May 2011

A Perfect Storm for a Regional Watershed Management Plan

An alternative to site-based LID

The small Colorado towns of Silver Cliff and Westcliffe, along with the Round Mountain Water and Sanitation District, and with the assistance of consultants Crabtree Group Inc. (CGI), civil engineers and town planners of Salida, CO, (members of the Congress for the New Urbanism) integrated cutting-edge town planning and rainwater treatment techniques to address acute flooding issues.

Because Bob Squire, the town manager for the Town of Westcliffe, garnered the support of adjacent jurisdictions, CGI was able to leverage that cooperative spirit into a regional watershed management plan, and that scale brought many more opportunities for watershed improvement than site-level-only approaches.

Regional Scale
Regional scale means that the largest applicable natural catchment area is used for analysis, and the largest feasible political boundary is used for administration and regulation. An EPA study (Figure 1), among many others, made it abundantly clear that larger-scaled approaches can result in significant advantages. First of all, rainfall follows natural laws and boundaries, not necessarily political arbitrations; and the watershed or catchment area is the unit of scientific analysis. Larger-scaled analyses provide the opportunity to more fully understand the natural processes and to capture the larger watershed impairments or opportunities. In this case, the cooperation of the towns and water district provided the vehicle for sweeping regulatory changes that encompassed most of the natural watershed.

Figure 1. EPA watershed graphic

Figure 2. Per-acre versus per-capita runoff rates

Figure 1 illustrates the importance and effect of differing settlement patterns on the watershed and shows that compact settlement patterns go hand in hand with preservation of natural and rural lands.

Figure 2 illustrates the importance of density and settlement patterns on the watershed—showing that per-capita rainwater impacts are significantly reduced as density increases. A per-acre analysis shows only part of the picture.

Figure 3. Pollutant loads per acre versus per capita (after John Jacob, Texas A&M)

Figure 3 illustrates a similar comparison for pollutant loadings: per-capita loadings reduce as urban density increases.

Analysis and design on the watershed scale facilitates the incorporation of this important per-capita-impacts factor.

Ecological Benchmarks
In the spirit of Ian McHarg’s 1968 groundbreaking book Design with Nature, a comprehensive aerial topographic mapping of the watersheds, combined with aerial photos and local GIS information and supplemented with field surveys and anecdotal evidence, provided a comprehensive layered base map and analysis (Figure 4).

Hydrology Analysis. The watersheds were analyzed hydrologically for the existing condition and also for the “natural cover” condition—which provide scientific benchmarks for comparison and potential emulation.

Figure 4. McHargian analysis conceptual illustrations

A hydrograph illustrates the rainwater runoff characteristics of a catchment area (Figure 5). The graph is plotted with runoff rate on the vertical axis and time on the horizontal axis. There are three basic characteristics to note with hydrographs: 1) runoff rate (Q) as measured in cubic feet per second; 2) volume (V) of runoff generated by the rainfall event, which is usually expressed in acre-feet and is represented graphically by the area under the curve; and 3) time of concentration (TC), the time at which the peak runoff rate occurs. In Figure 5, the lower line expresses the hydrograph for a naturally occurring catchment area, while the upper line represents the typical effects of conventional urbanization (adding impervious surface indiscriminately) as higher peak runoff rate, shorter time of concentration, and larger volume of runoff.

Figure 5. Hydrograph basics

The basic hydrologic metric for success for this case study was to develop a plan that would move the existing hydrograph toward the shape of the natural hydrograph over a period of 20 years or so, by mitigating impairments through exploitation of watershed opportunities. The shape of the hydrograph is expressed most basically by the Q, TC, and V parameters.

Impairments Versus Opportunities
The integrated cross-cutting McHargian analysis yielded a number of watershed impairments and opportunities. The lists were prioritized for impact and feasibility and matched up so that opportunities could be matched with, and address, impairments. Time horizons were designated for the opportunities so that immediate impact could be realized and long-term watershed restoration would occur. Both public and private impairments and opportunities were discovered.

Watershed Impairments. The ecological studies quantified a handful of key watershed impairments: buildings were being constructed in a flood hazard zone, downtown was subject to flash flooding, a portion of town had been built in a low-lying flood prone area, and the issues were escalating due to gradual urbanization without adequate stormwater regulations.

Sustainable Remedies Capitalize on Strategic Opportunities. Flood hazard zones were delineated to prevent the building of structures in a major drainage. A strategically located street (Hermit Lane) happened to have an unutilized 100-foot-wide right-of-way and a design was developed to convert this to a green street utilizing bioswales and rain gardens to create significant detention and retention capabilities within the existing right-of-way while incorporating street trees and pedestrian pathways to improve the overall streetscape. The reversion of a DOT stormwater diversion provided additional relief for downtown flooding issues. A green street design concept was developed for incremental upgrades of local streets. Rainwater regulations were drafted to address long-term goals.

