradas flowing transverse to its course.

The Intervalley appears to have been designed with several options in mind. In addition to its design capability to extend the watering season in the Moche Valley during low water times, the Intervalley had the capability to take up to 6% of the Chicama flow during peak flow months. As can be seen from Figure 6, the maximum Chicama flow rate can approach 80 m3/sec from March to April. Unfortunately, the inlet of the Intervalley is now destroyed so that it is not possible to determine if the same constant flow rate existed in the Intervalley Canal regardless of the flow rate in the Chicama River. The Chimu nevertheless had the option to extract 6% of the Chicama River flow when it probably could not have effectively been used for existing Chicama field systems (in March) but could (in part) effectively supply the Huanchaco Canal system. Since the Huanchaco field system was the intended benefactor of Intervalley (plus Vichansao) flow it would be most beneficial only if the system were fully utilized. That is, if the strategy of Intervalley operation was usage only during peak Chicama flow rate months, then only a month or two of use would be expected for the Huanchaco system. If (assuming 100% delivery efficiency) the maximum flow rate of 4.67 m3/sec flowed through the Intervalley from mid-October to mid-June then a full nine months' usage in the Moche Valley could be obtained from the Intervalley alone.

Excavations of the Huanchaco Canal (Ortloff 1981: Ortloff et al. 1981) reveal that parts of the late system were never operational. This could indicate several possibilities: (1) the Intervalley contribution was insufficient, requiring a contraction or abandonment of part of the Huanchaco canal network and further reliance upon an expanded Vichansao contribution; (2) long-term drought conditions prevailed and reduced Intervalley flow requiring a less extensive Huanchaco canal system; (3) outside invasion or political changes drew attention away from canal construction and maintenance. Option (1) appears to be the most likely. As will be discussed subsequently, the Intervalley probably never provided any water to the Moche Valley. This failure resulted in the need for an amplified Vichansao Canal to supply the Huanchaco and Esperanza canals. Most likely, the plans for Huanchaco expansion were contingent upon Intervalley waters supplement- ing Vichansao waters-canal segments built for this purpose therefore were never utilized. Fur- ther evidence for the hypothesis of dysfunction of the terminal stages of the Intervalley can be seen in the incomplete G-O system (Figure 1) near the divide, as well as the unused field systems north of the Quebrada del Oso.

One of the main reasons for failure of the Intervalley Canal appears to be tectonic warping of the channel slope. Prime evidence of the effects of tectonically induced coastal uplift are seen in canal segments north of Quebrada del Oso (Figure 1). South of this region, however, many canal segments appear to branch away from the main stem canal. Each such canal path represents a constant slope path across the then-present land surface; alternative canal paths adjacent to earlier ones hence represent constant slope paths on a land surface distorted by tectonic activity.

REPORTS 589

As canal slopes are typically a fraction of a degree, a relatively small ground slope change can require the re-engineering and relocating of a canal from an original position.

The major path to the intervalley pass is uphill and remains incomplete by standards set by upstream, fully lined canal construction. Thus, this final segment indicates the challenges of canal engineering on a coastal plain subject to tectonic effects throughout the many years required for canal construction. In final analysis, the Intervalley was ultimately abandoned due to difficulties with the southern portion. Although the Intervalley may have functioned off-design before tectonic effects caused its abandonment, the input to the Huanchaco system was nevertheless negligible at best. This led to the expansion of the Vichansao as the main supply canal for the Huanchaco system. Further evidence supporting option (1) is given in the next section.

Of the remaining options, (3) is contrary to the many years of stable Chimu coastal occupation and successful military expansion. Option (2) is more difficult to test, as evidence for decreasing water supply to the Rio Seco and Huanchaco systems over time can be given (Ortloff et al. 1981). This too is attributable to tectonically induced canal slope changes and the concomitant water flow rate decrease within the Moche Valley (Moseley 1981a).

By design, Intervalley waters would have certainly guaranteed ample water for the gardens and wells of the elite class at Chan Chan from October to June. This may reflect the desire to keep the capital city flourishing at the expense of valleys of lesser political importance. The potential capability for Chan Chan's agricultural survival under adverse climate conditions, as well as extension of the Moche Valley watering season, point to the dominance of Chan Chan in dictating policy to adjacent valleys; the Intervalley systems, by design, could have been used flexibly to im- plement the luxury status of Chan Chan's rulers as well as to guarantee continuance of this status throughout dry years by literally draining the Chicama dry.

Engineering Reality

Sighting from the northern dune region toward the Intervalley pass reveals a line-of-sight slope of - 0.007. The actual slope of this segment along the canal is, of course, less than the straight line slope. Since there was little margin for error over difficult terrain, and considerable terracing and aqueduct construction over the 20 km distance north of the Quebrada del Oso region were necessary, it is foreseeable that considerable iteration and correction would characterize this path. Examination of exposed canal profiles indicates a general trend toward decreasing the local slope of the canal in response to the need for less elevation loss with distance. Since there is sparse evidence for canal usage, such slope corrections are most likely a response to observations of tectonic activity in this area. Major fault lines are located throughout this region and to the south near Cerro Campana. Evidence of tectonic effects on canal systems in the immediate vicinity is shown by canal segment u-w (Figure 1). This segment shows extensive usage in the form of heavily oxidized silt layers-yet its present slope is in the uphill direction! Overlying this segment is a late canal of downhill slope, but indicating no usage. Obviously, significant tectonic effects have occurred within the time frame of the building of these canals (Moche V to Chimu-Inca). The fact that segment C-E of the Intervalley Canal runs uphill by current survey is indicative of the groundslope movement in the region south of Quebrada del Oso.

To assess the relative age of the Intervalley Canal at and north of Quebrada del Oso, a series of nine C-14 samples of associated organic matter were processed by the Desert Research Institute of the University of Nevada. The organic material is associated with early, middle, and late canal phases and, as such, bounds the earliest and latest times associated with canal building. Results indicate dates from A.D. 960 to 1420 (minus and plus la) with a mean date of A.D. 1170. These dates are compatible with Kus's findings for samples of organic matter taken in the region of Cerro Sausal farther upstream along the Intervalley Canal (Kus 1972, 1974). The downstream reaches of the canal indicate generally later dates, indicative of the many years of rework and reconstruction associated with the southern tectonically more active reaches of the Intervalley Canal. In the vicinity of Quebrada del Oso, for example, the height difference between lowest and


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REPORTS 591

highest phase canals is on the order of 8 m, attesting to the magitude of correction required by tectonically induced elevation changes in the southern reaches of the canal.

The Pampa Cabezon region (Figures 1, 7) is laced with sequential canal paths indicative of the response to ground slope changes in this southern zone. The fact that canal paths are abandoned for other paths after partial construction (without the benefit of a trial flow through them) represents the difficult process of engineering low slope canals through a geologically unstable area; the fact that hundreds of years may have transpired during the building process without apparent successful canal operation attests to the formidable problem facing Chimu engineers and their determination to obtain a solution.