Watershed Tuning
Watershed “tuning” consists of developing scenarios, testing their hydrological performance, and performing feedback loops on their implementation feasibility. The process is done iteratively until the project goals are realized.

Scenario Planning and Analysis. The scenario planning for this case study consisted of designing implementable scenarios, modeling hydrographs for those scenarios at various time horizons, and comparing them performatively with the benchmark ecological hydrograph.

Stormwater Management Zones Map. The rural-to-urban transect was used to integrate existing land-use zoning codes, existing land uses, and future land-use expectations into a stormwater management zones (SMZ) map (Figure 6). Hydrological values (coverage, density, etc.) were assigned to each of the SMZs, which parallel the rural-to-urban transect.

Figure 6. Stormwater management zones map

Rural-to-Urban Transect Integrates Key Watershed Principles
The rural-to-urban transect was developed by Duany Plater Zyberk and Company in the 1990s as a tool for understanding and integrating rural versus urban elements. Based on environmental transect theory, the rural-to-urban transect can be used to integrate regional scales down to the block size, reconcile several different zoning codes in one watershed, and provide a technique for developing context-sensitive designs and regulations. Let’s illustrate how this can work.

A natural environmental transect is a symbolic cut through nature in order to facilitate further study and analysis (Figure 7).

Figure 7. The natural transect

Patrick Geddes, biologist and urban planner, developed the Valley Section diagram in 1888, as an illustration of human interventions into the natural transect (Figure 8).

Figure 8. The human-use-of resources transect

The natural water cycle diagram (Figure 9) illustrates the precipitation, infiltration, runoff, and evapotranspiration processes that occur naturally.

Figure 9. The natural water cycle

Mankind, in its quest for survival, expresses major and minor interventions into the natural water cycle (Figure 10). Virtually all human works have an impact on the natural water cycle, while the natural water cycle has profound impacts on the civilization. Good solutions involve optimizing the positive traits while minimizing the negative traits of both natural and urban processes.

Figure 10. The human water cycle

The rural-to-urban transect can be used to integrate and resolve the conflicts between the natural water cycle and mankind’s interventions into nature (Figure 11).

Figure 11. Integration of natural water and human water cycles

Tuning of the Watershed Scenarios. Hydrographs were developed for the various scenarios and compared with the natural cover conditions of the watershed, then the regulating factors were adjusted to obtain an optimal long-term regulating scenario that would result in immediate and mid-term fixes for acute flooding conditions and a gradual improvement to the watershed as additional development occurs over the coming decades.

After a number of scenarios were analyzed iteratively, the resulting hydrographs showed an improvement that closely emulated the characteristics of the natural watershed. The example hydrograph (Figure 12) depicts the results for the 25-year rainfall event, showing the effects of the short-, mid-, and long-term implementations. The two-, five-, 10-, 50-, and 100-year rainfall event hydrographs showed similar characteristics.

Figure 12. Silver Cliff and Westcliffe hydrographs

Based on the hydrological conditions of the watershed, a simple relationship of “impervious area added= volume of onsite storage required” was developed for each SMZ. The relationships were based on incentivizing infill and higher density (which have a lower per-capita impact upon the watershed) through less stringent retention/storage requirements in higher SMZs, which were balanced with stiffer requirements in lower SMZs (Figure 13).

Figure 13. Transect-based rainwater regulations

The resulting regulations became a simple ratio for each SMZ. All one has to do, for infill or greenfield development, is look up the site on the SMZ map, refer to the storage volume/impervious surface ratio for that SMZ, and provide that storage volume by using a transect-appropriate Light Imprint technique. For more information on Light Imprint, see the article “Choosing a Green Infrastructure Framework?” in the March/April 2011 Stormwater or visit www.lightimprint.org. The Light Imprint matrix and handbook provide a number of transect-appropriate stormwater techniques, while the storage volume/impervious surface chart provides the regulating metric.

Implementation Steps
Short-term. The short term steps were:

1. A green street retrofit for Hermit Lane as a public works project (Figure 14)
2. Amending the Land Use Code stormwater regulations to prevent further building in floodplains, to adopt the SMZ map, and to implement transect-based rainwater regulations

Mid-Term. The mid-term step is to work with the COT to retrofit a diversion of runoff away from downtown.

Long-term. The new regulations require both public works and private developments to adhere to the new transect-based regulations, and to incrementally upgrade the streets and street frontages to the local green street pattern (Figure 15).

Figure 14. Hermit Lane cross-section

Figure 15. Local green street cross-section

Summary of Success
Community cooperation within a regional watershed, hydrological comparison with the natural cover condition of the watershed as an ecological benchmark, use of the rural-to-urban transect for context-sensitive regulation, and use of source-control BMPs such as Light Imprint result in implementable stormwater solutions for public and private works, and regulations that solve immediate flooding issues and promote the long-term health of the watershed while providing for smart urban planning.

Implementation Update. The towns received a Community Development Block Grant in 2010 to build the Hermit Lane green street retrofit, and design is underway, with construction to start in spring 2011.

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