Various strategies are represented among the canal paths of Pampa Cabezon. The easternmost path would require a cut through ridge H-2 while the westernmost route would require alluvial soil removal and low aqueducting. Apparently, while one path was in the process of construction, resurvey revealed the necessity for a new canal path requiring considerable rework of the upstream canal reaches, as evidenced by the many different elevation phases. With reference to Figure 6 it appears that a straight line from A to B has been surveyed from the peak of H-2 to far distant upstream and downstream points. The straight line path runs about 20 km and is meant to complete the approach to the intervalley divide. The region near the low ridge interrupting the A-B path is instructive as to the methods used to survey canals. Since a straight line appears to be surveyed through the hill it may be that large canal segments are laid out for the "feasibility" survey. The topographic details are then examined for local corrections to maintain the slope. Op- tions 1, 2, and 3 represent paths reflecting the difficulty of approach to the uplifting divide. Path D, in its approach to Quebrada del Oso, is some 12.3 m higher than any corresponding northside canal segment; further, south of hill H-2 this segment runs uphill. Paths C and D then run through a bowl-shaped depression centered about the ridge, with each end of these canals at higher elevations. Clearly paths C and D, once considered viable options on a differently contoured landscape, are presently unworkable. Path A-B is presently downhill and connects onto the last phase of the canal on the north side of Quebrada del Oso. This southside path retains the same slope as the average upstream canal segments and is obviously the last of the attempts to cross the intervalley divide. Near point A', and for all downstream reaches to the divide, the canal goes uphill and is therefore unworkable. The progression from paths C and D to A represents successively greater penetration in the southward direction of workable canal segments; however, none appear to have successfully negotiated the intervalley divide. It appears then that the system was abandoned and the Intervalley never reached its design potential.

Due to the large number of slope and cross-section geometry changes throughout the canal length, obvious difficulties would be encountered in crossing the pass with slope sufficient to permit near-critical flow. As the slope decreases in the southern reaches of the canal, the tendency toward subcritical flow increases with large attendant upstream influences, and canal flow rate decreases if this influence extends to the inlet point. The fact that the canal cross-sectional area and hydraulic radius may increase while flow rate remains constant indicates that the flow may be critical when the slope becomes shallower. This option exercised upon the A'-B segment would permit critical flow to exist to the Pampa Cabezon region, but further extension would require massive trenching and widening up to the divide. That such an option was never exercised attests to the labor investment involved-clearly, trenching to a depth of 15 m for 10 km would provide a workable canal (for a while). The prospect of yet-to-come re-engineering and rework clearly gave caution to the Chimu engineers to consider abandonment of the total system (as indeed proved the case).

In retrospect it is clear that the multiplicity of canal paths and strategies in this region was un- done by small ground-slope changes. Foreknowledge of nonfunction by resurvey led to alternate path selection under the constraints of slope and canal cross-sectional area necessary to produce critical flow in the canal. Any deviation from near criticality in the canal flow would tend to choke the entire system and render it useless with regard to levels of flow sufficient to justify the labor expenditure involved in its construction. Had the canal been choked by a low-slope downstream stretch, the entire canal would then have operated in a subcritical mode; to obtain the same flow rate as the critical mode, the entire system would then have to have been considerably widened, requiring terrace and aqueduct additions. Clearly, the hydraulic difficulties associated with the southern region near the divide are critical to the operation of the system in the design


mode. When it emerged that these problems could not be surmounted, the entire system must have been abandoned out of necessity.

Special Features

Several engineering features appear unique to the Intervalley Canal. At the base of a 25-m-deep, 30-m-long straight aqueduct (now destroyed) located about 5 km north of Quebrada del Oso are the vestiges of a porous culvert. Since the aqueducts block passage of quebrada flood- water they serve as inadvertent dams. The combination of water pressure and seepage into the aqueduct fill leads to failure if water is allowed to stand behind the "dam." Through the base of this aqueduct, size-graded large stones are piled transverse to the direction of the canal. Such a construction would have served as a porous path to transmit water through the base of the aqueduct. To date, this is the only such example found.

A survey of 10 aqueducts immediately north of the Quebrada del Oso zone reveals several interesting features. For aqueducts on the order of 30 m or less, a plan view indicates straight-line construction; for aqueducts averaging 60 m, a plan view indicates curved-line construction. Cur- vature is toward the up-quebrada direction, similar to an arch dam. Such a configuration will add additional strength to resist the hydrostatic pressure of water behind the aqueduct, due to the compressive force component acting toward the aqueduct center through the fill material. Most of the high canal terraces have extensive stepped-stone facings. In addition to maintaining slope stability, the facings prevent the tendency of the seepage flow to wash out fine soil particles at the point of exit from the embankment. In general, embankment slopes are on the order of 45 for ter- raced canals.

One further feature located near point M, Figure 1, consists of a narrow-width zone preceding a sharp canal bend. The narrowing is believed to induce a local hydraulic jump upstream of the bend, thereby reducing flow velocity and erosional effects in the canal bend. A bypass canal segment is found in the same region. Although this construction required massive filling of a quebrada, nevertheless the canal was shortened by some 200 m in length.

A number of lining types have been observed along the length of the Intervalley Canal. As mentioned earlier, many canal segments have been laboriously lined, only to be filled in for new canal phases within the same bed. Elevation of a canal is the result. Among the lining types are cobbled walls with/without a clay-rich plaster fill for the interstices between the cobbles. An unusual calcium sulfate lining occurs for about 2 km to the north of Quebrada del Oso. Further north in the dune region, a segment of lining consists of large basaltic slabs tightly fitted together. Approx- imate face area is on the order of 1 m2 for each slab. The most common lining appears to be quarried angular cobbles stacked tightly together on the sidewalls of the canal. Stone lining appears to run throughout the entire canal length to the north of Quebrada del Oso; to the south, lining appears intermittently. This further indicates that the process of ongoing construction on the south side failed to achieve a final version meriting stone lining.

In terms of new technology attempted, the Intervalley appears to be unique in that terracing and large aqueducts are employed extensively. Although Intervalley Canal slopes are approximately 100 times greater than those of the Vichansao Canal (Figure 1), small errors in slope calculation on the Intervalley nevertheless lead to large height deficiencies. For example, an error of 0.01 in slope can lead to a height shortfall of 12 m after the 70-km length of the Intervalley is traversed. In terms of labor investment the Intervalley Canal represents several orders of magnitude over that of the Vichansao Canal. In terms of technical sophistication it should be noted that the Vichansao is basically a subcritical system requiring gigantic cross-sectional area to provide adequate water supply to the Esperanza and Huanchaco systems. The fact that the Vichansao is subcritical and the Intervalley critical in design intent indicates that the designers understood how to minimize labor investment and obtain maximum flow rate in return. Were the Intervalley a subcritical system, the labor investment in construction for the same output flow rate would have been an order of magnitude greater than that for the existing system. Here, sophistication in hydraulic design clearly saves many valuable man-years of construction time and also leads to maximum possible agricultural production through large supply rates of water. Clearly, labor was not a commodity to be misspent even in ancient times.

The presence of complex choke/overflow systems on the Intervalley is without precedent

REPORTS 593

elsewhere in the Chicama-Moche Valley canal complex and represents considerable sophistication in open channel flow design. In terms of design intent it may be surmised that the Intervalley possessed an inlet designed to always flow full, i.e., the inlet was placed deep into the Chicama riverbank. In this manner, the design flow rate could be achieved and the sophisticated hydraulic controls made to function on-design.

SUMMARY AND CONCLUSIONS

Since the Intervalley Canal appears to be designed with knowledge of the critical slope necessary to transport water at a given optimal flow rate, it may be supposed that extensive surveying and calculation preceded its design. This implies that such variables as the absolute height from sea level of the Chicama Valley intake and the Moche Valley outlet points were known, and that slope measurements to less than 30' (0.009) were routine along canal length. Slope changes about the mean critical slope due to topographic variations produced the most sophisticated hydraulic countermeasures to keep the canal flow critical. It should be remembered that Chimu engineers could have selected any slope they wished for the Intervalley-the fact that they appear to have chosen a slope to transport water optimally speaks of foreknowledge of this technology. The intake point is some 44 km up the Chicama Valley-precise survey of the intake point in difficult foothill regions and determination of its absolute height from sea level are difficult problems even with present technology.

The sum total of this hydraulic and surveying expertise points to a most advanced technological society; indeed, the level of understanding of the complex relation between wall roughness, bed slope, hydraulic radius, and subcriticality and supercriticality as evidenced in the Intervalley were not known until the late nineteenth century in Europe and America (Rouse and Ince 1963; Sprague de Camp 1974).

It appears that the Intervalley Canal was not completed at its distal end due to severe problems of tectonic movement. At present, the southern uphill canal segment of the Intervalley stands as testimony to ongoing coastal uplift effects. It appears that on or before A.D. 1400 the Intervalley was abandoned without ever reaching its potential. Although parts of the Intervalley may have briefly functioned in a marginal off-design mode, nevertheless the absence of bed silt in the southern reaches is a clear indication that water never passed through this zone.

Although the techniques used by Chimu engineers to design canals are now lost, the fact nevertheless emerges that even by twentieth-century standards the Intervalley is close to optimum design, i.e., for transport of 4.67 m3/sec the same slope would be selected for critical flow. The use of local ib, n, Z, and cross-section variations to regulate Froude number show an understanding of hydraulic controls. The sum total of both surveying and hydraulics expertise points to a systematic science of observing, recording, and generalizing among Chimu engineers. Had the In- tervalley been designed for subcritical flow, then the system would have had a much wider and deeper cross-section to obtain the desired flow rate; had the canal been designed for supercritical flow, sidewall erosion would have been a problem and channel width would have had to be large. Also the required steep slope would have proven impractical to construct through the mountains. Only by a critical flow design, then, can both labor and hydraulic efficiency be served.

The Chimu thus appear to have had engineering skills worthy of maintaining an advanced agricultural system. Such skill implies a specialized branch of government devoted to collecting, analyzing, and abstracting hydraulic observations into a theoretical base which could be used for systems design. That the strategy of the Intervalley usage must necessarily be coupled to engineering design implies technical liaison activities coupled with Chimu administrative functions, much the same as in modern governments. Since flexibility of use of the Intervalley is a design feature, it is possible to envision multiple use strategies that could have been chosen depending upon climate, workforce levels, internal and external politics, and agricultural strategies. All of these conclusions suggest an advanced technology-based Chimu government and shed light on the techniques and strategies used by Chimu leadership to exercise dominance over wide areas of the north coast of Peru.

Acknowledgments. The research underlying this paper was supported by National Science Foundation grants BNS76-24538 and BNS77-24901. Assistance in the field was provided by members of the Programa Riego Antiguo, including Genaro Barr, Alfredo Narvaez, and Fred Nials.


The authors wish especially to acknowledge the comments and criticisms of Shelia Pozorski and Thomas Pozorski. Their manuscript (Pozorski and Pozorski 1981) forced us to reevaluate some of our assumptions and to make them more explicit. We also gratefully acknowledge the inclusion of data from their manuscript. The responsibility for our interpretations of those data, however, rests squarely with us.

GLOSSARY

Bed Slope (Ub): For a channel bed at an angle 0 with the horizontal, then 'b = tan 0. Downhill slopes are given as positive numbers, uphill as negative.

Critical Depth (Dc): Water depth at which the Froude number is unity. D80: Equivalent particle grain diameter size such that 80% of all particles have diameters less than this size. Flow Rate (Q): Number of cubic meters of water flowing by a fixed reference station in one second. Friction Velocity (V*): Defined as (To/p)1/2 where To is the shear stress at the fluid-wall interface and P is the

fluid density.

Froude Number (Fr): Defined as Fr = Vl (gD)1/2 where V is the mean water velocity, g the gravitational constant, and D is the hydraulic depth.

Hydraulic Jump: Transition zone between a supercritical and subcritical flow characterized by a height increase and velocity decrease of the subcritical flow.

Manning Roughness Factor (n): Empirical coefficient related to channel wall roughness. Normal Depth (Dn): Water depth at which uniform (constant depth) flow exists in a channel obtained by solu-

tion of the Manning equation, incorporating the bed slope for the head loss term. Reynolds Number: Defined as Re = VL/I,v where V is the mean water velocity, L is a characteristic length,

and v is the kinematic viscosity.

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